CD8A and viral infectious disease: Interestingly, the proportion of RO+ memory CD8+ T cells expressing PD-1 (the inhibitory receptor of programmed death), whose expression is associated with impaired T cell responses to viral infections in mice and humans and is also predictive of disease progression in HIV infected persons [38–40], was significantly higher in the blood versus the gut (mean of 40.9 versus 30.7, n = 33, p = 0.0016, Table 1).