Transcriptional and migration regulation of T follicular helper cell differentiation

© 2021 Amania Anwar SheikhT follicular helper cells (TFH) are specialised CD4+ T cells that promote B cells maturation into antibody secreting plasma and memory cells. Most of the current vaccines generate protection via the induction of long-term antibody responses and circulating TFH are reliable predictors of vaccine response. Conversely, dysfunctional TFH cells are associated with the pathogenesis of immunodeficiency, systemic autoimmunity and allergy. Despite their importance, we have an incomplete understanding of how TFH cells differentiate and function in distinct inflammatory settings. Answering this question has broad health implications to stimulate rational development of vaccines and therapeutics against diverse infections, autoimmune and allergic disease. In this thesis, I investigated the transcriptional and migration control of TFH differentiation during viral infection. In this setting, TFH cells differentiate in parallel with T helper 1 (TH1) CD4+ T effector cells. I investigated two canonical TH1 factors, T-bet and CXCR3, to understand their roles in TFH cells differentiation. Comparing two viral infections, I demonstrated a context-dependent role for T-bet in TFH differentiation and identified the cytokine and chemokine factors that underlie distinct T cell differentiation. Combined, this work demonstrates that there are multiple paths that direct TFH differentiation. This study has led to further investigations into pathogen-specific TFH programs, which will help us understand how TFH orchestrate tailored B cell responses in diverse infections

Assessing the role of the T-box transcription factor Eomes in B cell differentiation during either Th1 or Th2 cell-biased responses.

Successful T-dependent humoral responses require the production of antibody-secreting plasmablasts, as well as the formation of germinal centers which eventually form high-affinity B cell memory. The ability of B cells to differentiate into germinal center and plasma cells, as well as the ability to tailor responses to different pathogens, is driven by transcription factors. In T cells, the T-box transcription factors T-bet and Eomesodermin (Eomes) regulate effector and memory T cell differentiation, respectively. While T-bet has a critical role in regulating anti-viral B cell responses, a role for Eomes in B cells has yet to be described. We therefore investigated whether Eomes was required for B cell differentiation during either Th1 or Th2 cell-biased immune responses. Here, we demonstrate that deletion of Eomes specifically in B cells did not affect B cell differentiation in response to vaccination, as well as following viral or helminth infection. In contrast to its established role in CD8+ T cells, Eomes did not influence memory B cell differentiation. Finally, the use of an Eomes reporter mouse confirmed the lack of Eomes expression during immune responses. Thus, germinal center and plasma cell differentiation and the formation of isotype-switched memory B cells in response to infection are independent of Eomes expression

Context-dependent role for T-bet in T follicular helper differentiation and germinal center function following viral infection

Following infection, inflammatory cues upregulate core transcriptional programs to establish pathogen-specific protection. In viral infections, T follicular helper (TFH) cells express the prototypical T helper 1 transcription factor T-bet. Several studies have demonstrated essential but conflicting roles for T-bet in TFH biology. Understanding the basis of this controversy is crucial, as modulation of T-bet expression instructs TFH differentiation and ultimately protective antibody responses. Comparing influenza and LCMV viral infections, we demonstrate that the role of T-bet is contingent on the environmental setting of TFH differentiation, IL-2 signaling, and T cell competition. Furthermore, we demonstrate that T-bet expression by either TFH or GC B cells independently drives antibody isotype class switching. Specifically, T cell-specific loss of T-bet promotes IgG1, whereas B cell-specific loss of T-bet inhibits IgG2a/c switching. Combined, this work highlights that the context-dependent induction of T-bet instructs the development of protective, neutralizing antibodies following viral infection or vaccination. Shiekh et al. show that, in influenza and LCMV infections, the role of the transcription factor T-bet in TFH differentiation is contingent on environmental cues, IL-2 signaling, and T cell competition. Cell-specific T-bet expression independently drives antibody isotype class switching. Therefore T-bet instructs immune protection in a context-dependent manner