CEACAM1 Inhibits Toll-like Receptor 2-triggered Antibacterial Responses of Human Pulmonary Epithelial Cells

Although Moraxella catarrhalis and Neisseria meningitidis are important human pathogens, they often colonize the human respiratory tract without causing overt clinical symptoms. Both pathogens express structurally unrelated proteins that share the ability to stimulate the adhesion molecule CEACAM1 expressed on human cells. Here we demonstrate that the interaction of CEACAM1 with ubiquitous surface protein A1 expressed on M. catarrhalis or with opacity-associated proteins on N. meningitidis resulted in reduced Toll-like receptor 2-initiated transcription factor NF-kappaB-dependent inflammatory responses of primary pulmonary epithelial cells. These inhibitory effects were mediated by tyrosine phosphorylation of the immunoreceptor tyrosine-based inhibitory motif of CEACAM1 and by recruitment of the phosphatase SHP-1, which negatively regulated Toll-like receptor 2-dependent activation of the phosphatidylinositol 3-OH kinase-Akt kinase pathway. Our results identify a CEACAM1-dependent immune-evasion strategy.