CD4 and infection: Spleens were collected on day 8 post-infection (Fig. 2A), splenocytes were stimulated with select SARS-CoV-2 peptides, immunolabeled for cell surface markers, intracellular cytokines, and the degranulation marker CD107a, and the frequencies of activated (CD44+ CD62L−) effector CD8+ and CD4+ T cells were quantified by flow cytometry (Fig. S2).