Helper T Cells (Activation AND Differentiation)
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p186
2025-09-15
404
Helper T cell (TH ) activation and differentiation is the central event that regulates both the components of immune response; CMI and AMI.
Activation of Helper T Cells
Signal Generation
Activation of TH cells requires generation of three specific signals (Fig. 1).
1. Antigen-specific signal: It involves binding of antigenic peptide present in the groove of MHC-II on APCs to TCR (T cell receptor) present on surface of TH cells. CD4 molecules of TH cells also interact with β2 domain of MHC-II
2. Costimulatory signal: It involves binding of CD28 molecule on TH cells to B7 molecules on APCs
3. Cytokine signal: APCs (macrophages) secrete interleukin-1 (IL-1) which acts on TH cells.
Fig1. Activation of TH cell by interacting with APC. Abbreviations: APC, antigen-presenting cells; MHC, major histocompatibility complex; TCR, T cell receptor.
Signal Transduction
Following induction of signal, its transmission is essential for TH cell activation. Signal transduction is initiated at CD4 molecule which interacts with CD3 complex, which in turn transmit the signal leading to activation of TH cells.
Differentiation of Helper T Cells
Activated TH cells secrete increased amount of IL-2 as well as IL-2 receptor (IL2R or CD25). IL-2 binds to its receptors on the same TH cell and also on other TH cells and induces the naive TH cells to proliferate and differentiate. TH cells get activated and become lymphoblast cells which subsequently differentiate into memory and effector TH cells.
Effector TH Cells
They are derived either from the naive TH cells or pre-existing memory TH cells following antigenic stimulus. They are short lived (few days to weeks). They further differentiate into either TH 1 or TH 2 subsets. This differentiation is very crucial as they secrete distinct cytokines that further mediate specific functions.
Cytokines secreted by TH 1 cell stimulate cytotoxic T cells and induce cell mediated immune response; while cytokines secreted by TH 2 cell stimulate B cells producing different classes of antibodies (humoral immune responses). IL12 secreted by macrophage plays an important role in the differentiation of TH cells. It promotes TH 1 subset proliferation.
TH 1 cells produce IL-2, interferon-γ (IFN- γ) and tumor necrosis factor-β (TNF-β); each has specific function (Table 1) TH 2 cells secrete IL-4, IL-5, IL-6, IL-10 and IL-13. They activate the B cells to transform into plasma cells which in turn secrete antibodies (Table 1).
Table1. Role of cytokines secreted by TH 1 and TH 2 cells.
Memory T Cells
They are derived from activated TH cell. They have longer life span (months to years). They are in resting stage, but following subsequent antigenic stimulus, they become activated and differentiated into effector TH cells. They express CD45RO isoform of common leukocyte antigen CD45, as compared to naive T cells which express CD45RA.
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