Heat shock protein gp96 controls regulatory T cell lineage stability via IL-2/pSTAT5 signaling

Abstract Gp96 is an essential endoplasmic reticulum chaperone for various proteins including GARP, TLRs, integrins, Wnt co-receptor LRP6 and the platelet glycoprotein Ib/IX/V complex. Our laboratory has previously shown that loss of gp96 in murine Foxp3+ regulatory T cells (Tregs) results in their instability as determined by FOXP3 downregulation, impaired immunosuppressive function and production of effector cytokines (Zhang, et al., J Clin Invest., 2015, 125: 859–869). However, the mechanism underlying Treg dysfunction upon gp96 deletion remains unknown. Using tamoxifen-inducible Treg-specific gp96 gene (encoded by Hsp90b1) knockout (KO) mice (Foxp3-ERT2creHsp90b1f/f), we first phenotypically characterized gp96-deleted Tregs in mouse spleen and peripheral lymph nodes (pLNs). Gp96 deficient Tregs showed decreased frequency of TCF1−CD62L− effector population, but were enriched with short-lived KLRG1+ and Helios+ subsets, in both spleen and pLNs. We also confirmed that the expression of Foxp3 was decreased in splenic gp96-deleted Tregs, which correlated with a reduction of cell surface CD25. Intriguingly, both Foxp3 and surface CD25 levels stayed high in gp96-null Tregs in pLNs. In addition, gp96-null Tregs in mouse spleen, but not in pLNs, demonstrated compromised suppressive function. We further found that IL-2/CD25 signaling, as assessed by STAT5 phosphorylation, was more activated in gp96-null Tregs in pLNs, compared to that in spleen. In vitro IL-2 treatment was able to rescue the attenuated Foxp3 expression in gp96-null Tregs. Our work indicates that IL-2/pSTAT-5 signaling may be a key factor in maintaining Treg Foxp3 expression and its lineage stability upon gp96 deletion.