Cytotoxic functionality of T cells can be inhibited by several regulatory mechanisms.31,32 In particular, upregulated expression of programmed cell death protein 1 (PD-1) has been shown to impede viral control by CD8+ T cells during infection.33–35 Infant intestinal CD8+ T cells (Tn cells excluded) had increased PD-1 expression at steady state (15%, IQR 12–25%) compared to adult intestinal CD8+ T cells (2.9%, IQR 1.5–5.2%, P < 0.01) (Fig. 6a, b). This evidence concerns the gene PDCD1 and infection.