CD8A and viral infectious disease: Since upregulation of PD-L1 expression is evident on parenchymal cells (e.g., microglia, astrocytes, and oligodendrocytes) following different types of viral infections in the CNS (223–227), it is reasonable to speculate that PD-1 expression by brain CD8+ TRM cells maintains a tolerable balance between immunopathology and immune control of the virus in the CNS (190).