Differential induction of prostaglandin E2-dependent and -independent immune suppressor cells by tumor-derived GM-CSF and M-CSF.

Both prostaglandin E2 (PGE2)-dependent (indomethacin-sensitive) and PGE2-independent (indomethacin-insensitive) suppressor cell activities that inhibited mitogenic T cell blastogenesis appeared in the bone marrow and spleen of mice on days 20 to 30 following transplantation of NFSA fibrosarcoma molecularly expressing mRNA for both macrophage (M) and granulocyte-macrophage (GM) colony-stimulating factors (CSFs). The present study was done to characterize the two different suppressor cells isolated from NFSA tumor-bearing mice and to verify a role of CSFs in the induction of suppressor cells in vitro. Whereas PGE2-releasing suppressor cells were found in bone marrow and spleen cells isolated from tumor-bearing mice, indomethacin-insensitive suppressor cells in both tissues were localized predominantly in adherent cell fractions. An increase in Mac-1+ and Mac-2+ spleen cell populations with two to three times larger cell volumes was observed, and both showed strong PGE2-releasing capacity and indomethacin-sensitive suppressor cell activity. However, after elimination of Mac-1+ or Mac-2+ cells, bone marrow cells still showed higher PGE2-releasing capacity and indomethacin-sensitive suppressor activity. The in vitro cultures of normal bone marrow and spleen cells with NFSA cell conditioned medium (NFSA-CM) induced heterogeneous mixtures of indomethacin-sensitive and - insensitive suppressor cells like those observed in cultures with the combination of M-CSF and GM-CSF. However, cultures with either GM-CSF or M-CSF resulted in the induction of indomethacin-sensitive suppressor cells by GM-CSF and of indomethacin-insensitive suppressor cells by M-CSF. In addition, NFSA-CM pretreated with anti-GM-CSF antibody induced indomethacin-insensitive suppressor cells in in vitro cultures of bone marrow and spleen cells. These results suggest that two distinctly different suppressor cells developed under hemopoiesis of myelomonocytic lineage cells are regulated differentially by the two macrophage growth factors, M-CSF and GM-CSF.