Analysis of splenic Gr-1(int) immature myeloid cells in tumor-bearing mice

It is known that the number of ImC, expressing myeloid markers, CD11band Gr-1, increase with tumor growth and ImC play a role in the escape of tumor cells from immunosurveillance in tumor-bearing mice and cancer patients. However, the mechanisms by which ImC suppress immune responses in tumor-bearing mice have not been completely elucidated. In the present study, we investigated the function of splenic ImC freshly isolated from tumor-bearing mice and splenic ImC differentiated in vitro by GM-CSF. Freshly isolated splenic ImC were divided into two groups depending on Gr-1 expression, Gr-1 high (Gr-1(hi)) and intermediate (Gr-1(int)). Freshly isolated splenic Gr-1(int) ImC, but not Gr-1(hi) ImC, from tumor-bearing mice reduced production of IFN-gamma in CD8(+) T cells, but neither splenic Gr-1(int) ImC nor Gr-1(hi) ImC isolated from naive mice did. Both Gr-1(int) and Gr-1(hi) ImC differentiated in vitro by GM-CSF inhibited production of IFN-gamma in both CD8(+) and CD4(+) T cells. In addition, the differentiated Gr-1(int) ImC, one-third of which were CD11c(+)F4/80(+) cells, and their culture supernatants suppressed proliferative responses of T cells stimulated by CD3 ligation, but the differentiated Gr-1(hi) ImC and their culture supernatants did not. These results suggest that Gr-1(int) ImC are altered to immune-suppressive cells in tumor circumstances and that they are differentiated by GM-CSF progressively into CD11c(+)F4/80(+) cells with further suppressive activity against T cells.