I. Phase I clinical trials involving the use of swine tissues have begun. However, pigs carry endogenous retroviruses, called porcine endogenous retrovirus or PERV. We have previously shown that activation of this normally latent retrovirus occurs after exposure of pig primary peripheral blood mononuclear cells (PBMC) to mitogenic agents. The virus released from the activated cells is infectious for porcine and human cell lines. We have now cloned and sequenced the envelope of this virus isolate and found that is a class A envelope, previously defined as a human-tropic envelope. In addition, we surveyed a number of human-derived cells and cell lines to determine the tropism of the virus. Importantly, we found that human PBMC were resistant to infection by PERV under the in vitro conditions utilized. This result has implications for developing guidelines to monitor recipients of porcine xenografts for evidence of PERV infection. We also examined a range of non-human cell lines as an initial step developing a rational animal model that is permissive to infection. We found that mink and feline-derived cell lines support productive viral infection, while cell lines derived from other species examined did not. Finally, we have begun applying our methods developed to study PERV towards analysis of porcine plasma-derived products. In a study of porcine factor VIII, trade name Hyate C, we were able to detect PERV sequences and PERV reverse transcriptase activity in the final product. However, infectivity studies did not provide any evidence for presence of infectious virus. Hence, CBER was able to make an informed decision regarding the risk to recipients, and allowed Hyate C to remain on the market. II. The outermost protein of the retrovirus, the envelope, carries the primary determinant of viral tropism through its specific interaction with a cell surface receptor. We are studying type C retroviruses that are currently used as retroviral vectors in human gene therapy clinical trials as models. These studies will increase our understanding of virus-receptor interactions and how they dictate the host range of a retrovirus. This year, in collaboration with Ira Berkower, we were able to isolate recombinant envelope glycoprotein from amphotropic murine leukemia virus. This reagent will be used to develop a binding assay to elucidate regions of the receptor involved in virus attachment vs. viral entry. In addition, we collaborated with Maribeth Eiden to study a strain of gibbon ape leukemia virus that cannot infect a Chinese hamster cell line, even though the cells express functional receptors for this virus. This finding suggests that type C retroviruses may also require a "coreceptor" for entry into cells, which may be a unifying feature of retroviral entry.