In vivo cytotoxicity of T cells produced in tumor-bearing hosts. Rag2^{− /−} mice were challenged with tumor cells or left unchallenged. Two days later, the mice were reconstituted with T-depleted bone marrow cells from P1CTL mice. At 4 weeks after reconstitution, they received an intravenous injection of a mixture of P1A-pulsed (CFSE^hi) and control peptide-pulsed (CFSE^lo) syngeneic BALB/c spleen cells (5 × 10⁶ each). The spleens were harvested and analyzed by flow cytometry for distribution of fluorescence. Data shown are dot plots, showing the forward scatters and fluorescence intensity. CFSE^hi and CFSE^lo cells are gated as indicated. The number shown in the gates are the percent of gated cells, whereas the number shown underneath the gates are means and SDs of the ratios of CFSE^lo/CFSE^hi cells. The data indicate drastically stronger cytolysis of P1A-pulsed targets by CTLs in tumor-bearing hosts. The recipients used were: (A) control RAG-2–deficient mice without tumor challenge; (B) P1CTL mice without tumor challenge; (C) unchallenged RAG-2–deficient mice reconstituted with T-depleted P1CTL bone marrow; and (D) tumor-challenged RAG-2–deficient mice reconstituted with T-depleted P1CTL bone marrow. Data shown are representative of 2 independent experiments involving a total of 3 to 5 mice per group. Two-sided Student t tests indicate a very significant difference between the cytolysis found in tumor-bearing hosts versus all other groups (P < .001). None of the other differences are statistically significant.