CTLA4 and COVID-19: Exhausted COVID-19 CD8+ T cells express elevated levels of PD-1, CTLA-4, TIGIT, and Tim-3 (122, 123), but Kusnadi et al. showed that the exhausted phenotype in memory CD8+ T cells was more pronounced in mild disease, as compared to severe COVID-19, where CD8+ T cells appeared to be more polyfunctional and exhibit a pro-survival signature (124).