STAT4 Regulates the CD8 Regulatory T Cell/T Follicular Helper Cell Axis and Promotes Atherogenesis in Insulin-Resistant Mice.

The metabolic syndrome and diabetic conditions support atherosclerosis, but the exact mechanisms for accelerated atherogenesis remain unclear. Although the proinflammatory role of STAT4 in atherosclerosis and diet-induced insulin resistance (IR) was recently established, the impact of STAT4 on atherogenesis in conditions of IR is not known. In this study, we generated mice that were fed a diabetogenic diet with added cholesterol (DDC). DDC-fed mice demonstrated improved glucose tolerance, insulin sensitivity, and a 36% reduction in atherosclerosis compared with controls. Interestingly, we detected a reduction in T follicular helper (Tfh) cells and plasma B cells but a sharp elevation in CD8 regulatory T cells (Tregs) in spleens and aortas of mice compared with mice. Similarly, STAT4 deficiency supported CD8 Treg differentiation in vitro. STAT4-deficient CD8 Tregs suppressed Tfh cell and germinal center B cell development upon immunization with keyhole limpet hemocyanin, indicating an important role for STAT4 in CD8 Treg functions in vivo. Furthermore, adoptive transfer of CD8 Tregs versus CD8 Tregs resulted in a significant reduction in plaque burden and suppression of Tfh cell and germinal center B cells in DDC-fed recipients. STAT4 expression in macrophages (MΦs) also affected the Tfh/CD8 Treg axis, because conditioned media from MΦs supported CD8 Treg differentiation, but not Tfh cell differentiation, in a TGF-β-dependent manner. These findings suggest a novel mechanism by which STAT4 supports atherosclerosis in IR mice via STAT4-dependent MΦs, as well as cell-intrinsic suppression of CD8 Treg generation and functions and maintenance of Tfh cell generation and the accompanying humoral immune response.