Dissecting the innate immune networks regulating CD4+ T cell differentitation

Abstract

The mammalian immune system has evolved distinct strategies for combatting a wide variety of pathogens, which range from minute single-cell organisms (bacteria and yeast) to large parasitic worms (helminths). A central component of tailoring immune responses towards different pathogens involves the differentiation of naïve CD4+ T helper (Th) cells into functionally distinct sub-classes of effector Th cells that are distinguishable by their unique cytokine profiles. Intracellular bacteria promote the differentiation of IFNγ+ Th1 cells, whereas helminth and fungal material promote the differentiation of IL-4/IL-13+ Th2 cells and IL-17+ Th17 cells, respectively. Furthermore, a proportion of Th cells differentiate into IL-21+ T follicular helper (Tfh) cells that required for B cell activation, germinal centre formation and immunoglobulin class-switching. The processes that regulate the differentiation of CD4+ T cells are multifactorial and are currently incompletely understood.

Antigen presenting cells (APC), including dendritic cells (DC) and monocytes, are required for the initiation of CD4+ T cell responses and are thought to play a central role in regulating CD4+ T cell differentiation in lymph nodes (LNs). In particular, migratory DC enable the presentation of tissue-derived antigen to CD4+ T cells by actively transporting antigen from the tissue to local LNs via the afferent lymphatics. APC can be resolved into many developmentally and phenotypically distinct subsets, which have been proposed to specialise in initiating a particular CD4+ T cell differentiation programme upon activation. However, signals derived from the antigen itself or from surrounding cells in the tissue environment have the potential to influence APC function. Therefore, functional conditioning of APC by antigenic or environmental cues could also play an important role in regulating CD4+ T cell differentiation.

This thesis dissects the roles of distinct APC populations and APC conditioning signals in the regulation of CD4+ T cell differentiation. Using an intradermal immunisation model, the requirements for CD4+ T cell responses to bacterial (M. smegmatis; Ms), helminth (N. brasiliensis; Nb) and fungal (C. albicans; Ca) antigens were assessed in a systematic fashion. Injection of fluorescently-labelled antigen revealed that migratory CD326- CD103- DC (DC2) and monocytes were the predominant APC subsets associated with antigen uptake. Restricting LN homing of migratory DC2 using an Itgax-cre Irf4-flox mouse model significantly reduced the CD4+ T cell response to all immunogens, whereas depletion of monocytes was associated with a reduced CD4+ T cell response to Ms. These data show that DC2 are required for optimal Th2 and Th17 responses and act in concert with monocytes to support optimal Th1 differentiation. Importantly, transcriptional analysis revealed that antigen-carrying DC2 and monocytes expressed context-specific transcriptional profiles, suggesting that conditioning of APC by antigenic or environmental signals was involved in regulating CD4+ T cell polarisation. Indeed, IFNγ-conditioning of DC2 and monocytes was found to be a key component in promoting effector Th1 responses, while restricting Tfh cell development. On the other hand, IFN-I-conditioning of DC2 was required to amplify CD4+ T cell responses to Nb, while thymic stromal lymphopoietin (TSLP) specifically promoted Th2 differentiation. Together, this thesis highlights the importance of APC conditioning in the regulation of CD4+ T cell differentiation, providing vital information for the development of new immunotherapeutic strategies.