Regression of established mouse leukemia by GM-CSF-transduced tumor vaccine: Implications for cytotoxic T lymphocyte responses and tumor burdens

This study investigated the therapeutic effects of granulocyte-macrophage colony-stimulating factor (GMCSF) on a mouse leukemia model, By using a retroviral vector, mouse GM-CSF cDNA was transduced into a highly tumorigenic T leukemia cell line, RL male 1, Injection of GM-CSF-secreting RL male 1 cells into syngeneic BALB/c mice elicited protective immunity in the animals, which could regress preestablished tumors introduced either by a subcutaneous or in an intravenous route, However, the therapeutic effects were less prominent in the mice inoculated with a large tumor load or in mice treated later, Winn tests further demonstrated that the splenocytes from the late-treated group conferred poorer protective effects in terms of reducing the growth of parental RL male 1 cells in naive mice than the splenocytes from the early-treated group, Nonetheless, upon stimulation in vitro, the activity of tumor-specific cytotoxic T lymphocytes (CTL) was comparable in the splenocytes of both groups of mice, Histological analysis also indicated that the CD8(+) T cells appeared as early as 3 days following vaccination at the vaccine sites and at the tumor sites in both groups of mice, Above observations implied that the T cells in the animals bearing large tumors appeared to be in a state of suppression or anergy, Systematic histological analyses for 2 weeks provided further insight into various infiltrates at the vaccine sites and at the tumor sites in response to the inoculation of GM-CSF-secreting tumor vaccine.