Memory B cell subsets have divergent developmental origins that are coupled to distinct imprinted epigenetic states and recall responses

The establishment of immune memory in response to initial pathogen exposure is a defining feature of adaptive immunity and underlies the success of vaccines (Kalia et al., 2006). Memory B cells (MBCs) generated from initial exposure respond more quickly to antigen than their naïve B cell (NBC) counterparts and are important for control of infection in specific contexts, including infection of malaria, influenza, CMV, and SARS-CoV-2 (Goel et al., 2022; Klenovsek et al., 2007; Krishnamurty et al., 2016; Li et al., 2012; Muecksch et al., 2022; Wang et al., 2022; Wrammert et al., 2011). In addition, some pathogens have developed mechanisms to circumvent or suppress MBC responses, further emphasizing their importance (Moir et al., 2008; Nothelfer et al., 2015). Despite decades of work, heterogeneity among MBCs has only recently been appreciated; how subsets of MBCs contribute to recall responses and subsequent pathogen clearance is therefore poorly understood. MBC subsets can be defined in mice based on expression of the surface markers CD80 and PD-L2 (Anderson et al., 2007; Tomayko et al., 2010; Zuccarino-Catania et al., 2014). MBCs double-negative (DN) for CD80 and PD-L2 preferentially seed germinal centers upon restimulation. MBCs double-positive (DP) for both CD80 and PD-L2 quickly differentiate into antibody-secreting plasmablasts, with no detectable germinal centers. A PD-L2 single-positive (PD-L2SP) subset showed an intermediate phenotype. Though first defined in hapten-carrier alum immunized mice, other groups have identified these MBC subsets in different systems. However, no intrinsic mechanisms underlying their distinct developmental pathways or functions have been established. In Chapter 3 of this dissertation, we will first discuss how all MBCs are intrinsically rewired compared to NBCs, followed by a discussion between MBC subsets themselves. We report that DN and DP MBCs are generated by distinct proliferating precursors, resulting in imprinted transcriptional and epigenetic states. Additionally, we define new subsets of DP MBCs that are defined by germinal center-independent or germinal center-dependent development, retain transcriptional and epigenetic elements of their respective precursor cells, and have different propensities for plasmablast differentiation upon re-activation. These results provide insight into how cellular memory is stored. We continue these findings in Chapter 4 with a discussion of how the epigenetic differences found in Chapter 3 may prime MBCs for rapid reactivation compared to NBCs, and also may explain the different functions of MBC subsets. By subjecting NBCs and MBCs to various stimuli over time, we were able to determine genes that were differentially induced in each cell type, and found that differential induction was often associated with differential chromatin accessibility as described in Chapter 3. To summarize, the findings reported in this dissertation further advance our understanding of how MBCs store memory of previous activation, and also how this stored memory may be utilized to rapidly respond upon secondary challenge to stimuli.