MKI67 and infection: As expected, comparison of T cells from naive versus LCMV-infected mice revealed the emergence of proliferating T cell clusters marked by Ki67 expression with infection, as well as that of exhausted CD8+ T cells and T helper 1 (TH1)-like CD4+ T cells, marked by high expression of Tox and Pdcd1 (Fig. 2a and Extended Data Fig. 2b–d).