A reservoir of on-going HIV replication persists in essentially all patients receiving HAART. Early studies identified a unique CD4+CD25+ T cell as a potential target of productive HIV infection. Similarly, CD4+CD25+ but not CD4+CD25- T cells from FIV-infected cats support a productive FIV infection in vitro and in vivo. These CD4+CD25+ cells have the characteristics of T regulatory (Treg) cells in that they are anergic and suppress T cell proliferative responses to mitogen. These experiments will define the cytokines and intracellular signaling pathways that regulate Treg cells and determine how these molecules may promote FIV replication. PBMC and LN CD4+ T cells from FIV+ and FIV- cats will be analyzed by multi-color flow cytometry with specific antibodies (e.g. CD25, B7.1, B7.2, CTLA4, MHCII and TGFbeta) to identify specific subsets of CD4+ cells. Multiparameter high speed cell sorting using mAb to these receptors (e.g. alphaCD4 to alphaCD25 to Beta7.1) will provide CD4+CD25+ and CD4+CD25- T cell subsets from FIV+ cats for determination of latent or productive infection by PCR and p24 analysis. Purified CD4+ T cell subsets from FIV+ and FIV- cats will also be analyzed for cytokine (e.g. IL10, IL2, TGFbeta, IL4 and IFNgamma) expression by TaqMan PCR and flow cytometry staining. Antibody blocking studies with alphalL10 and alphaTGFbeta will determine their role in maintaining the CD4+CD25+ anergic state and FIV replication capability. Effects of in vivo and in vitro FIV infection on CD4+CD25+ Treg cell function and viability will be established by correlating virus burden with a) anergy (PI staining, 3HTdR uptake); b) susceptibility to apoptosis (Anexin V, TUNEL); and c) suppressor activity against mitogen- or FIV peptide-stimulated CD4+CD25- Th cells; and longevity in culture (CFDA SE staining). Finally, CD4+CD25+ and CD4+CD25- will be analyzed for expression of p21 cip1 and p27 kip1 cdk inhibitors of G1 cell cycling and ATF and AP-1 transcription factor to correlate expression of these protein with CD4+CD25+ T cell anergy and FIV replication. Modulation of expression of these proteins will establish cause and effect relationships between proteins regulating anergy and FIV replication. Cats with acute FIV infection will be analyzed temporally for CD4+CD25+ activation and establishment of a FIV reservoir. These studies should yield insights into the cytokines and signaling pathways that maintain the anergic state of Treg cells and help define the molecular environment that promotes FIV replication and how soon CD4+CD25+ Treg cells are activated and establish a FIV reservoir after infection.