T Cell Adolescence: Maturation Events Beyond Positive Selection

Abstract

Single positive (SP) thymocytes that successfully complete positive and negative selection must still undergo one final step, generally termed T cell maturation, before they gain functional competency and enter the long-lived T cell pool. Maturation initiates after positive selection in SP thymocytes, and continues in the periphery in recent thymic emigrants (RTEs), before these newly produced T cells gain functional competency and are ready to participate in the immune response as peripheral naïve T cells. Recent work using genetically altered mice demonstrates that T cell maturation is not a single process, but a series of steps that occur independently and sequentially after positive selection. This review will focus on the changes that occur during T maturation, and the molecules and pathways that are critical at each step.

Generation of Tumor Antigen-Specific iPSC-Derived Thymic Emigrants Using a 3D Thymic Culture System

“…Day 18 Pmel-iPSC-derived cells from OP9/DLL1 co-culture were stimulated with irradiated splenocytes in the presence of cognate hgp100 peptide for 8 days. As with naive CD8 + T cells, Pmel-iPSC-derived T cells expanded specifically in response to cognate peptide and released tumor necrosis factor (TNF) after re-stimulation with anti-CD3/CD28 (Figures S2G and S2H), indicating that they were proliferation-competent and functional, which are markers of T cell maturity (Hogquist et al, 2015). However, most cells lost surface expression of CD8β and obtained a CD8αα + or DN phenotype during expansion (Figure 1E), similar to when immature self-reactive T cells bind strongly to self-antigens (Yamagata et al, 2004).…”

“…An investigation of genes within these pathways revealed that extrathymic T cells had decreased expression of classical MHC class I ( H2-D1 and H2-K1 ) and non-classical MHC genes ( H2-Q4 , H2-T3 , H2-T10 , H2-Q7 , H2-Ob , H2-DMb1 , and H2-DMa ) as well as increased expression of NK receptors ( Klrc1 , Klrc2 , and Klrc3 ) (Figure 1H). The down-regulation of MHC class I genes as well the upregulation of Ptcra , Rag1 , Rag2 , and Rorc suggest that extrathymic T cells have an immature T cell phenotype similar to immature CD8SP (CD8iSP) T cells, potentially because of an inadequate positive selection, impaired TCR signaling, or an unfinished allelic exclusion program (Figure 1I; Carpenter and Bosselut, 2010; Hogquist et al, 2015; von Boehmer, 2005). However, NK receptor expression also confirms potential innate-like features among the extrathymic T cells.…”

“…Flow cytometry was used to measure markers important for T cell differentiation and maturation, including MHC class I, L-selectin (CD62L), and CD69 (Figures 3A–3C). iTEs showed a downregulation of the activation marker CD69 and high expression of MHC class I and CD62L, markers associated with high proliferative competency, cytokine production, peripheral survival, and lymphoid homing (Hogquist et al, 2015; Rosen, 2004; Vivier et al, 2011). This phenotype was consistent with M2 SP thymocytes, the most mature population of SP T cells in the thymus (Hogquist et al, 2015), suggesting that iTEs have transitioned through a normal thymic development program.…”

“…iTEs showed a downregulation of the activation marker CD69 and high expression of MHC class I and CD62L, markers associated with high proliferative competency, cytokine production, peripheral survival, and lymphoid homing (Hogquist et al, 2015; Rosen, 2004; Vivier et al, 2011). This phenotype was consistent with M2 SP thymocytes, the most mature population of SP T cells in the thymus (Hogquist et al, 2015), suggesting that iTEs have transitioned through a normal thymic development program. Moreover, when stimulated in vitro with cognate peptide, Pmel-iTEs rapidly expanded (>1,000-fold), analogous to naive CD8 + T cells, whereas extrathymic T cells lacked robust expansion after cognate peptide stimulation (Figures 3D and S4A).…”

“…Cell functionality was also compared with both primary CD8SP thymocytes and naive T cells (Figure S4E). It is perhaps notable that thymically educated iTEs had cytokine release patterns consistent with those seen in CD8 + T cells that have not yet achieved full effector function because full-blown CD8 + T cell effector differentiation is marked by reduction or elimination of IL-2 production and acquisition of robust levels of IFN-γ (Gattinoni et al, 2011; Hogquist et al, 2015). …”