Immunity in schistosomiasis: a holistic view

Identification of larval cross-reactive and egg-specific antigens involved in granuloma formation in murine schistosomiasis mansoni

Cross-reactive humoral immune responses between antigens of different developmental stages of the worm Schistosoma mansoni have previously been demonstrated. In contrast, information on antigenic cross-reactivity at the T-cell level is still very sparse. The present study examined the cross-reactive T-cell responses to eggs and crude and fractionated soluble egg antigens (SEA) in infected mice prior to (from 0 to 4 weeks of infection) and after (5 weeks and onwards) egg deposition. Splenic lymphocyte proliferation to unfractionated SEA was detected as early as 2 weeks postinfection and increased rapidly by 4 weeks postinfection. Injections of live eggs into the lungs of infected mice at 4 weeks postinfection demonstrated enhanced granuloma formation, indicating the presence of primed T cells that respond to egg antigens. Further experiments with the artificial granuloma model and polyacrylamide gel electrophoresis-separated SEA fractions demonstrated that in mice infected for 4 weeks the 60- to 66-, 93- to 125-, and greater than 200-kDa SEA fraction-coated beads elicited significant pulmonary granulomas. By 6 weeks postinfection, when eggs are deposited in the livers, in addition to the cross-reactive fractions (60 to 66, 93 to 125, and greater than 200 kDa), beads coated with fractions of 25 to 30, 32 to 38, and 70 to 90 kDa also elicited significant granulomatous reactions. These antigenic fractions are considered to have elicited egg stage-specific T-cell responsiveness. In addition hepatic granuloma T cells from the 6th week of infection demonstrated the strongest blastogenic response to the 60- to 66-kDa cross-reactive fraction. Thus, in vitro and in vivo experiments demonstrated T-cell cross-reactivity between the larval and egg stages of the worm. On the basis of these observations, the appearance of the primary circumovum granulomatous response in infected mice is considered to represent the sum of larval cross-reactive and egg-specific T-cell responsiveness.

Immunopathology of Schistosoma mansoni infection

Schistosomiasis mansoni is a chronic helminthic disease that affects about 100 million people in the tropics. The worms have a life span of 5 to 10 years, and they live in the mesenteric veins of the host. Lightly infected individuals are asymptomatic or manifest mild intestinal symptoms. Heavily infected individuals often develop severe morbidity with hepatosplenomegaly, sometimes with a fatal outcome. Morbidity is attributed to the strong humoral and T-cell-mediated host immune responses developed to a variety of parasite antigens and expressed as tissue inflammations. The immunopathology includes dermatitis, immune complex-mediated kidney disease, and, chiefly, T-cell-mediated granuloma formation and fibrosis around disseminated parasite eggs. This review describes the mechanisms of induction and expression of immunopathology in infected persons and experimental animals. Immunoregulatory mechanisms that modulate the enhanced immune responses and may ameliorate excessive morbidity are discussed.

Limitations to schistosome growth and maturation in nonpermissive hosts

The life cycle of schistosomes is reviewed in its various steps, both in permissive hosts (in which the cycle is completed) and in nonpermissive hosts (which excrete no viable eggs as a result of the infection). A large worm loss occurs at (or after) the lung stage in both types of hosts ("normal attrition") and some nonpermissive hosts (like the rat) have an additional elimination of worms from the portal circulation. Worm growth and reproductive maturation are also impaired in several nonpermissive hosts and the possible host-parasite interactions leading to such limitations are discussed, with special reference to hormonal influences. Attention is also given to peculiar phenomena occurring in some hosts, like the late portal worm elimination in rhesus monkeys, the migration from mesenteric veins to lungs in rats, and the block to egg excretion in guinea pigs. The steps of the schistosome life cycle which appear vulnerable in several hosts are contrasted with the steps which are carried out successfully in the majority of hosts studied.