IgM and IgD B cell receptors differentially respond to endogenous antigens and control B cell fate.

Naive B cells co-express two BCR isotypes, IgM and IgD, with identical antigen-binding domains but distinct constant regions. IgM but not IgD is downregulated on autoreactive B cells. Because these isotypes are presumed to be redundant, it is unknown how this could impose tolerance. We introduced the Nur77-eGFP reporter of BCR signaling into mice that express each BCR isotype alone. Despite signaling strongly in vitro, IgD is less sensitive than IgM to endogenous antigen in vivo and developmental fate decisions are skewed accordingly. IgD-only Lyn-/- B cells cannot generate autoantibodies and short-lived plasma cells (SLPCs) in vivo, a fate thought to be driven by intense BCR signaling induced by endogenous antigens. Similarly, IgD-only B cells generate normal germinal center, but impaired IgG1+ SLPC responses to T-dependent immunization. We propose a role for IgD in maintaining the quiescence of autoreactive B cells and restricting their differentiation into autoantibody secreting cells

Non-redundant functions of IgM and IgD B cell receptors in B cell biology

Naïve B cells co-express two different BCR isotypes, IgM and IgD, which have identical antigen binding domains but distinct membrane proximal regions. Despite decades of investigation, it is still unclear why B cells co-express both isotypes. Initial studies of IgD- and IgM-deficient mice concluded that IgM and IgD can largely substitute for each other. However, the isotypes differ in structure and expression pattern. IgD is highly expressed on the surface of all naïve B cells, but surface IgM expression is downregulated on autoreactive B cells. Here we demonstrate that IgD is less sensitive than IgM to self-antigen, and the isotypes differ in their ability to drive rapid antibody responses. We generated competitive chimeras in which B cells expressed either IgD or IgM alone. IgD-expressing cells adopted a developmental pattern consistent with reduced self-antigen recognition; they did not enter the B1a compartment but efficiently generated marginal zone cells. We crossed a reporter of endogenous antigen signaling, Nur77-eGFP, onto IgM- and IgD-deficient backgrounds. Differences in reporter expression suggest that IgD senses endogenous antigens more weakly than IgM in vivo. Lyn-/- mice lack a kinase essential for inhibition of BCR signaling, resulting in inappropriate B cell responses to self-antigen and aberrant short-lived plasma cell (SLPC) generation. Lyn-/- B cells expressing exclusively IgD did not differentiate into SLPCs, but were competent to generate germinal center (GC) responses. Similarly, B cells expressing exclusively IgD exhibit normal GC but impaired IgG1+ SLPC generation in response to T-dependent immunization. We propose a model in which autoreactive cells are excluded from rapid antibody responses due to predominant IgD and low IgM expression, but these cells can still participate in immune responses by entering a GC response and undergoing somatic mutation

Optimal Development of Mature B Cells Requires Recognition of Endogenous Antigens.

It has long been appreciated that highly autoreactive BCRs are actively removed from the developing B cell repertoire by Ag-dependent receptor editing and deletion. However, there is persistent debate about whether mild autoreactivity is simply tolerated or positively selected into the mature B cell repertoire as well as at what stage, to what extent, under what conditions, and into which compartments this occurs. In this study, we describe two minor, trackable populations of B cells in B1-8i Ig transgenic mice that express the VH186.2 H chain and recognize a common foreign Ag (the hapten 4-hydroxy-3-nitrophenylacetyl) but differ in L chain expression. We use the Nur77-eGFP reporter of BCR signaling to define their reactivity toward endogenous Ags. The less autoreactive of these two populations is strongly counterselected during the development of mature B1a, follicular, and marginal zone B cells. By genetically manipulating the strength of BCR signal transduction via the titration of surface CD45 expression, we demonstrate that this B cell population is not negatively selected but instead displays characteristics of impaired positive selection. We demonstrate that mild self-reactivity improves the developmental fitness of B cell clones in the context of a diverse population of B cells, and positive selection by endogenous Ags shapes the mature B cell repertoire

An extracatalytic function of CD45 in B cells is mediated by CD22.

The receptor-like tyrosine phosphatase CD45 regulates antigen receptor signaling by dephosphorylating the C-terminal inhibitory tyrosine of the src family kinases. However, despite its abundance, the function of the large, alternatively spliced extracellular domain of CD45 has remained elusive. We used normally spliced CD45 transgenes either incorporating a phosphatase-inactivating point mutation or lacking the cytoplasmic domain to uncouple the enzymatic and noncatalytic functions of CD45 in lymphocytes. Although these transgenes did not alter T-cell signaling or development irrespective of endogenous CD45 expression, both partially rescued the phenotype of CD45-deficient B cells. We identify a noncatalytic role for CD45 in regulating tonic, but not antigen-mediated, B-cell antigen receptor (BCR) signaling through modulation of the function of the inhibitory coreceptor CD22. This finding has important implications for understanding how naïve B cells maintain tonic BCR signaling while restraining inappropriate antigen-dependent activation to preserve clonal "ignorance.