PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression. by Day, C, Kaufmann, D, Kiepiela, P, Brown, J, Moodley, E, Reddy, S, Mackey, E, Miller, J, Leslie, A, DePierres, C, Mncube, Z, Duraiswamy, J, Zhu, B, Eichbaum, Q, Altfeld, M, Wherry, E, Coovadia, H, Goulder, P, Klenerman, P, Ahmed, R, Freeman, G, Walker, B