Chapter 17.2: The Immune Response (PDF)

CHAPTER 17 The Immune Response PART 2 – final revised B-cells, T-cells and Dendritic cells: The Adaptive Immune Response 17.2-1 Details are FYI only 17.2-2 The Adaptive Immune Response: Humoral immunity Antibo...

CHAPTER 17 The Immune Response PART 2 – final revised B-cells, T-cells and Dendritic cells: The Adaptive Immune Response 17.2-1 Details are FYI only 17.2-2 The Adaptive Immune Response: Humoral immunity Antibodies: 1: bind to and neutralize a bacterial toxin (also viruses and bacteria) 2: Coat the pathogen (opsonization) which promotes phagocytosis 3: activate complement (IgG and IgM) Ultimate goal: target pathogens and their products for elimination by phagocytes 17.2-3 Antibodies are the secreted form of the BCR and are specific Are the B-cell receptors expressed on a B cell identical or different? Are antibodies specific or non- specific for targets? Epitopes for B cells tend to be on the surface of pathogens 17.2-4 Antibody structure Antibodies consist of: 2 identical light chains (~ 25kD) 2 identical heavy chain (50 kD or more) Each light chain is joined to a heavy chain by a disulfide bond (as well as by noncovalent linkages) Each light chain/heavy chain dimer is joined to an identical light chain/heavy chain dimer by disulfide bonds Each light chain contains one variable region and one constant region (of one domain) Each heavy chain contains one variable region and one constant region (of 3 or 4 domains) Antibodies are highly folded 17.2-5 Antibody structure Fab fragment: (fragment antigen binding) composed of the light chain and part of the heavy chain Fc fragment: (fragment crystalizable) a portion of the constant region of the heavy chain Fc region of Antibody molecules bind to Fc receptors on cells 17.2-6 Antibodies (immunoglobulins) are the secreted form of the B-cell receptor and are specific for one antigen binding site (epitope) Epitope: (antigenic determinant) the portion of an antigenic molecule that is: bound by an antibody An antigen can have many epitopes When a B-cell recognizes its antigen it will multiply and produce clones of itself – all of which will secrete the identical antibody to the identical epitope on the antigen During an infection different B-cells will recognize the same antigen but different epitopes on that antigen B-cells tend to recognize external epitopes 17.2-7 Antibodies are the secreted form of the BCR and are specific Hypervariable regions (5 to 10 amino acids) are also known as complementary-determining regions (CDRs) CDRs are flanked by less variable framework regions The six hypervariable regions of heavy chain and light chain form the antigen binding site 17.2-8 Generation of B cell antigen recognition diversity: Step 1 = Somatic Recombination FYI On a germ-line level, to generate the variable region in a light chain: one V segment and one J segment are joined To generate the variable region in a heavy chain: One V segment, one D segment and one J segment are joined There are multiple V (D) J segments to randomly choose 17.2-9 Generation of B cell antigen recognition diversity Step 2 = Junctional Diversity FYI How much diversity is generated during somatic recombination? k light chain: 3Vk x 5Jk segments = 175 different possible combinations l light chain: 30 Vkl x 4Jl segments = 120 different possible combinations Heavy chain: (40V x 23D x6J) = 5520 combinations But there is actually more diversity added during the joining V(D)J segments: 2) Junctional Diversity Addition of new and random nucleotides at the V and J segments of light chain and D and J segments of heavy chain The randomly chosen V gene segment codes CDR1 and CDR2 Junctional diversity codes CDR3 17.2-10 Generation of B cell antigen recognition diversity Step 3 = Combinatorial Diversity FYI In the bone marrow: In a B cell - Heavy chain is generated first somatic recombination junctional diversity Additon of  contant region B cells with an intact heavy chain undergo clonal expansion Light chain generation then takes place and though each of these clones has the same heavy chain the light chain can be different Combinatorial diversity Different light chains combine the already generated heavy chain Ultimately these three steps lead to billions of circulating B-cells with unique specificity Before B-cells leave the bone marrow they undergo a selection process so that they do not have strong recognition of self Remember B-cells that leave the bone marrow are expressing are IgM B-cells that do not encounter antigen will undergo apoptosis 17.2-11  There are five classes of antibodies based on the constant region of their heavy chain A clone of mature B-cells released from the bone marrow have one antigen specificity and express IgM receptors (the  heavy chain is added first ) This means that the first class of antibodies produced during an infection are IgM class antibodies Class switching takes place in lymph nodes following B cell activation and with 17.2-12 help from T cells What happens when B-cells leave the bone marrow They circulate between blood and lymph If they encounter antigen they phagocytose that antigen and are considered “activated” But in order to be able to secrete antibody (becoming effector cells) they need T-cell help This takes place in a lymph node B-cells will phagocytose the antigen and present pieces of it to a helper T-cell in the context of an MHC molecule 17.2-13 The first class of antibody the B-cell will secrete is IgM  What are MHC molecules Class II MHC molecules: are expressed on professional Class I MHC molecules: are expressed antigen presenting cells (and a on all nucleated cells few others) Generally present viruses (as peptides) Generally present extracellular that have infected a cell to a cytotoxic T pathogens that must be cell (CD8+) and the cytotoxic T cell will phagocytosed before presentation then kill the virus infected cell Class I MHC: one Class II MHC: transmembrane two transmembrane  chain of three chains –  and  domains that are non-covalently complexed to 2 microglobulin 17.2-14 Antigen processing and presentation FYI involves the endomembrane system Pieces of pathogens end up on the cell surface in the binding site of either Class I or Class II MHC molecules 17.2-15 The T-cell receptor only recognizes antigen when antigen is presented by an MHC molecule that the T cell also recognizes MHC Class I are recognized by CD8+ cytotoxic T-cell receptors 17.2-16 MHC Class II are recognized by CD4+ helper T-cell receptors Lets cover a little bit about T-cells now Cells of the immune system-lymphocytes (T cells) T lymphocytes (T cells): three main classes upon activation and differentiation Cytotoxic T cells: (CD8+) kill cells infected with virus Helper T cells: (CD4 +) regulate the activities of other white blood cells help activate macrophages, B cells and cytotoxic T cells (many types of helper T cells) FYI Only: Regulatory T cells: suppress the activity of other lymphocytes and help to control immune responses 17.2-17 In this course we will only discuss helper T-cells in the context of helping B-cells produce antibodies (TFH cells) What are MHC molecules? MHC molecules were first Six MHC class I isotypes identified by their role in immune response to transplanted tissue and are encoded by a large cluster of genes called the major histocompatibility complex (MHC) All (nucleated) cells of higher vertebrates express MHC proteins In humans, MHC proteins are called HLA antigens (human- leucocyte-associated antigens) Class I MHC and Class II MHC have basic structural differences Note the polymorphism Five (?) MHC class II isotypes 17.2-18 Polymorphism in MHC genes MHC proteins are encoded by stable germline genes from 3 families that encode: MHC class I heavy chain isotypes MHC class II  chains isotypes MHC class II  chains isotypes But within these families are many alternative forms (alleles) of a gene The protein encoded by an allele is the allotype While human populations contain many different alleles for each isotype: you inherit one allele from each parent They may be different alleles They may be the same allele So what do you think the chances are that anyone else in this room expresses the same complement of MHC proteins (on every nucleated cell in their body) that you do? 17.2-19 Note that the diversity in isotypes and allotypes: the term isoform can be used for any particular MHC protein The MHC is the most highly polymorphic gene system in the body (and therefore the population) Every cell that expresses MHC molecules expresses proteins transcribed from both the maternal and paternal chromosome You will express up to 12 MHC Class I allotypes (6 maternal and 6 paternal) So why are only 9 MHC Class I molecules on the surface of this cell? IMPORTANT: Each MHC molecule displays a range of peptides - that is, not just one peptide will fit into the binding site but thousands of peptides will and each of these peptides is the antigen presented to a T-cell with an epitope the T-cell receptor might recognize 17.2-20 The more the heterogeneity the more peptides that can be presented Q: What if you expressed only 6 MCH class I allotypes? What does this mean with respect to inheritance? What if you expressed only 8 class I MHC allotypes MHC association with resistance to disease Seroconversion = when an individual infected with HIV begins to make detectible levels of antibody The onset of overt symptoms of AIDS occurs years after seroconversion and how many years that takes is associated with heterozygosity at MHC 17.2-21 Red = heterozygous for all polymorphic HLA Yellow = homozygous for one locus Blue = homozygous for more than one locus Back to T cells T-cells function by making contact with other cells T-cell receptors recognize antigen in the context of either a Class I MHC molecule or a Class II MHC molecule and inducing changes in those cells 17.2-22 T cells originate in the bone marrow and mature in the thymus T-cell precursors from the bone marrow do not express CD4 or CD8 They trickle through the thymus and those that pass a stringent set of requirements at multiple points emerge as either CD4+ or CD8+ cells 17.2-23 They continue to re-circulate between blood and lymph and if they recognize self-MHC with pathogen in a lymph node (which is presented by a dendritic cell) they will be activated What happens to T-cells in lymph nodes The adaptive immune response does NOT occur at the site of infection Dendritic cells carry antigens from sites of infection to secondary lymphoid tissue where they become present antigen to T-cells T-cells (both CD4 and CD8) are always activated in a lymph node by dendritic cells first and after this activation by dendritic cells can go on to perform their effector functions After T-cells are activated by dendric cells: CD4+ (helper T-cells) can stay in the lymph node and help B- cells make antibody molecules CD8+ (cytotoxic T-cells) can leave the lymph node and kill virus infected cells 17.2-24 What happens when Naïve T cells first encounter antigen presented by dendritic cells in secondary lymphoid organs Thousands (more?) of T cells will scan the dendritic cells for their antigen The T-cell receptors scan the peptide:MHC complexes on the dendritic cell surface looking for their specific antigen 17.2-25 If T-cells don’t encounter their antigen on a dendritic cell they will continue to circulate between blood and lymph If they never encounter antigen they will undergo apoptosis T-cells binding to dendritic cells are activated by signal transduction pathways Clustering of the T-cell receptor and a co-receptor initiates signaling within the T-cell ITAMS: immunoreceptor tyrosine-based activation motifs Phosphorylated Lck: (lymphocyte specific iTAMS in yellow boxes tyrosine in red protein kinase) a tyrosine kinase associated with the cytoplasmic tails of CD4 and CD8 Upon binding of MHC and TCR , Lck phosphorylates the ITAMS ZAP-70 (another tyrosine kinase -  chain associated protein of 70 kD) then binds to phosphorylated tyrosine on the  chain of the TCR complex Lck then phosphorylates T cell activation initially leads to proliferation of T cells ZAP-70 17.2-26 What do effector T-cells do 17.2-27  How are B-cells helped by T-cells that have been activated by a dendritic cell in the lymph node? B-cells circulate between blood and lymph If they encounter antigen in the lymph node, B-cells will phagocytose that antigen But in order to be able to secrete antibody they need T-cell help This also takes place in a lymph node A B-cell will phagocytose the antigen and present pieces of it to a helper T-cell in the context of an MHC molecule 17.2-28 The first class of antibody the B-cell will secrete is IgM TFH cells determine whether B cells will differentiate into plasma or memory cells Class switching occurs first, then differentiation into plasma or memory cells Initial need is for antibody secreting plasma cells As the infection subsides the same cytokines direct the formation of memory cells ICOS: inducible T cell costimulator Some of these plasma cells remain in the lymph node to produce antibody that binds antigens arriving from infected tissue – are short lived Some plasma cells migrate to the bone marrow to provide a systemic source of antibody circulating between blood and lymph – long lived 17.2-34 Summary of Naïve B cells and plasma cells and what antibodies do 17.2-37 Back to T cells: the T-cell receptor only recognizes antigen when antigen is presented by an MHC molecule that the T cell also recognizes MHC Class I are recognized by CD8 + cytotoxic T-cell receptors MHC Class II are recognized by CD4+ helper T-cell receptors 17.2-38 Antigen presenting cells (dendritic cells, B-cells and Macrophages) have Class II MHC on their cell surface All other nucleated cells have Class I MHC Dentritic cells also have Class I MHC on their cell surface in order to activate CD8+ cells CD8+ T-cell killing 17.2-39 CD8+ T-cell killing 17.2-40 Primary and secondary antibody response Memory T cells are also formed 17.2-41 Learning objectives: Know the three effector functions of antibody molecules Be very clear on the structure of antibody molecules – all of it – including the details of the antigen binding site and all the terminology associated with B-cells and antibodies Know the names of the five classes of antibodies and what differentiates them Know what an MHC molecule is, the difference between the two class, both structural and distribution Understand the polymorphism of MHC and the implications of that Understand in detail how a helper T-cells (Folicular Helper T cell) helps to activate a B-cell to secrete antibody Understand that IgM is the first class secreted in an antibody response Understand the role of a CD8+ cell and how it recognizes a virus infected cell 17.2-42