Autoreactive antibodies control blood glucose by regulating insulin homeostasis.

Homeostasis of metabolism by hormone production is crucial for maintaining physiological integrity, as disbalance can cause severe metabolic disorders such as diabetes mellitus. Here, we show that antibody-deficient mice and immunodeficiency patients have subphysiological blood glucose concentrations. Restoring blood glucose physiology required total IgG injections and insulin-specific IgG antibodies detected in total IgG preparations and in the serum of healthy individuals. In addition to the insulin-neutralizing anti-insulin IgG, we identified two fractions of anti-insulin IgM in the serum of healthy individuals. These autoreactive IgM fractions differ in their affinity to insulin. Interestingly, the low-affinity IgM fraction (anti-insulin IgMlow) neutralizes insulin and leads to increased blood glucose, whereas the high-affinity IgM fraction (anti-insulin IgMhigh) protects insulin from neutralization by anti-insulin IgG, thereby preventing blood glucose dysregulation. To demonstrate that anti-insulin IgMhigh acts as a protector of insulin and counteracts insulin neutralization by anti-insulin IgG, we expressed the variable regions of a high-affinity anti-insulin antibody as IgG and IgM. Remarkably, the recombinant anti-insulin IgMhigh normalized insulin function and prevented IgG-mediated insulin neutralization. These results suggest that autoreactive antibodies recognizing insulin are key regulators of blood glucose and metabolism, as they control the concentration of insulin in the blood. Moreover, our data suggest that preventing autoimmune damage and maintaining physiological homeostasis requires adaptive tolerance mechanisms generating high-affinity autoreactive IgM antibodies during memory responses.

Primary Immune Responses and Affinity Maturation Are Controlled by IgD.

Mature B cells co-express IgM and IgD B cell antigen receptors (BCR) on their surface. While IgM BCR expression is already essential at early stages of development, the role of the IgD-class BCR remains unclear as most B cell functions appeared unchanged in IgD-deficient mice. Here, we show that IgD-deficient mice have an accelerated rate of B cell responsiveness as they activate antibody production within 24h after immunization, whereas wildtype (WT) animals required 3 days to activate primary antibody responses. Strikingly, soluble monovalent antigen suppresses IgG antibody production induced by multivalent antigen in WT mice. In contrast, IgD-deficient mice were not able to modulate IgG responses suggesting that IgD controls the activation rate of B cells and subsequent antibody production by sensing and distinguishing antigen-valences. Using an insulin-derived peptide we tested the role of IgD in autoimmunity. We show that primary autoreactive antibody responses are generated in WT and in IgD-deficient mice. However, insulin-specific autoantibodies were detected earlier and caused more severe symptoms of autoimmune diabetes in IgD-deficient mice as compared to WT mice. The rapid control of autoimmune diabetes in WT animals was associated with the generation of high-affinity IgM that protects insulin from autoimmune degradation. In IgD-deficient mice, however, the generation of high-affinity protective IgM is delayed resulting in prolonged autoimmune diabetes. Our data suggest that IgD is required for the transition from primary, highly autoreactive, to secondary antigen-specific antibody responses generated by affinity maturation.

Pten controls B-cell responsiveness and germinal center reaction by regulating the expression of IgD BCR.

In contrast to other B-cell antigen receptor (BCR) classes, the function of IgD BCR on mature B cells remains largely elusive as mature B cells co-express IgM, which is sufficient for development, survival, and activation of B cells. Here, we show that IgD expression is regulated by the forkhead box transcription factor FoxO1, thereby shifting the responsiveness of mature B cells towards recognition of multivalent antigen. FoxO1 is repressed by phosphoinositide 3-kinase (PI3K) signaling and requires the lipid phosphatase Pten for its activation. Consequently, Pten-deficient B cells expressing knock-ins for BCR heavy and light chain genes are unable to upregulate IgD. Furthermore, in the presence of autoantigen, Pten-deficient B cells cannot eliminate the autoreactive BCR specificity by secondary light chain gene recombination. Instead, Pten-deficient B cells downregulate BCR expression and become unresponsive to further BCR-mediated stimulation. Notably, we observed a delayed germinal center (GC) reaction by IgD-deficient B cells after immunization with trinitrophenyl-ovalbumin (TNP-Ova), a commonly used antigen for T-cell-dependent antibody responses. Together, our data suggest that the activation of IgD expression by Pten/FoxO1 results in mature B cells that are selectively responsive to multivalent antigen and are capable of initiating rapid GC reactions and T-cell-dependent antibody responses.

PI3K-Mediated Blimp-1 Activation Controls B Cell Selection and Homeostasis.

Activation of phosphoinositide 3-kinase (PI3K) signaling plays a central role in regulating proliferation and survival of B cells. Here, we tested the hypothesis that B cell receptor (BCR)-mediated activation of PI3K induces the terminal differentiation factor Blimp-1 that interferes with proliferation and survival, thereby controlling the expansion of activated B cells. In fact, B-cell-specific inactivation of Pten, the negative regulator of PI3K signaling, leads to deregulated PI3K activity and elevated Blimp-1 expression. Combined deficiency for Pten and Blimp-1 results in abnormal expansion of B-1 B cells and splenomegaly. Interestingly, Blimp-1 also acts at early stages of B cell development to regulate B cell selection, as Blimp-1 deficiency results in an increased proportion of autoreactive B cells. Together, our data suggest that the combined requirement of deregulated PI3K signaling in addition to defective terminal differentiation represents the basis for proper selection and expansion of developing B cells.

PI3K induces B-cell development and regulates B cell identity.

Phosphoinositide-3 kinase (PI3K) signaling is important for the survival of numerous cell types and class IA of PI3K is specifically required for the development of B cells but not for T cell development. Here, we show that class IA PI3K-mediated signals induce the expression of the transcription factor Pax5, which plays a central role in B cell commitment and differentiation by activating the expression of central B cell-specific signaling proteins such as SLP-65 and CD19. Defective class IA PI3K function leads to reduction in Pax5 expression and prevents B cell development beyond the stage expressing the precursor B cell receptor (pre-BCR). Investigating the mechanism of PI3K-induced Pax5 expression revealed that it involves a network of transcription factors including FoxO1 and Irf4 that directly binds to the Pax5 gene. Together, our results suggest that PI3K signaling links survival and differentiation of developing B cells with B cell identity and that decreased PI3K activity in pre-B cells results in reduced Pax5 expression and lineage plasticity.