5.1 T cell development in the thymus

T cell development in the thymus is a crucial process that shapes our immune system. It's like a training camp where immature T cells learn to recognize threats while avoiding attacking our own body. This process ensures we have a diverse army of T cells ready to fight infections.

The thymus acts as a strict screening center, eliminating T cells that might cause trouble. Through positive and negative selection, it creates a balanced T cell repertoire that can effectively combat pathogens without harming our own tissues.

T Cell Development in the Thymus

T cell maturation in thymus

• T cell progenitors from bone marrow enter thymus lacking CD4 and CD8 markers (double-negative stage)
• Thymic microenvironment supports T cell development through specialized structures
• Cortical thymic epithelial cells (cTECs) provide Notch ligands for T cell lineage commitment and express unique proteasome subunit β5t for peptide generation
• Medullary thymic epithelial cells (mTECs) express AIRE for tissue-specific antigen presentation crucial for self-tolerance
• Thymic stromal cells produce cytokines (IL-7) promoting T cell survival and proliferation
• T cell maturation progresses through stages: double-negative (DN, CD4- CD8-), double-positive (DP, CD4+ CD8+), and single-positive (SP, CD4+ or CD8+)
• TCR gene rearrangement occurs during DN stage generating diverse receptor repertoire
• Positive and negative selection processes shape functional T cell repertoire eliminating non-functional or self-reactive cells

Positive vs negative selection

• Positive selection occurs in thymic cortex mediated by cTECs presenting self-peptide-MHC complexes
• Process selects T cells with functional TCRs capable of recognizing self-MHC promoting survival of potentially useful T cells
• Negative selection takes place in thymic medulla mediated by mTECs and dendritic cells
• Eliminates T cells with high affinity for self-peptide-MHC complexes preventing autoimmunity
• Selection processes ensure T cell functionality balance self-tolerance with pathogen recognition ability
• Creates diverse T cell repertoire capable of responding to wide range of antigens
• Establishes central tolerance mechanism reducing risk of autoimmune reactions in periphery

TCR and self-peptide-MHC interactions

• TCR composed of α and β chains (or γ and δ chains for γδ T cells) associated with CD3 complex for signal transduction
• V(D)J recombination generates diverse TCR repertoire during DN stage of T cell development
• Low affinity TCR-peptide-MHC interactions promote positive selection ensuring MHC restriction
• High affinity interactions lead to negative selection eliminating potentially autoreactive T cells
• Co-receptors CD4 and CD8 bind MHC class II and class I molecules respectively enhancing TCR signaling
• Signaling thresholds determine cell fate during selection processes influencing lineage commitment (CD4 vs CD8)
• TCR diversity allows recognition of vast array of foreign antigens while maintaining self-tolerance

CD4+ vs CD8+ T cell lineages

• Lineage commitment influenced by strength and duration of TCR signaling and cytokine environment in thymus
• CD4+ T cells recognize peptides presented on MHC class II molecules express transcription factor ThPOK
• CD4+ cells differentiate into helper T cell subsets (Th1, Th2, Th17, Treg) coordinating immune responses
• CD8+ T cells recognize peptides presented on MHC class I molecules express transcription factor Runx3
• CD8+ cells differentiate into cytotoxic T lymphocytes (CTLs) directly killing infected or abnormal cells
• Functional differences: CD4+ T cells provide help to other immune cells while CD8+ T cells execute cytotoxic functions
• Post-thymic maturation involves naive T cell migration to secondary lymphoid organs (lymph nodes, spleen)
• Further differentiation occurs upon antigen encounter in periphery leading to effector and memory T cell generation