Biliary antibody response in rats infected with rodent Giardia duodenalis isolates

Using a sensitive ELISA, specific serum and bile anti-Giardia IgM and IgA responses were studied in rats infected with two strains of Giardia duodenalis: a rat isolate which produces a chronic infection and a mouse isolate which produces a self-limiting infection. Paired samples of serum and bile were collected from groups of DA (RT1avl) rats at various times during primary and secondary infections. Antibody responses to both organisms were similar. Only IgA anti-Giardia antibodies were detected in bile whereas both IgM and IgA antibodies were detected in serum. Biliary IgA antibody titres increased throughout the course of the primary infection and remained at high levels for at least 10 weeks. Biliary IgA titres increased 16-fold during the secondary infection with both isolates. Serum IgA anti-Giardia titres also increased but more slowly than the titres in bile. Serum IgM antibody responses were observed against both organisms during the primary and secondary infections. Trophozoites harvested from the intestinal lumen during primary infections were examined for surface-bound IgA by immunofluorescence microscopy. IgA was detected on 3% of trophozoites on day 7 after infection but on over 70% of trophozoites by the 10th day. The data demonstrate the occurrence of a secretory IgA immune response in rats infected with both G. duodenalis isolates, some of which is directed against surface antigens of the trophozoites.