tompkins- t-cell development, activation, regulation and effector function

TCR structure

transmembrane heterodimers composed of either an α and a β chain or a γ and a δ chain

αβ T cell functions

‘normal’ functions

γδ T cells

might bind carbohydrates, expressed in odd places, various fucntions

what determines the function/specificity of the t cell? how many transmembrane polypeptides are in one?

TCR complex with 4 transmembrane polypeptides collectively called CD3

describe CD3

NOT a receptor, does not bind ligand but is essential for intracellular signalling

CD4 or CD8 will associate with TCR except which kind of T cells?

γδ T cells

how is diversity/variable region of the TCR generated?

gene rearrangement (unique to t and b lymphocytes during development)

describe the gene rearrangement process of TCR/BCR

random rearrangement of germline segments of genes encoding TCR and BCR, requires enzymes (RAG1 and RAG2), occurs independent of antigen (antigen specific receptors are not generated in response to antigens)

species specific gene segment numbers

each species has a unique number of each gene segement, within the species there should be the same number

what are the letter segments of germline arrangement of TCR genes?

v (variable), d (diversity), j (joining), c (constant)

describe combinational diversity

β-chain genes are rearranges by random joining of segments (D,J) which then links with a V; α-chain genes are rearranged by random (V,J) segments joining

what is junctional diversity?

during the process of combinational diversity, random nucleotides are removed or added between the segments creating MORE diversity (intentional sloppy ligation)

what does non-homologous DNA end joining enable?

thousands of possible sequences per joining

what does defective VDJ recombination result in?

profound immunodeficiencies

what is SCID?

severe combines immunodeficiency

describe the steps of gene rearrangement in α:β T cells

DN thymocyte (no CD4 or CD8 present), rearrange beta chain, pre-t alpha + beta + CD3 are ran through the pre-TCR checkpoint, rearrangement is stopped and beta chain proliferation happens, DP thymocyte (CD4 and CD8 present now), alpha chain is rearranged, functional alpha beta TCR is ready for selection

what is another name for t cell selection in the thymus?

central tolerance

what are the 4 random mutations scanned for in t cell selection in the thymus?

nonsense (no receptor), non-functional (useless), self-rective (dangerous), and non-self reactive (good guys)

where does positive selection of t cells occur?

thymic cortex (DP thymocytes interact with the cortical thymic epithelial cells that express both MHC I and MHC II)

what is the purpose of positive selection?

determines CD4 or CD8 expression and eliminates non-functional TCRs

what must happen for a DP thymocyte to be given survival signals? to die by apoptosis? which is more common?

must interact with MHC-peptide complexes, do not interact with MHC-peptide complexes, up to 96% of vells die by apoptosis at this stage

what happens to DP thymocytes that bind with MHC I?

lose CD4 expression and become CD8+ t cells

what happens to DP thymocytes that bind well with MHC II?

lose CD8 expression and become CD4 t cells

where does negative selection occur?

thymic medulla

what is negative selection/central tolerance?

eliminates self reactive TCRs

what are the two types of CD4+ T cells?

effector t cells and regulatory t cells

what is the function of effector t cells?

help fight infections by coordinating immune responses

what is the function of regulatory t cells?

helps control the immune response and prevents it from becoming excessive or attacking the body, immunosuppresive t cells

why can effector t cells become a problem?

some effector t cells can retain the capacity for self-reactivity and may cause autoimmune disease or alloreactive responses during pregnancy or after organ/stem cell transplantation

if the affinity of TCR binding to self-peptide-PHC complexes is very high, what is the outcome?

apoptosis/negative selection and removal of self-reactive T cells

if the affinity of TCR binding to self-peptide-PHC complexes is higher, what is the outcome?

positive selection and differentiation into regulatory t cells

if the affinity of TCR binding to self-peptide-PHC complexes is lower, what is the outcome?

positive selection and differentiation into effector t cells

if the affinity of TCR binding to self-peptide-PHC complexes is very low, what is the outcome?

no survival signals and death by neglect

what is peripheral tolerance?

a mechanism that takes place outside primary lymphoid organs that prevent immune responses against self ot harmless materials like food (perfrormed by regulatory t cells)

what species have a majority of  γ/δ T cells? were are they usually found?

ruminants and chickens, mucosally associated

how is a gamma delta t cell made?

gamma and delta are rearranged before TCR beta

why can gamma delta and alpha beta not be expressed at the same time?

when an alpha chain is rearranged, TCR delta is deleted

do gamma/delta t cells require MHC?

no, antigen interaction does not require MHC and can recognize unprocessed antigen

what makes gamma/delta t cells less diverse and what function do they have?

few V gene segments, cytotoxic function

what are the 3 signals necessary for t cell activation?

antigen, co-stimulation, cytokines

describe signal one of t cell activation

naive CD4 and CD8 cells that have exited the thymus pigrate to lymph nodes, ONLY activated by an APC (usually dendritic cell) presenting its specific antigen

in signal one, which t cell is responsible for extracellular Ags? intracellular Ags?

CD4- extracellular Ags, presented on MHC II molecules, CD8- intracellular Ags, presented on MHC I

what APC can also take up extracellular antiegns to cross present to CD8+ T cells?

dentdritic cells

describe signal two of t cell activation

costimulation, CD80/86 molecules on the APC must interact with CD28 molecules on the T cell

where is CD80/86 expressed?

only on APCs and in low levels unless APCs are activated by cytokines, pathogens, or t cells

what happens if naive t cell encounters antigen (signal 1) in the absence of co-stimulation (signal 2)?

triggers apoptosis or anergy (a state of non-responsiveness) of the cell— acts as another check to reduce self-reactivity and maintain peripheral balance

what is another name for rhe CD80/86?

B7

describe signal three of T cell activation

cytokines in the environment or produced by the APC will provide signal 3 and help determine the fate of the T cell by providing context

which signal of t cell activation determines the phenotype of the T cell?

signal 3

explain the importance of signal three

naive t cells have a low affinity version of interleukin 2 (IL-2) receptor, after signals 1 and 2 the t cells express a high affinity version of IL-2 receptor and begin secretion of IL-2, once bound to IL-2 the high affinity IL-2 receptor induces t-cell proliferation

what is responsible for priming naive t cells?

activated dendritic cells

what are the general steps in naive t cell activation in lymphoid organs?

APC (DC) presents specific peptide to antigen, provides cotimulation, naive t cell proliferation, IL-2 produced by t cells and driving autocrine proliferation and differentiation

what are the general steps in naive t cell activation in periphery?

local immune responses direct CD4 and CD8 t cells to site of infection, activated CD8 t cells recognize peptides presented on MHC I then kill infected cells while activated CD4 t cells recognize peptides presented on MHC II by macrophages and other APCs and help via cytokines and costimulatory molecules

what is clonal expansion?

preferential proliferation of antigen specific lymphocytes— cells that recognize the Ag produce IL-2 and also respond to it to ensure the Ag-specific t cells are the ones that proliferate (explains the lack of bystander proliferation)

clonal expansion of CD8+ t cells

50,000 fold expansion

clonal expansion of CD4+ t cells

100 fold expansion

the cytokines from signal 3 of t cell activation determine what kind of effector a CD4+ t helper cell will be. what are the most common?

Th1, Th2, Th17, Tfh (t follicular helper), and Treg

what is another name for naive t cells?

Th0

what determines which cytokines are secreted, and thus which type of t cells are generated?

the nature of the innate immune response

Th1 cell functions

intracellular pathogens, produces the cytokine IFN-γ (opsonization), TNF-β (induces macrophages, kills chronically infected cells)

Th2 cell functions

parasites, extracellular pathogens, allergy and asthma— IL-4 (induces b cell differentiation by neutralizing IgG antibodies, IgE antibodies induces mast cell degranulation), IL-5 (stimulates eosinophil activation)

Th17 cell functions

inflammasome (bacteria, fungi, parasites)— IL-17 attracts beutrophils and macrophages, IL-17 synergizes with other cytokines to augment inflammation

Treg cells and mechanisms of immunosuppression

1. Bind to and deplete IL-2 from their surroundings

2. Express cytotoxic T lymphocyte antigen 4 (CTLA-4), which binds to CD80 and CD86 on APCs, transmitting suppressive signals

3. Binding and sequestration of CD80/CD86 from APCs to Treg cells, disrupting priming and/or activation of Teff cells

4. Production of immunosuppressive cytokines, including IL-10, IL-35, & TGF-β, downregulating the activity of APCs and Teff cells.

5. Release of ATP, which is converted to adenosine that can provide immunosuppressive signals to Teff cells and APCs via adenosine A2A receptor

Tfh cells

activates and improves b cells— CXCR5 is the main cytokine produces, associated with germinal genter reactions in lymph nodes, helps differentiation into plasma cells, signals for B cell affinity maturation and class switching

which T cell has a higher threshold for activation?

naive CD8+ T cells need more co-stimulation

when are DCs alone sufficient for CD8+ T cell activation?

high levels of co-stimulatory molecules

when there are low levels of co-stimulatory molecules, what is needed to help with CD8+ t cell activation?

CD4+ t cells

where can IL-2 come from?

APC, CD4+, or CD8+ t cells

describe CD8+ killing by release of cytotoxic granules

perforin forms pore, granzyme induces apoptosis, cell dies

CD8+ killing by Fas/FasL binding

activated CTLs express Fas ligand, FasL binds death receptor Fas (expressed on many cell types), Fas ligation signals activation of caspases, leading to target cell apoptosis

what percent of subsets are memory cell effectors?

5-10%

how long can memory t cells specific to an antigen persist for?

years or a lifetime

T/F: memory requires CD4 T cell help for generation

true

what causes a decline of t cell responses? why?

elimination of antigen (microorganism), less IL-2 available, cells die, pool of antigen activated lymphocytes contracts

how do t cells decline after such increases?

death by apoptosis— neglect, negative signalling

what is the end result of the decline in t cells?

95% if cell numbers are deleted