The role of T cell receptor signal intensity in T helper 17 cell development

Abstract

T-helper (Th) 17 cells are a subset of CD4+ T cells defined through the release of the cytokine interleukin-17a (IL-17a). Activation of these cells is critical for protection against some extracellular bacterial and fungal pathogens. However, a dysregulated Th17 response targeted against self is thought to play an important role in the immunopathology of a number of autoimmune conditions including Inflammatory Bowel Disease (IBD), Multiple Sclerosis (MS) or inflammatory arthritides. Further understanding of the mechanisms that influence the development of Th17 cells may aid future therapeutic targeting of these cells. Whilst the role of the cytokine milieu in Th cell polarisation is relatively well characterised, the degree of signalling through the TCR can also shape the form of the Th cell response. Both the density of antigenic peptide available and the affinity of the antigenic peptide for a particular TCR can contribute to the degree of TCR signalling. The hypothesis of this project was that TCR signal intensity could alter the development of Th17 cells from a naive precursor population. In particular, it was of interest to determine how citrullination of a putative TCR contact amino acid in an antigenic peptide could alter the Th cell response observed. The 5/4E8 T-cell receptor transgenic (TCR Tg) mouse provides a model in which >80% of T-cells specifically recognise an immunodominant epitope derived from the G1 domain of aggrecan – peptide-84-103 (p84-103). This model allowed for the examination of these factors and the underlying mechanism ex vivo using a purified naive CD4+ T cell population in coculture with LPS-matured dendritic cells (mDCs). The data presented in this thesis show the activation, proliferation and effector responses of naive 5/4E8 TCR Tg T cells to alterations in both cognate peptide (p89- 103) density and affinity through citrullination of a putative TCR contact residue (R93Cit). Interestingly, by reducing TCR signal strength the observed response shifts from one dominated by the Th2 phenotype to Th17 cells. Linking the degree of TCR activation to Th cell phenotype was the intensity of IL-2 signalling that in turn shaped the balance between phosphorylated STAT3 and STAT5. Compared to p89-103-primed T cells, T cells responding to R93Cit produced less IL-2, expressed lower levels of the ILiii 2 receptor subunit CD25, and showed reduced levels of STAT5 phosphorylation, whilst STAT3 activation was unaltered. IL-2 blockade in p89-103-primed T-cells selectively reduced STAT5 but not STAT3 phosphorylation, and concomitantly enhanced Th17 development. In summary, this work indicates the impact that changes to the intensity of TCR signalling can have on the murine Th17 response. Indeed, these data illustrate how a disease-relevant post-translational modification such as citrullination can promote Th17 development by altering the balance between STAT5 and STAT3 activation in responding T cells.