Transcriptional control of CD4+ T lymphocytes

Sammanfattning: The immune system can respond to various types of pathogens such as bacteria, viruses, tumors and parasites. The ability to provide protection against different types of pathogens partly relies on development of alternative T effector cells with separate effector functions. The discovery of new T helper subsets, provide insight to the great flexibility of the immune system, and raises questions about how alternative cell fates are regulated. Epigenetic regulation of genes, i.e. modulations to chromatin that affects transcription in a heritable manner without affecting primary DNA, has proven essential for the regulation of alternative T helper cell development. Many aspects of epigenetic regulation in T cells is however still unexplored, especially in the human immune system as most studies regarding T cells and epigenetics are performed with murine T cells. The aim of this thesis was to explore epigenetic and transcriptional mechanisms involved in regulating differentiation and homeostasis of CD4+ T lymphocytes. Furthermore, the aim was to investigate if altered epigenetic regulation can be observed in clinical situations associated with dysregulated CD4+ T lymphocytes. Initially, we investigated the methylation profile of cytokine and transcription factor loci known to be important for CD4+ cell differentiation and/or effector function. We show that complete demethylation of the FOXP3 promoter is only observed in committed T regulatory cells, whereas nonregulatory T cells exhibiting transient FOXP3 expression are partially methylated at the FOXP3 promoter. Furthermore we describe interspecies differences in regulation of IFN-gamma expression, where the human IFNG gene is not ´poised` for rapid onset of IFN-gamma expression. When investigating IFNG methylation in tumor infiltrating lymphocytes, we show that these cells are subjected to immunosuppression mediated by CpG hypermethylation of the IFNG promoter. This thesis also includes development of a method for analysis of commitment to any of the known CD4+ T cell subsets; Th1, Th2, Treg and Th17. When using this method to profile the CD4+ T cell population in autoimmune diseases we observe deviations from normal immune homeostasis, demonstrating how epigenetic regulation of T cell effector fates has great relevance for clinical immunology. Finally, we show that a micro-RNA, miR-155, regulates T cell proliferative responses by targeting the cytotoxic T lymphocyte antigen 4 (CTLA-4), a negative regulator of immune responses. In conclusion, the work presented here highlights the importance of transcriptional control in development and homeostasis of CD4+ T lymphocytes.