Immunoglobulin subclass distribution and diagnostic value of Leishmania donovani antigen-specific immunoglobulin G3 in Indian kala-azar patients

Visceral leishmaniasis, or kala-azar, a fatal tropical disease, remains pro blematic, as early diagnosis is difficult and treatment often results in dr ug resistance and relapse. We have developed a sensitive enzyme-linked immu nosorbent assay (ELISA), using leishmanial membrane antigenic extracts (LAg ) to detect specific antibody responses in 25 untreated Indian visceral lei shmaniasis patients. To investigate the pathogenetic significance of isotyp e markers in kala-azar, relative levels of specific immunoglobulin G (IgG), IgM, IgA, IgE, and IgG subclasses were analyzed under clinically establish ed diseased conditions. Since LAg showed higher sensitivity for specific Ig G than lysate, the immunoglobulin isotype responses were evaluated, with LA g as antigen. Compared to 60 controls, which included patients with malaria , tuberculosis, leprosy, and typhoid and healthy subjects, visceral leishma niasis patients showed significantly higher IgG (100% sensitivity, 85% spec ificity), IgM (48% sensitivity, 100% specificity), and IgE (44% sensitivity , 98.3% specificity) responses, Low levels of IgA in visceral leishmaniasis patients contrasted with a 13-fold-higher reactivity in sera from patients with leprosy. Among IgG subclasses, IgG1, -3, and -4 responses were signif icantly higher in visceral leishmaniasis patients than in the controls. IgG 2 response, however, was significantly higher (twofold) in leprosy than eve n visceral leishmaniasis patients. The rank orders for sensitivity (IgG = I gG1 = IgG3 = IgG4 > IgG2 > IgM > IgE > IgA) and specificity (IgM = IgG3 > I gE > IgG4 > IgG2 > IgG > IgG1 > IgA) for LAg-specific antibody responses su ggest the potentiality of IgG3 as a diagnostic marker for visceral leishman iasis.