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Human schistosomiasis is associated with IgE and eosinophilia, feature of a type 2 response. In experimental investigations, murine model has been widely used in order to dissect the immune responses involved in the expression of protective immunity or disease in Schistosoma mansoni infection. Collectively, observations made in this model and in humans demonstrated a strong contrast since a Th2 response in infected mice is involved in the expression of pathology, however, in infected humans the same type of response is rather beneficial for the host. This review will consider the relevance of extrapolating studies of immune responses from experimentally infected rats a semi-permissive host, to studies on S. mansoni infected humans.

We have previously demonstrated in rat experimental schistosomiasis an upregulation of IL-4 expression at the mRNA and protein levels which could explain, at least in part, the increased IgE production observed during infection. Using this model, we have investigated the expression of IL-13 which is also involved in the induction of the IgE response. In the present study, we have shown a significant increase in IL-13 mRNA expression in spleen, liver and lungs following primary and secondary infection. IL-13 protein was detected by intracellular staining in spleen cells from infected rats, and in the supernatants of antigen-stimulated spleen cells. Furthermore, circulating levels of IL-13 were increased in sera from infected rats as compared to those from non-infected control animals. These findings show that, similarly to IL-4, IL-13 is upregulated and secreted during rat schistosomiasis, suggesting an involvement of both cytokines in IgE induction. In the in vivo experiments, only rats cotreated with neutralizing anti-IL-4 and anti-IL-13 antibodies showed significant decrease in the IgE levels. Moreover, administration of IL-13 enhanced total IgE levels. These results demonstrate the implication of IL-4 and IL-13 in vivo in IgE production, and provide a relevant animal model for a better understanding of the role of IL-4 and IL-13 in humans.

In contrast to most mouse strains, rats eliminate the primary schistosome burden around 4 weeks postinfection and subsequently develop protective immunity to reinfection. In rat schistosomiasis, we have shown predominant expression of a Th2-type cytokine response at the mRNA level after primary infection. In the present study, we showed a significant increase in interleukin-4 (IL-4) mRNA expression in inguinal lymph nodes early after a secondary infection. IL-5 mRNA expression showed a significant increase at days 2 and 4 postreinfection in the spleen and lymph nodes, respectively. We did not detect any gamma interferon (IFN-gamma) mRNA after a challenge infection. Analysis of cytokine secretion by stimulated spleen cells after a primary infection showed predominant expression of IL-4 with maximum production on day 21, accompanied by production of IL-5 from day 11 to day 67. A significant increase in IFN-gamma secretion was detected at day 21. Analysis of immunoglobulin G2b (IgG2b) and IgG2c (Th1-related isotypes) showed undetectable levels of IgG2b, but detectable levels of specific IgG2c antibodies were observed from day 42. The analysis of Th2-related isotypes showed high specific IgG1 and IgG2a antibody titers from day 29. After a secondary infection, only IL-4 and IL-5 secretion was sustained. This is supported by the increased production of Th2-related isotypes. These findings showed that S. mansoni infection can drive Th2 responses in rats in the absence of egg production which is required to induce a Th2 response in mice and are in favor of the role of Th2-type cytokines in protective immunity against reinfection.

A cDNA coding for rat IL-5 was obtained by RT-PCR from total spleen RNA. With the exception of a single a.a. replacement at position 85 (L-P), it is identical to the published sequence obtained by retroviral gene transfer. This cDNA was used to express biologically active recombinant IL-5 in E. coli and in insect cells using a baculovirus system. Rat IL-5 is more active on B13, an IL-5 dependent cell line, when produced in insect cells (specific activity 1.47 x 10(11)UI/mg compared to 4.28 x 10(6)UI/mg). This increased activity seems to be associated with the presence of IL-5 homodimers in recombinant protein preparations. A rabbit antiserum raised against recombinant bacterial IL-5 specifically inhibited B13 proliferation induced by bacterial and baculoviral IL-5. The availability of such reagents should facilitate studying the role of IL-5 in different infectious diseases, experimental allergic encephalomyelitis and in transplantation biology where the rat represents a more suitable model than mice.

The nucleotide sequences containing the rat interleukin-12 p40 gene was determined. Sequencing revealed the presence of six exons and five introns. Analysis of the 5' non-coding region showed the presence of several possible sites involved in cytokine gene regulation at the transcriptional level. Comparison of the deduced amino acid sequence of rat IL-12 p40 with that of the mouse and of human p40, showed 92% and 65% identity respectively.

As an animal model, rat schistosomiasis mansoni has provided considerable knowledge of immune mechanisms involved in the expulsion of worms and in a subsequent development of immunity to reinfection. Although it is clear that ADCC mechanisms participate in immunity to reinfection; the nature of the cytokines involved in immunity is unknown. To analyse the pattern of cytokines involved, the mRNA levels of different cytokines were assessed by RT-PCR as they occur within tissues during the course of infection. In spleens from infected rats, a significant elevation in IL-2 and IL-5 mRNA was observed during the early phase of infection (day 7). Analysis of pulmonary cytokine responses showed a dramatic increase in IL-4 and IL-5 on day 7. This was accompanied with a low but significant increase in IL-2 (day 11) and IL-12 (day 7) in the absence of augmented IFN-gamma expression. The cytokine expression patterns of draining lymph nodes (LN) from infected rats showed a significant increase of IL-2, IL-4 and IL-5 on day 21. Analysis of IL-10 expression showed exclusively a significant increase in LN on day 11, IFN-gamma mRNA was not detected in any tissue sample. Thus, rats develop a predominately Th2-type cytokine response during a primary infection which may be involved at least in part, in the expression of immunity against Schistosoma mansoni infection.

A new baculovirus (BacTen) was constructed in order to generate p10 recombinant expression vectors at high frequency. This virus is an AcMNPV derivative, with the polyhedrin gene deleted and thus exhibiting p10 promoter as a single strong late promoter. The polyhedrin coding sequence was re-inserted subsequently under the control of the p10 promoter, in place of the p10 coding sequence. Two flanking Bsu36I restriction sites were inserted together with the polyhedrin coding region. BacTen can, therefore, be efficiently restricted at the p10 locus and used in co-transfection experiments along with p10 transfer vectors carrying the foreign gene to be expressed. It is shown with three independent transfer vectors, that the proportion of recombinants in the viral progeny can be as high as 80% The BacTen baculovirus represents a new powerful tool for the generation of p10 promoter based expression vectors. in a system without the background of considerable production of very late proteins.