PDCD1 and COVID-19: (2021) (136) a higher percentage of the HERV-W-env protein was seen in leukocytes, especially in TCD3+ lymphocytes of COVID-19 patients, which was also correlated with the expression of programmed cell death 1 (PD1), an exhaustion marker, and in CD8+ T cells, with the expression of CD57, a senescence marker.