A negative-feedback function of PKCβ in the formation and accumulation of signaling-active B cell receptor microclusters within B cell immunological synapse

A novel transgenic mouse strain expressing PKCβII demonstrates expansion of B1 and marginal zone B cell populations

Protein kinase Cβ (PKCβ) expressed in mammalian cells as two splice variants, PKCβI and PKCβII, functions in the B cell receptor (BCR) signaling pathway and contributes to B cell development. We investigated the relative role of PKCβII in B cells by generating transgenic mice where expression of the transgene is directed to these cells using the Eµ promoter (Eµ-PKCβIItg). Our findings demonstrate that homozygous Eµ-PKCβIItg mice displayed a shift from IgD⁺IgM^dim toward IgD^dimIgM⁺ B cell populations in spleen, peritoneum and peripheral blood. Closer examination of these tissues revealed respective expansion of marginal zone (MZ)-like B cells (IgD⁺IgM⁺CD43^negCD21⁺CD24⁺), increased populations of B-1 cells (B220⁺IgD^dimIgM⁺CD43⁺CD24⁺CD5⁺), and higher numbers of immature B cells (IgD^dimIgM^dimCD21^neg) at the expense of mature B cells (IgD⁺IgM⁺CD21⁺). Therefore, the overexpression of PKCβII, which is a phenotypic feature of chronic lymphocytic leukaemia cells, can skew B cell development in mice, most likely as a result of a regulatory influence on BCR signaling.

Discs large homolog 1 regulates B-cell proliferation and antibody production

Antibody production results from B-cell activation and proliferation upon antigen binding. Discs large homolog 1 (Dlg1), a scaffold protein from the membrane-associated guanylate kinase family, has been shown to regulate the antigen receptor signaling and cell polarity in lymphocytes; however, the physiological function of Dlg1 in humoral responses is not completely clear. Here, we addressed this question using a conditional knockout (KO) mouse model with Dlg1 deficiency in different B-cell subsets by crossing dlg1fl/fl mice with either mb1cre/+ or aicdacre/+ mice, respectively. In both mouse models, we observed that Dlg1 deficiency in B cells (Dlg1-KO B cells) led to obvious hyper-antibody responses upon immunization, the effect of which was more obvious in antigen-recall responses. Mechanistically, we found that Dlg1-KO B cells exhibited hyper-proliferation compared with wild-type B cells upon antigen stimulation, suggesting that the hyper-antibody responses are likely induced by the hyper-proliferation of Dlg1-KO B cells. Indeed, further studies demonstrated that Dlg1 deficiency in B cells led to the down-regulation of a tumor suppressor, FoxO1. Thus, all these results reveal an unexpected function of Dlg1 in restraining hyper-antibody responses through the inhibition of FoxO1 and thus antigen-binding-induced proliferation in B cells.

Profiling the origin, dynamics, and function of traction force in B cell activation

B lymphocytes use B cell receptors (BCRs) to recognize membrane-bound antigens to further initiate cell spreading and contraction responses during B cell activation. We combined traction force microscopy and live-cell imaging to profile the origin, dynamics, and function of traction force generation in these responses. We showed that B cell activation required the generation of 10 to 20 nN of traction force when encountering antigens presented by substrates with stiffness values from 0.5 to 1 kPa, which mimic the rigidity of antigen-presenting cells in vivo. Perturbation experiments revealed that F-actin remodeling and myosin- and dynein-mediated contractility contributed to traction force generation and B cell activation. Moreover, membrane-proximal BCR signaling molecules (including Lyn, Syk, Btk, PLC-γ2, BLNK, and Vav3) and adaptor molecules (Grb2, Cbl, and Dok-3) linking BCR microclusters and motor proteins were also required for the sustained generation of these traction forces. We found a positive correlation between the strength of the traction force and the mean fluorescence intensity of the BCR microclusters. Furthermore, we demonstrated that isotype-switched memory B cells expressing immunoglobulin G (IgG)-BCRs generated greater traction forces than did mature naïve B cells expressing IgM-BCRs during B cell activation. Last, we observed that primary B cells from patients with rheumatoid arthritis generated greater traction forces than did B cells from healthy donors in response to antigen stimulation. Together, these data delineate the origin, dynamics, and function of traction force during B cell activation.