IMMUNE EVASION BY TRYPANOSOMATIDAE - NORMAL AGGREGATED IMMUNOGLOBULINPROTECTS AGAINST LYSIS BY THE ALTERNATIVE COMPLEMENT PATHWAY

Immunoglobulin Fc receptors (FcRs), present in Trypanosomatidae pathog enic for mammals, may be a mechanism by which these parasites escape t he host immune response. We studies the possible role of these recepto rs 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 incu bated with fresh normal guinea pig serum were shown to be more resista nt to lysis. When compared to log phase Leishmania promastigotes, this resistance was at least 4.5-fold greater in parasites harvested in th e stationary growth phase. EDTA and EGTA plus MgCl2 inhibited the cyto toxic effect of serum, suggesting the participation of the alternative complement pathway. The distribution of FcRs among genera of Trypanos omatidae that are pathogenic, infective or noninfective for mammals an d their affinity for mammalian and fowl immunoglobulin were also exami ned. These receptors are present only in species infective or pathogen ic for mammals, a finding that suggests that this structure is essenti al for the establishment of infection but is not necessarily a virulen ce factor. Furthermore, the ligand specificity is limited to the immun oglobulin of mammalian but not of fowl origin.