Vaccine induced immunity to Schistosoma mansoni: spleen cell proliferative responses before and after challenge in BALB/c mice given irradiated or normal schistosomula.

Spleen cell proliferative responses in BALB/c mice were assessed at varying intervals after vaccination or primary infection and subsequent cercarial challenge. Mice were vaccinated with 500 50-Krad-irradiated Schistosoma mansoni schistosomula or infected with 20 normal schistosomula. Prior to challenge, splenic responses in the two test groups to phytohemagglutinin (PHA) declined progressively while schistosomula (SMA)-driven responses increased. After challenge, PHA responses increased in both groups on day 3 then declined to significantly lower levels compared to normal controls. On day 3 after challenge, SMA responses in vaccinated mice were vigorous, and greater than twice the responses in infected mice. Thereafter, responses in vaccinated mice declined while responses in infected mice increased on days 7 through 25 but dropped markedly by day 39. For the infected group, in vitro depletion of plastic adherent cells or Lyt 2.2+ lymphocytes resulted in augmented SMA responses 3 days post-challenge by greater than 400% and greater than 100%, respectively. Depletion of either cell population in the vaccinated group had no significant effect. Protection assessed by total worm burdens showed a reduction of 62% in vaccinated mice and 43% reduction in infected mice. The post-challenge results indicate that these two models of anti-schistosomula immunity differed in the dynamics of their splenocyte antigen-specific proliferative responses. These findings may contribute to an understanding of the mechanisms by which resistance to S. mansoni is induced.