Indirect Evidence of Circulating Parasite Hapten Immune Complexes in Visceral Leishmaniasis.

Background: Hypergammaglobulinemia is present in visceral leishmaniasis (VL), inducing the formation of immune complexes (ICs), which interferes in conventional serology. Parasitic haptens block antibodies, making it difficult to identify and detect them. ICs could be determined indirectly by acid dissociative ELISA (DE) seroconversion in natural and experimental VL.

Methods: We determined the frequency of samples that seroconverted in DE or presented a 10% increase in DE (ΔDE) in 3 types of VLs-hamster, canine, and human samples-with larger antigen determination by direct antigen capture in experimental samples.

Results: The ΔDE frequency is increased in all VL models: human (34%), canine (27%), and hamster (25%) samples. Seroconversion was present in hamster (14%), dog (1%), and human (6%) samples. During experimental infection, higher frequencies (28%) of circulating antigens were observed at the 30th day, associated with higher ΔDE (47%) and seroconversion (22%), with lower frequencies in other periods.

Conclusions: The frequency of ΔED and seroconversion samples found in natural and experimental infection suggests that specific antibodies can be blocked by low molecular weight antigens that interfere qualitatively (seroconversion) or quantitatively (ΔDE) in serology. Several antigen types may be involved, as high molecular weight proteins and low molecular weight glycoconjugates. The higher frequency of those indirect demonstrations of antibody-blocking antigen or haptens that could be acid-removed in VL has implications for the development of assays for detection of circulating or antibody-bound 1- to 3-kDa antigens, which could interfere in diagnosis and also in the immune response of the host.