Cancer and Transplant Immunology ILA 2024 PDF

Objectives Explain the concept of immune surveillance and the evidence that supports the immune system’s role in cancer suppression. Describe how antigenic variation occurs in cancer cells and how this is used by the host immune system to target cancer cells. Starting with APC presentation of cancer...

Objectives Explain the concept of immune surveillance and the evidence that supports the immune system’s role in cancer suppression. Describe how antigenic variation occurs in cancer cells and how this is used by the host immune system to target cancer cells. Starting with APC presentation of cancer antigens, explain the steps required for lymphocyte activation against cancer cells in both sterile and non-sterile tumors. List the common ways cancer cells evade and suppress the host immune system. Explain why MHC must be matched closely to ensure the best chance of a transplant being accepted. Describe the three major types of transplant rejection. While it might seem odd to cover transplant and cancer immunology in the same ILA, they provide an interesting contrast to one another. Recall that the immune system must distinguish between self and non-self. If something is not you (referred to as non-self such as food, cosmetics, drugs, etc.) the immune system must decide if it warrants a strong immunological response or not. If your immune system responds to a non-self antigen excessively, then that can cause unnecessary, severe morbidity (allergies). However, excessive responses even to threats, like pathogens, can cause severe tissue damage to the point that it can endanger a patient’s life. Clinical Correlate: Acute respiratory distress syndrome and sepsis is a consequence of massive cytokine production initiated by the immune system. But are there instances where your immune system should attack your own cells? With cancer, cells of your own body have accumulated mutations leading to hyper proliferation, invasion into flanking tissue, and sometimes metastasis. In this instance, the immune system plays a critical role in preventing and slowing the progress of cancer. Immune Surveillance So what is the evidence that the immune system prevents cancer? The concept of immune surveillance is that the host immune system is constantly searching for cancerous and pre-cancerous cells and eliminating them prior to it becoming an issue. However, even seemly healthy, immunocompetent patients get cancer, so like most systems, this one is not failproof. Here is some of the evidence for immune surveillance: Figure 2: Common evidence for immune surveillance. In humans, lymphocytes infiltrate tumors and the draining lymph nodes surrounding the tumors are frequently enlarged, indicating a focused response against the tumor. Indeed, lymphocytes harvested from patients with cancer have TCR (T cells) and antibodies (B cells) specific for tumor-related antigens. As highlighted in the congenital immunodeficiency lecture, patients with immunosuppression have an increased risk of cancer, indicating the critical role the immune system plays in cancer prevention. Also, stimulating the immune system by blocking inhibitory receptors, like PD-1 and CTLA-4, leads to regression in some cancer types. So if the immune system is responding to cancer, why can’t it cure you? Unfortunately, cancer cells have access to your entire playbook (genome). Mutations in cancer cells allow them to obtain immunosuppressive and immunoevasive functions. Tumors produce immunosuppressive cytokines (IL10, TGF-ß) and recruit regulatory cells, like Treg, to tumor sites to suppress the immune system. In essence, cancer turns your own immune system against itself. Once established, even in a healthy patient, cancer is very difficult for the host immune system to deal with. Figure 3: Patients born with or who acquired immunodeficiencies are at a higher risk for certain types of cancer. After transplantation, immunosuppressants are given to prevent rejection of the transplant. The drawback is that this immunosuppression causes an increased risk of cancer, highlighting the immune system’s role in preventing cancer. The Immune System’s Defense Against Cancer So how does the immune system recognize cancer cells as foreign and attack them if they are so similar to your own cells? Due to mutations, cancer cells express proteins that are structurally distinct from host cells. Recall that DNA sequences determine a protein’s sequence and structure. If substantial DNA damage occurs, then the affected alleles will produce altered proteins that the immune system have not been tolerated to via positive and negative selection. In addition, proteins may not necessarily be altered in structure but in expression. Overexpression of host proteins can also provide a target for the immune system as well. Figure 4: Various ways in which cancer alters protein structure and expression, which can facilitate an immune response against the cancer cells. While not always the case, some cancers are associated with viruses. Virtually all cervical cancers are caused by human papillomavirus (HPV). Cervical cancer cells will have viral antigens within the cells which can be targeted by the immune system, indirectly leading to the cancer cells being targeted by the host immune response. Figure 5: Some viruses commonly associated with cancer. With most cancers, it seems that the main way the immune system tries to eliminate cancer cells is through direct killing by cytotoxic T cell (CTL AKA CD8 T cells). While some tumors are associated with viruses, most aren’t. This is somewhat problematic since activation of APCs needs to occur to upregulate costimulatory molecules and traffic to the lymph nodes to activate lymphocytes. Typically, this signal is provided by pathogen associated molecular pattern (PAMPs) during an infection. However, since tumors can be sterile, other signals must be produced to activate APCs. In addition to PAMPs, damage associated molecular pattens (DAMPs) can also activate local tissue APCs, causing them to migrate to the local lymph nodes to present antigens to lymphocytes. While it’s not completely clear what is activating APCs during cancer, it is hypothesized that the damage caused by cancer (nutrient hogs, invasions/destruction of health tissue) triggers the release of DAMPs, which is all local APCs need to activate and traffic to the lymph nodes. DAMPs are components of the extracellular matrix, cytosol, nucleus, mitochondria and other parts of the cells that are released in damaged or dying cells. DAMPs are recognized by APCs by receptors they possess. This leads to the APC being activated, upregulate costimulation, and traffic to the lymph tissues. T and B cells can now be activated by the APCs. Figure 6: Activation of local tissues APCs, like dendritic cells, by PAMPs or DAMPs allow for initiations of the host immune response against cancer-associated antigens. Cytotoxic lymphocytes (CD8 T cells) play a critical role in the response, but other cell types are also critical as well. Cross presentation will ensure that cancer antigens will be presented via MHC I and MHC II, ensuring activation of both CD4 and CD8 T cells. Cancer’s Suppression of the Immune System So this sounds great…why do we still get cancer, and why does the cancer frequently overwhelm our immune system? As mentioned earlier, cancer is extremely difficult to deal with since a series of mutations commonly make cancer cells resistant to elimination by the host immune system. Clinical Correlate: Cancer cells are constantly mutating to adapt to the host immune response. Cells that mutate and successfully evade or suppress the host immune response become the predominate type of that cancer cell in the host. By nature, cancer cells divide rapidly. This can outpace the rate at which the immune system can kill the cancer cells. Tumors will also stop expressing antigens that are being targeted by the host immune response. This makes T and B cells generated against that antigen unable to kill the cancer cells. Some cancers just stop expressing MHC I all together. Recall that antigen recognition in MHC I is how CTLs destroy target cells. No MHC I, no CTL killing. Figure 7: Mechanisms of killing of infected cells by CD8 + cytotoxic T lymphocytes (CTLs). CTLs recognize class I major histocompatibility complex (MHC)–associated peptides of cytoplasmic microbes (or cancer) in infected cells and form tight adhesions (conjugates) with these cells. Adhesion molecules such as integrins stabilize the binding of the CTLs to infected cells (not shown). The CTLs are activated to release (exocytose) their granule contents (perforin and granzymes) toward the infected cell, referred to as the target cell. Granzymes are delivered to the cytosol of the target cell by a perforin-dependent mechanism. Granzymes then induce apoptosis. ICAM-1, Intercellular adhesion molecule 1; LFA-1, leukocyte function–associated antigen 1. Loss of MHC I is commonly seen in more invasive and metastatic cancers. The loss of MHC I can induce killing of said cells by NK cells. Recall that NK cells check for MHC I on cells. If it is absent, this potentially targets the cells for destruction. In addition to downregulating MHC I, cancers will frequently express inhibitory receptors that block T cell activation. For example, PD-1 and CTLA-4 are frequently expressed on tumors. Recall from your basic immunology lectures, that CTLA-4 removes B7 from APCs. B7 is critical for an APC to be able to activate T cells. So CTLA-4 blocks B7, which leads to poor activation of T cells. PD-1 expression blocks the activation signal of TCR/MHC and B7/CD28 signaling. Figure 8: CTLA-4 and PD-1 are critical receptors needed to control the host immune system and keep it in check (tolerance). However, cancer cells frequently use them to suppress immune responses against them. Cancer uses our own immunosuppression tools against us. Other common mechanism used to evade the host immune system is expression of CD47. CD47 is expressed on cancer cells and provides an inhibitory signal to phagocytes. In essence CD47 is a cell’s way of telling phagocytes to not eat them. If cancer cells resist phagocytosis, antigen presentation will be affected. In addition, cancer cells commonly secrete inhibitory cytokines, such as IL-10 and tumor growth factor ß, to inhibit the inflammatory response. Tregs, which suppress lymphocytes functions, are also commonly found in high numbers inside tumors. Cancerous cells can secrete CCL22, which attracts Tregs to the tumor. Those Tregs, in turn, inhibit effector lymphocytes in the tumor environment. FasL can also be secreted by tumor, leading to the death of lymphocytes through FasL/Fas pathway. The previous mechanisms of immunosuppression are by far not comprehensive. But they do highlight the variety of ways that cancer cells can suppress and destroy the host immune system. So what are some of the ways researchers are trying to utilize the host immune response to combat cancer? While traditional cancer treatments (surgery, chemotherapy, radiation) have saved countless lives, these techniques have serious side effects. More options to treat cancer, including stimulating the host’s own immune system to fight back, are now arising as viable therapies for some types of cancer. However, like all cancer treatments, they too suffer from inconsistent results and serious side effects. In general, there three main approaches to modern immunotherapy currently: Passive immunotherapy with monoclonal antibodies Adoptive transfer of anti-tumor T cells Stimulation of the host’s immune response With therapies that utilize passive immunotherapy, monoclonal antibodies are injected into patients. These antibodies bind the surface of cancer cells, targeting them for elimination by the host immune response. Other monoclonal antibody therapies don’t destroy the cancer cells directly, but block growth factor receptors or angiogenesis signaling, inhibiting tumor growth and invasion. Some examples of monoclonal antibodies utilized in cancer treatments are. Anti-CD20: Utilized in B cell lymphomas. Targets B cells for destruction, host immune system is supplemented with pooled immunoglobulin since even health B cells are destroyed as well. Anti-Her2/Neu: Growth factors receptor is blocked in breast cancers, inhibiting spread. Anti-VEGF: Blocks angiogenesis, used to treat multiple types of cancer. Adoptive transfer of anti-tumor T cells is a strategy used to enhance the host’s T cell response, but it has had mixed results. As highlighted above, the tumor microenvironment is extremely immunosuppressive and activated T cells have an extremely hard time functioning with the multilayer immunosuppression caused by cancer. T cells are harvested from the patient, stimulated ex vivo, and placed back in the patient. This allows the T cells to divide and activate in an environment conductive to activation prior to being introduced back into the patient. The results of this type of therapy are very mixed. More recently however, viral vectors have been utilized to alter T cell receptors themselves to recognize cancer cells more efficiently. These chimeric antigen receptor (CAR) expressing T cells are generated by transfecting the host T cells with a viral vector that expresses a new receptor that recognizes surface antigens on cancer cells. These T cells are able to recognize cancerous cells more easily than normal host T cells and do not require MHC/TCR interactions to activate. However, a major side effect of this therapy is cytokine release syndrome. Cytokine release syndrome is mediated by massive amounts of inflammatory cytokines, including IL-6, interferon-γ, and others, that are released because all the injected T cells recognize and are activated by the patient’s tumor cells. These cytokines cause high fever, hypotension, tissue edema, neurologic derangements, and multi-organ failure. Figure 9: CAR T-cell therapy. A type of treatment in which a patient’s T cells (a type of immune cell) are changed in the laboratory so they will bind to cancer cells and kill them. Blood from a vein in the patient’s arm flows through a tube to an apheresis machine (not shown), which removes the white blood cells, including the T cells, and sends the rest of the blood back to the patient. Then, the gene for a special receptor called a chimeric antigen receptor (CAR) is inserted into the T cells in the laboratory. Millions of the CAR T cells are grown in the laboratory and then given to the patient by infusion. The CAR T cells are able to bind to an antigen on the cancer cells and kill them. Cancer.gov Finally, the last category of immunotherapy is by stimulating the patient immune response without removing cells for ex vivo stimulation. There are two broad ways this is being accomplished. First, blocking inhibitory receptors on T cells or their ligands is utilized to stimulate the antitumor immune responses. The principle of this strategy is to boost host immune responses against tumors by blocking normal inhibitory signals for T cells, thus removing the brakes (checkpoints) on the immune response. This has been accomplished by blocking monoclonal antibodies specific for the T cell inhibitory molecules CTLA-4 and PD-1, first approved for treating metastatic melanoma in 2011 and 2014, respectively. Since then, the use of anti-PD-1 or anti-CTLA-4 antibodies has expanded to many different cancer types. The most remarkable feature of these therapies is that they have dramatically improved the chances of survival of patients with advanced, widely metastatic tumors, which previously were almost 100% lethal within months to a few years. Although the efficacy of checkpoint blockade therapies for many advanced tumors is superior to any previous form of therapy, only a subset of patients (25% to 40% at most) respond to this treatment. The reasons for this poor response are not well understood. The second way the patient’s immune response is stimulated is through vaccines. One way of stimulating active immunity against tumors is to vaccinate patients with their own tumor cells or with antigens from these cells. An important reason for defining tumor antigens is to produce and use these antigens to vaccinate individuals against their own tumors. Vaccines may be administered as recombinant proteins with adjuvants. In another approach, a tumor patient’s dendritic cells are expanded ex vivo from blood precursors, the dendritic cells are exposed to tumor cells or a defined tumor antigen, and these tumorantigen-pulsed dendritic cells are used as vaccines. It is hoped that the dendritic cells bearing tumor antigens will mimic the normal pathway of cross-presentation and will generate CTLs against the tumor cells. Tumor vaccines have achieved only modest success, perhaps because these are therapeutic vaccines that are administered to patients in whom tumors may have established mechanisms that suppress immune responses. Many years ago, Dr. Herrmann tried a tumor vaccine with her bluetick coonhound Stephen who suffered from an osteosarcoma. Read the news story if you interested: https://www.greenvilleonline.com/story/news/2016/07/28/cancer-vaccine-used-dogs-may-one-day-helptheir-owners/87633378/ Tumors caused by oncogenic viruses can be prevented by vaccinating against these viruses. Two vaccines that are proving to be remarkably effective are against hepatitis B virus (the cause of a form of liver cancer) and human papillomavirus (the cause of cervical cancer). These are preventive vaccines given to individuals before they are infected, and thus prevent infection (like all preventive vaccines for infections). Again, prevention is the best cure. Transplantation Immunology In a lot of ways, transplantation is the other side of the immune response. So in cancer you have modified self antigens you want to mount an immune response against, but in a transplant patient, you’re dealing with immune responses against non-self antigens that you really don’t want to respond to (risk of damaging graft). It’s rough being an immune system… Before we start, let’s get some basic terminology out of the way. In transplants, the donor is the person giving the tissue, while the recipient is the person receiving the tissue. Syngeneic transplants are between animals identical to one another (twins in humans or clones in inbred animals). Allogenic transplants are between animals of the same species but not genetically identical, in contrast to syngeneic transplants. Xenogeneic transplants are between different species. While the later may seem odd, we’re already doing it. You may have heard recently about pig heart transplant to a human. Currently, the American College of Cardiology and the American Heart Association recommend mechanical valves for people under age 50 and biologic (tissue) valves for those over 70. Tissue valves, which are made from pig heart valves or cow heart-sac tissue, typically last about 15 years. But they usually don't require the lifelong use of anti-clotting drugs. The tissue itself also has its own terminology. Allografts are tissue from another individual of the same species, while xenografts are tissue from a different species, like Wilber up there. Autografts are when your own tissue is used in a graph. Transplantation has a unique set of challenges for it to work, namely doing your best to ensure that the host immune systems doesn’t destroy the transplant. This is an instance where the immune system’s response backfires on us. It will blindly destroy a life-saving transplant because nature never figured that organs would just jump from one person to another. Recall from our basic immunology lectures that all nucleated cells express MHC I and have 1000’s of them on their surface. MHC II is mainly found on professional APCs (dendritic cells, B cells, macrophages), but can be induced in other cell types. MHC alleles are co-expressed, meaning both mom and dad’s alleles that you inherited will be expressed. This is beneficial since the more diversity in MHC I and II you have, the broader range of antigens you can present. Poor antigen presentation would lead to poor lymphocyte activation and a weak immune system. While this diversity is fantastic and helps fight off the broad range of pathogens we encounter, it’s awful for transplants. Major histocompatibility complex (MHC) gets its name because it is the major cause of rejection if not properly matched before transplantation. If MHC is poorly matched between donor and recipient, then the organ will most likely be rejected rapidly. Non-MHC antigens that induce graft rejection are called minor histocompatibility antigens, and most are normal cellular proteins that differ in sequence between donor and recipient. The rejection reactions that minor histocompatibility antigens elicit usually are not as strong as reactions against foreign MHC proteins. Two clinical situations in which minor antigens are important targets of rejection are blood transfusion (ABO blood groups, Rh: Dr. Kennedy/Dr. Alston’s lecture) and hematopoietic stem cell transplantation, discussed later. Getting perfect donor/recipient matches are not always possible. In the event that rejection occurs, antigens from the graft are phagocytized by APCs, transported to local lymphoid organs, and used to activate T cells. If the antigens are from a graft from another human, they are referred to as alloantigens. If the graft is from a different animal species, then the antigens are referred to as xenoantigens. Figure 11: Immune response against transplants. Graft antigens that are expressed on donor dendritic cells or captured by recipient dendritic cells are transported to peripheral lymphoid organs where alloantigen-specific T cells are activated (the sensitization step). The T cells migrate back into the graft and destroy graft cells (rejection). Antibodies are also produced against graft antigens and can contribute to rejection (not shown). The example shown is that of a kidney graft, but the same general principles apply to all organ grafts. To further complicate matters, the APCs may come from either the recipient or are APCs from the donor organ. Depending upon where the APC comes from, recipient or donor, can change the recipient’s immune response and recognition of graft antigens. Direct allorecognition. Most tissues contain dendritic cells, and when the tissues are transplanted, the dendritic cells are carried in the graft. When T cells in the recipient recognize donor allogeneic MHC molecules on graft dendritic cells, the T cells are activated; this process is called direct recognition (or direct presentation) of alloantigens. Direct recognition stimulates the development of alloreactive T cells (e.g., CTLs) that recognize and attack the cells of the graft. Indirect allorecognition. If graft cells (or alloantigens) are ingested by recipient dendritic cells (notice from Figure 12 below this is what differentiates direct from indirect), donor alloantigens are processed and presented by the self MHC molecules on recipient APCs. This process is called indirect recognition (or indirect presentation). Alloreactive CTLs are induced by the indirect pathway, these CTLs are specific for donor alloantigens displayed by the recipient’s self MHC molecules on the recipient’s APCs, so they cannot recognize and kill cells in the graft (which, of course, express donor MHC molecules). When graft alloantigens are recognized by the indirect pathway, the subsequent rejection of the graft likely is mediated mainly by alloreactive CD4 + T cells. These T cells may enter the graft together with host APCs, recognize graft antigens that are picked up and displayed by these APCs, and secrete cytokines that injure the graft by an inflammatory reaction. Figure 12: T cells may recognize allogeneic MHC molecules in the graft displayed by donor dendritic cells in the graft, or graft alloantigens may be processed and presented by the host’s dendritic cells. Graft rejection is classified into 3 categories: hyperacute, acute, and chronic rejection. The features of each are defined below. Hyperacute rejection occurs within minutes of transplantation and is characterized by thrombosis of graft vessels and ischemic necrosis of the graft. Hyperacute rejection is mediated by circulating antibodies that are specific for antigens on graft endothelial cells and that are present before transplantation. These preformed antibodies may be natural IgM antibodies specific for blood group antigens, or they may be antibodies specific for allogeneic MHC molecules that are induced by exposure to allogeneic cells due to previous blood transfusions, pregnancy, or organ transplantation. Almost immediately after transplantation, the antibodies bind to antigens on the graft vascular endothelium and activate the complement and clotting systems, leading to injury to the endothelium and thrombus formation. Hyperacute rejection is not a common problem in clinical transplantation, because every donor and recipient are matched for blood type and potential recipients are tested for antibodies against the cells of the prospective donor. (The test for antibodies is called a cross-match.) However, hyperacute rejection is the major barrier to xenotransplantation. Acute rejection occurs within days or weeks after transplantation and is the principal cause of early graft failure. Acute rejection is mediated by T cells and antibodies specific for alloantigens in the graft. The T cells may be CD8 + CTLs that directly destroy graft cells or CD4 + cells that secrete cytokines and induce inflammation, which destroys the graft. T cells may also react against cells in graft vessels, leading to vascular damage. Antibodies contribute especially to the vascular component of acute rejection. Antibody-mediated injury to graft vessels is caused mainly by complement activation by the classical pathway. Clinical Correlate: Current immunosuppressive therapy is designed mainly to prevent and reduce acute rejection by blocking the activation of alloreactive T cells. Chronic rejection is an indolent form of graft damage that occurs over months or years, leading to progressive loss of graft function. Chronic rejection may be manifested as fibrosis of the graft and by gradual narrowing of graft blood vessels, called graft arteriosclerosis. In both lesions, the culprits are believed to be T cells that react against graft alloantigens and secrete cytokines, which stimulate the proliferation and activities of fibroblasts and vascular smooth muscle cells in the graft. Alloantibodies also contribute to chronic rejection. Clinical Correlate: Although treatments to prevent or curtail acute rejection have steadily improved, leading to better 1-year survival of transplants, chronic rejection is refractory to most of these therapies and is becoming the principal cause of graft failure. When the host immune system attacks a graft, this is called host verses graft disease. Figure 13: The 3 major types of graft rejection along with some common pathological findings. Since blood transfusions have already been covered, we’ll spend the remaining time in this ILA talking about a specific type of transplant that has a fairly unique complication. In a hematopoietic stem cell transplants, the recipient’s bone marrow is eliminated by chemotherapy and/or radiation and replaced with a donor’s healthy stem cells. Bone marrow transplants can be used to treat a wide range of diseases, including cancer and congenital immunodeficiencies. Since the host immune system must be eliminated prior to transplantation, there is a window after the transplant that the patient is severely immunocompromised and is at high risk for infections. Even benign everyday infections can be lethal to patients who have recently received bone marrow transplants. It can take weeks after a transplant for the bone marrow to be able to produce enough cells to return to normal levels. During this time shortly after transplantation, the patient is severely immunocompromised and must be isolated. One bizarre complication of bone marrow transplants is that the recipient’s new lymphocytes generated from the stem cells of the donor recognize the recipient’s cells as foreign and attack them. Your defective old bone marrow was replaced…with a new bone marrow that is producing lymphocytes that are attacking you. This is referred to as graft verse host disease (GvHD). GvHD can be either acute or chronic. With acute GvHD, symptoms usually occur within the first 100 days post-transplant. The first signs are usually rash, burning, and redness of the skin of the hands and feet. This can spread all over the body followed by nausea, vomiting, diarrhea, jaundice, and weight loss. With chronic GvHD, symptoms usually occur after 100 days post-transplant. Symptoms of chronic GvHD are very similar to acute GvHD, but occur more progressively as opposed to the fulminant onset typically seen with acute GvHD. However, both chronic and acute GvHD can quickly become lifethreatening if not addressed. Dermatological symptoms are the usual first sign of chronic GvHD. Symptoms include… Decreased appetite Diarrhea Abdominal (belly) cramps Weight loss Yellowing of the skin and eyes (jaundice) Enlarged liver Bloated abdomen (belly) Pain in the upper right part of the abdomen (belly) Increased levels of liver enzymes in the blood (seen on blood tests) Skin that feels tight Dry, burning eyes Dryness or painful sores in the mouth Burning sensations when eating acidic foods Bacterial infections Blockages in the smaller airways of the lungs With both acute and chronic GvHD, immunosuppressants are given to suppress the immune system and prevent tissue damage. Unfortunately, this common leads to increased risk of severe infections and cancer. ILA Checkpoint! 1. What is immune surveillance and what evidence exists demonstrating the immune system suppresses cancer? 2. How are the antigens in cancer cells distinct from host antigens even though cancer cells arise from host tissue? How is this used by the immune system to target cancer cells? 3. How are APCs activated in cancers associated with viruses? How about sterile cancers? 4. How do cancer cells commonly evade/suppress the host immune response? Be able to explain the following in relation to this topic: MHC, PD-1, CTLA-4, CD47, Tregs, CCL22, IL-10, TGF-ß, and FasL/Fas. 5. What are some of the ways research and approved therapies modulate the immune system to fight cancer? (Passive immunotherapy, adoptive transfer, stimulation of host immune response) 6. Define the following terms used in transplant immunology: donor, recipient, syngeneic transplant, allogenic transplant, xenogeneic transplants, allografts, autografts, and xenografts. 7. What receptor is critical to match closely between donors and recipient to ensure the graft has the best chance of not being rejected? 8. How are direct and indirect allorecognition different? Which cell type mediates the rejection in each case predominantly? 9. What are the 3 main types of graft rejection and how are they distinct from one another? 10. What is the difference between graft versus host disease and host verse graft disease? 11. What is the difference between acute and chronic GvHD? Hint: time.