CD8A and infection: Before infection, the old ChRM had a severe immunosenescence phenotype, as demonstrated by the significantly lower frequency of naïve CD4+ T cells (25.78%) and naïve CD8+ T cells (18.50%), and the insignificantly lower frequency of naïve B cells (29.54%) compared with that in young macaques (47.20%, 34.33% and 42.28%; P = 0.0047, 0.0127 and 0.0813, respectively) (Fig. 5a).