Identification and assessment of schistosome candidate vaccine antigens has relied, and continues to rely, heavily on studies of naturally acquired immune responses in human populations [20, 38, 39]. However these studies have been hampered by the lack of defined single antigens, particularly for S. haematobium, and by methodological difficulties in detecting responses associated with resistance to infection/re-infection in field studies of human populations [40, 41]. In this study, Sh13, the putative orthologue of the S. mansoni tegumental protein Sm13 was cloned from a cDNA library of adult schistosome worms . There are no reported homologues from other organisms and as yet, there have been no functional studies of Sm13.
The recombinant Sh13 protein was recognised by mouse sera following immunisation (data not shown) and was reactive predominantly with IgG3 and also with IgG1 antibodies in sera from the Zimbabwean population exposed to S. haematobium infection. The pattern of reactivity of the sera against Sh13 differed from that against adult worm crude antigens, where the predominant isotypes were IgG1 and IgG4. Responses to crude antigens are more heterogeneous as they are directed against multiple epitopes present on numerous uncharacterised antigens and therefore may mask the relationship between infection levels and responses to specific antigens .
The epidemiology of schistosome infection in this population followed a convex shape of rising infection levels (intensity and prevalence) peaking in childhood and declining with age thereafter, a pattern which has been attributed to cumulative exposure to infection and the development of gradually acquired resistance to schistosomes [8, 11, 14, 20, 42]. The relationship between infection intensity and anti-Sh13IgG3 levels varied significantly with age. The low levels of infection intensity in the youngest age group were matched by low levels of the antibody, presumably reflecting a low cumulative exposure to adult parasite antigens in these young children. This is consistent with an earlier report on children resident in a S. haematobium endemic area in Zimbabwe showing that the IgG3 response against adult worm antigens developed slowly in young children 4 months- 6 years old . Infection intensity and prevalence increased significantly to peak in children 11–12 years old. This significant change in infection levels was matched by a significant increase in anti-Sh13IgG3. Correlation analysis showed a significant positive association between infection intensity and anti-Sh13IgG3 levels in the age group presenting the highest egg counts allowing for the conjecture that the anti-Sh13 IgG3 response is stimulated by the increasing parasite burden. While infection intensity declined significantly in the oldest age group, anti-Sh13 IgG3 levels remained high so that the relationship between levels of this response and infection intensity is uncoupled as illustrated by the changes in the correlation between infection intensity and levels of the antibody in this age group.
The changing relationship between anti-Sh13 IgG3 and infection intensity with host age observed here was confirmed by the significant change in the correlation coefficient between the two variables with host age, and is consistent with that predicted for immune responses associated with resistance to infection . Theoretical work following the profile of a protective immune response triggered by and directed against antigens in the adult worms in a population, younger age groups as observed here . Furthermore, this correlation is predicted to fall in age groups with declining infection levels, so that a significant change in correlation coefficients with age is expected to occur as observed here. Similar changes have previously been reported in immuno-epidemiological studies of Necator americanus IgG responses against adult worm excretory/secretory antigens , and interpreted as suggesting the development of protective parasite-specific acquired immunity [8, 44].
There are several competing explanations for the falling infection intensity/prevalence and uncoupling of S. haematobium infection intensity and antibody levels observed here. These include changes in exposure to infection resulting in lower worm burdens and physiological changes resulting in lower susceptibility to infection. In this population data from the initial questionnaire studies did not show a significant change in exposure to infective water throughout the age range of the participants. In addition, our previous studies and those of other groups working on S. haematobium show that reduction in water contact with increasing age is in itself insufficient explanation for the observed decline in infection levels [14, 45–48]. Age-related physiological changes have also been suggested as a possible explanation for the development of innate resistance to infection, but evidence for this from field studies is still lacking. The alternative explanation is the development of schistosome-specific acquired immunity, [14, 20, 42] and the patterns of infection and antibody responses in our study are consistent with the effects of acquired immunity against the parasite.
If the expression pattern of Sh13 is similar to that of Sm13, which is expressed only in adult worms , then the anti-Sh13 response is being elicited by adult worms (both living and dying) and is directed against adult worms. In S. mansoni, the protein has been immunolocalised to the tegument and therefore is accessible to the host's immune system. Work to characterise the expression of Sh13 is now underway. From the present work, we can conclude that the profile of anti-Sh13 IgG3 is consistent with that of a protective response, but we have yet to establish that the anti-Sh13 response contributes to, rather than simply, reflects the development protective immunity against schistosome infections. We have obtained encouraging results from our preliminary vaccination studies in 10 hamsters which have shown that Sh13 stimulates an antibody response in hamsters and that vaccinated hamsters have little or no infection compared to unvaccinated controls which had heavy infection and liver pathology (data not shown). These studies are now being repeated using a larger sample size. Clearly, protective immunity will be multifaceted involving different cellular and humoral responses to several antigens.