ALPHA-2-MACROGLOBULIN BINDS TO THE SURFACE OF TRYPANOSOMA-CRUZI

Trypanosoma cruzi, the causative agent of Chagas' disease, infects ver tebrate cells after an initial step of parasite/host-cell recognition. Alpha-2-macroglobulin (A2M), an important type of physiological prote inase inhibitor found in tissues and in the plasma of mammals, inhibit s cell invasion by T. cruzi and accumulates in sites of the inflamed m yocardium associated with parasite antigens. To study whether A2M woul d bind to T. cruzi, an indirect immunofluorescence reaction was perfor med using two different anti-mouse A2M sera. Intense labeling was obse rved in the membrane lining the cell body and the flagellum of bloodst ream trypomastigotes obtained from experimentally infected mice in the peak of parasitemia, suggesting that the antisera recognize plasma A2 M associated with the parasite surface. Metacyclic trypomastigotes obt ained in a serum-free defined medium reacted with anti-A2M only after previous incubation with purified human A2M. Enzyme-linked immunosorbe nt assay (ELISA) studies were applied to characterize better the bindi ng of native (N-A2M) and of proteinase-complexed (P-A2M) forms of A2M. The ''in vitro'' binding of N-A2M to trypomastigotes was better at pH 5.0, followed by pH 10.0 and pH 7.4. Cysteinyl and serine proteinase inhibitors, E-64 and STI, respectively, inhibited the reaction. P-A2M also bound to T. cruzi in a dose-dependent way. Flow-cytometry studies showed that about 80% of the parasites stained with fluorescein isoth iocyanate (FITC)-labeled P-A2M (50 mu g/ml) with high affinity at pH 7 .4 (but also at pH 10.0) in a process that was reverted by the additio n of unlabeled P-A2M or the calcium-chelator agent EDTA and also by in cubation at an acid pH (4.0). These results suggest that (a) native-A2 M binds to T. cruzi proteinase(s) and (b) T. cruzi expresses a recepto r(s) that binds proteinase-complexed A2M.