Experimental schistosomiasis japonica in the pig: Immunohistology of the hepatic egg granuloma

Use of the pig as an animal model in schistosomiasis research is increasing, but knowledge of the porcine immune response to schistosome infection is still very limited. We investigated the immunohistology of different maturation stages of the Schistosoma japonicum egg granuloma in pigs. Liver sections from pigs experimentally infected with S. japonicum for 9, 12 or 21 weeks were examined by immunohistochemistry using a three-step streptavidin-biotin-complex/immunoperoxidase method or a two-step alkaline phosphatase-mediated system. All granulomas showed marked expression of major histocom-patibility complex (MHC) class II in epithelioid macrophages and were dominated by T lymphocytes, comprising both CD4⁺ and CD8⁺ phenotypes, with consistently higher proportions noted for CD8⁺ cells. B lymphocytes, as identified by expression of CD21, were confined to lymphoid nodular structures primarily associated with mature granulomas. Early and mature granulomas contained numerous immunoglobulin (Ig)G⁺ plasma cells. Significant differences in immunohistology related to duration of infection were not observed. The results indicate that all stages of the hepatic S. japonicum egg granuloma in the pig manifests MHC class II-dependent CD4⁺ T cell activity concomitant with infiltration of CD8⁺ T cells. B cell activity preceding the effector cell stage appears to occur in granuloma-associated lymphoid nodules, whereas antibody, mainly IgG, is produced within the granuloma.