Helper T Cells

-Aka intraepithelial lymphocytes -receptors that are expressed on a subset of T cells that are found mostly in epithelial location -recognize native antigen rather than peptide-MHC complex -displays limited antigenic diversity -in ruminants 60% of T cells carry these receptors (compared to less than 10% in nonruminants) -role is poorly understood

-aka TH and CD4+ -help B cells produce antibodies to protein antigens -promote activation and proliferation of other T cells

aka Tc, CD8+ -causes lysis of antigen-bearing target cells

respond only to processed antigen when it is presented by professional APC's in association with self-MHC class II molecules -Requires two signals, 1) TCR complex and CD4 recogintion of peptide MHC complexes on the APC 2) costimulation

Antigen recognition by helper T cell induces the expression of CD40L. CD40L binds to CD40 on the APC and stimulates the expression of B7 molecules which bind to CD28 on the helper T cell and the secretion of cytokines that activate the helper T-Cell

-The best co-stimulators for naive TH cells -absent or expressed at low levels on resting APCs -enhanced expression occurs when APC's are stimulated by endotoxin [LPS], interferon-ϒ, and binding of T cell CD40L to CD40 on the the APC

T cell receptor for B7 proteins. -signaling by this receptor enhances several TH cell responses to antigen including production of cytokines i.e IL-2 and differentiation of naive T cells into effector and memory cells

aka autocrine growth factor Source: naive CD4 T cells and TH1 cells; CD8 T cells produce and secrete low levels Activites: stimulate growth and differentiation of T cells, B cells, and NK cells

Antigen-activated naive TH cells synthesize IL-2 and IL-2 receptors. If the receptors are bound by IL-2 the cell undergoes clonal expansion and differentiation into effector cells (mostly) and memory cells

-more sensitive to TCR/CD4 recognition of peptide-MHC complexes and therefore do not need costimulation -able to respond to peptide-MHC complexes displayed on APC's that lack B7 molecule -once activated - synthesize various membrane bound or soluble effector molecules (CD40L & cytokines)

binds to CD40 on macrophages and B cells and activates them

-long lived cells -functionally inactive until reexposure to antigen induces them to undergo clonal expansion and differentiation -do not need co-stimulation to induce full activation

-TH1, and TH2 -develop from the same naive CD4 T lymphocytes in the peripheral lymphoid organs -based on their production and response to specfici cytokines -differentiation from CD4 T cells is basesd on cytokines produced in innate immune response to microorganisms or early in the adaptive immune response -have the ability to amplify itself and crossregulate the other

promotes the production of TH1-promoting transcription factor T-bet

promotes the production of the TH2 promoting transcription factor GATA-3 -produced in response to helminth parasites and other pathogens -promotes TH2 differentiation whilst inhibiting proliferation of TH1 cells

responsible for classical cell-mediated immune functions, eg delayed-type hypersensitivity and activation of cytolytic T lymphocytes

functions mostly as a helper B cell activation and antibody production