Schistosomiasis, due to Schistosoma mansoni, is a major health problem in many subtropical countries, and major efforts are being made to define a vaccine. In this regard, we have reported that sera from subjects with low susceptibility to infection by S. mansoni react with a major larval surface antigen (P-37), having an apparent molecular mass of 37 kD, against which sera of susceptible individuals show little reactivity. We have now cloned the cDNA for this antigen by screening a schistosome cDNA expression library with antibodies against the purified protein. The selected cDNAs encode a protein that is specifically identified by immune human sera containing antibodies against P-37, while sera exhibiting low or no reactivity toward P-37 fail to recognize the recombinant protein. The cloned cDNAs hybridize with a 1.2-kb RNA that is the transcript of a single copy gene. This RNA directs the synthesis of a 36.5-kD polypeptide that is precipitated by sera from the most resistant subjects. The amino acid sequence of the encoded polypeptide shows homology with the glycolytic enzyme Glyceraldehyde-3P-dehydrogenase (72.5% of positional identity with human Glyceraldehyde-3P-dehydrogenase). Antibodies against the recombinant protein identified P-37 on the larva. These findings, together with other reports, indicate that a number of conserved proteins may be major targets of host-protective immunity against S. mansoni. The hypothesis is discussed that genetic restriction of the immune response to these antigens may occur in heterogeneous human populations because of the limited number of T cell epitopes carried by these host-like proteins. Such genetic effects might allow parasite transmission through nonresponder (susceptible) individuals. This hypothesis and the protective properties of P-37 can now be tested using the recombinant protein and synthetic peptides derived from selected regions of the polypeptide chain.

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