Novel CD8 molecule on macrophages and mast cells: Expression, function andsignaling

Background We previously identified, using flow cytometry and in situ RT-PC R, a novel CD8 molecule on rat alveolar macrophages (AM) and mast cells (MC ). RT-PCR also demonstrated that mouse AM express CD8 mRNA. Functional stud ies on rat AM determined that ligation of CD8 alpha- and beta-chains induce d inducible nitric oxide synthase (iNOS) upregulation, nitric oxide (NO), T NF-cr and IL-1 beta (CD8 alpha only) secretion. However, CD8 did not induce AM phagocytosis of IgG-opsonized or unopsonized particles. Rat MC stimulat ed through CD8 secreted NO and TNF-a, but not histamine. Because of its pot ential role in regulating cell function, we investigated the signaling path ways involved in macrophage CD8 stimulation. Methods and Results: Inhibitor of src family kinases (PP1) significantly (p<0.05) inhibited CD8a (OX8 ant ibody)-induced iNOS upregulation, NO, TNF-alpha and IL-1 beta production in rat AM. In addition, Ro 31-8220 (a PKC inhibitor) inhibited OX8-induced iN OS upregulation, NO and IL-1 beta production, but did not inhibit TNF-alpha production. Using Syk antisense, we further determined that OX8 stimulatio n of NO is Syk kinase dependent. Conclusion: Studies on the signaling mecha nisms of CD8 determined that src family kinases, PKC, and Syk kinase are in volved in CD8 signaling. Additionally, CD8 may have differential signaling pathways, as an inhibitor to PKC downregulated OX8-induced IL-1 beta, but n ot TNF-alpha release. Our studies demonstrate that AM CD8 is similar to T l ymphocyte CD8 in that src kinases are involved in CD8-mediated signaling. H owever, p56(Ick), which is expressed in T lymphocytes, has not been found i n macrophages, suggesting that other src family kinases may be involved in AM and MC CD8 signaling.