Immunological cross reactivity between Schistosoma mansoni and cholera toxin

SUMMARYIntranasal administration of schistosome antigens in com-bination with appropriate adjuvant may be an effectiveroute for immunization against schistosomes, since thelungs represent an important site of elimination of schisto-somulae. Our previous studies have shown that in miceintranasal administration of cholera toxin (CT) beforeinfection with Schistosoma mansoni results in an enhance-ment of the worm burden in comparison to nontreatedinfected animals. In the present study, it was shown thatmice treated intranasally with CT displayed high numbers ofschistosome-reactive IgM-secreting cells in the spleen aswell as high levels of schistosome-reactive serum IgMantibodies, whereas no significant immunological responseagainst two other antigens, ovalbumin (OVA) or keyholelimpet haemocyanin (KLH) was noted. Sera from micetreated intranasally with CT recognized a 22kDa antigenon SWAP blots. This band was not demonstrable afterabsorption of the sera with SWAP. These findings indicatea possible cross reactivity between cholera toxin andschistosome antigens. Further analysis by Western blotrevealed that a 22kDa antigen was detected on CT blotsby sera from mice and humans infected with S. mansoni.This band was not demonstrable after absorption of themouse or the human sera with CT. The 22kDa cross reactiveantigen was heat-stable. The antibodies against the 22kDaantigen were only found within the IgM class but not withinother Ig isotypes. Our findings also indicate that the 22kDaantigen detected by anti-S. mansoni antibodies representsthe A1 fragment of the cholera toxin.Keywords Schistosoma mansoni, cholera toxin, crossreactivity, immune responseINTRODUCTIONSchistosomiasis mansoni is a helminth infection of man,causing a major public health problem in the tropics anddevelopment of an effective vaccine against this disease is amajor challenge. Since the lungs may represent an importantsite of elimination of schistosomulae (Wilson et al. 1986),we recently employed intranasal administration of schisto-some antigens in combination with cholera toxin in anattempt to achieve protective immunity. In these studieswe found that schistosome infected mice which had beentreated intranasally with cholera toxin (CT) displayed asignificant increase in worm burden as compared to non-treated mice, whereas intranasal administration of solubleadult worm antigen (SWAP) in combination with CT had nosuch effect (Akhiani et al. 1993). Furthermore, it wasdemonstrated that administration of CT to mice led toincreased production of SWAP-reactive IgM serum anti-bodies. Since these findings suggested an antigenic relation-ship between CT and schistosome worm antigen, the presentstudy was undertaken in order to elucidate this possibility.MATERIALS AND METHODSExperimental designThe possible existence of an antigenic structure shared by S.mansoni and cholera toxin (CT) was studied. FemaleC57BL/6 mice (ALAB, Sollentuna, Sweden) aged six toten weeks were anaesthetized by inhalation of ether. Theanimals were then exposed by the intranasal route to 50mlvolumes of PBS containing soluble adult worm antigen(SWAP, 2mg), 10mg of CT (List Biological Laboratories,Inc., Campbell, CA, USA) or PBS alone on days 1 and 14.The animals were then infected percutaneously with 100–150 S. mansoni cercariae on day 21. All groups of mice werebled before and six weeks after challenge infection. Serawere collected and analysed by ELISA and Western blot.Some mice from the CT and the PBS groups were sacrificedon day 17(before infection) and on day 28 (1 week afterinfection) and spleen cells were collected and used to detect