Granulocyte-macrophage colony-stimulating factor increases the infectivity of Leishmania amazonensis by protecting promastigotes from heat-induced death.

We have studied the effect of granulocyte-macrophage colony-stimulating factor (GM-CSF) on the infectivity of promastigotes of Leishmania amazonensis, an obligate intramacrophage parasite. We measured the capacity of the promastigotes to infect macrophages after preincubation at different temperatures (28, 34, and 37 degrees C) with recombinant murine GM-CSF, as well as the effect of an anti-murine GM-CSF antibody on the in vitro and in vivo infectivity of the parasite. GM-CSF increases the capacity of the promastigotes to infect cells when preincubated at 34 and 37 degrees C, whereas the anti-GM-CSF antibody exerts the opposite effect: it decreases the internalization rate and the progression of infection in macrophage cultures and slows the growth of the lesion in infected BALB/c mice. Neither of the described effects were observed when the in vitro and in vivo infections were made with amastigotes. Promastigotes die in a time-dependent manner when incubated at temperatures higher than 28 degrees C in the absence of GM-CSF. They are protected from this heat-induced death by incubation with the recombinant hormone. Our interpretation of these data is that the increase in the infectivity of promastigotes when incubated with GM-CSF at the temperatures at which infection occurs (34 and 37 degrees C) is due to the larger number of surviving forms within the infecting population. The decrease in infectivity when they are incubated with the antibody is due to inhibition of the protection conferred by the GM-CSF produced by the macrophages during the in vitro and in vivo infections.