Schistosoma mansoni: Immunization with cercarial glycocalyx preparation increases the adult worm burden

Abstract

Cercariae are covered by a thick glycocalyx (GCX) that activates complement by the alternative pathway and is recognized by antibodies in the serum of infected animals (Standen 1952; Samuelson and Caulfield 1985). Further, it contains antigens reactive with protective monoclonal antibodies produced in rats and mice (Grzych et al., 1982; Ham et al. 1984). Recently, Veira et al. (1986) found that cercarial transformation fluid, which contained GCX and other cercarial components, suppressed the in vitro proliferative response of human peripheral blood lymphocytes to both schistosome antigens and mitogens. Despite this evidence that GCX is immunologically relevant in schistosome infections, its effect as a primary immunogen has not been studied. We have recently characterized GCX prepared with sodium dodecyl sulfate (SDS) or guanidine hydrochloride (GuHCl) as solubilizing agents (Caulfield et al. 1987). Both preparations were rich in carbohydrate, 40% for GuHCl-GCX and 80% for SDS-GCX, with fucose as the major sugar. Here we injected mice with pooled fractions of either preparation of GCX, using phosphate-buffered saline (PBS), Maalox, or Bacille Calmette-Guerin (BCG) as carriers and/or adjuvants. We assessed the effect of immunization with GCX by comparing the numbers of adult worms recovered (worm burden) be mesenteric vein perfusion (Stirewah et al. 1951; Warren and Peters 1967) in GCXand sham-immunized mice 6 weeks postcercarial challenge. The number of adult worms recovered from mice was dramatically different between GuHCl-GCX and sham-immunized groups. As shown in Table I, animals injected with GuHCl-GCX in either PBS or Maalox had increased adult worm burdens in ah four experiments. The data were compared statistically using a stratified Wilcoxon’s test (Lagakos 1982; Table I). The statistical significance of the difference between the GuHCl-GCX-immunized and sham-immunized controls was 0.02 < P < 0.01. Thus, GuHCl-GCX when administered with PBS or Maalox significantly increased the number of surviving adult worms at 6 weeks postinfection when compared to shamimmunized controls. When BCG was administered with GuHCI-GCX, we found that the adult worm burdens were the same as in mice which received BCG alone. Mice immunized with SDS-GCX gave very different results from mice immunized with GuHCl-GCX. In two separate experiments with SDS-GCX, the adult worm burdens of SDS-GCX-immunized and shamimmunized controls were not significantly different when PBS or Maalox were used as adjuvants. Thus, immunization with SDS-GCX did not affect the number of surviving adult worms when compared to control-immunized mice. Several immunological mechanisms could explain the increase in numbers of adult worms recovered in mice immunized with GuHCl-GCX. Immunization with GuHCl-GCX may have suppressed cellular immunity, as suggested by Veira et al. (1986). Other studies have also demonstrated that schistosome materials suppressed the proliferative response of peripheral blood lymphocytes in vitro (Dessaint et al. 1977; Colley et al. 1979; Camus et al. 1981). Significantly, egg antigens, some of which cross-react with the cercarial surface (Ham et al. 1984), have been implicated in the suppression of the cell-mediated immune response (Boros et al. 1975; Colley 1975; Naggar and Colley 1982). Alternatively, GuHCl-GCX-immunized mice may have produced blocking antibodies which would bind to and protect invading larval parasites. Blocking antibodies to cercarial tegumental epitopes have been demonstrated in the rat model, both in vitro and in vivo (Grzych et al. 1982; Khahfe et al. 1986). Lastly, immunization with GuHCl-GCX may have induced a state of tolerance with respect to the carbohydrate antigens in GCX, analogous to that induced