Bim regulates the survival and suppressive capability of CD8+ FOXP3+ regulatory T cells during murine GVHD

Abstract

CD8⁺ Foxp3⁺ T cells (Tregs) are a potent regulatory population whose functional and ontological similarities to CD4⁺ Fox3⁺ T cells have not been well delineated. Using an experimental model of graft-versus-host disease (GVHD), we observed that CD8⁺ Tregs were significantly less potent than CD4⁺ Tregs for the suppression of GVHD. To define the mechanistic basis for this observation, we examined the T-cell repertoire and the transcriptional profile of in vivo-derived CD4⁺ and CD8⁺ Tregs that emerged early during this disease. Polyclonal and alloantigen-induced CD8⁺ Tregs had repertoire diversity that was similar to that of conventional CD8⁺ T cells, indicating that a restricted repertoire was not the proximate cause of decreased suppression. Transcriptional profiling revealed that CD8⁺ Tregs possessed a canonical Treg transcriptional signature that was similar to that observed in CD4⁺ Tregs, yet distinct from conventional CD8⁺ T cells. Pathway analysis, however, demonstrated that CD8⁺ Tregs had differential gene expression in pathways involved in cell death and survival. This was further confirmed by detailed mRNA sequence analysis and protein expression studies, which demonstrated that CD8⁺ Tregs had increased expression of Bim and reduced expression of Mcl-1. Transplantation with CD8⁺ Foxp3⁺ Bim^-/- Tregs resulted in prolonged Treg survival and reduced GVHD lethality compared with wild-type CD8⁺ Tregs, providing functional confirmation that increased expression of Bim was responsible for reduced in vivo efficacy. Thus, Bim regulates the survival and suppressive capability of CD8⁺ Tregs, which may have implications for their use in regulatory T-cell therapy.