T lymphocytes are the central mediators of cell-mediated immunity, responsible for killing infected cells, helping B cells produce antibodies, and regulating immune responses. Their development in the thymus and activation in peripheral lymphoid organs are tightly controlled processes essential for effective immunity and self-tolerance.
T Cell Development in the Thymus
T cell progenitors originate from hematopoietic stem cells in the bone marrow and migrate to the thymus, where they undergo a programmed series of developmental stages. Early thymocytes lack CD4 and CD8 expression and are termed double-negative (DN) cells, which progress through DN1 to DN4 stages characterized by ordered rearrangement of the T cell receptor (TCR) β-chain locus. Successful β-chain rearrangement pairs with a pre-Tα chain to form the pre-TCR, which signals to promote proliferation, CD4 and CD8 co-receptor expression (double-positive stage), and α-chain rearrangement. Double-positive thymocytes expressing a mature αβ TCR undergo positive selection in the thymic cortex: cells whose TCR recognizes self-peptide presented by MHC molecules with adequate affinity receive survival signals, while cells whose TCR fails to recognize self-MHC die by neglect. Positively selected cells then migrate to the medulla for negative selection, where thymic medullary epithelial cells expressing AIRE (autoimmune regulator) present a broad repertoire of tissue-specific self-antigens; thymocytes with high-affinity reactivity to self-antigens are eliminated by apoptosis to establish central tolerance.
T Cell Receptor Structure and Diversity
The αβ TCR is a heterodimer of α and β chains, each containing variable (V) and constant (C) regions. The enormous diversity of the TCR repertoire, estimated at 10¹⁵–10²⁰ possible receptors, is generated by somatic recombination of V, D (for β chain only), and J gene segments through V(D)J recombination mediated by RAG1 and RAG2 recombinases. Junctional diversity is further increased by the addition of template-independent N-nucleotides by terminal deoxynucleotidyl transferase (TdT) and by exonuclease trimming. The CDR3 region, spanning the V(D)J junction, is the most variable and directly contacts the peptide presented by MHC, determining antigen specificity.
T Cell Activation by Antigen-Presenting Cells
Naive T cells circulate through secondary lymphoid organs (lymph nodes, spleen, Peyer patches) scanning dendritic cells for their cognate antigen. T cell activation requires three signals. Signal 1 is TCR recognition of peptide-MHC complexes, with CD8+ T cells recognizing peptides on MHC class I and CD4+ T cells recognizing peptides on MHC class II. Signal 2 is co-stimulation, primarily the interaction of CD28 on T cells with CD80/CD86 (B7) on dendritic cells, which is essential for full activation and prevents anergy (functional unresponsiveness). Signal 3 consists of polarizing cytokines that direct effector differentiation, as determined by the cytokine milieu produced by the activated dendritic cell.
CD4+ T Helper Cell Subsets
Upon activation, CD4+ T cells differentiate into distinct effector subsets directed by the cytokine environment. Th1 cells, driven by IL-12 and IFN-γ via STAT4 and T-bet, produce IFN-γ and TNF-α and are essential for immunity against intracellular bacteria and viruses by activating macrophages. Th2 cells, driven by IL-4 via STAT6 and GATA3, produce IL-4, IL-5, and IL-13 and mediate immunity against helminths and allergic inflammation. Th17 cells, driven by TGF-β, IL-6, and IL-23 via STAT3 and RORγt, produce IL-17A, IL-17F, and IL-22 and are critical for antifungal immunity and neutrophil recruitment at mucosal surfaces. Regulatory T cells (Treg), driven by TGF-β via FoxP3, produce IL-10 and TGF-β and suppress immune responses to maintain tolerance and prevent autoimmunity. Follicular helper T cells (Tfh), driven by IL-6 and IL-21 via Bcl6, localize to B cell follicles and provide essential help for germinal center formation and antibody production.
CD8+ Cytotoxic T Cell Activation
Activation of naive CD8+ T cells requires recognition of peptide-MHC class I complexes and co-stimulation, which is optimally provided by dendritic cells that cross-present exogenous antigens on MHC class I. Fully activated CD8+ T cells proliferate extensively and differentiate into cytotoxic T lymphocytes (CTLs) that kill target cells through two principal mechanisms. The granule exocytosis pathway releases perforin, which forms pores in the target cell membrane, and granzymes (serine proteases) that enter through these pores and activate caspases to induce apoptosis. The Fas-FasL pathway involves upregulation of Fas ligand on activated CTLs, which engages Fas (CD95) on target cells, recruiting FADD and caspase-8 to form the death-inducing signaling complex. CTLs also secrete IFN-γ and TNF-α that enhance antigen presentation and amplify the immune response.
Memory T Cells
After antigen clearance, the majority of effector T cells die by apoptosis, but a small fraction persists as long-lived memory T cells capable of rapid response upon re-exposure. Central memory T cells (TCM) express CCR7 and CD62L, recirculate through lymphoid organs, and have high proliferative capacity. Effector memory T cells (TEM) lack lymphoid homing receptors, reside in peripheral tissues, and provide immediate effector function. Tissue-resident memory T cells (TRM) permanently reside in barrier tissues such as skin, lung, and gut, providing first-line defense at sites of pathogen entry. Stem cell-like memory T cells (TSCM) have enhanced self-renewal capacity and multipotency.
