Proliferative responses of T cells isolated from tumor tissues after the treatment of anti-ckit antibodies. (A) Proliferative response of T cells derived from tumor tissues of anti-ckit– or control Ig–treated mice. The anti-CD3/anti-CD28–mediated proliferative responses of the T cells derived from tumor tissues were assessed in a standard [³H]-thymidine incorporation assay. Splenic T cells purified from naive mice were used as positive control. Stimulation index (SI; cpm in the presence of anti-CD3/anti-CD28 divided by cpm in the absence of anti-CD3/anti-CD28) of the result is presented. (B) Decreased percentage of Gr-1⁺CD115⁺ MDSCs in bone marrow of tumor-bearing mice treated with anti-ckit. BM Percoll Fraction 2 cells from tumor-bearing mice treated with anti-ckit or control rat Ig were stained with anti–Gr-1-APC plus anti–CD115-PE or isotype control antibodies followed by flow cytometric analysis. A significantly lower percentage of MDSCs was observed in tumor-bearing mice treated with anti-ckit compared with those treated with control rat Ig (P < .001 by Student t test). Numbers in quadrants represent percent positive. (C) Reduced suppressive activity of MDSCs isolated from tumor-bearing mice treated with anti-ckit. The suppressive activities of MDSCs were assessed using HA peptide-mediated proliferation at various ratios of CD4 HA TCR transgenic splenocytes and MDSCs. The MDSCs isolated from mice treated with anti-ckit exhibited lower suppressive activity compared with those from mice receiving control rat Ig. Error bars represent standard deviation.