CTLA4 and COVID-19: T cells in COVID-19 patients often show a hyperactive and exhausted phenotype with coinhibitory molecule expression (e.g., PD-1, T cell immunoreceptor with Ig and ITIM domains (TIGIT), and Cytotoxic T-lymphocyte associated protein 4 (CTLA-4)) and increased populations of potentially pathogenic Th17 cells.