Granulocyte-macrophage colony-stimulating factor (GM-CSF) has opposing effects on the capacity of monocytes versus monocyte-derived dendritic cells to stimulate the antigen-specific proliferation of a human T cell clone.

GM-CSF is widely used in combination with IL-4 to differentiate monocytes into potent T cell stimulatory cells, referred to as monocyte-derived dendritic cells (MoDC). These cytokines further increased the stimulatory function of MoDC when present during their incubation with antigen, as determined by the proliferative response of an allergen-specific T cell clone. Conversely, the incubation of freshly isolated monocytes with antigen in the presence of GM-CSF or GM-CSF and IL-4 strongly inhibited the specific stimulation of the T cells, compared with monocytes pulsed in the absence of cytokines. This suppression was partly due to the secretion of prostaglandin E2 (PGE2) and IL-10 by GM-CSF-treated monocytes, since the combined use of indomethacin and anti-IL-10 antibodies during GM-CSF incubation and antigen pulsing restored T cell growth to about 65% of control levels. As confirmed by culture supernatant transfer experiments, maximal inhibition of T cell stimulation was also dependent on the direct contact between the T cells and GM-CSF-treated monocytes during antigen presentation. Collectively, these results imply that GM-CSF can either inhibit or enhance the re-stimulation of primed T cells by antigen-presenting monocytes or MoDC, respectively.