CD8A and infection: High levels of PD-1 expression on virus-specific CD8+ T cells during chronic human (human immunodeficiency virus [HIV], hepatitis B virus [HBV]), monkey (simian immunodeficiency virus [SIV]), and murine (lymphocytic choriomeningitis virus [LCMV]) infections have been associated with T cell dysfunction and failure to control viral replication [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].