The mouse major histocompatibility complex (MHC) class I sequence was detected in 8-week-old Schistosoma mansoni by in situ polymerase chain reaction (in situ PCR). The signals to the mouse class I MHC sequence were observed in the nuclei of the mesenchymal and reproductive cells of S. mansoni. Signals were also observed in the cytoplasm of the tegumental tubercles. This finding suggested the possibility of MHC gene transfer from the host to schistosomes. Furthermore, the class I MHC sequence was detected in the DNA extracted from the cercariae of S. mansoni by nested PCR. Neither the nucleotide sequence of class I MHC detected in adult worm DNA nor that of class I MHC detected in the host (mouse) DNA was identical with that of class I MHC detected in the cercarial DNA. From the data we assumed that S. mansoni may have retained their own mouse class I MHC sequence in their genome throughout their life-cycle.
