(G) Comparison of CD38hiCD138+cells (%) within IgG+MBCs and IgM+MBCs as in (F)

(G) Comparison of CD38hiCD138+cells (%) within IgG+MBCs and IgM+MBCs as in (F). switched IgG+and unswitched IgM+MBCs; however, whether these MBC subpopulations are comparative in their response to B cell receptor cross-linking and their producing fates is usually incompletely understood. Here, we show that IgG+and IgM+MBCs can be distinguished based on their response to -specific monoclonal antibodies of differing affinities. IgG+MBCs responded only to high-affinity anti- and differentiated almost exclusively toward PC fates. In contrast, IgM+MBCs were eliminated by apoptosis by high-affinity anti- but responded to low-affinity anti- by differentiating toward GC B cell fates. These results suggest that IgG+and IgM+MBCs may play unique yet complementary functions in response to pathogen challenge ensuring the immediate production of high-affinity antibodies to homologous and closely related challenges and the generation of variant-specific MBCs through GC reactions. The adaptive human immune system is amazing in its ability to provide lifelong protection against pathogen reinfection in individuals who either survived an initial infection or were exposed to a less virulent or attenuated form of the pathogen through vaccination. Immunity confers protection to homologous antigen challenge, and protection can lengthen to mutant variants of the original pathogen depending on exposure history and antigenic contexts of contamination versus vaccination as observed in the current COVID pandemic (1). Antibodies play an essential effector role in immunity to pathogens, and we now understand that at the cellular level, antibody immunity is KPT 335 dependent around the antigen-driven acquisition of both long-lived plasma cells (LLPCs) and memory B cells (MBCs) (25). Both LLPCs and MBCs differentiate from naive B cell precursors in highly specialized microenvironments of germinal centers (GCs) in secondary lymphoid tissues in which GC B cells undergo somatic hypermutation (SHM) and T celldependent affinity selection (6,7). As recently reviewed, most of our present knowledge of the composition and function of the mammalian immune system has come from studies of mice Mouse monoclonal to PRKDC (8,9) including molecular details of MBC reactivation upon secondary challenge (4). It is not yet obvious how results from mice studies can be reliably translated to results of human studies, but it would not be unexpected that contradiction would arise. Consequently, it will be important to appreciate studies done both KPT 335 in mice and humans in interpreting new data. LLPCs are generated late in GC responses and are highly antigen selected and capable of generating high-affinity, isotype-switched antibodies KPT 335 specific for the homologous pathogen (10). LLPCs take up residency in the bone marrow and secrete large amounts of antibodies into the blood circulation that persist for years, potentially for an individuals lifetime, in the absence of antigen or a secondary challenge (11,12). Thus, LLPC-produced antibodies act as a first line of defense against homologous or closely related pathogen challenge (2,4). Immune individuals also acquire long-lived MBCs, but as compared to LLPCs, MBCs are generated early in GC responses KPT 335 and appear to be less highly somatically mutated and less selected by antigen and as a consequence maintain broadly reactive B cell repertoires (3,5,13). Indeed, the repertoire of MBCs and LLPCs does not appear to fully overlap (10,14,15), leading to the hypothesis that the greater diversity of the MBCs compartment enables MBCs to respond to variant heterologous pathogens that escape acknowledgement by LLPC-produced antibodies (1517). In both mice and humans, upon antigen challenge by vaccine improving or pathogen reinfection, MBCs appear to predominantly undergo three fates: either differentiating into PCs that produce high-affinity neutralizing antibodies to homologous antigen challenge; differentiating toward GC B cells capable of reentering GCs and undergoing SHM and antigen-affinity selection providing additional long-lasting protection against heterologous variant antigens; or undergoing apoptosis (3,18,19). The ability of MBCs generated through main immunization to rapidly and efficiently differentiate into PCs.