Abstract
Background
Schistosomiasis continues to be one of the most prevalent parasitic diseases in the world. Despite the existence of a highly effective antischistosome drug, the disease is spreading into new areas, and national control programs do not arrive to complete their tasks particularly in low endemic areas. The availability of a vaccine could represent an additional component to chemotherapy. Experimental vaccination studies are however necessary to identify parasite molecules that would serve as vaccine candidates. In the present work, C57BL/6 female mice were subcutaneously immunized with an n-butanol extract of the adult worm particulate membranous fraction (AWBE) and its protective effect against a S. mansoni challenge infection was evaluated.
Methodology and Findings
Water-saturated n-butanol release into the aqueous phase a set of membrane-associated (glyco)proteins that are variably recognized by antibodies in schistosome-infected patients; among the previously identified AWBE antigens there is Alkaline Phosphatase (SmAP) which has been associated with resistance to the infection in mice. As compared to control, a significantly lower number of perfuse parasites was obtained in the immunized/challenged mouse group (P<0.05, t test); and consequently, a lower number of eggs and granulomas (with reduced sizes), overall decreasing pathology. Immunized mice produced high levels of sera anti-AWBE IgG recognizing antigens of ∼190-, 130-, 98-, 47-, 28-23, 14-, and 9-kDa. The ∼130-kDa band (the AP dimer) exhibited in situ SmAP activity after addition of AP substrate and the activity was not apparently inhibited by host antibodies. A preliminary proteomic analysis of the 25-, 27-, and 28-kDa bands in the immunodominant 28–23 kDa region suggested that they are composed of actin.
Conclusions
Immunization with AWBE induced the production of specific antibodies to various adult worm membrane molecules (including AP) and a partial (43%) protection against a challenging S. mansoni infection by mechanism(s) that still has to be elucidated.
Schistosomiasis is a neglected disease affecting more than 200 million people globally, especially in sub-Saharan Africa. The mainstay of control of schistosomiasis is Praziquantel, but the mass administration of this drug is unsustainable due to the high rates of re-infection after treatment. These high rates of re-infection point towards the potential emergence of schistosoma drug resistance, making the anti-schistosome vaccine an essential component for the future control of schistosomiasis, as an adjunct to chemotherapy. Multiple strategies have been used to develop an anti-schistosome vaccine with different levels of success. These studies found that the tegument is the most important source of protective antigens; a logical assumption considering this structure represents the surface where the parasite and host interact. In our laboratory, we have isolated a (glyco)protein extract (AWBE) from the whole membrane fraction of adult worms, which is enriched by enzymatic and somatic antigens. Some of these antigens are recognized by infected patients and by mice immunized with irradiated cercariae. Given this context, we tested the possible protective effect of AWBE in mice. The results showed that immunization with AWBE induced a strong humoral response (IgG) with 43% protection against a challenge infection. The AWBE-vaccinated mice showed specific recognition of epitopes in identified proteins, such as schistosome phosphatase and probably actin, pointing to a possible association of these antigens with immunoprotection. These antigens may join the gallery of candidate proteins for vaccination against the infection by schistosomes.
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