Superantigens are microbial proteins that strongly stimulate T cells. We described previously that the Epstein-Barr virus (EBV) transactivates a superantigen encoded by the human endogenous retrovirus, HERV-K18. We now report that the transactivation is dependent upon the EBV latent cycle proteins. Moreover, LMP-2A is sufficient for induction of HERV-K18 superantigen activity.Superantigens are pathogen-derived proteins that elicit a strong primary T-cell response from the host (reviewed in references 25 and 27). Superantigens are presented to T cells by major histocompatibility complex (MHC) class II molecules on antigen-presenting cells. They differ from conventional peptide antigens by binding solely to the V portion of the T-cell receptor (TCRBV) outside of the peptide-binding groove, thus forming a bridge between the T cell and the antigen-presenting cell (29). This bridging transduces a signal to the T cell, causing it to secrete cytokines that can further activate surrounding T cells. The hallmark of a superantigen response is the rapid and strong primary T-cell activation, which is MHC class II dependent and TCRBV restricted. In addition, antigen processing into peptides is not required. Both bacteria and viruses encode superantigens. The bacterial superantigens are mainly enterotoxins, which are secreted and bind externally to MHC class II molecules for presentation (34). In contrast, viral superantigens are glycosylated proteins that are endogenously produced in the infected cells.There are three families of viruses that are associated with superantigen or superantigen-like activity: Retroviridae, Rhabdoviridae, and Herpesviridae. Retroviral superantigens were first depicted in the B-type virus group in mouse mammary tumor viruses and are found in both infectious mouse mammary tumor viruses and endogenous proviruses (14,18,39,63). It has been previously shown that the env gene of HERV-K18, a defective human endogenous provirus located on chromosome 1, encodes a superantigen activity (51, 52). The HERV-K family is closely related to the B-type retroviruses based on amino acid similarity in the reverse transcriptase gene (48). HERV-K18 is a relatively recent integrant in the genome, as it is found in Old World primates but not in New World primates, indicating that it was acquired subsequent to the evolutionary divergence of these species (28). A few years ago, it was reported that Epstein-Barr virus (EBV) is associated with TCRBV13-specific superantigen activity, which is MHC class II dependent and not due to a recall antigen response (53). More recently, it was demonstrated that the superantigen activity is due to EBV transactivation of HERV-K18 env (52). We show here that this activity is dependent upon the major EBV latent gene transactivator EBNA-2, which upregulates most of the other EBV latent genes, all of which have the ability to transactivate host cell genes. In accordance with this finding, we show that the EBV latent membrane protein LMP-2A is sufficient for transactivation of HERV-K18 env.E...