Dr. Tompkins - antigens and T cells

what is an antigen?

any sybstance that binds specifically to an antigen receptor

where/what can an antigen receptor be?

antibody, B cell receptor, T cell receptor

T/F: all antigens are recognized by specific lymphocyte receptors/antibodies but only some antigens are capable of activating lymphocytes

TRUE

what is an immunogen?

an antigen that can induce an immune response from a B or T cell

what is a hapten?

an antigen that cannot induce an immune response alone, but when attached to a more complex molecule becomes immunogenic

what are components of a “good” immunogen?

complex, organic, degradable, large, foreignness in context

what influences immunogenicity?

size, complexity and foreignness, the more complex the more immunogenic

what is an epitope?

the part of the antigen that is recognized by the TCR or BCR, can also be called the determinant or antigenic determinant

characteristics of BCR and bound epitopes

intact antigen, can be proteins, carbs, lipids, etc, can be linear or conformational and do not need to be processed

characteristics of TCR and bound epitopes

antigen must be processed, usually peptides and need to be a linear sequence bound to MHC

what are neoantigens?

important for antiviral and anticancer immunity, are antigens expressed in cancer cells, can also be mutated antigens

what must an antigen go through to be considered a T cell epitope?

must be processed into peptides and then presented on MHC of APC

what are the types of MHC?

class 1 and class 2

what are characteristics of class I MHC

expressed on all nucleated cells, presents peptides to CD8+ (cytotoxic) T cells, intracellular antigens, and binds short amino acid peptides

what are characteristics of MHC class II?

expressed only on APCs, presents peptides to CD4+ (helper) cells, extracellular antigens and binds larger aminoacid peptides

how do we get MHCs?

genetically, they are inherited

what does it mean that MHC molecules are polygenic?

several MHC genes in one individual

how are MHC genes co-dominant?

maternal and paternal alleles are expressed

what does polymorphic mean in regards to the MHC?

there are many alleles within the population

how do MHC work?

they bond peptides and present them to TCRs to activate T cells, the structure of binding pocket determines which peptides it can bind

T/F: only the peptide has to match with a TCR for the T cell to bind to the MHC and APC

FALSE; both antigen AND MHC must match/fit

what is the peptide source for MHC I?

intracellular

what is the peptide source for MHC II?

extracellular

what organelle acts as a cellular garbage disposal?

proteosome

how does a proteosome work?

will degrade proteins into peptides, this includes misfolded, mutant, damaged, or targeted proteins

what are the steps in class 1 antigen presentation?

1. proteosome degrades protein into peptides

2. peptides are transported by TAP from cytoplasm to ER

3. peptide loaded onto MHC I proteins

4. class I is transported to cell surface to present to CD8 cells

what cells present peptides on MHC class 1?

all nucleated cells, provides survey of everything going on within a cell

what are the steps to MHC class 2 antigen presentation?

1. extracellular proteins endocytosed and endosomes fuse with lysosomes to degrade proteins inside

2. class 2 proteins are folded into ER but dont load peptide

3. invariant chain blocks the binding groove and targets class 2 to endosomal compartments

4. in endosome the invariant chain is degraded and peptides produced in endosome are loaded onto class 2

5. class 2 is transported to the cell surface where it presents peptide to CD4

what prevents MHC 2 from behaving like MHC 1?

the binding of the invariant chain

what are professional APCs?

dendritic cells, macrophages, and B cells, all can endocytose, responding/recognizing specific insult, present extracellular peptides to CD4 T cells

how much of MHC 2 expression is there in macrophages?

low levels, induced by pamps, damps, and cytokines, is upregulated upon activation

how much of MHC 2 expression is there in dendritic cells?

always expressed, upregulated upon activation

how much of MHC 2 expression is there in B cells?

always expressed, upregulated upon activation

what type of antigen do macrophages present?

extracellular antigens via MHC 2

what type of antigens do dendritic cells present?

intracellular via class 1 and extracellular via class 2

what type of antigen do B cells present?

extracellular antigens via MHC 2

where are macrophages found?

lymphoid tissue, connective tissue, body cavities, tissue resident

where are DCs found?

lymphoid tissue, connective tissue, epithelium, peripheral tissues

where are B cells found?

lymphoid tissue, blood

what is another function of dendritic cells?

activates naive T cells

how do dendritic cells mature?

DC cells in periphery capture antigen, then move to lymph nodes and then are activated

what does it mean that a DC can cross-present?

can present antigens on MHC class 1 or 2

what is immunodominance?

T cells can only respond strongly to a subset of peptides from antigen, these peptides are the ones were processed and bind best to MHC

what are problems associated with alloreactive T cell activation and superantigen t cell activation?

MHC-TCR si not necessarily specific, and this reduced specificity results in an over-activation of T cells and a more exaggerated immune response

what is different between alloreactivity and superantigens?

alloreactivity involves both CD8 and CD4 T cells, while superantigens only activate T cells with MHC class 2 (CD4)

how does MHC impact organ donations and skin grafts?

we each have our own “self” MHCs, when our body doesn’t recognize the MHC there is a reaction as a result

T/F: each lymphocyte bears antigen receptor of single specificity

TRUE

what are the two types of structures for the TCR?

alpha-beta TCR or gamma-delta TCR

what is CD3?

polypeptides/transmembrane proteins that form a complex with TCR that help transduce signals to activate T cell

T/F: CD3 is a receptor and binds a ligans

FALSE

what is the predominant type of TCR?

alpha-beta

what type of TCR do cattle have predominantly?

gamma-delta

how do we generate variable regions in B and T cells?

gene rearrangement, a process that requires protein complexes like RAG1 and 2

T/F: gene rearrangement of TCRs is independent of antigen

TRUE

what are the segments of germline rearrangement for TCR genes?

V (variable) D (diversity) J (joining) C (constant)

what germline segments do alpha chains have?

VJC

what germline segments do beta chains have?

VDJC

how are beta chains rearranged?

by the random joining of D and J segments linked with V segment (VDJ+C)

how are alpha chain genes rearranged?

by random joining of V and J segments (VJ+C)

what is junctional diversity?

intentionally sloppy ligation of genetic material, non-homologous end joining and defective VDJ recombination, random nucleotides are added or deleted at joining of segments to create additional diversity

what happens when a person has a RAG deficiency?

cannot make TCRs, are immunocompromised

what are the GENERAL stages of gene rearrangement of alpha-beta TCRs?

1. beta gene rearrangement first, then expression with CD3

2. alpha rearrangement starts

3. CD4 and CD8 are expressed (double positive)

4. alpha gene rearrangement

5. once alpha chain is functional, pairs with beta chain and then T cell goes to thymus

how are T cells selected in the thymus?

once in the thymus the immature T cells interact with thymic epithelial cells that express both MHC class 1 and 2 that express TCRs are tested for reactivity (none, just enough, too much) based on how well TCRs react with MHC 1 or 2, will differentiate into CD8 or CD4

what is negative selection?

thymic selection in which thymocytes (immature T cells) bind too well to APC and will be eliminated because of self-reactivity

what are t regulatory cells?

a subset of self-reactive thymocytes that have survived, are immunosuppressive that help with autoimmune responses, once bound to self antigen, secretes cytokines to inhibit reactive T cell

what are characteristics of gamma/delta TCRs?

do not express CD4 or CD8, don’t always require MHC, less diverse, and cytotoxic in function

what are the requirements for T cell activation?

antigen, costimulation and cytokines

what is the 1st signal for T cell activation?

naive T cells have exited the thymus and are in the lymph nodes, are activated only by an APC that presents a specific ANTIGEN

what type of antigens are presented for CD4 (helper) T cells?

extracellular antigens presented on class 2 MHC

what type of antigens are presented for CD8 (cytotoxic) T cells?

intracellular antigens presented on MHC class 1

what is the 2nd signal for T cell activation?

co-stimulation, via CD80/86 and CD28 interacting

what is CD80/86?

only expressed on APCs in higher amounts once APCs are activated

what is CD28?

expressed on T cell, must interact with CD80/86 for co-stimulation

what happens if naive T cell encounters antigen but there is no co-stimulation?

triggers apoptosis or anergy (non-responsiveness) of the T cell

what is the 3rd signal for T cell activation?

cytokines in the environment or produced by APC and will determine phenotype of T cell

what is IL-2?

a cytokine; after signals 1 and 2, T cells already present low-affinity receptor is converted into a high-affinity version IL-2 receptor and will bind the cytokine and trigger T cell proliferation and differentiation

how do activated CD8 cells respond in periphery?

recognize peptides presented on MHC 1 by infected cells and kill the infected cells

how do activated CD4 cells respond in periphery?

recognize peptides presented on MHC 2 by macrophages and other APCs, provide T cell help via cytokines and costimulatory molecules

what helps determine what kind of helper a CD4 T cell will be?

cytokines

what are some common types of T helper cells?

Th1, Th2. Th17, Tfh, Treg

what do Th1 cells produce?

cytokines IFN-y and TNF-B that help with intracellular pathogens

what does IFn-y do?

helps activate macrophages, enhances B cell proliferation and differentiation (opsonizing antibodies)

what does TNF-B do?

helps kill chronically infected cells and releasing bacteria to be killed by macrophages, induce macrophages to exit blood vessels at site of infection

what are Th2 cell functions?

helps with parasites and extracellular pathogens, is also associated with allergies and asthma

what cytokines does Th2 produce?

IL-4 and IL-5

what does IL-4 do?

induces B cell differentiation, neutralizes IgG antibodies and IgE antibodies

what does IL-5 do?

stimulates eosinophil activation

what is Th17 functions?

inflammation, bacteria, fungi, and parasites

what cytokine does Th17 produce?

IL-17

what does IL-17 do?

attracts neutrophils and macrophages, synergizes with other cytokines to augment inflammation, can sometimes be immunosuppressive

how are Treg cells immunosuppressive?

bind and deplete IL-2, and express cytotoxic antigen that binds to APC and transmits suppressive signals, disrupting priming and activation of T cell, immunosuppressive cytokines are produced to slow activity of APC and T cell

what do Tfh cells do?

activate and improve B cells

what is the main cytokine produced by Tfh cells?

CXCR5

T/F: naive CD8 cells need more co-stimulation than CD4 cells

TRUE

where does help for co-stimulation come from for CD8 cells?

CD4 t cells

what is perforin?

a way that CD8 cytotoxic t cells kill infected cells, forms a pore in infected cell causing apoptosis

what is FasL?

ligand expressed on activated cytotoxic t-cell, binds the death receptor (Fas) that is expressed on cell types

what is Fas?

death receptor that when bound signals activation of caspases, leading to cell apoptosis

what are memory cells?

t cells specific for an antigen that may persist for years or a lifetime, provide long-lasting protection against previously encountered microorganisms

what do memory cells require for generation?

CD4 t cell help