Pertussis toxin–treated CD4⁺ T cells fail to traffic into tumors or induce proliferation of CD8⁺ T cells. TDLN CD4⁺ T cells were untreated, irradiated, or treated with pertussis toxin and labeled with CFSE. They were mixed with TDLN CD8⁺ T cells labeled with CFSE and adoptively transferred to irradiated hosts bearing 10-day pulmonary metastases. Lungs were harvested 3 days later and cells were stained for CD4 or CD8 expression and analyzed by FACS. The percentage of cells demonstrating at least 4-fold reduction in CFSE intensity is indicated in each graph.