Regulation of human T helper 17 cell responses

Abstract

T helper 17 (Th17) cells potently produce interleukin (IL)-17, which is essential for Th17 cell-mediated pathogen clearance. Failure to regulate Th17 cells can increase Th17 cell numbers and IL-17 production, and is associated with autoimmune disease pathology. Therefore, understanding how Th17 cell responses are controlled may improve treatments in instances of Th17 cell dysregulation. Investigations in mice and humans have mainly studied the cytokine signals that determine Th17 cell responses. However, the strength of TCR signalling has previously been shown to be a further factor capable of determining effector T cell development. The central hypothesis of my thesis is, therefore, that the strength of TCR stimulation is also capable of regulating Th17 cell responses. I also investigated if T-cell density, a parameter often overlooked in investigations, can also affect Th17 cell responses. Cell density has been shown previously to be capable of modulating many parameters including the expression of certain Th17 cell-related transcription factors. To assess the effect of T-cell stimulation strength on Th17 cell responses, human CD4+ T-cells were activated with high or low strength stimuli administered by bead-bound antibodies which activate the TCR/CD3 complex and the costimulatory molecule CD28, or by monocyte-derived dendritic cells pulsed with decreasing superantigen concentrations. Experiments were performed in the presence of pro-Th17 cell cytokines IL-1β, TGFβ and IL-23. In both systems low strength TCR stimulation profoundly and significantly promoted Th17 cell responses, both proportionately and absolutely. The enhancement of Th17 cell responses by low TCR stimulation only occurred in the presence of co-stimulation through CD28. Furthermore, it was revealed by chromatin immunoprecipitation that low strength stimulation promoted Th17 cell responses by allowing binding of a Ca2+ regulated transcription factor NFATc1 to the IL-17 promoter in a Ca2+ dependent manner. To investigate how low strength T-cell stimulation might promote human Th17 cell responses in vivo, 20 healthy donors were genotyped for a single nucleotide polymorphism within the gene Protein Tyrosine Phosphatase-N22 (PTPN22), which is highly associated with the autoimmune diseases type I diabetes and rheumatoid arthritis. PTPN22 encodes a TCR signalling molecule, Lyp, which in minor allele carriers confers both reduced TCR and Ca2+ signalling. Culture of genotyped memory CD4+ T-cells with anti-CD3/anti-CD28 stimulation and pro-Th17 cell cytokines revealed a trend indicating that the presence of the minor T allele promoted both IL-17 and IFN- production but decreased regulatory IL-10 secretion. Regulation of Th17 cells by T-cell density was explored by culturing memory CD4+ T-cells at decreasing T-cell densities in the presence of either pro-Th17 or pro-Th1 cell cytokines. Low T-cell densities profoundly promoted Th17 cell responses both proportionately and absolutely. No effect was observed on the IFNy response within cultures containing pro-Th1 cell cytokines, suggesting that T-cell density specifically affects Th17 cell responses. STAT3 activation, important for IL-17 expression, can be regulated by cell density. Analysis of STAT3 activation by western blot revealed higher STAT3 activation in low density cultured T-cells compared to high density cultured T-cells, which may provide an explanation for the increased Th17 cell responses observed. The data within this thesis provide interesting and novel mechanisms by which human Th17 cell responses are regulated. I have demonstrated that Th17 cell responses are favoured by both low strength TCR stimulation and low T-cell density. These data highlight the diversity of factors capable of affecting Th17 cell responses in vitro; factors of which in the majority of studies have been overlooked.