CD4 and COVID-19: After stimulation and incubation, cells were fixed and stained with a combination of surface lineage markers CD45, CD3, CD4, and CD8, cell activation markers CD45RO and CD69 and intracellular cytokines including IFN-γ, TNFα, IL-17A and granzyme B. Both CD4+ T helper cells and CD8+ cytotoxic T cells were activated by pepS in all participants; subjects with a history of COVID-19 also responded to pepN (Fig. 2c).