STAT4 Regulates the CD8 + Regulatory T Cell/T Follicular Helper Cell Axis and Promotes Atherogenesis in Insulin-Resistant Ldlr -/- 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 Stat4 ^-/- Ldlr ^-/- mice that were fed a diabetogenic diet with added cholesterol (DDC). DDC-fed Stat4 ^-/- Ldlr ^-/- mice demonstrated improved glucose tolerance, insulin sensitivity, and a 36% reduction in atherosclerosis compared with Ldlr ^-/- 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 Stat4 ^-/- Ldlr ^-/- mice compared with Ldlr ^-/- 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 Stat4 ^-/- Ldlr ^-/- CD8 ⁺ Tregs versus Ldlr ^-/- CD8 ⁺ Tregs resulted in a significant reduction in plaque burden and suppression of Tfh cell and germinal center B cells in DDC-fed Ldlr ^-/- recipients. STAT4 expression in macrophages (MΦs) also affected the Tfh/CD8 ⁺ Treg axis, because conditioned media from Stat4 ^-/- Ldlr ^-/- 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 Ldlr ^-/- 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.
(Copyright © 2017 by The American Association of Immunologists, Inc.)