CD4+CD25+ T regulatory cells from FIV+ cats induce a unique anergic profile in CD8+ lymphocyte targets

Background: Using the FIV model, we reported previously that CD4(+)CD25(+) T regulatory (Treg) cells from FIV+ cats are constitutively activated and suppress CD4(+)CD25(-) and CD8(+) T cell immune responses. In an effort to further explore Treg-mediated suppression, we asked whether Treg cells induce anergy through the alteration of production of cyclins, cyclin dependent kinases and their inhibitors. Results: Lymphocytes were obtained from control or FIV+ cats and sorted by FACS into CD4(+)CD25(+) and CD8(+) populations. Following co-culture with CD4(+)CD25(+) cells, CD8(+) targets were examined by Western blot for changes in cyclins D-3, E and A, retinoblastoma (Rb) protein, as well as the cyclin dependent kinase inhibitor p21(cip1). Following co-culture with CD4(+)CD25(+) cells, we observed up-regulation of p21(cip1) and cyclin E, with down-regulation of cyclin D-3, in CD8(+) cells from FIV+ cats. As expected, CD8(+) targets from control cats were quiescent with little up-regulation of p21(cip1) and cyclin E. There was also a lack of Rb phosphorylation in CD8(+) targets consistent with late G(1) cell cycle arrest. Further, IL-2 mRNA was down regulated in CD8(+) cells after co-culture with CD4(+)CD25(+) Treg cells. Following CD4(+)CD25(+) co-culture, CD8(+) targets from FIV+ cats also had increased Foxp3 mRNA expression; however, these CD8(+)Foxp3(+) cells did not exhibit suppressor function. Conclusions: Collectively, these data suggest that CD4(+)CD25(+) Treg cells from FIV+ cats induce CD8(+) anergy by disruption of normal G(1) to S cell cycle progression.