We discovered a fundamental role for B cells in the pathogenesis of obesity associated insulin resistance. Previous studies identified other immune cells as metabolic controllers with pathogenic potential in obesity. In healthy non-obese individuals, VAT-resident regulatory T cells and Th2 cells play a beneficial role by reducing VAT inflammation. During DIO, these cells are overwhelmed by proinflammatory CD8+
and Th1 cells, which promote insulin resistance and glucose intolerance11
. B lymphocytes can now be added to the list of immune cells participating in this process, where they activate CD8+
and Th1 cells and release pathogenic antibodies.
Consistent with a previous report15
, we show that B cell accumulation in VAT occurs early (by 4 weeks). B cells worsen glucose tolerance, in part, by inducing MHC dependent proinflammatory cytokine production by both CD4+
T cells. Similar mechanisms occur in models of cancer, infection30
, and autoimmunity28
. Since B and T cells are recruited early to VAT in response to HFD10,15
, the data support a role for B cells in modulating T cell function in DIO VAT. Alternatively, T cells could function by inducing IgG class switching in B cells. Indeed, we observed elevated levels of pro-inflammatory IgG2c in serum and VAT of HFD mice. It is also possible that cytokines or antibodies produced by B cells can directly interact with and affect insulin sensitivity in adipocytes.
B cells also exacerbate metabolic disease through production of IgG. Although autoantibodies have not been previously recognized as playing a critical role in T2D, an estimated 10% of T2D patients have antibodies to islet cell antigens, and these antibodies correlated with the need for insulin therapy31
. Almost one third of patients with advanced T2D have autoantibodies that inhibit endothelial cell function32
. Antibodies to GFAP, which predict insulin resistance in our array (), are also reported at higher rates in T2D33
We show that transfer of IgG from DIO mice to HFD Bnull
mice induces rapid local and systemic changes in inflammatory cytokine production, and skews VAT macrophages to a pro-inflammatory M1 phenotype. These effects required exposure to a HFD, suggesting that factors related to diet, possibly diet induced conditioning or induction of target autoantigens, are required for antibodies to exert their effects on glucose metabolism. Furthermore, we show that IgG antibodies induce insulin resistance through an Fc mediated process. Since DIO VAT is a site of increased apoptotic/necrotic load, and antibodies concentrate in regions of VAT CLSs, it is conceivable that interactions between antibodies and FcRs on macrophages occur in VAT and promote clearance of apoptotic/necrotic debris and inflammation34,35
. Identification of the precise FcR responsible for IgG effects on glucose metabolism warrants further investigation. Antibodies also fix complement and recently, the complement protein C3a and its receptor C3aR on macrophages, were identified as important mediators of insulin resistance36
We further demonstrate that insulin resistance in obese humans is linked to autoantibodies directed against a specific profile of self-antigens. Antibodies to one of the top three hits, GFAP, occur in approximately 30% of T2D patients33
. Interestingly, we detected antibodies to GOSR1 in more than 70% of obese insulin resistant males (P
=0.0000854). GOSR1 is an essential component of the Golgi SNAP receptor complex and functions in trafficking proteins among the endoplasmic reticulum (ER) and the Golgi37
. It is unknown whether levels of this protein change in response to ER stress, which is thought to be a prominent initiator of insulin resistance38
. Our array data also show distinct antigenic targets associated with insulin sensitivity raising the possibility that some IgG antibodies may be protective. It will be important to validate antigenic targets in larger cohorts.
Finally, we show that depletion of B cells with a CD20 mAb early in disease has therapeutic benefit in abnormal glucose metabolism. These results are consistent with a role for B cells early in disease pathogenesis, similar to observations in several autoimmune diseases39
. Recently, CD20 mAb was used in the treatment of atherosclerotic lesions in Apoenull
. In CD20 mAb experiments, beneficial effects are linked to reduced T cell activation. Consistently, we observed reduced levels of pro-inflammatory IFN-γ and TNF-α in VAT after CD20 mAb treatment.
Interestingly, total serum levels of IgM and IgG following CD20 mAb treatment were not drastically changed despite a prominent therapeutic benefit, consistent with other reports41
. One possible explanation for this finding is that in our studies, CD20 mAb was administered by 6–7 weeks after HFD, just a few weeks after B cells significantly infiltrate VAT. Since antibodies become more pathogenic with longer exposure to HFD (), it is possible that the antibodies present after early treatment were not fully pathogenic. This was verified by the inability of antibodies from animals treated with CD20 mAb to transfer metabolic disease. The lack of pathogenicity of IgG at this time point could be due to reduced affinity maturation and/or class switching41
Rituximab, an anti-human CD20 mAb, used in the treatment of rheumatoid arthritis as well as B cell malignancies, can cause both hyperglycemia and severe hypoglycemia42
. Other B cell and antibody modulating agents are either approved for human use or in clinical trials, including IvIg, TACI fusion proteins, and antibodies or small molecule inhibitors to CD19, CD22, CD79a,b, BLyS, Syk, and APRIL. Our findings suggest new possible uses for such agents, and agents that modulate FcR function and signaling, in the management of obesity related glucose abnormalities.
Collectively, our data support a model wherein early recruitment of B cells promote VAT T cell activation and pro-inflammatory cytokine production which potentiate M1 macrophage polarization and insulin resistance. B cells can also exert their detrimental effects systemically through production of pathogenic IgG antibodies, which target distinct clusters of self proteins. Comparative mass sequencing of T- and B- cell antigen receptors in obesity related insulin resistance is underway, and promises to yield additional insights into the fundamental cause of this pervasive disease.