New insights into the significance of the BCR repertoire in B-1 cell development and function

As an important player of innate immunity, B-1 cells have been characterized by natural antibody production and rapid immune response. Based on their expression or lack of CD5, B-1 cells are further separated into B-1a and B-1b subsets.¹ Although extensive studies have demonstrated a pivotal role of conventional B cells in adaptive immunity, there is increasing evidence indicating the close associations of B-1a cells with self-reactivity, autoimmunity, infection, and even leukemia.^2–4 Here, we describe recent advances that lead to a clearer understanding of B-1 cell development with a focus on new insights into the significance of the B-cell receptor (BCR) repertoire in B-1a cell biology and discuss the potential strategies for specific targeting of B-1a cells under pathological settings.

The repertoire of B-1a cells is shaped by self-antigens and evolves in adulthood

B-1a cells have long been known to produce the “natural” antibodies against both self-antigens and general motifs of microbial antigens that are present prior to an infection, but exactly how the BCR repertoire in B-1a cells is selected and shaped remains largely unclear. Using high-dimensional fluorescence-activated cell sorting with quantitative immunoglobulin heavy chain (IgH) deep-sequencing technologies, Yang et al.⁵ extensively studied the third IgH complementarity-determining region of B-1 and B-2 populations from various organs of mice at different ages. Although the de novo development of B-1 cells in adulthood is largely restricted, the BCR repertoire in B-1a cells from peritoneal cavities and spleens was found to dynamically evolve throughout their ontogeny, reflecting an ongoing selection and/or clonal expansion of the B-1a cell repertoire pool throughout adulthood. Similar repertoire changes in B-1a cells were observed among different individuals even when mice were reared in the absence of microbiota, as revealed by the striking evidence that the B-1a cell repertoire from mice housed in specific pathogen-free and germ-free conditions was indistinguishable. Equally interesting was that activation-induced cytidine deaminase-mediated somatic hypermutation (SHM) was observed after weaning and cumulatively increased with age. This process was associated with class switch recombination but was not driven by microbiota-derived antigens. Since B-1a cells are known to produce anti-self antibodies, these data provide further evidence suggesting that endogenous (self) antigens are possibly the major driving force for repertoire selection. These observations not only support a role for B-1a-derived natural antibodies in maintaining homeostasis but also identify a molecular basis for the notion that age-dependent accumulative SHM may increase the risk of autoimmune disease due to pathogenic high-affinity autoreactive antibodies. From this perspective, the participation of B-1a cells in the first line of host defense may partially result from their cross-reactivity with microorganism-derived antigens, such as the hemagglutinin glycoprotein of influenza virus membrane.⁶ The sequence data presented in this study offer significant insights into understanding the origins and behaviors of B-1 cells in both hemostasis and autoimmunity, in which increased production of autoantibodies is implicated in the pathogenesis of autoimmune diseases.

Recently, Graf et al.⁷ elegantly demonstrated the importance of BCR specificity in driving and maintaining the fate of murine B-1 cells. The authors generated two lines of BCR heavy-chain locus transgenic mice in which the V_H12 gene that is exclusively expressed by B-1 cells with specificity against phosphatidylcholine and the B1-8 IgH gene, a typical IgH gene segment of conventional B-2 cells, were fused head to head and flanked by inverted loxP sites at the IgH locus. By expressing Cre recombinase in B cells, the inversion of the inserted V_H cassette generated B cells that switched from expression of the B1-8/Vk4 BCR to the V_H12/Vκ4 BCR or vice versa. Using a variety of in vitro, in vivo, and adoptive transfer approaches, they showed that B-1 cells in the V_H12/Vκ4 mice did not adopt a B-2 phenotype when induced to express the B1-8/Vκ4 BCR. This finding suggests that strong BCR engagement in early postnatal life has “hardwired” a typical feature of B-1 cells. In contrast, a subset of mature B-2 and marginal zone B cells originally expressing the B1-8/Vκ4 BCR and then being induced to express the V_H12/Vκ4 BCR acquired a B-1-like phenotype and transcriptome profile, and functioned like B-1 cells, including spontaneous production of IgM, self-renewal and homing to pleural and peritoneal cavities. Thus, engraftment of the autoreactive, B-1 cell-typical BCR appeared to reprogram the fate decision of mature B-2 cells. This study still leaves many questions unanswered. For example, how generalizable these findings are for different BCRs, and the effect of BCR antigen specificity on B-cell subset fate and maintenance of that fate. Indeed, deficiency of the transcription factor Bhlhe41 led to selective loss of B-1a cells and the V_H12/Vκ4 BCR. However, transgenic expression of prearranged V_H12/Vκ4 BCR failed to rescue the altered phenotype, revealing that multiple mechanisms have evolved in controlling B-1a cell differentiation beyond the BCR repertoire and antigenic specificity.^8,9 Nevertheless, these seminal findings shed new light on understanding the significance of BCR usage in B cell fate decisions.

Autoreactive B-1a cells initiate autoimmune pathogenesis

In line with a potential pathogenic role of the B-1a repertoire in autoimmune pathogenesis, a study by Diana et al.¹⁰ assessed how activated B-1a cells initiate autoimmune diabetes by secreting autoreactive antibodies. In this study, they observed that, parallel to the wave of physiological beta cell death during the first postnatal week, a large population of B-1a cells transiently infiltrated into the pancreatic islets of non-obese diabetic (NOD) mice. Consistent with the existing data on the skewing of the B-1a cell repertoire to mostly autoreactive specificities, pancreatic infiltrating B-1a cells secreted IgGs specific for double-stranded DNA (dsDNA), which subsequently activated neutrophils to release DNA-binding cathelicidin-related antimicrobial peptide (CRAMP). Then, dsDNA, dsDNA-specific IgG, and CRAMP activated plasmacytoid dendritic cells (pDCs) through the toll-like receptor 9-myeloid differentiation factor 88 pathway, leading to interferon-α (IFN-α) production in pancreatic islets, and ultimate onset of type I diabetes (T1D). By performing peritoneal lavages to deplete B-1a cells from pancreatic islets of NOD mice, the frequency of IFN-α-secreting pDCs and the subsequent diabetogenic T cell response within the pancreatic lymph nodes and islets of prediabetic NOD mice were profoundly reduced, unveiling a critical role of pancreatic B-1a cells in the activation of pDCs and T1D initiation. In addition to their potential to differentiate into antibody-producing plasma cells, B-1 cells can also modulate local immune responses through secretion of soluble products. We also found that B-1a cells participated in the development of autoimmune arthritis by producing the receptor activator of NF-κB ligand during disease progression.¹¹ Together, these findings indicate new therapeutic avenues directed toward the targeting of B-1a cells for the treatment of autoimmune diseases.

Manipulation of cell metabolism to target pathogenic B-1a cells

In a recent study by Clarke et al.,¹² murine peritoneal B-1a cells were described as being more metabolically active than follicular B cells and relying on lipophagy for their homeostasis and self-renewal. Treatment with a fatty acid synthase inhibitor led to a dramatic depletion of peritoneal B-1a cells, with no significant effect on peritoneal B-2 cells or other subsets in the spleen. They further found that the autophagy process functioned critically in maintaining B-1a cell metabolic homeostasis by providing metabolites such as free fatty acids. This could explain the preferential decrease of the B-1a cell subset among B cell populations under Atg7 deficiency in this study and Atg5 deficiency in a previous study.¹³ The discovery of this unique pathway in B-1a cells represents an important advance in this field, which opens up the possibility to explore the pharmacological agents that inhibit glycolysis or autophagy to target pathogenic B-1a cells in various disease settings.

Competing interests

The authors declare no competing interests.