Innate stimulation of B1a cells enhances the autoreactive IgM repertoire in the NOD mouse: implications for type 1 diabetes.

We sought to determine whether the presence of natural autoreactive antibodies of B1a cell origin would play a role in the initiation of type 1 diabetes. We compared IgM repertoires and B1a cell compartments in NOD and C57BL/6 mice. Serum IgM autoreactivity profiles were determined by ELISA and the secretory properties and activation status of B1a cells were characterised by enzyme-linked immunosorbent spot (ELISPOT) assay and flow cytometry. B1a cell response to innate activation was analysed by gene expression assays, ELISA and [(3)H]thymidine incorporation. The effect of NOD IgM produced by B1a cells on NOD.severe combined immunodeficient (SCID) beta cells was examined in co-cultures: IgM binding was measured by flow cytometry and real-time PCR was used to study oxidative stress responses. NOD mice displayed increased levels of serum anti-insulin IgM that were independent of the H2 locus, that were maintained up to prediabetic stages and that correlated with the NOD B1a cell secretion profile. NOD B1a cells had a naturally increased pattern of activation, expressed higher levels of toll-like-receptors (Tlrs) and responded to TLR stimulation in vitro with higher proliferation and increased capacity to secrete anti-type-1-diabetes-related IgM, but produced lower amounts of IL10. IgM of NOD B1a cell origin was able to bind to pancreatic beta cells in vitro and induce expression of inducible nitric oxide synthase (Nos2). NOD B1a cells had a lower innate activation threshold for secretion of autoreactive IgM capable of triggering oxidative stress responses on binding to pancreatic beta cells; this provides an early mechanism that contributes to diabetes in a mouse model of type 1 diabetes.