Inhibition by cholera toxin of clonal growth of murine granulocyte/macrophage progenitor cells in soft agar cultures.

Proliferation in vitro of the committed granulocyte-macrophage progenitor cell (CFUC) is inhibited by cholera toxin (CT) in a dose-dependent manner. This inhibition is reduced and counteracted by higher doses of colony stimulating factor (CSF), an obligatory growth stimulator of CFUC. Mixing of CT with its specific receptor, the monosialoganglioside, GM1, before exposure to bone marrow (BM) cells, blocks the toxin's effect, and a restoration of colony formation is achieved. A dose-dependent inhibition of CSF-induced colony formation is also observed in the presence of choleragenoid, the B subunit of the toxin which binds to the specific receptor on the cell surface, but is biologically inactive. Incubation of BM cells with CT prior to cloning in soft agar cultures supplemented with CSF inhibits clonal proliferation of CFUC, whereas a brief in vitro exposure of BM cells to CSF prior to CT protects the cells against subsequent CT inhibition. The ability of CT to inhibit the CSF-induced clonal proliferation of CFUC and the effectiveness of CSF in reducing and even counteracting CT inhibition suggests that CT, by binding to BM cells through the B subunit, might either directly or indirectly interfere with the stimulatory activity of CSF upon its target cells.