(provided: This proposal is an R21 application entitled "Regulation of PD-1 as a strategy against chronic HIV-1 infection". The current antiretroviral therapy (ART) has shown tremendous success in reducing the mortality and morbidity of HIV-1 infected individuals. However, ART has failed to eradicate the virus since the virus is suppressed rather than truly eradicated. Programmed death 1 (PD-1) has emerged as a potential target to facilitate HIV eradication because it is associated with T cell exhaustion. PD-1 is an inhibitory immune- checkpoint receptor expressed on a variety of immune cells. PD-1 was shown to adversely hinder the immune responses to chronic viral infections, such as HIV-1 infections. Elevated PD-1 level positively correlates with T cell exhaustion and latently-infected HIV-1 in memory T cells. The ability of T cells to recover from exhaustion was shown to be inversely correlated with PD-1 level. Furthermore, animal studies indicated that anti-PD-1 antibody was well-tolerated and led to increased SIV-specific CD8 and CD4 T-cell functions and an improved survival in SIV-infected animals. Thus, targeting PD-1 to regulate immune responses is a promising strategy that can be developed into a novel therapeutic approach against chronic HIV-1 infection. Although there are antibodies and fusion proteins that can effectively modulate the function of PD-1, no small molecules have been reported to regulate PD-1 expression and function. Therefore, the goal of this study is to identify small molecules that can selectively suppress the expression of PD-1. The rationale of this study is that inhibition of PD-1 expression will prevent or rescue CD8+ T cells from exhaustion. In an initial screening, we discovered that triterpene derivatives, such as dihydrobetulinic acid (d-BA) and a d-BA derivative D9-1, could selectively suppress the PD-1 expression on human peripheral blood mononuclear cells at low micro molar concentrations without affecting T cell activation. Based on these encouraging preliminary results, we hypothesize that small molecules can be synthesized with the ability of selectively suppressing PD-1 expression and used to prevent or rescue T cells from exhaustion. The following two specific aims are proposed to test this hypothesis: (1) to obtain potent PD-1 suppressors through lead optimization based on our preliminary results;(2) to identify the target protein of the PD-1 suppressors using biotin-tagged molecular probe with photoaffinity labeling functionality. PD-1 is in the same family of inhibitory receptors such as CTLA-4. Ipilimumab, a CTLA-4 monoclonal antibody, was recently approved by FDA for anti-melanoma therapy. The effectiveness of PD-1 blockade was successfully demonstrated in a SIV/macaque model. Therefore, PD-1 is a valid drug target, and accomplishing the proposed studies will lay a foundation for further development of this class of small molecules as immunotherapeutic drug candidates for potential HIV-1 eradication.