HAVCR2 and COVID-19: Additionally, co-inhibitory molecules such as PD-1, PD-L1, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and T cell immunoglobulin domain and mucin domain-3 (TIM-3) are found to be overexpressed in CD4+ and CD8+ T cells, and high levels of PD-L1 expression are associated with lymphocytopenia and increased mortality in COVID-19 patients (211, 213).