s perforin protein (Fig. 1, A–D). Relative to its expression in naive T cells, perforin (Prf1) mRNA expression did not increase appreciably at day 2 but showed a reproducible decrease at day 4, followed by robust reexpression between days 4 and 8 (Fig. 1, A–D). In contrast, granzyme B (Gzmb) mRNA was low or undetectable in naive T