Granulocyte-monocyte colony-stimulating factor upregulates HIV-1 replication in monocyte-derived macrophages cultured at low density

The effects that granulocyte-monocyte colony-stimulating factor (GM-CSF) has on HIV-1 replication in monocyte-derived macrophage are controversial. We noted that groups reporting that GM-CSF inhibits HIV-1 replication performed their experiments at relatively high cell densities. To address this issue, we performed experiments at different macrophage densities. In cultures seeded at low cell densities, we find that adding GM-CSF during the first week of culture (ie, before infection, during maturation) increased viral replication compared with that in untreated controls in 10 of 11 donors with quantifiable HIV-1 replication. (No effects were observed if GM-CSF was added after the first week of culture.) In cultures seeded at the higher cell densities representative of those in some previous studies, adding GM-CSF during the first week reduced subsequent viral replication in 8 of 12 donors. In all cases in which GM-CSF reduced viral replication, however, the pH in the wells containing GM-CSF-treated cells dropped dramatically. Macrophages in these acidified cultures had numerous dark granules, suggesting that they were under stress. We conclude, contrary to previous reports, that GM-CSF usually enhances viral replication when cells are grown at low densities in which excessive medium acidification can be prevented. Our results illustrate the dramatic effects that in vitro tissue culture conditions can have when studying the effect of cytokines on HIV-1 replication