Molecular cloning and characterization of the porcine CD18 leukocyte adhesion molecule

Leukocyte adhesion molecules play an important role in regulating immunocyte trafficking into peripheral tissues. CD18 is the common β-chain of the β₂-integrin class of leukocyte adhesion molecules. Porcine CD18 cDNA was cloned using reverse transcriptase-polymerase chain reaction (RT-PCR), subcloned, and sequenced (GenBank accession number U13941). The deduced 769 amino acid sequence for porcine CD18 contains a 22 amino acid signal sequence, a cysteine-rich domain, and a 23 amino acid transmembrane domain. Porcine CD18 showed 79.3% cDNA sequence identity and 83.2% amino acid sequence identity with human CD18. Northern blot analysis revealed that porcine CD18 mRNA was expressed constitutively at high levels in freshly isolated alveolar macrophages, and levels declined approximately 50% over a 12 hr culture period. Peripheral blood cells and spleen cells expressed lower amounts of CD18 mRNA. Anti-human and anti-canine CD18 antibodies were tested for xeno-reactivity against porcine CD18, and more than 62% freshly isolated unstimulated alveolar macrophages were stained with anti-CD18 antibodies. Porcine macrophage CD18 transcript levels and cell surface expression detected by flow cytometry were not significantly altered following stimulation with LPS or human proinflammatory cytokines, including interferon-γ (IFN-γ), interleukin-α (IL-1a), IL-1β, tumor necrosis factor-α (TNFα), and IL-6. Thus, CD18 gene expression by porcine alveolar macrophages appears not to be regulated by human proinflammatory cytokines. These reagents may be useful for studying the role of adhesion molecules in the immunobiology of porcine models of allo- and xenotransplantation.