Fibronectin enhances macrophage association with invasive forms of Trypanosoma cruzi

Treatment of either mouse peritoneal macrophages (MPH) or invasive blood forms of Trypanosoma cruzi with human plasma fibronectin (FN) significantly enhanced their association (a term to mean surface attachment and parasite internalization) with the untreated counterpart in a dose-dependent manner. This effect involved increases in the percentage of MPH that associated with the parasites and in the number of parasites per MPH. By using indirect immunofluorescence, the percentages of FN-positive MPH and FN-positive parasites found in preparations of these cells were 26 and 13%, respectively, and increased to 70 and 73%, respectively, after incubation with FN for 60 min and multiple washings. These results demonstrated the presence of FN itself and FN-binding sites on the surface of MPH and T. cruzi. Incubation of FN-treated MPH and FN-treated parasites with gelatin, for which FN has a binding site, significantly reduced the stimulatory effect of FN. A reduction was also seen when FN-treated MPH were incubated with anti-FN antibody before adding the parasites. These observations suggested that FN might enhance association by bridging the interacting cells. The presence of excess soluble FN during MPH-parasite interaction also inhibited the association, possibly by blocking FN receptors on the MPH and parasite surfaces. Pretreatment of the MPH with FN enhanced the capacity of these cells to associate with either untreated latex beads or killed T. cruzi. These findings indicated, on the one hand, that the FN-mediated enhancement was not unique to living T. cruzi and, on the other, that this enhancement was not likely due to an FN-induced alteration of the MPH membrane that would render it more susceptible to active penetration by the parasites. Taken together, these results suggest that FN, produced by MPH, may play a role in infection of this cell type by T. cruzi.