Vaccination with Leukemia Cell Expression Membrane Bound GM-CSF Overcomes the Host Immunosuppression Induced by Leukemia

Immunotherapy of leukemia involves stimulating host-cell mediated immunity by facilitating immune recognition of leukemia cells, which are normally weakly immunogenic. We previously showed that vaccination with membrane bound GM-CSF leukemic cells protects mice from leukemia challenge (Ling et al., Oncogene, 2007). In these studies, after addition of a transmembrane domain to the original GM-CSF coding sequence (tmGM-CSF), the construct was transduced into murine leukemia cells (WEHI-3B), which was shown to be more than 98% on the cell surface. Vaccination with lethally irradiated tmGM-CSF expressing murine leukemia cells prevents leukemia in immunocompetent mice (BALB/c), as 100% of vaccinated BALB/c mice were protected from leukemia (Ling et al, Oncogene 2007). No protection was observed by vaccination of nude mice, indicating that adaptive immunity is involved in the protective response. In the present studies, we extended our original observation and provided evidence to show that leukemic mice undergo immunosuppression and that vaccination with leukemia cells expressing cell surface tmGM-CSF overcomes immunosuppression. Vaccination with lethally irradiated leukemia cells expressing cell surface tmGM-CSF overcame the immunosuppression induced by leukemia development, as normal levels of CD4+/CD25+/Foxp3+ T-regulatory (Treg) cells were maintained in spleens and thymus after challenge with leukemia cells. In contrast, the Treg population was significantly increased in leukemic mice vaccinated with leukemia cells lacking cell surface tmGM-CSF (p<0.001) after leukemia challenge, and these mice had a lower CD8+/Treg cell ratio (p<0.01). The ratio of CD8+/Treg cells was higher in tmGM-CSF/GFP vaccinated mice than in GFP vaccinated mice (p<0.001), which in-turn leads to a more effective CD8+ T-cell response. DC levels were also increased from normal levels in mice vaccinated with tmGM-CSF+ leukemia cells compared to control vaccinations. These results suggest that vaccination with leukemia cells expressing GM-CSF on their cell surface leads to an effective cell-mediated immune response in the vaccinated host by overcoming an impaired host cellular immunity induced up-regulation of Treg cells caused by the leukemia process. This strategy has potential for use in the treatment of various human leukemias.