B7-H4 regulates β cell mass and insulin secretion by modulating cholesterol metabolism

Chronic islet inflammation is a hallmark of type 2 diabetes (T2D) and involves in the dysfunction of β cells. However, how β cells participate in this process remains unclear. Here, we report that the immune checkpoint molecule B7-H4(B7S1, B7x, VTCN1) expressed in β cells is critical to maintain β cell mass and insulin secretion. Lesion of B7-H4 in β cells results in glucose intolerance due to less β cell mass and deficient insulin secretion with upregulated cytokines and activated signal transducer and activator of transcription 5 (Stat5) signaling, while overexpression of B7-H4 in β cells ameliorates glucose intolerance in high-fat diet (HFD)-treated mice. Mechanistically, B7-H4 deficiency actives the Stat5 signaling, which inhibits the expression of Apolipoprotein F (ApoF), leading to reduced cholesterol efflux and accumulated cholesterol in β cells, thereby impairing the insulin processing and secretion. Inhibiting Stat5 activity or overexpression of ApoF in β cells can rescue the glucose intolerance and insulin secretion deficiency in β-cell-specific B7-H4 knockout (B7-H4 cKO) mice. Our study demonstrates that β cell expressed immune checkpoint molecule B7-H4 is essential for islet immune homeostasis and β cell function maintenance, and for the first time unravels the mechanism by which B7-H4 regulates insulin secretion through regulating cholesterol metabolism via Stat5 signaling, which may shed new light on the development of novel strategies for T2D treatment. B7-H4 fl/fl mice, in which exon 2 of the Vtcn1 gene is flanked by two loxP sites, were generated by Cyagen Biosciences (Suzhou, China) with CRISPER/Cas9 system. B6.FVB-Tg(Pdx1-cre)6Tuv/J , commonly called Pdx-Cre mice in which Cre recombinase is under the transcriptional control of the mouse Pdx1 promoter, and B6.Cg-Tg(Ins2-cre)25Mgn/J, commonly called RIP-Cre mice in which Cre recombinase is under the control of the rat insulin II promoter were purchased from the Jackson Laboratory. Mice homozygous for the floxed B7-H4 allele (F/F) were crossed with Pdx-Cre or Rip-Cre transgenic mice to create β-cell–specific B7-H4 knockout mice (Pdx-Cre; B7-H4 fl/fl ; PB7H4KO or RIP-Cre; B7-H4 fl/fl ; βB7H4KO) and their control littermates. To elucidate the therapeutic effect of B7-H4 on obesity associated metabolic dysfunction, 6 weeks old WT mice fed with 8 weeks of NCD or HFD were then injected with adeno-associated virus serotype 8 vector (AAV8)–rat Insulin2 promoter (RIP)–specific B7-H4-ZSGreen or with control virus (Hanbio Biotechnology Co.Ltd., Shanghai, China). To clarify the role of ApoF in β cell function affected by β-cell–specific B7-H4 knockout, 6 weeks old flox and βB7H4KO mice fed with 8 weeks of HFD were injected with AAV8-Apof-ZSgreen or control virus (Genomeditech, Shanghai, China).The viruses (1012 genome copy particles/mL) were infused at a rate of 6 μL/min by pancreatic intraductal viral infusion as described previously.