PD-1 signaling in HIV and chronic viral infection--potential for therapeutic intervention?

Programmed death-1 (PD-1) is a negative immunoregulatory cell surface receptor molecule whose interaction with its ligands PD-L1 and PD-L2 downmodulates T-cell immune responses. Originally investigated in the context of self-tolerance, PD-1 has more recently been discovered to be upregulated on T cells of HIV-infected individuals. High levels of PD-1 on HIV-infected T cells are correlated with viral load and with a state of cellular anergy, or ' exhaustion' that results in decreased cellular proliferation, cytotoxic function and cytokine secretion. The finding that interruption of PD-1 with its ligand PD-L1 rescues HIV-infected cells from this state of anergy or ' exhaustion' presents the promise for therapeutic intervention. Understanding the molecular signaling pathway(s) of PD-1 may provide opportunities for therapeutic intervention, that may serve as adjunctive therapies to HIV vaccine development. Evidence to date suggests that PD-1 exerts its regulatory effect by interfering with T cell receptor signaling. While certain molecular signals in the PD-1 pathway have been identified, their precise roles and mechanisms of action remain poorly understood. This article reviews what is currently known about PD-1 signaling in human T cells, and more specifically in T cells of individuals chronically infected with certain viruses such as HIV.