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

Background: We previously identified, using flow cytometry and in situ RT-PCR, a novel CD8 molecule on rat alveolar macrophages (AM) and mast cells (MC). RT-PCR also demonstrated that mouse AM express CD8 mRNA. Functional studies on rat AM determined that ligation of CD8 α- and β-chains induced inducible nitric oxide synthase (iNOS) upregulation, nitric oxide (NO), TNF-α and IL-1β (CD8α only) secretion. However, CD8 did not induce AM phagocytosis of IgG-opsonized or unopsonized particles. Rat MC stimulated through CD8 secreted NO and TNF-α, but not histamine. Because of its potential role in regulating cell function, we investigated the signaling pathways involved in macrophage CD8 stimulation. Methods and Results: Inhibitor of src family kinases (PP1) significantly (p < 0.05) inhibited CD8α (OX8 antibody)-induced iNOS upregulation, NO, TNF-α and IL-1β production in rat AM. In addition, Ro 318220 (a PKC inhibitor) inhibited OX8-induced iNOS upregulation, NO and IL-1β production, but did not inhibit TNF-α production. Using Syk antisense, we further determined that OX8 stimulation of NO is Syk kinase dependent. Conclusion: Studies on the signaling mechanisms of CD8 determined that src family kinases, PKC, and Syk kinase are involved in CD8 signaling. Additionally, CD8 may have differential signaling pathways, as an inhibitor to PKC downregulated OX8-induced IL-1β, but not TNF-α release. Our studies demonstrate that AM CD8 is similar to T lymphocyte CD8 in that src kinases are involved in CD8-mediated signaling. However, p56(lck), which is expressed in T lymphocytes, has not been found in macrophages, suggesting that other src family kinases may be involved in AM and MC CD8 signaling.