Studies on Human Cellular Immune Responses in an Endemic Area for Schistosomiasis Japonica

ZHAOSONG Zhang, LEI Shen, GUANLING Wu, et al.
Laboratory for Molecular Immunoparasitology, Institute for Molecular Biology of Medicine, Nanjing Medical University, Nanjing 210029, China

Chemotherapy remains the cornerstone of intervention, but reinfection necessitates frequent retreatment. There is thus a high priority need for the development of effective anti-schistosomiasis vaccines. Identifying the immunological mechanisms responsible for host resistance against schistosome is a fundamental step in rational vaccine design.

In rat, primate and human schistosomiasis, it is widely agreed that the mechanism of antibody-dependent, cell-mediated cytotoxicity (ADCC) plays a major role in the expression of acquired resistance. In all animal models and in humans, elevated immunoglobulin E (IgE) concentrations and blood and tissue eosinophilia are hall-marks of schistosomiasis. The protective role of IgE antibody was observed initially in rats by passive transfer experiments in vivo, and confirmed by several immunoepidemiological studies in human infection.

A major conceptual revolution in immunology by dividing mouse T helper cells into two populations with contrasting and cross-regulating cytokine profiles, deeply influenced our understanding of the immunity to schistosomes in human populations and in experimental models. It is universally accepted that in humans, primates and rats, protective immunity to schistosomes is associated with Th2 responses( IL-4, IL-5, IgE, IgG4 and eosinophils). IgE responses and eosinophils in schistosomiasis and their role in protective immunity have been regarded as part of a Th2-dependent regulatory circuit.

Protective immunity induced by immunisation with irradiated schistosomes is dependent on Th1-type responses, with interferon-gamma having a central role. By contrast, non-attenuated infections do not seem to induce significant levels of specific immunity, and the onset of egg production results in a switch to Th2 cytokine production. The skewing of Th1 and Th2 cytokine responses may in fact be a strategy evolved by the parasite to limit an effective host immune response. In fact, the differences observed in different hosts and even sometimes in the same host, indicate that the complex interaction between host and parasite cannot simply be restricted to the context of Th1 and Th2 cells and the effects of their products. It is likely that expression of immunity against schistosomes is the result of the combination and appropriate balance of cytokines and effector cells, leading in some instances to a successful immune response. The most recent studies of Jankovic et al, using B-cell-deficient and IFN-gamma knockout mice, have strongly reinforced the belief that effective vaccination against schistosomes depends on the simultaneous induction of both humoral and cell-mediated immunity. Therefore, it is necessary to find the candidate antigens expressing epitopes which can stimulate specific T cells and/or B cells for a cocktail vaccine.

The present study was designed to explore the characteristics of human cellular immune response and the association between cellular immunity and humoral immunity in the endemic area with schistosomiasis japonica and to find more B-cell and T-cell epitopes from some recombinant antigens to participate in the vaccine candidates. A group of 129 individuals aged 14-41 years old in Nanshan Island of Poyang Lake were selected. The levels of IL-4, IL-5, IL-10 and IFN-g stimulated by SEA and SWAP were detected in the supernatant of whole blood culture by Sandwich ELISA. Of the 129 individuals, 93 ones who were confirmed to be fully cleared the infection by praziquantel were tested again 45 days after chemotherapy and were observed the relationship between reinfection,exposure and cytokine levels. The correlation between the cytokine levels and age, infection intensity was analyzed and the cytokine levels before treatment were compared with those after treatment. At the same time, a panel of recombinant antigens(rSj14, rSj22.6, rSj23, rSj26, rSj28, rSj62, rSj97, TPI) were tested to observe their potential capacity to induce specific cytokine levels in vitro.

The main results of the study are as follows:

1. The results showed that age was a significant determination of SEA-specific IL-4, IL-5, IL-10 and IFN-g and SWAP-specific IL-10 levels and there was a significant inverse correlation between age and these cytokine levels by Spearman Rank Correlation analysis. It may be suggested that age-dependent is an epidemiological character of human cellular immune responses, which is consistent with the age-dependent acquired resistance and isotypic antibodies distribution.
2. There were non-responders whose cytokine, especially IFN-gamma levels were lower than 15.6ng/l, when stimulated by SWAP or SEA. The proportion of non-responders in egg-positive group (EPG>0 ) was higher than egg-negative group (EPG=0) . The mean level of each stimulated cytokine was significantly higher in EPG=0 group than in EPG>0 group, there was a significant difference between the two groups (P < 0.05). This suggests that the cell-mediated immune response of the human population in endemic area with schistosomiasis japonica was down-regulated in general rather than to control selectively Th1 or Th2 subset, especially in egg positive individuals.
3. In EPG>0 group, the non-responders were 59.1%-88.6% of total subjects before treatment and there were still some non-responders after chemotherapy, but the percentage of non-responders in EPG>0 group reduced very significantly. Comparison of the mean cytokine levels before and 45 days after praziquantel treatment of 44 subjects with original egg positive indicates that after chemotherapy all cytokine levels significantly increased, except the level of IL-4 stimulated by SEA. It may be due to the restoration of the down-regulated immune response after sterilizing the parasites or to large amount of adult worm antigen release because of chemotherapy, resulting in the increase of corresponding cytokine level stimulated by the antigens in vitro.
4. The levels of IL-4 and IL-5 stimulated by all the recombinant antigens were not detectable, while the control antigens PHA, SKSD and PPD could induce IL-4 and IL-5 responses. But all the recombinant antigens could stimulate IL-10 and IFN-g responses. This suggests that Th2 cytokines can not be induced by the recombinant antigens, which may be due to the simple structure of the small peptides without the predominant Th2-epitopes. This may explain the limited success of most subunit vaccine protocols designed to preferentially induce either Th1 or Th2 responses.
5. Multiple and non-conditional logistic regression analysis showed that individuals with the higher levels of SEA-specific IFN-gamma were 6.5 times less likely to become reinfected after chemotherapy than those with the lower levels. This suggests IFN-gamma may associated with acquired immunity.

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created 21/12/00