Serial determinations of fluorescent cercarial antibody titers were performed in mice experiencing full-scale Schistosoma mansoni infections, and in mice infected, respectively, with (a) irradiated cercariae, (b) purified eggs and (c) irradiated cercariae followed by purified eggs. The findings were correlated with worm and egg burdens and with the pathological events at successive stages of infection.
All four of the above types of infection induced antibody titers of varying levels and durations. Mild elevations of titer were first noted two to three weeks after primary stimulation, regardless of the type of schistosome antigen used. Successive stimulation with pure worms and with pure eggs resulted in an accelerated antibody response. Infections with pure worms or eggs, alone or in succession, resulted in much lower antibody titers than did full-scale infection with normal cercariae. The antibody titers and the degrees of host cell response, or lymphoreticular activity, showed a good correlation in all experimental situations.
In full-scale schistosome infection the earliest antibody rise was due to worm antigens, since it occurred prior to the onset of oviposition. The period of rapid egg-deposition in host tissue, between the 4th and 7th week of infection, was accompanied by a steep rise in titer, followed by leveling off of both titer and eggs/g of liver tissue toward the 10th week of infection. This florid phase of schistosome infection can be interpreted as a secondary immunological host response to an escalatory challenge with eggs, and marks the maximal stage of host sensitization and immunological reactivity.
Correlation of data on antigen destruction and on antibody formation in experimental schistosome pseudotubercles, which have been obtained with a uniform immunofluorescent system, has shown that catabolism of stainable schistosome antigen and granuloma formation precede the appearance of fluorescent antibody in host serum. Antigen remains sequestered in the granuloma past the time of the rise and fall of antibody titer, so that a gradient of antigen-concentration between the granuloma center and the milieu interne of the host is maintained during much of the primary host reaction to schistosome eggs.
The significance of the findings is discussed, and avenues for further studies are proposed.
WHO Fellow, Harvard Medical School. Present address: Hospital San Juan de Dios, Santa Ana, El Salvador, Central America.