The study of antibody avidity changes during infection has improved the understanding of the pathologic processes involved in several infectious diseases. In some infections, like toxoplasmosis, this information is being used for diagnostic purposes. Results of the evolution of antibody avidity for different specific antigens in Trypanosome cruzi-infected rats are presented. A Western blotting technique, combined with avidity analysis to identify antigens that elicit high-avidity antibodies, is suggested. In this system, antibodies showed high avidity values only during the chronic phase of infection and only in relation to antibodies against 21-, 33-, 41-, 42-, 56-, 58-, 66-, and 72-kDa antigens. Finally, a 97-kDa T. cruzi antigen, which was recognized by high-avidity antibodies and occurred in noninfected rats, was identified. These results allow us to evaluate the different antigens in chagasic infection. Our results show that with the correct choice of antigen it is possible to detect differences in maturation of antibodies and to discriminate, in an experimental model, between recent (acute) and chronic infections.Chagas' disease is caused by the hematic protozoan Trypanosoma cruzi. The infectious process in humans is characterized by an initial phase with large quantities of parasites in peripheral blood. The presence of the parasite in the bloodstream is partially controlled by the immune response. In most cases, the infection evolves toward a chronic phase characterized by high titers of immunoglobulin G (IgG) antibodies, by low and intermittent parasitemia, and very frequently by the absence of symptoms.As in other parasitic and infectious diseases, the monitoring of antibody responses and antigens involved in different stages of the infection may give information for the comprehension of the mechanisms of immune system control over the parasite (1,14,17,20,24). In different infectious processes the maturation of antibodies, measured in terms of avidity, has been determined in order to find a correlation with the evolution of the infection (7,8,10,18,25).An optimization process of the antibody binding properties has been described in long-term infection (19): somatic hypermutation of sequences encoding the variable region of light and heavy chains of immunoglobulins occurs, and B lymphocytes producing better quality antibodies, mainly in terms of avidity, are then selected. During this stage, some serum immunoglobulins evolve from low to high avidity. On the basis of this mechanism, methods have been developed to age a variety of infective diseases by using a simple method based on the selective unbinding of antibodies with chaotropic agents and the assessment of the evolution of their functional avidity (4, 9, 21). Another type of information that it is possible to obtain from the determination of the avidity index is the discrimination between autoantibodies and cross-reacting antibodies generated by an infectious process (17).In this work, the evolution of the antibody response to different T. cruzi ...

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Serological diagnosis of Trypanosoma cruzi infection is hampered by issues related to test specificity due to the cross-reactivity of most antigens with proteins of related parasites such as Leishmania spp. The recombinant calflagins are considered relevant antigens for the diagnosis of infection by Trypanosoma cruzi. In the present work, we describe two genes coding for putative calflagins in Leishmania major with the N-terminal moieties presenting high similarity with T. cruzi genes. This fact raised questions about their role in some cross-recognition of this antigen by sera from Leishmania spp.-infected individuals. The complete T. cruzi calflagin and two fragments of the protein, consisting of 146 amino acids of the N-terminal and 65 amino acids of the C-terminal regions, were expressed and evaluated against a panel of sera, which included wellcharacterized samples from T. cruzi, and Leishmania-infected patients. We were able to show that sera from Leishmania (Viannia) braziliensis-infected individuals recognized the recombinant full-length calflagin. Both the N-terminal and the complete protein presented the same high sensitivity (98.5% of sera from T. cruziinfected patients was detected) but different specificities (94% and 98%, respectively, when evaluated against sera from people not infected by T. cruzi, including 15 sera from people infected with L. braziliensis). The C-terminal fragment presented low sensitivity (70%) but 100% specificity. We propose the use of these antigens in two sequential assays to optimize the serological diagnosis of T. cruzi infection in humans in geographic areas where Leishmania spp. infection is coendemic.