PDCD1 and COVID-19: Interestingly, the elevated serum concentration of TNF-α, IL-6, and IL-10 detected in severe COVID-19 patients is associated with a reduced number of circulating T cells that, in addition, show an exhausted phenotype characterized by high expression of the immune checkpoint molecules programmed cell death protein 1 (PD-1) and T-cell immunoglobulin and mucin-domain containing-3 (Tim-3) [50].