The open reading frame (ORF) in the long terminal repeat (LTR) of mouse mammary tumor virus (MMTV) has recently been shown to encode multiple products including a negative acting factor (Naf) and a superantigen (Sag). Expression of superantigens from endogenous MMTV loci in the mouse results in the deletion of whole classes of T cells. In a PCR approach, with primers to the MMTV ORF and hybridization to MMTV specific probes, we have identified three human sequences. Direct sequencing of PCR products revealed that one of these products is related to a human autoantigen that is conserved among many species and is expressed in testes and sperm. The second sequence that we have identified is novel, and no evidence for expression of this sequence could be obtained. Finally, the third ORF-like sequence is a new member of a previously described family of human endogenous retroviruses (RTVL-I). This sequence is transcribed in several human cell lines, including B lymphoblastoid cells, and is thus the first demonstration that an RTVL-I-related sequence can be expressed. Taken together, these findings raise the intriguing possibility that the human genome contains superantigen-like sequences, some of which are also related to endogenous retroviruses, that may influence the T cell repertoire.
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