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T cell activation requires the engagement of both the T cell receptor (TCR) and the costimulatory receptor, CD28. CD80 or B7.1 is a cell surface molecule on antigen presenting cells that interacts with CD28 on T cells and initiates the costimulatory signal pathway. We have cloned and characterized the CD80 gene from porcine lymphocytes. Porcine CD80 was found to be upregulated at 72 hours on lymphocytes upon stimulation with E. coli lipopolysaccharide (LPS). This cell surface expression was detected by flow cytometry using CTLA4-Ig and an anti-human CD80 antibody that we have found to cross-react with porcine CD80. Sequence alignments between the CD80 genes from various species revealed a highly conserved region in the extracellular domain, which shares homology with the immunoglobulin constant (Ig-C) domain. Degenerate primers were used to clone this segment from a pig genomic DNA library. Rapid amplification of complementary ends (RACE) techniques was then employed to clone the porcine gene using sequence information from the Ig-C domain. Amino acid sequence analysis of porcine CD80 shows 53% sequence identity and 70% sequence homology with human CD80. The highest degree of homology (80%) as expected from sequence conservation between species is found in the Ig-C domain followed by the Ig-V domain (75%). The amino acid comparison between the porcine CD80 and human CD80 also shows that all of the amino acid residues that have been demonstrated as being important for CD28/CTLA4 interactions are conserved. This observation taken together with the high degree of homology suggests that porcine CD80 may be capable of interacting with human CD28/CTLA4. To test this, the porcine CD80 gene was then transfected into COS cells to test for cross reactivity and function. The results indicate that porcine CD80 transfected COS cells can cross activate human T cells. When considered with previous findings from this laboratory that porcine endothelial cells can be induced to upregulate the B7's by exposure to human T and NK cells suggests that porcine B7's may play a role in the human anti-porcine cellular response.