Immunology Past Paper PDF - 09/12/2021

Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 IMMUNE MEMORY AND VACCINE - PT II Going back to the previous lesson, we’ve already discussed about herd immunity, but in this image, it is clear how viruses spread in vaccinated an...

Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 IMMUNE MEMORY AND VACCINE - PT II Going back to the previous lesson, we’ve already discussed about herd immunity, but in this image, it is clear how viruses spread in vaccinated and not vaccinated people. Of course, if the majority of the population is vaccinated, the virus is still spreading but in a minor way because the vaccinated ones are able to block it and not be more infectious for the others; in this way even the ones that are immunocompromise or immunodeficient can be protected by the vaccinated ones. TRIUMPHS AND DEFEATS OF VACCINES Several vaccines work to provide an effective protection against the infection (vaccines that are subminister in the pediatric age), against pathogen that usually elicit the acute inflammatory response and a strong memory response. Most of them, especially those against polio and smallpox, induce the eradication of the virus and so the vaccination is no longer necessary. Pathogens like Leishmania, Schistosoma or the parasites of Malaria, that in nature induce a more chronic response, are very difficult to fight with a vaccine: they have different cell lines cycles and different maturation stages; this is why if you design a vaccine against a protein that is present in the early stage of maturation, and that characterized the event of entrance in our body, the vaccine won’t be so effective because after entering our blood, pathogens will change their conformation. 1 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 Also the vaccines against flu and hepatis, which mutate a lot in their life, are very difficult to create. The flu vaccine, for example need an ex novo design each years because we have different strain of the virus; the presence of two hemispheres is helpful: the epidemy of flu starting in the other hemisphere during our summer, will allows us to create the new vaccine that we’ll use during the winter and vice versa. Vaccines are very important also for veterinary medicine against different pathogens. Sometimes there is a significant hostility against vaccinations mostly due to religious and ethical beliefs or to lack of information in the population. Calendar used in Italy: Most vaccines need several boosts to obtain the efficacy required. Other are needed to be injected during the pediatric age. First experimental against smallpox: people were afraid to become cows because the vaccine was creating starting from cow’s material. Vaccination is a typical act of preventive medicine and so it is administered when people are healthy in order to avoid hypothetical risk of contagion. An alterate perception of the risk-benefit ratio can make unacceptable the risk and discomfort associated to vaccination. The main problem is that people is not so able to evaluate the balance between risks and benefits. Vaccine means no disease, however, if the disease is not present we cannot appreciate the benefits of fighting the disease → if the disease is not present all the risk is on the vaccine itself. Unfortunately, the hesitancy lead people to avoid the vaccination and when the immunization rate drop, “it’s always the measles that comes back first”. “Measles is the canary in the coal mine” When people start to not vaccine the measles incidence grow up. 2 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 In this image we can see the percentage of people that think that the vaccines are safe in different countries of the world. - Red/brown: 30-39% of people think that the vaccines are safe → less than half of the population - Yellow: 90-100% of people think that vaccine are safe We can see that some years ago (in France and Ukraine) people stopped to take the vaccine. In this second image we can see the increase in measles cases: Most of the hesitancy is due to fake news, in fact people are not really informed. Fake news run very fast in social networks, in TV and this case, even in the scientific literature. An English physician (Andrew Jeremy Wakefield) pretend to demonstrate the correlation between the trivalent vaccine against measles, mumps and rubella, the inflammatory intestinal disease and autism. He published his study on “The Lancet”, which is an important medical journal, reporting that 12 children that have received the vaccine have developed autism and the intestinal inflammatory disease. As a consequence, all the scientific world was afraid that this could be true, so they performed different clinical trials. One big study was made in Denmark which demonstrates that Wakefield was completely wrong and that he manipulated the data → in fact, between the 12 children, 3 of them were not autistic and 5 were already autistic before the vaccination. The consequences of Wakefield were that parents were afraid to vaccinate children. 3 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 In this graph we can see that in 2004 the percentage of vaccinated dropped of more than 15%; as a consequence, at the same time there was an increase in the confirmed cases of measles. Another fake news is that vaccines contain toxic substances as adjuvant: - Formaldehyde: it is still present in a small amount in the tetanus and diphtheria vaccines. However, our body too produces a small quantity of formaldehyde. A 2 month child contains around 1.1 – 1.2 mg of formaldehyde in his blood. With the vaccine we take 0.1 mg of formaldehyde per dose → total 0.3 mg (we need three boots of the vaccine). Furthermore, a pear contains 60.000 of µg. - Mercury: it is no more present in any vaccine. People confused the ETIL mercury (not toxic) with the METIL mercury (toxic). - Aluminum: it is present in the water, in food, etc. Of course, there are very strict rules that establish that the water must not contain more than 0.2 mg of aluminum in a liter. In total we ingest around 700 mg of aluminum per year by drinking water. In the vaccine, the quantity of aluminum is 0.5 mg. It is very important to consider the total cost of the Health National Service: - The cost of drugs in 2015 was 28,9 billion of euro. - The cost of vaccine is only 318 million of euro - Three times more (923,3 million) was the cost of antiacids. - Sofosbuvir (a drug against hepatitis C) has a cost of 1,2 billion of euro. This table compare the cost of the vaccines against hepatitis B and C with the cost of the therapy to treat people infected by the hepatitis: It is obvious that the cost of the therapy is much higher. For people that just have the cirrhosis the cost always more than the one of vaccines. For the transplantation (often needed when there is the chronic hepatitis) the cost is around 90.000 euro followed by a cost of around 17.000 euro per year for the follow up. 4 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 Women and men react differently to the vaccine. In this table it is reported the presence of adverse events in male and female and the intensity of the response to different types of vaccines. Independently to the use of attenuated viruses, dead viruses or just proteins in different ages of the life, usually females respond greater than males. As a consequence, they also have a higher probability to develop and adverse reaction. Only in few cases there is a greater response in male. For example, it is greater the response in aged adults for the pneumococcal and tetanus vaccines. Why do females usually have a greater response to the vaccines? Many cells both in the innate and adaptive immune cells have the receptor for the sexual hormones. For example, the beta-estradiol, which has a receptor on the phagocytes, the dendritic cells, NK, T cells, B cells, activates the female phagocytes against the pathogen and increases the cytotoxicity on the NK cells. Low doses of the beta-estradiol increase the inflammatory cytokines, while high doses decrease it → the beta-estradiol act on the release of cytokines. Furthermore, the beta-estradiol induces a more proliferative rate. In female T cells and B cells. Usually, females have much more antibodies compared to males. 5 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 Another sexual hormone that has the receptor on the immune cells is progesterone. However, it has the opposite effect respect to the estradiol. For example, on the phagocytes (in particular on macrophages) it induces the differentiation of macrophages into M2 (the most important anti- inflammatory macrophages which switch off the inflammatory response) while it decreases their differentiation into M1. Furthermore, progesterone also decreases the production of pro-inflammatory cytokines and the cytotoxicity of the NK cells. The progesterone also affects the T cells differentiation because it upregulates the switch to the Th2 cells and regulatory T cells (Treg) while it decreases the switch into Th17. As for the androgen (testosterone) we know that they have too the receptors on the cells of the innate and adaptive systems. Usually, androgen gives a more anti-inflammatory profile; in fact, it down-modulates the expression of Toll Like receptors on phagocytes, it decreases the secretion of pro-inflammatory cytokines (TNF-alpha and IL1) while it increases a lot the secretion of anti- inflammatory cytokines (IL10 and TGF-beta). The androgen also influences the Th activation and differentiation. It has been demonstrated that men with low levels of testosterone have and increase in CD4 and CD8 → there is an experimental demonstration performed with the castration; in fact, mice that were castrated showed more CD4 and CD8 T cells in the blood (exactly as female) compared with the non-castrated mice. 6 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 Some of these features depend on the stage of the age → for example they can appear after the puberty. This is a confirmation that these features really depend on the presence of sexual hormones. Some features such as the up- regulation (abundance of the CD4 compared to the CD8) in the female, as we can see in this image, are present during all the women’s life → this means that also the presence of sexual chromosomes is important. Indeed, many of the molecules that are important for the activation of the immune response are on the X chromosome. Some examples are the Toll like receptor 7, CTLA4, HOXP3. The presence of the double chromosome in the women allow them to escape the silence (one of the two chromosomes in somatic cells is silent). As a consequence, we have a double amount of Toll Like receptors 7, CTLA4; this affects the behavior of the immune cells. https://video.corriere.it/cultura/vaccini-9-lezioniscienza-trailer-film-elisabetta-sgarbi/2d198992- 0e21-11ea-8033-a2d631aa9706 This link is for a docu-film of Elisabetta Sgarbi about the vaccines. THE CENTRAL AND PERIPHERAL TOLERANCE TOLERANCE = non-response to an antigen (self or non-self) which should induce an immune response. Tolerance is defined during the maturation of the adaptive system (T and B cells). However, we have also many mechanisms in the periphery, so in the secondary lymphoid organs, that prevent the activation of T and B cells against antigens. The antigens can be self or non-self → in both cases they induce tolerance. A central issue of adaptive immunity is the discrimination between antigens to be tolerated and protected and antigens to be destroyed. This discrimination (tolerance vs. immune aggression) is not genetically regulated. Instead, it is learned by the immune system. How is it possible that our immune system, especially the adaptive one, learns to discriminate what is self and what is non-self? 1. CENTRAL TOLERANCE The T and B cells, during the maturation, learn what is self from non-self, so what should be tolerated and what should not. A B and T cell encountering a cognate antigen during their differentiation in the bone marrow and thymus will undergo one of the various forms of tolerance. Any antigen interacting with developing T and B cells can be immunogenic or tolerogenic. This tolerance will take place equally towards genetically self and genetically foreign molecules. 7 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 Most of the antigen in the central lymphoid organs are self; in fact, in the thymus or in the bone marrow there are no foreign antigens. The immune system learns the molecular features of the body. During their maturation in the bone marrow, B CELLS that have generated a BCR interacting at high affinity with antigens expressed by bone marrow stromal cells may: - Re-edit their BCR (Receptor editing): they can recombenate the light chain using the variable segment that remain during the first recombination. By changing this variable region of the light chain they can vary a bit the BCR. With the editing of the receptor it is possible that the new receptor has less affinity for the self-antigen. - Die by apoptosis (Clonal deletion): all the clone are deleted and there is no possibility that some of the B cells exit from the bone marrow. - Become anergic: it means that the B cells are not responsive, even maintaining the same BCR. T CELLS arriving in the thymus from the bone marrow start to recombine the TCR genes. The TCR is then confronted with the membrane antigens expressed by thymic cells. A special transcription factor (AIRE) induces thymic cells to express several antigens of the body. A maturing T cell with a TCR interacting at high affinity with self HLA-p may: - Die by apoptosis (Clonal deletion) - Become a regulator (Treg cell) - Become anergic. The generation of TCR and BCR is a random process → we don’t know which specific antigen receptor is built from the precursors. Each of us eliminate from this immense repertoire T and B cells reacting against self (negative selection). This is called the black hole which, in the T and B cell repertoire, defines what is self and what is not-self. Because of the polymorphism and the different haplotype (different HLA class I and II molecules) each of us has a different repertoire that includes this black hole (we know that more than 85% of T and B cells are deleted with the negative selection). In the population it is important to have the biggest repertoire as possible to fight the appearance of new pathogens. The negative selection is different in each person because each of us has different HLA complex. NEGATIVE SELECTION OF SELF-REACTING B CELLS The mature B cells, with the IgM on the cell surface, are prone to apoptosis. Their death or survival depend on the binding of the BCR. The one that interact strongly with the self-antigen will die, while the other one will survive and exit. The clonal deletion has been also demonstrated thanks to experimental models. For example, as we can see in the image, we can use a mouse that is engineered to express a specific BCR/Ig against H2K (a class I molecule specific for the mouse). This mouse has most of the B cells?? (the white circles) with a BCR specific for these HLA molecules. - If we cross this mouse with a similar one which has not the H2K on the cell surface, it happens that all the progenies maintain most of the B cells able to recognize the HLA class I. 8 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 - Instead, if we cross this mouse with one that express the specific HLA class I molecules, the progenies have very few B cells with a BCR specific for H2K. This is because the B cells in the bone marrow of these mice can see the presence of the HLA class I molecules, so the one that bind with high affinity these molecules has been eliminated from the bone marrow. This was the proof of the clonal deletion based on the presence of the antigen. The clonal deletion is one of the two mechanisms to tach what is self and what is non-self to the B cell. The other one, as we said, is the editing of the receptor. For the soluble antigen released from the stromal bone marrow cells, instead, there is the third mechanism. In this case, the B cells with the BCR highly specific for the soluble antigen become anergic. As we can see in the image, becoming anergic means that they express less receptors and they cannot change the receptor (it has already been recombined). As a consequence, they do not have the amount of BCR necessary to get triggered by the antigen in the periphery; so, they remain anergic and non-responsive in exiting the bone marrow. This is due because, in this case the BCR, but also the TCR, is degraded by the proteasome after the ubiquitination. 9 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 We have different pathway, represented in the image, that lead to the activation of the Calcium channel, protein kinase c or map kinases which lead to the activation of NF-kB, NFAT and AP-1. Instead of the kinase usually recruited for the B cells in SYK, in the anergic B cell it is the phosphatase that is recruited to the receptor. These phosphatases are inhibiting all the three signalling pathways, so no more transcriptional factors reach the nucleus and induce the transcription of new genes. NEGATIVE SELECTION OF THE SELF-REACTIVE T CELLS The T cell make a travel starting from the cortex, then they go to the medulla in which they go through different steps of selection. During the maturation, the T cells, differently from the B cells, recombines the TCR, tests the beta-chain with a surrogate alpha-chain, proliferates and then starts the recombination of the alpha-chain. At this point, they pass the negative selection because if they bind with too much affinity they will die. After that, they go to the medulla where the APC and the medullary thymic epithelial cells have an important role in making the positive selection. So, the T cells go through two types of selection: the negative (such as the B cells) and the positive selection which is very important for the tolerance. The immature thymic T cells progenitor receive all the trigger signals for the expression of the TCR, they have different beta-chains. All the thymocytes that made a TCR unable to bind the histocompatibility antigens on the surface of thymus cells undergo apoptosis. This is also true for the B cells. Then, they complete the receptor and those that have a TCR binding with high affinity hystocompatibility molecules on the surface of thymus cells undergo apoptosis (mitochondria). The others become the regulatory T cells. The negative selection of the T cells depends on: - The presence of the antigen - The affinity with which the TCR binds the antigen The antigen for the positive selection in the thymus is presented especially by the medullary thymic epithelial cells (MTEC cells). These cells are also able to express proteins that are not specific for the thymus but are specific in our body. To provide a broader negative selection of potentially autoreactive T cells, thymic epithelial cells express the Auto Immune Regulator (AIRE) gene. This gene encodes a transcriptional factor enabling epithelial thymic cells to express proteins (ex. Insulin, troponin) that normally are expressed only by peculiar peripheral tissues. 10 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 In this way, they make possible the selection of the T cells that, if recognize with too much affinity the self-peptide presented by these cells, will be deleted. Why there is a more stringent control of T cell auto-reactivity compared to the B cells? Because the B cells that recognize the antigen with the BCR will be activated but they endocyte the complex. The antigen will be exposed to the MHC class II and it is necessary for the conjugation of B and T cells in the lymphoid organ. We have the activation of the B and the pre-activation of the T cells in different area. Then, the B cells primed from the antigen (antigen-primed B cells) need to physically encounter the T cells, so they express the peptide on the surface through the HLA class II. In this way they there can be the interaction between the CD40 on the B cell and the CD40 ligand on the T cell that allows the differentiation in memory or plasma cells. Without this interaction the B cells will undergo to apoptosis. This means that if a B cell is activated from a self-antigen, the lack of T cell in enough to avoid the full activation of the B cells → this is why the central tolerance is more stringent for the T cells compared with the B cells. 11 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 2. PERIPHERAL TOLERANCE Not all B cells that have generated a BCR reacting at high affinity with self molecules are eliminated or inactivated by the central control (Central tolerance) during cell maturation. Self-reactive mature B cells escaping central tolerance do not produce high affinity antibodies against self antigens (auto-antibodies) because of the Peripheral tolerance. In the B cells, the peripheral tolerance can be represented by: - The absence of auto-reactive T cells - The presence of regulatory T cells that can switch off the potential auto-reactive T and B cells. - The induction of an anergic response - Other negative control mechanisms induced by the cytokines. What happen for soluble antigens? The experimental model has to learn how and for how long the B cell anergy is reached. In this example we have a mouse which is transgenic for the lysozyme, so it expresses a lot of it. We cross this mouse with another which is transgenic for having a BCR specific for the lysozyme, so it produces a lot of anti-lysozyme antibodies. From this cross we obtain a mouse that expresses a lot of lysozymes and without the anti-lysozyme antibodies. To understand if this anergy (inability to respond to this antigen) was due to the B or T cells they took the B cells and they injected them in another mouse. Then they combined the T cells coming from a completely different mouse (they were normal T cells, not transgenic for the lysozyme). At this point, they irradiated this mouse in other to switch off the original B and T cells, they give to this mouse the lysozyme and they found again that no anti-lysozyme antibodies were seen. This means that the anergy is independent from the T cells and that the B cells anergy is more fast but less long lasting than the one of the T cells. In fact, they saw that giving the antigen after some weeks there was the production of the antibodies. This experiment is represented in the image below: 12 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 B cell anergy: - It implies a change of the level of IgM (not a change in the receptor). In the experiment of the mouse, by checking the B cells during the anergy we can find that they have very few IgM on the surface. - Tents to have a short duration - It requires continue presence of the antigen. If we stop for a while to give the antigen, the expression of the BCR starts again and the B cells lose the anergy The receptors CD22 or Fc-gamma-RIIB are the inhibitory receptors → when they bind the immunoglobulin they switch off the pathways, so the receptor needs again the phosphatase instead of the kinase. MATURE T CELL UNRESPONSIVNESS Usually, the T cells become effector and memory cells after the presentation which occur with a (??) dendritic cell, the one that express the co-stimulatory molecules. If this doesn’t occur, for example if the co-stimulatory molecule is missing, the dendritic cell is not so well activated by the antigen so it does not express the B7 on the cell surface. This dendritic cell is called tolerogenic. During the presentation the T cells can also be inhibited thanks to the presence of regulatory T cells. The third mechanism to inhibit the T cells is the deletion, this can occur during the chronic infection. The continue presence of the antigen can exhaust the T cells leading the cell death. ANERGY When a virgin T and B cell is stimulated by the antigen in the absence of costimulatory signals, the lymphocyte becomes “anergic”. Anergic cells are alive in circulation but are unable to mount an immune response against their target antigen even when the antigen is re-presented along with the correct costimulatory signals. The functional unresponsiveness of anergic cells depends on a blockade of the receptor signalling pathway and requires a continuous presence of the antigen. Phosphatases are recruited at the TCR or its degradation is increased; ubiquitine ligases are increased and therefore its proteasomal degradation. Otherwise anergy may be lost or overcome by special cytokine combinations. If we stimulate the anergic cells with a cocktail of cytokines, after a while they restart to proliferate and differentiate. 13 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 The anergic T cells are long lasting compared to the B cells but the anergy starts a bit later. Anergic cells can survive for long periods or die by apoptosis. INHIBITION/DELETION Mature T cell inhibition can also be caused to the recruitment of inhibitory receptors (which are called immune checkpoint) belonging to the CD28 family: - CTLA4 (Cytotoxic T Lymphocyte-associated Antigen 4) - PD1 (Programmed Cell Death 1) These receptors which off the activation, so the T cells are no more able to perform their function. Mature T cell can be inhibited by the presence of antigen-specific Treg IMMUNE CHECK POINTS CTLA-4 and PD-1 are two key cell-surface coreceptors that, when bound by their ligands, trigger inhibitory pathways and dampen T-cell activity. CTLA-4 (Cytotoxic T-lymphocyte-associated protein 4, CD152) is a T cell receptor that interacts with B7 ligands (B7.1/CD80; B7.2/CD86) with a much higher affinity than CD28. The CD28 is present during the activation of the T cells. After the activation, the cells express the CTLA-4; this is a way to physiologically decrease the activation of the T cells because, as we said, the CTLA-4 binds with high affinity the B7 before that this one is bound by the CD28. Defective CTLA-4 signal transduction is associated with insulindependent diabetes mellitus, Graves’ disease, Hashimoto’s thyroiditis, celiac disease, systemic lupus erythematosus and other autoimmune diseases. PD1 (Programmed cell death 1, CD279) is a receptor of the CD28 family that interacts with two different ligands: PD-L1 and PD-L2 (Programmed death ligand 1 and 2, CD274) and B7.1 (CD80). PD1 – PD-L1 interaction plays a major role in suppressing T cell response to self antigens and during pregnancy. The PD-1 is more important for switching of the T cells in the periphery, while the CTLA-4 is important in the lymphoid organ to switch off the activation → this is due to the fact that during the T cell activation, B7 is present in the APCs, it reacts with the CD28 and then the T cell starts to express the CTLA4 and the PD-1. Both receptors have an important role in inhibiting autoimmune activation of T cells. As we can see in the image, the CTLA4 has two different mechanisms to switch off the activation: - Intrinsic mechanism: the CTLA4 win the competition with the CD28 to bind the B7 and it switch off the activation - Extrinsic mechanism: it is due to the high presence of CTLA4 on the regulatory T cells. The Treg may are important in switching off and maintaining the physiological tolerance for the self-antigen in the periphery. The Treg constitutively express the CTLA4 so they win the competition compared to the naïve T cells that have the CD28 (it binds the B7 with less affinity). - The CTLA4, after binding the B7, is inducing the internalization of itself bound with the B7. So, the APCs have less B7 available for the normal autoreactive T cells. 14 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 In this picture we can see in the middle the normal T cell which express PD-1, the inhibition due to the presence of the PD-L1 on the APCs and the tumour cells. The inhibition of the T cell activation by the PD-1 is one of the mechanisms that the tumour cells act to escape the immunosurveillance from the T cells. In this other picture we can see the T cells with the CTLA4 and the presence of B7 on the APCs. People that won the Nobel price for the CTLA4 and PD-1 were developing antibodies against the CTLA4. The presence of the antibody inhibits the PD-L1 and the CTLA4. 15 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 REGULATORY T CELLS Two types: - Native Treg cells - Induced Treg cells 1. NATURAL Treg CELLS Natural Treg cells: If a T cell maturing in the thymus generates a TCR interacting at high affinity with self HLA molecules presenting self peptides the cell may undergo apoptosis or become a natural Treg cell. Natural Treg cells are CD4+, CD25+, and Foxp3. They interact at high affinity with self HLA molecules presenting self peptides. Once activated natural Treg cells suppress the activity of other surrounding T cells by releasing TGF-b and IL-10. In this way, Treg cells limit the extent of immune reactions and prevent autoimmunity. FoxP3 is indispensable for developing and functioning of most Treg. Its absence determines autoimmunity in experimental models and human IPEX (Immune Dysregulation, Polyendocrinopathy, Enteropathy, X-linked) is caused by mutation in FoxP3 gene. Treg express high levels of IL2Ra, therefore rely on IL2 and not IL7 to survive, and high levels of CTLA-4 These regulatory T cells coming from the thymus are activated as all the other T cells and they can expand. Then they inhibit the naïve T cells activation (through the CTLA4), the expansion of effector T cells and the B cells and NK cells. 16 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 ROLE OF IL2 IN MAINTAINING REGULATORY T CELLS The image below represents what we anticipated before: the competition for IL2 We can see a T cell and a regulatory T cell which can be either autoreactive or not. When these cells encounter the peptide on the APCs (in this case dendritic cells), the Treg expand faster because they use the IL2 produced by the normal T cells and they can inhibit the activation of the effector cells. Mice knock out for gene coding the IL2Ra or b chain develop an autoimmune syndrome like mice that do not have the FoxP3 (inflammatory bowel disease, autoimmune hemolytic anemia and autoantibody production). They do not have FoxP3 Treg. 2. INDUCED Treg CELLS Differentiation of Effector Th cells The peculiar set of cytokines secreted by the APC presenting the HLA-p and the distinctive molecules expressed on their cell membrane guide virgin Th0 cell activation and its polarization towards Th1,Th2, Th17, T follicular helper (TFh) cell, induced Treg… The repertoire of cytokines released by an APC is dictated by the kind of antigen they have taken up. An APC displaying antigen peptides along with the secretion of a high amount of Transforming Growth Factor-b (TGF-b) in the absence of IL-6 triggers the differentiation of a virgin CD4+ T0 cell towards an Induced CD4+ Treg cell (CD4+, CD25+, and Foxp3+). They can be specific for self as well not-self antigens. Induced Treg cells may trigger the production of indoleamine dioxygenase (IDO) by APC. IDO breaks down tryptophan, and the lack of tryptophan is another way to block the activation of the lymphocytes present in the micro-environment. IDO-DC in turn trigger the induction of Treg. As we can see in the image, in the lymphoid organ, after the presentation, there is the induction of the FOXP3 (regulatory T cells). These cells, then inhibit the activation of T cells and other B and innate cells. The absence of TGFbeta (released by the dendritic cells) in mice leads to a decrease of Treg and development of an inflammatory systemic disease. 17 Michelle Guichardaz / Lucia Griva – Lesson 18 Immunology (Prof.ssa Cappello) – 09/12/2021 TRANSFORMING GROWTH FACTOR-b: A MULTIFUNCTIONAL REGULATORY CYTOKINE IL10: A SUPPRESSOR CYTOKINE ROLE OF Treg IN SELF TOLERANCE AND AUTOIMMUNITY Treg can be expanded in vitro and re-infused into the patients Are ongoing clinical trials to assess the use of Treg in limiting or avoiding the transplant rejection, graft versus host disease (GVHD), autoimmune diseases (bowel disease, diabetes type I, multiple sclerosis, etc..), and inflammatory diseases. Other trials are trying to induce in vivo this population through the administration of IL2. Dose and route ideal for Treg induction. 18 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 TOLERANCE PART 2 IMMUNO-PRIVILEGED SITES Immuno-privileged sites are very important organs for our own life, but also for population survival. Some examples are the eye, the testis and the central nervous system, especially brain. They are called immuno-privileged sites because they are protected by the immune system, whose response is usually induced to protect our cells by invaders and by the damage tissue that can lead to the loss of function of many organs. Sometimes the immune system can be harmful for our normal cells because the mechanism for killing the infected cells can also kill the normal cells. In some kinds of organs this would be a mess, for example a Th1 response against neurons means loss of neuronal function and consequently loss of other functions. In these protected organs, the immune response is usually deviated to a less harmful one. For example, if there is an aggressive immune response against the retina it causes loss of vision; therefore, in these organs the Th1 response (usually induced by pathogens) is deviated to a Th2 response. So instead of having a cytotoxic cell directly killing the cells, an immune response is made prevalent by the humoral response, so the production of antibodies, which need the involvement of the complement or other kinds of innate cells (NK, neutrophils, macrophages and so on). This is possible because many of these tissues are populated by specific dendritic cells. The immature Langerhans cells that we have in the skin that are very powerful, like antigen presenting cells which can be licenced by the CD4 Th1 cells to cross present the antigen to the CD4, and therefore make the differentiation of cytotoxic T cells. On the other hand, there are more tolerogenic dendritic cells. The placenta is one of the tissues very rich in Th2 cytokines, because, in a specific cycle of life when the placenta surrounds the foetus, the maternal immune response against the new-born is dampened from the Th2 response. The Th2 response, thanks to the effect of the most relevant cytokine, the IFN-γ, causes the activation of macrophages and NK cells. IMMUNE IGNORANCE In some cases, antigens are so little represented on the groove of MHC class I and II molecules, that they are not able to efficiently activate the T cells, because they are not able to trigger the TCRs. In order to activate the T cell, hundreds of thousands of molecules of TCR on the surface need to be triggered simultaneously and activated by a piston movement. If the complex antigen-MHC class I or class II is little represented, it’s not able to trigger this process efficiently. This does not mean that the MHC molecules are not expressed; they are on the surface of the antigen presenting cell as well as the other tissue cells. It’s not a defect related to the presence of MHC class I or II molecules, but it’s the presence of the peptide related to a certain antigen that is too few represented. When a cell is infected by a virus, among all the peptides and self-peptides presented on the cell surface, the ones coming from the virus are more represented, therefore they are able to trigger the TCR and activate the T cells. Ignorance means that the antigen is few represented in our cells. This is enough to maintain the tolerance. For example, even if some autoreactive T cells exit the thymus, because the antigen is little represented on the MHC class I or II, it doesn’t activate these autoreactive T cells. This is another mechanism to maintain tolerance. 1 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 EXPERIMENTAL TOLERANCE All the mechanisms which have been discussed can be exploited to induce a tolerance used as a therapeutic approach. For many allergic atopic people or people with autoimmune diseases, it’s possible to try to induce the tolerance against a specific antigen. Many of the aspects about the mechanism of the tolerance, which have been discussed in the last lesson, come from the experimental tolerance. Tolerance does not mean suppression, because tolerance is antigen specific. Therefore, tolerance can be insulin specific, troponin specific, etc. On the other hand, suppression is a general condition in which the immune system is not responsive. This means that if more cytokines are administered, the suppression is gone and the autoreactive T cells or the cells recognizing the allergens can again mount a normal response. It has been demonstrated that almost any kind of protein can be a conventional antigen. This means that if a mouse is injected with an antigen there is a certain kind of response, the primary response, that can be both cellular and humoral. If the mouse is injected with the same antigen a second time, it is able to induce the secondary response, which is faster and more intense than the first one. The same antigen can be transformed in a tolerogenic antigen (also called tolerogen), in order to impair the primary, but also the secondary response. If an antigen “A” which was able to induce the normal response is modified to cause a tolerogenic one, when a mouse is injected the first time there is no antibody response or T cell activation. If a mouse is injected a second time, there is still no response. When macrophages, but especially dendritic cells because they are the professional antigen presenting cells, are not efficiently triggered by the antigen, they cannot be so efficient in the presentation and activation of T cells. How is it possible to transform the antigen into a tolerogenic one? By changing the chemical or structural features of the protein. For example, when big proteins are made by L amino acids, they are more immunogenic, while if the same polymer is synthesized with D amino acids, the result is a tolerogenic protein. Glycosylation. Glycosylation is necessary to the stabilization of the protein. When a protein is more stable it’s more immunogenic. Solubility. Size. It’s not well understood why, but if a molecule is too small, it’s not able to induce the immune response; probably because the dendritic cells during the phagocytosis are usually triggered by big complexes, big molecules, or the entire pathogen. Whether or not the antigen is in a complex with the antibodies. The complex is more immunogenic than the soluble one. This happens because immunocomplex can be 2 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 recognized by the FCγ receptor on the innate cells. This triggers a signal inside, inducing degranulation, release of cytokines and so on. Dose. This is also connected with the vaccine procedure and approach, because for each antigen a low-zone tolerance and a high-zone tolerance have described. In this experiment, some animals have been injected with increasing doses of the same antigen (starting with 0 and finishing with 100 μg), and then boosted again with the most powerful dose (1 μg). 1 μg has been chosen for the boost because it represents the peak in antibody production against the specific antigen. It means that very few doses of the antigen are not able to induce the humoral response (low tolerance zone), while around 1 – 10 μg the peak of production is reached, then the production declines again (high tolerance zone). So even the dose of the antigen that you can use is important for the induction or not of the tolerance against that antigen. It’s important to decide if the antigen will be an immunogenic one or a tolerogenic one. It’s possible to transfer the tolerogenic T or B cell from one mouse to the other. In this case we should keep in mind that tolerant T cells are immediately induced by the tolerogenic antigen and are long lasting too. Therefore, a couple of hours after the injection and the induction of tolerant T cells is enough to create a clone of tolerogenic T cells that can be transferred in another animal. Said animal will be able to be injected with the same antigen without having any response or activation. They are also long lasting, so they can persist in the host even for 1 year and a half. On the other hand, B cells are a bit slower in the induction of the tolerance, because they have to feel all the BCR before. They are also less persisting in the body because after a couple of months if the same antigen is injected, there is a normal activation of the B cells. 3 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 HYPERSENSITIVITY REACTIONS Hypersensitivity reactions (as the name suggests) are a sort of secondary exacerbated immune response. They hypersensitivity reaction consists of 2 phases: the sensitization phase and the effector phase. “Sensitization phase” means primary immune response, in which the T cell or B cell have to recognize the antigen, activate and differentiate in memory cells. During the secondary response the memory cells will react faster and produce much more antibodies or kill the cells. In some cases, some kinds of antigens that for many people are harmless (so normal proteins entering our body but not inducing any kind of response), for some individuals become very harmful and induce this exacerbated secondary response. Some of these hypersensitivity reactions are the ones involved in allergies, atopic reactions, transplantation rejections, or even autoimmune diseases. They are normal immune reactions, they need the presentation of the antigen, the differentiation in effector cells and memory cells and the activation of memory cells. What changes is usually the antigen, which can be an environmental antigen (usually harmless), or worse, a self-antigen. They constitute like normal immune reactions in effector mechanisms, which can be related to antibody production or activation of T cells. The type of antigen can be cell associated (usually these are the self-antigens) or soluble (usually other antigens like allergen). All types of this reaction always consist of 2 phases: sensitization phase (primary response) and effector phase (secondary response). Hypersensitivity reactions are divided in 4 different types, which depend on - the effector - the immunological reactant: what is inducing this type of reaction - the antigen - the effector phase: the mechanism by which this kind of reaction induces the damage of our cells. The following table shows the main differences between each type. 4 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 TYPE I HYPERSENSITIVITY The type I hypersensitivity reactions are induced by the presence of IgE. IgE are usually very few in circulation, they have a short half-life and at least they do not interact with FCε receptor on the mast cells or basophils. The IgE that are bound to the mast cells can stay for months in the body and be triggered any time due to the presence of the specific antigen. Usually the antigen that induced the IgE is a soluble antigen called allergen, because 40% of the allergy and atopic reactions (both systemic, like anaphylactic shock or local, like rhinitis) are induced by this kind of immunoglobulin. This is because the IgE activate mast cells and basophils. a. Sensitization phase In this image, an antigen passes through a physical barrier (skin, airway mucosa, eye mucosa…) and it’s taken out by dendritic cells. After phagocytosis, these dendritic cells will move into the lymphoid organ and there they present the antigen to the Th2. The differentiation of Th2 is induced, but before some T cells are activated, which will recognize this antigen. The expanded Th2 can reach the site in which the allergen has entered through the blood. Here they recognize the antigen again (which is expressed by resident dendritic cells or even some B cells) and so a lot of Th2 cytokines are released. In B cells the production IgE is induced. The IgE locally find the mast cells, because mastocytes are the most abundant residential phagocytes in our tissues. Basically, during the sensitization phase, some of the B cells will release the antigen or reach the bone marrow where they produced the IgE. Therefore, the mast cells and basophils circulating in the blood, that can encounter the IgE, will remained armed. They can be considered armed and trigger to shoot all their content. 5 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 b. Effector phase In the type I hypersensitivity reaction we recognize 2 effector phases: the immediate phase (which occurs within 30 minutes of the entrance of the allergen) and then the late phase. During the immediate phase the triggered and armed mastocytes form the sensitization phase, encountering again the antigen. Considering pollen, every day allergic people encounter the same antigen for some days, therefore the mastocytes in the tissue will encounter very easily the antigen and they will degranulate. They degranulate all the content in the pre-formed granules (histamine, serotonin, immediate mediators of the inflammatory response). The consequence is an increased permeability, especially of the vessels in the area, which will cause leakage of the liquid outside and local swelling. 6 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 During the late phase, the mast cells that have been activated from the antigen start to synthesize ex novo some other chemoattractant (like chemokines, prostaglandins, leukotrienes), which will induce the recruitment of other innate cells. In the late phase the inflammatory response is enhanced, and with the amplification of the innate cell activity, the tissue will be exaggeratedly injured. All the common allergy symptoms (itching, coughing, asthma, diarrhoea, vomiting…) are due to all the steps of the inflammatory response: - Increased vascular permeability - Vasodilatation - Smooth muscle contraction: bronchi and viscera (peristalsis) - Local inflammation: recruitment of circulating eosinophils and neutrophils Different symptoms are based on different antigens or different tissues triggered by the antigen. The hypersensitivity reaction can also be systemic, in which the cytokines produced by the innate cells cause the obstruction of the airways by the mucus. Consequently, beside the airway obstruction, there is also the collapse of the heart, so it’s important to immediately have the drug for the compensation. The following table shows different syndromes that are induced by hypersensitivity reactions type I. 7 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 What makes the antigen an allergen? Usually these antigens are not an antigens for many of us, because they enter our body without inducing the activation of dendritic cells, so they don’t properly induce the maturation of antigen presenting cells, because they don’t activate PAMPs or DAMPs. The proteins don’t cause activation because they are processed through the lysosome or the proteasomal pathway: they are not presenting properly to the T cell during the sensitization phase. Co-stimulation is necessary for the proper activation. If dendritic cells are not triggered by PAMPs, they are tolerogenic. This has happened in many of us for almost all the proteins. We have 12.000 proteins and only about 200 families of proteins can be allergenic for someone. So most of them are presented, but not seen as harmful from our immune system. What makes an individual atopic? Allergic means that a person is reactive against a specific allergen (eggs, pollen, etc) Atopic is a person who has the same symptoms for allergies, but against an unknown antigen It means that many kinds of protein can induce this kind of reaction. An individual can be atopic due to many factors: Genetic factors It has been demonstrated that 50% of the children with both allergic parents are also allergic, compared to the 19% of the children born form not allergic people. It means that the genetic component is important. At the same time, studies on enrolling homozygous twins have demonstrated that not always both twins are allergic and not always they are allergic for the same antigen. So genetic factors are not sufficient to induce atopies or other allergies. Many of the genes involved belong to the MHC complex (HLA), but especially to the Th2 cytokines. For example, some polymorphisms (or SNPs), in the IL4 receptor or the IL13 receptor predispose to some allergies. The same goes for some PRR genes. Especially Toll Like receptor because usually the allergens do not trigger the response, so they don’t induce the efficient presentation. In some individuals the hyperactivation of the Toll Like receptor 2 or 4 induces the response to the allergens too. In some cases, the allergen is able to trigger the Toll Like receptor, like the lipoproteins do. Environmental factors Environmental factors are important to predispose the individual to allergies or atopies. For example, one of the hypotheses is that excessive hygiene could cause allergies, because already at the beginning of the nineteenth century allergies were described as rich people diseases. Usually in poor developed countries allergic people are very infrequent, compared to high income countries. 8 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 This is probably because, during the first year of life, babies from richer families are protected from pathogens. Usually, they are vaccinated and even protected by mothers that clean and sterilize everything that comes in contact with them. Families are also smaller, there aren’t many siblings in the same family that are passing the spoon or any kind of toys from mouth to mouth. Many kids also aren’t used to frequent kindergarten, so it’s the same as not being contact with other people. Immunologically speaking, we are not teaching to our system to recognize the pathogens that usually induces the Th1 response. At birth babies already have more Th2 (because in the placenta they have Th2 cytokines). If children are too protected and not exposed to certain pathogens (not so dangerous ones), they will not learn how to induce the Th1 response. It seems that the immune system remains more prevalent on a Th2 response. The same is also because houses are much more insulated, so there’s no exchange with the air outside, so there’s no contact with pathogens or pollution. Trigger factors: necessary for the reaction. The 2 previous factors are not enough to explain the appearance or the susceptibility to allergies. Sometimes the hypersensitivity reactions are triggered by other events, like stress events, chronic inflammation, or pathogen exposure. Some autoimmune diseases increase after an infection. Even hormone fluctuation or the deficit in nutrition can explain different predisposition in the immune system to induce a Th2 response compared to a Th1. So, it explains the activation of this kind of reaction. The following table contains some genes related to predisposition. THERAPIES 1) The conventional therapies (mostly used) are represented by antihistamines or bronchodilators, or corticosteroids, which inhibit the inflammatory response. The antihistamine acts for the early phase, the inhibition of prostaglandins and other inflammatory mediators act for the late phase. 2) Another type of therapy is represented by hypo-sensitization, which is based on the experimental tolerance. Very young people (usually between 5 and 9 years old) are injected for years with the potential allergen at low does, in order to induce the tolerance against this antigen. 9 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 It’s a procedure which needs to be performed in the hospital, so under the physician control, because the allergen can also induce anaphylaxis, so it’s important that it’s administered under strict control. This is based on experimental tolerization. 3) Lastly, there are immunotherapies which are based on the use of different antibodies (monoclonal antibodies) against Th2 cytokines. The most targeted cytokines are IL4, IL5, (which induce the isotypic switch in these cells, so they are the promoters of IgE production), TNF-α, (released by mast cells to recruit and amplify late effector phase). These therapies work to inhibit or lower the symptoms, but they can’t cure the allergies. Only hypo- sensitization can do that. TYPE II HYPERSENSITIVITY Type II hypersensitivity reaction is mediated by the presence of IgG. Hypersensitivities are not induced by the normal level of IgG in our circulation, it’s induced by the abundance of them. This provokes the exaggerated response against the surface proteins, for example some of the thrombocytopenia is induced by this kind of reaction. Some autoimmune diseases against the normal surface proteins are induced by the hypersensitivity type II, because the binding of the IgG on the surface of the cells induces the activation of the phagocytosis by macrophages or the antibody dependent cellular cytotoxicity by the NK. Below are listed some pathologies caused by this type of hypersensitivity During a normal reaction, when the antibodies are bound to the red blood cells, they are transported in the circulation to the liver or the spleen, where the Kupffer cells or the macrophages then degrade these antibodies and also all the red cells. On the other hand, something different happens when an antigen expressed on the epithelial cells is recognized by the antibody. The binding of antibody induces the slow activation of the complement, but since IgG are not so abundant, the alternative pathway will be activated and not the classical one (which needs a lot of IgG to bind the C1Q). 10 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 In this case the C3 is exposing the thioester bond, so it can bind the IgG present on the cell surface. Consequently, it will activate for example the neutrophils to recognize the antibody, the C3B bound to the antibody, or directly the NK cells. In this case the degranulation of phagocytes is triggered by the FCγ receptor, or the complement receptor and NK activation. This can happen in any kind of tissue (kidney, gut, intestine, lung, everywhere). The following are some reactions to type II hypersensitivity - Autoimmune disease elicited by antibodies against tissue antigen - Goodpasture Syndrome (nephritis) - Reaction against blood transfusion (antibodies against blood groups) - Reactions against Rh antigen (foetal erythroblastosis) - Haemolytic diseases - Hyperacute rejection TYPE III HYPERSENSITIVITY Type III hypersensitivity is characterized by abundant IgG as well, but they bind a soluble antigen. Therefore, the prevalent activation is the complement activation and not the NK activation as before. The Latex theory explains that, based on the concentration of an antigen and antibody, there can be small complexes, which are innocuous, or bigger immunocomplexes. 11 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 They activate the complement, so usually, because they are circulating, they destroy the injury of the endothelial cells. These immunocomplexes, only when they are very big, can activate the complement and destroy endothelial cells. This happens especially in the capillary, where the calibre of the vessel is very small: these immunocomplexes are practically stuck in the capillary. The disruption of the endothelial layer allows the entrance of the immunocomplex in the interstitial tissue: this activates more complement which will induce the formation of Mac, because the cells are adherent to each other. So, it’s not only inducing the C3B, but it allows the formation of the effector Mac and so it destroys the cells. This activation of the complement also induces the release of the anaphylatoxins, so C3A or C5A, which are very chemoattractant for phagocytes, so they will recruit in the site of injure. Because of the activation against tissue cells, the phagocytes will encounter the “frustrated” phagocytosis. It means that they will try to phagocyte the cells on which they recognize the antibodies or the complement, but because they are in tissue cells, they will start degranulating or induce oxidative burst causing death of other tissue cells. TYPE IV HYPERSENSITIVITY Type IV hypersensitivity reaction is called “delay type hypersensitivity”, because it occurs 2-3 days after the encounter of the antigen. This happens because this reaction is mediated by T cells. which need at least 1 day to react against the recognized antigen. 12 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 It can be triggered by surface or soluble antigens and induce both T helpers and cytotoxic T cells. These cells are then responsible for a few pathologies: - Contact dermatitis - Chronic rejection of the transplant - Formation of granulomas - Hypersensitivity pneumonitis. The hypersensitivity type 4 reaction can be induced, for example, by persistent pathogens., which induce granuloma formation. Even some persisting non-infectious chemical agents that are not catabolized by our cells, so they induce the hyperactivation of T cells and then innate cells (e.g. beryllium, silica or carbon). In the presence of some very persisting pathogens or chemical reactants, the antigen presenting cells try to take out these threats. However, they are difficult to be degraded by the phagocytes, so these will present the antigen to the memory T cells that are circulating. In this case knockoff stimulatory signals are mandatory, so the TDTH will expand into the tissue. The production of IFNγ (but also other cytokines like TNF or chemokines) induce the recruitment and then the activation of macrophages that reach the tissues or become hyperactivated, because they start to phagocyte the antigen. Then they try to clear as usual, but they cannot, so the presence of TPH and these macrophages will induce the formation of the granuloma. The pathogen or the chemical reactant will be isolated from the tissues to preserve the tissue cells. These hyperactivating macrophages into the granuloma are also changing their shape, because they fuse together, so they become foamy cells because of the production and the release of the granules: this causes death of some tissue cells. The fibroblasts into the tissue will build a sort of capsule around this granuloma in order to avoid the destruction of all the tissue around. This is quite common after the mycobacterial infection: X-Rays show dots (granulomas) in the lungs of infected people. The formation of the granuloma is the hypersensitivity reaction in response to these kinds of pathogens induced by the Th1 specific T cell. 13 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 In other types of delay type hypersensitivity reactions, the main actor can be cytotoxic T cells. Some substance may change the protein expressed by our skin (in this case, poisoning of a nerve). Some poison is chemically binding our surface proteins, so a neo-antigen is created. The CTL, that has been triggered as usual by the Langerhans cells, becomes activated and it recognizes this neo-antigen on our keratinocytes. Therefore, the CTL will start to degranulate and destroy these keratinocytes and provoke the contact dermatitis which entails dry skin or scratch lesions because of the death of the keratinocytes. All these reactions will also amplify or start the innate reaction, because the disruption of the keratinocytes will release DAMPS and the CTL will release chemokines: both will activate innate cells. Usually, the contact dermatitis causes symptoms for a few days, until the inflammatory response is switched off (thanks to corticosteroids or atopic gel, for example). The following table shows some syndromes related to type IV hypersensitivity. 14 Lucia Griva/Francesco Grossi – Lesson 19 Immunology (Prof.ssa Cappello) – 14/12/2021 The DTH response is usually exploited to see if a person has been in contact with the mycobacteria or vaccinated. A test is performed, in which the tuberculin (which is one of the substances released by mycobacteria) is administered intra-dermally. If people have never been in contact with mycobacteria, they don’t have the memory lymphocytes recognizing the tuberculin, so the physician will just see the dot of the syringe. On the other hand, if a person has come in contact with mycobacteria, they have a lot of memory T cells that become activated with the injection of tuberculin, so it will induce the disruption of keratinocytes in the site of injection. This will induce the big inflammatory response, recognized as the presence of memory T cells. Type IV hypersensitivity is induced especially by cytokines release (by Th1 or Th2) or even by CTL response, which induces the endothelial cell activation (swelling in the contact dermatitis). 15