T Regulatory Cell Suppression of CD8+ Lymphocytes During FIV Infection is Likely Mediated by Membrane TGF-β / TGF-β Receptor II Interactions
Data suggest that auto-reactive CD4+ and CD8+ cells, as well as excessive immune responses to pathogens, are controlled by CD4+CD25+ T regulatory (Treg) cells, which regulate the duration and magnitude of immune responses. Recent data suggest that Treg cells mediate their suppressive function through TGF-β expressed on their surface. We previously reported that Treg cells are constitutively activated and suppress CD4+CD25- T cell responses in feline immunodeficiency virus infection (FIV). Subsequently, we demonstrated that these immunosuppressive Treg cells express membrane TGF-β (mTGF-β). The following experiments explore the effect of CD4+CD25+ Treg cells on CD8+ T cell responses in FIV infected cats and the possible role of mTGF-β in mediating CD8+ immune suppression. SPF cats were infected with the NCSU1 isolate of FIV, and peripheral lymph nodes (LN) and blood were collected at intervals during the acute and asymptomatic phases of the infection. Feline specific ELISpot assays demonstrated that Treg cells from both acutely and chronically infected cats suppressed CD8+ IFN-γ production in response to immune stimulation. LN cells and PBMCs were analyzed by flow cytometry for surface phenotype, including TGF-β and TGF-β receptor II (TGF-βRII) expression. Flow cytometric analysis revealed a low percentage of Treg cells expressed mTGF-β during the acute phase of FIV. However, analysis of cells from the same cats during the chronic phase of infection demonstrated that 40-50% of the Treg cells were mTGF-β+. In contrast, Treg cells from FIV- cats exhibited little or no mTGF-β expression. CD8+ lymphocytes from FIV+ cats also displayed greater surface TGF-β RII expression when compared to FIV- cats. Treatment of CD8+ cells with anti-TGF-β RII antibody and Treg cells with anti-TGF-β antibody prior to the IFN-γ ELISpot assay inhibited Treg mediated immune suppression. Furthermore, feline CD8+ lymphocytes exhibited up regulation of Smad 2/3 phosphorylation and down regulation of IFN-γ message when treated with soluble TGF-β, suggesting that CD4+CD25+mTGF-β+ Treg cells mediated CD8+ suppression via the TGF-β / TGF-βRII signaling pathway.
Originally presented at the Keystone Symposia--TGF beta Family in Homeostasis and Disease, Santa Fe, NM.