Investigating the roles of the alternative Isoforms of the preTCR alpha (pTα) chain in T cell development

Abstract

The pre-T cell receptor (preTCR) is required for αβ T cell development and hence efficient adaptive immunity. Composed of a rearranged TCRβ chain paired with the invariant preTCR α chain (pTα), it drives immature thymocytes through the “β-selection” checkpoint, promoting differentiation, proliferation and survival. Although two functional splice isoforms of pTα exist, non-redundant roles for pTαa and pTαb have yet to be ascribed. This thesis demonstrates that pTαa and pTαb display non-identical expression patterns in immature thymocyte subsets. Moreover, it demonstrates that preTCRa and preTCRb promote divergent T cell development; preTCRa drives prolonged expansion of post-β-selection thymocytes, while preTCRb drives rapid maturation of TCRαβ(+) cells. Importantly, by mutating charged residues in the extracellular domain of pTα, it is shown that pTα oligomerization is not required for preTCR signalling per se, as had previously been proposed. Instead, the capacity for oligomerization regulates surface preTCR levels, which in turn dictates subsequent developmental potential. This thesis also presents preliminary evidence for a novel role for the preTCR; that preTCR signalling sensitises the extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase pathway that in turn regulates the signalling thresholds for positive and negative selection in CD4(+)CD8(+) double positive (DP) cells. Indeed, it is suggested that this process may be critical for central tolerance in the thymus. Finally, this thesis describes the generation and initial characterisation of pTαa and pTαb BAC transgenic mice that will facilitate the separate investigation of pTαa and pTαb in a pTα-deficient background in vivo. Collectively, the results presented in this thesis ascribe non-redundant roles for the two isoforms of pTα and necessitate a fresh examination of the mechanism by which the preTCR initiates signalling. The implications of these results for αβ T cell development and the immune system as a whole are discussed.