Immune evasion by Trypanosomatidae: normal aggregated immunoglobulin protects against lysis by the alternative...

Immunoglobulin Fc receptors (FcRs), present in Trypanosomatidae pathogenic for mammals, may be a mechanism by which these parasites escape the host immune response. We studied the possible role of these receptors in evasion by the alternative complement pathway. Promastigotes of Leishmania amazonensis and trypsinized trypomastigotes of Trypanosoma cruzi treated with heat-aggregated normal gamma globulin and then incubated with fresh normal guinea pig serum were shown to be more resistant to lysis. When compared to log phase Leishmania promastigotes, this resistance was at least 4.5-fold greater in parasites harvested in the stationary growth phase. EDTA and EGTA plus MgCl2 inhibited the cytotoxic effect of serum, suggesting the participation of the alternative complement pathway. The distribution of FcRs among genera of Trypanosomatidae that are pathogenic, infective or noninfective for mammals and their affinity for mammalian and fowl immunoglobulin were also examined. These receptors are present only in species infective or pathogenic for mammals, a finding that suggests that this structure is essential for the establishment of infection but is not necessarily a virulence factor. Furthermore, the ligand specificity is limited to the immunoglobulin of mammalian but not of fowl origin.