Intrinsic heterogeneity in the survival and proliferation capacities of naïve CD8⁺ T cells

Abstract

This thesis describes the identification and characterization of a novel 'layer' of intrinsic non-genetic functional heterogeneity within the seemingly homogeneous naive CD8⁺ T cell population in their survival and proliferation capacities. This heterogeneity is predictably marked by the surface level of CD5. NaYve CD8⁺ T cells that are also CD5hi have a greater intrinsic capacity to proliferate both in response to IL7 alone or identical stimulation of the TCR, pathway (same dose of PMA/ionomycin) as compared to CD51' cells. In contrast, CD510 cells survive better in conditions of cytokine deprivation. The selective proliferation in response to IL7 as well as the relative CD5 level is preserved even after several rounds of activation-induced proliferation and differentiation into memory-like cells in vitro, suggesting that the relative CD5 level could be used as a lineage marker to predict the proliferation capacity of CD8⁺ T cells. Microarray analysis of two naive TCR transgenic CD8⁺ T cells, from the same genetic background, but with marked differences in CD5 levels, namely OT-1 and F5 Rag-/- T cells, revealed consistent differences in their cell survival, proliferation and metabolic pathways. Analysis of upstream regulatory networks suggests that the E2A family of transcription factors and miR-181 are likely involved in setting the proliferation and survival capacities of naive T cells, possibly during thymic development. Our estimates of spMHC-induced signals in OT-1 (CD5hi) and F5 (CD50) cells, based on cytosolic Ca2⁺ influx measurements, suggest that the differences in their lymphophenia-induced proliferation, which were previously attributed to putative corresponding differences in their strength of interaction with self-peptide MHC, may also largely be a result of intrinsic differences in their proliferation capacities in response to 1L7. Further, the potential of exogenous IL7 therapy to skew the CD8⁺ T cell repertoire towards the CD5hi phenotype was demonstrated with in vivo studies in mice. Conversely, antibody-mediated depletion of IL7 has the opposite result.