DECODING LYMPHOCYTE DEVELOPMENT AND DIFFERENTIATION

Abstract

Mammals are equipped with a complex adaptive immune system that provides them protection against foreign pathogens, toxins or allergenic agents and also tumors. B and T lymphocytes comprise critical building blocks of this system and are essential in establishing effective humoral and cellular immune responses. In this thesis, we first introduced the reader to key processes involved at multiple stages of lymphocyte development and differentiation. In the next part, we demonstrated that transition from Pro- to Pre-B cells stage during early development of B lymphocytes is exclusively dependent on the Ig heavy chain (IgHC) and not directed by a non-coding function of the Ig heavy mRNA (IgHR). We also highlighted the capacity of progenitor B cells in sensing very low amounts of IgHC, that licenses further development. Phenotypically and functionally, lymphocytes display great heterogeneity and plasticity. This cellular plasticity is accounted for by dynamic changes in their epigenetic landscape including post translational modifications of histone. Across distinct developmental and differentiation stages, the genome of a lymphocyte undergoes many of these epigenetic modifications. In subsequent sections of the thesis, we highlighted the functional significance of an epigenetic writer, DOT1L in lymphocytes biology. We demonstrated that DOT1L plays a critical role in establishing germinal center B cells, a B cell differentiation state that is closely associated with lymphoma formation. Furthermore, we showed that DOT1L prevents premature differentiation of B cells into plasma-like cells. In later part of the thesis, we provided a detailed characterization of CD8+ T lymphocytes that lack DOT1L. We demonstrated that DOT1L is a key epigenetic writer that safeguards the epigenetic identity of naïve CD8+ T lymphocytes. In absence of DOT1L, CD8+ T lymphocytes prematurely differentiate towards antigen-independent memory T cells. Apart from physiological significance, lymphocytes especially B cells also represent as a unique biological system that have programmed DNA double strand break (DSBs). These DSBs provide the basis of key processes that are related with their functionality. With their capacity of regulating DSBs repair, B cells can be used to study the role of proteins that may become involved in DSBs repair process. In next part of the thesis, with the aim to check the proposed direct role of poorly characterized protein CAAP1 in regulating apoptosis and delaying DNA double strand break (DSBs) repair, we described a new mouse model that lack CAAP1. Comprehensive analysis involving different cellular systems including B lymphocytes that lack CAAP1 we disproved the proposed role of CAAP1 and suggested that new approaches should be adopted to unravel significance of CAAP1 in biological system. At the end, we summarized and discussed key findings from the thesis and also presents an outlook for the future research