Identification of leishmanial antigens in the sera of patients with American visceral leishmaniasis.

Circulating immune complexes are present in the sera of patients with visceral leishmaniasis caused by Leishmania donovani chagasi. In order to determine whether these complexes contain parasite antigens, sera were collected from Brazilian patients with visceral leishmaniasis and from hospitalized control subjects with other diagnoses. High-molecular-weight complexes were precipitated from pooled sera with 2.5% polyethylene glycol. Approximately 140-fold-more protein was precipitated from patient sera than from control sera; 12% of the total patient serum protein was precipitated. Patient serum precipitates contained immunoglobulins G (525 mg/dl), M (27 mg/dl), and A (8 mg/dl). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the patient serum precipitates revealed multiple bands, including a prominent band at 70 kilodaltons, that were not seen in precipitates of control sera. The 70-kilodalton band was recognized by human and hamster sera with antileishmanial antibodies, but not by control sera. Finally, immunization of BALB/c mice with the high-molecular-weight precipitates from patients elicited antileishmanial antibodies against L. donovani chagasi antigens as detected by enzyme-linked immunosorbent assay and Western blot (immunoblot) assay. In summary, sera of patients with American visceral leishmaniasis were found to have high-molecular-weight complexes that contained one or more parasite antigens. These complexes may play a role in the immunology of the disease, and detection of circulating parasite antigens has potential diagnostic importance.