T and B cell collaboration in response to vaccination

Abstract

The invention of vaccines has profoundly reduced the burden of infectious disease worldwide. However, for some infectious diseases including malaria, HIV and tuberculosis we have struggled to develop vaccines that give enduring protection. Long lasting antibody responses depend on the collaboration of T cells with B cells to form memory and bone marrow plasma cells. Thus, my thesis aimed at addressing the basic questions in T and B cell biology, hoping to provide new insights in vaccination strategy. In my thesis I make three major contributions to this area.

Firstly, I reported that Tfh17 cells are superior in immunological memory maintenance. Memory Tfh cells are known to contribute to protection by enhancing humoral responses upon antigen re-exposure but how they are maintained is poorly understood. Memory Tfh are divided into Tfh1, Tfh2 and Tfh17 cells by the expression of CXCR3 and CCR6 respectively. Using a method to differentiate murine Tfh1, Tfh2 and Tfh17-like (iTfh1, iTfh2 and iTfh17) cells in vitro, I found iTfh17 cells were superior to iTfh1 and iTfh2 cells with survival and proliferative advantages. Importantly, analysis of multiple human cohorts that received different vaccines for HBV, influenza virus, tetanus toxin or measles revealed that vaccine-specific Tfh17 cells outcompeted Tfh1 or Tfh2 cells for the persistence in memory phase.

Next, I investigated the role of a poorly characterized population of B cells called atypical B cells (ABCs) in vaccination. These cells are abundant in chronic disease and autoimmune conditions, but also seen in responses to vaccination. However, the factors driving their development and maintenance are unknown. Here I show that zinc finger E-box binding homeobox 2 (ZEB2) was identified as the lineage defining transcriptional factor (TF) of ABCs. I generated a human tonsil single-cell RNA-seq (scRNA-seq) dataset and showed that ABCs had high ZEB2 expression. ABC formation was severely impaired among CD23Cre/+Zeb2fl/fl B cells after Plasmodium infection or immunization in mice. In humans I found that ZEB2 haploinsufficient Mowat Wilson syndrome patients also had low numbers of circulating ABCs. Surprisingly, Zeb2 deficiency had no impact on other aspects of the B cell response in both mice and humans, suggesting a minimal role for ABCs in healthy immune responses.

Finally, I investigated how B cells obtain antigen from complex pathogens for presentation for T cells. While B cell responses to recombinant proteins and small viruses are well understood, the interaction with complex eukaryotic pathogens is understudied. Here I discovered antigen specific B cells used a unique "surface pinching" mechanism to extract antigens from the plasmodium sporozoites (SPZs). By visualizing the interaction of Plasmodium falciparum circumsporozoite protein (PfCSP) specific Ighg2A10 B cells with PfCSP expressing SPZs (PfCSP-SPZs), I found Ighg2A10 B cells directly "pinched off" surface antigens from PfCSP-SPZs. scRNA-seq analysis confirmed the repertoire of Ighg2A10 helping Tfh cells were mainly PfCSP specific upon SPZ immunization. I further validated this by using the SPZs strains expressing OVA on spatially different locus and showed that Ighg2A10 B cells could only be helped by OT-II cells when OVA was located on the surface but not interior of the SPZs.