Histological examination of the cellular reactions around schistosomula of Schistosoma mansoni in the lungs of sublethally irradiated and unirradiated, immune and control rats

Why the radiation-attenuated cercarial immunization studies failed to guide the road for an effective schistosomiasis vaccine: A review

Schistosomiasis is a debilitating parasitic disease caused by platyhelminthes of the genus Schistosoma, notably Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum. Pioneer researchers used radiation-attenuated (RA) schistosome larvae to immunize laboratory rodent and non-human primate hosts. Significant and reproducible reduction in challenge worm burden varying from 30% to 90% was achieved, providing a sound proof that vaccination against this infection is feasible. Extensive histopathological, tissue mincing and incubation, autoradiographic tracking, parasitological, and immunological studies led to defining conditions and settings for achieving optimal protection and delineating the resistance underlying mechanisms. The present review aims to summarize these findings and draw the lessons that should have guided the development of an effective schistosomiasis vaccine.

Radiation-attenuated schistosome vaccination – a brief historical perspective

The high level of protection which can be induced by vaccination of a range of hosts, from rodents to primates, with live radiation-attenuated schistosome larvae offers great promise for development of a human schistosome vaccine. Studies of the irradiated vaccine models benefitted from significant funding during the 1970–90s and much was learned concerning the inducers, targets and mechanisms of immunity. Less progress was made in definition of the protective antigens involved. The application of new techniques for identifying membrane and secreted antigens has recently provided new vaccine candidates and a new impetus for schistosome vaccine development. This article is intended as an overview of some of the main lessons learned from the studies of the irradiated vaccines as a backdrop to renewed interest in schistosome vaccine development.

Immunity induced by the radiation-attenuated schistosome vaccine

As a paradigm for the development of a vaccine against human schistosomiasis, the radiation-attenuated (RA) vaccine has enabled the dissection of different immune responses as putative effector mechanisms. This review considers advances made in the past, and updates our knowledge with reference to recent studies that have provided new information relevant particularly to the early innate events after vaccination, and to the nature of the protective effector mechanism. Priming of a protective response by RA larvae is a highly co-ordinated series of events starting in the skin, draining lymph nodes and lungs, leading to the development of various effector responses, ranging from Th1-associated cell-mediated activity, to anti-parasitic antibodies, all of which contribute to the elimination of challenge larvae to varying extents. In this respect, the RA vaccine elicits a multifaceted immune response, from which we can derive valuable insights relevant to the future design of novel delivery systems and adjuvants for recombinant and subunit vaccines.

Mechanisms involved in the loss of antibody-mediated adherence of macrophages to lung-stage schistosomula of Schistosoma mansoni in vitro

Sera from rabbits vaccinated with irradiated cercariae mediated cell (P388D1 or mouse peritoneal macrophage) adherence to lung-stage schistosomula (LS) but such antibody-mediated cell adherence was short-lived in contrast to cell adherence to mechanically transformed schistosomula (MS). Thus LS lost 50% of their adherent cells within 3-6 h in culture and up to 90% by 24 h, whereas adherence to MS was undiminished during this time. Rapid loss of adherent cells was unique to schistosomula that had developed to the lung stage because schistosomula recovered from the skin up to 3 days post-infection did not exhibit the rapid cell loss shown by 3-day LS. To determine whether cell loss was caused by loss of surface antigenicity during culture LS were cultured on their own for up to 24 h and at various intervals samples of schistosomula were tested for antigenicity by addition of immune serum and cells. Levels of adherence to both MS and LS were maintained throughout the incubation period. When antibody-opsonized schistosomula were washed and indicator cells added at progressive intervals, persistence of adherence was again demonstrated, showing that antibody binding to LS had not promoted surface antigen loss or degradation of bound antibody. It was then shown, by adding fresh macrophages to cultures up to 24 h old that LS which had lost their adherent cells nevertheless retained bound antibody, and comparison of adherence of 'used' and 'fresh' cells to MS and LS showed that the cytoadherence properties of macrophages were not significantly reduced during their culture with LS from which cells had been lost.(ABSTRACT TRUNCATED AT 250 WORDS)

Schistosoma mansoni: acquired immunity in mice after the use of oxamniquine at the evolutive skin and pulmonary phases

Mice infected with 350 cercariae of Schistosoma mansoni (LE strain) were treated with oxamniquine, at the dose of 400 mg/kg, 24, 48, 72, and 96 h after infection. Forty days after the treatment, the animals were submitted to a challenge infection with 80 cercariae, through the abdominal and ear skins. The number of immature worms in the animal groups treated 24 and 96 h after the first infection was found to be lower than that in the control group, thus showing that the death of schistosomes by chemotherapy, at the skin and pulmonary phases, causes an acquired resistance state.

Schistosoma mansoni: histological analysis of the synergistic interaction between vaccine immunity and praziquantel therapy in the lungs of mice

Naive CBA mice and mice vaccinated 4 weeks previously with gamma-irradiated cercariae of S. mansoni were challenged percutaneously with normal cercariae and then treated with 500 mg/kg body weight of Praziquantel (Pzq). The drug was administered intradermally on day 1 or intramuscularly on day 6, thus targeting against skin stage or lung stage challenge larvae respectively. The skin site of challenge and/or the lungs were removed at various time points to provide samples for histological examination. As reported elsewhere (Flisser, Delgado & McLaren 1989) the efficacy of Pzq was significantly enhanced in vaccinated mice and was influenced by the treatment regime. Histological analysis revealed that when Pzq was administered I/D on day 1 to vaccinated mice, the inflammatory response to challenge differed in extent but not nature from that seen in vaccinated but untreated cohorts. This correlates with worm recovery data showing no (this study), or only marginal synergy between drug treatment and immunity using this regimen of drug treatment (Flisser et al. 1989). Following the day 6 protocol of drug delivery, however, lungs from treated vaccinated mice exhibited many large inflammatory reactions containing trapped challenge larvae. In contrast, lungs from untreated vaccinated mice had only few foci which were small and rarely contained trapped larvae. These data again correlate well with worm recovery data showing that there is a highly significant synergy between vaccination and drug treatment administered at this time (Flisser et al. 1989; this study). It would seem, therefore, that Pzq exacerbates lung phase immunity in the NIMR vaccine mouse model where skin phase immunity predominates and pulmonary attrition is normally minimal. The results are discussed in the light of published data concerning the effector mechanisms thought to characterize skin and lung phase vaccine resistance in the murine model.

Schistosoma mansoni: evidence that site-dependent host responses determine when and where vaccine immunity is expressed in different rodent species

Laboratory rodents vaccinated with highly irradiated cercariae of Schistosoma mansoni develop significant levels of specific acquired resistance yet effect challenge elimination in different organs. Mice and guinea-pigs are at opposite ends of the spectrum in this respect since, in our hands, vaccinated mice kill challenge parasites in the skin whereas vaccinated guinea-pigs kill challenge parasites predominantly in the liver. To determine whether this phenomenon is host-dependent (site) or parasite-dependent (stage), we have transferred worms harvested from mice or guinea-pigs into vaccinated recipient guinea-pigs. The results show that mouse-derived 5-day lung worms and 9-day liver worms that are essentially refractory to vaccine resistance in mice are indeed susceptible to vaccine resistance in guinea-pigs. Identical levels of susceptibility were recorded for lung-stage larvae introduced via the foot vein so as to experience lung and liver mechanisms, or via the mesenteric vein to bypass the lung, thereby confirming that vaccine resistance in guinea-pigs operates in the liver. Mouse worms and guinea-pig worms exhibited equivalent levels of susceptibility at all stages of development. Thirteen-day-old larvae from either donor species were on the border-line of vulnerability, while 20-day-old worms were totally refractory to vaccine immunity in guinea-pigs. These data show that vaccine immunity in different rodent species is a site-dependent, rather than a stage-dependent phenomenon. There is, however, an upper age limit of schistosome vulnerability which is common to worms harvested from different donor species.

Schistosome migration in primates: a study in the olive baboon (Papio anubis)

The migration of isotopically labelled schistosomula of Schistosoma mansoni from the skin to the hepatic portal system of baboons was tracked by autoradiography of compressed organ preparations. Passage of schistosomula from skin to lungs was rapid between days 2 and 5, 72% of applied parasites being detected in the lungs at the latter time. There was an equally rapid migration from lungs to liver, 60% of the adult worm burden having arrived by day 9. Perfusion data indicated a final maturation of not less than 78%. No evidence was found to support or refute a systemic phase of migration. Thus, in comparison to rodents, schistosome migration in baboons appears to be faster and more successful, possibly because the lungs do not represent a significant obstacle to migration.

Immunity to Schistosoma mansoni in vivo: contradiction or clarification?

In recent years controversy and contradiction have hindered the elucidation of the immune effector mechanisms that are most effective against Schistosoma mansoni - an essential goal for the development of an effective vaccine. However, recent in-vivo studies have clarified the relative contributions of such mechanisms to protection. Here, Dario Vignali and colleagues summarize current evidence that suggests that both antibody and CD4+ T cells, in cooperation with macrophages, are crucial for the development of an effective response. In addition, a model is presented that may account for some of the discrepancies observed and which could be used as a basis for future research.