NK cells express increased GzmB protein after short-term exposure to the latent MuHV-4 environment in vivo. Latently infected mice (28 days after infection with MuHV-4) were injected intraperitoneally with CFSE-labeled splenocytes from B6.RAG1^−/− donors as a source of naive NK cells. Mock-infected and naive recipient mice were used as controls. At 72 hours peritoneal cells were harvested and analyzed for CFSE and GzmB expression in NK1.1⁺CD3⁻ NK cells. (A) Representative flow cytometric density plots illustrating the expression of GzmB protein in transferred (CFSE⁺) versus endogenous (CFSE⁻) NK cells. (B) Pooled data from 2 independent experiments showing the mean ± SD percentage of NK cells that express GzmB. *P = .027 and **P < .001 (MuHV-4 vs control, 11-14 mice per group). (C) Correlation of GzmB protein expression between transferred versus endogenous NK cells in each mouse. Each dot represents one mouse. Correlation coefficient (r²) = 0.9 in latently infected hosts.