In vitro studies suggest an important role for CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1) in infection by multiple gram-negative bacteria. However, in vivo evidence supporting this role is lacking, largely because the bacterial adhesins involved in this host-microbe association do not bind to murine-derived CEACAM1. One of several adhesins expressed by nontypeable Haemophilus influenzae (NTHI), the outer membrane protein P5-homologous adhesin (or P5), is essential for colonization of the chinchilla nasopharynx and infection of the middle ear. Here we reveal that NTHI P5 binds to the chinchilla homologue of CEACAM1 and that rabbit anti-human carcinoembryonic antigen blocks NTHI colonization of the chinchilla nasopharynx, providing the first demonstration of a role for CEACAM receptor binding by any bacterial pathogen in vivo.The carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs) are a subset of the immunoglobulin superfamily that mediate intercellular binding by homophilic and heterophilic interactions which influence a variety of normal and pathogenic processes, cellular growth, differentiation, and activation (reviewed in references 13, 16, 25, and 35). In vitro studies from several laboratories have now clearly established that adhesins expressed by a variety of important human-restricted bacterial pathogens bind specific members of the CEACAM family. For example, the evolutionarily unrelated colony opacity-associated (Opa) family of adhesins expressed by pathogenic and commensal Neisseria spp. (8, 10-12, 14, 15, 51-53), the Haemophilus influenzae outer membrane protein (OMP) P5 (20), and the Moraxella catarrhalis ubiquitous surface protein UspA1 (21) each bind CEACAMs. This interaction appears to mediate both attachment and engulfment of the bound bacteria by CEACAM-expressing epithelia (52, 54), endothelia (33, 34), and phagocytes (10,14,29,46,47,52). In the context of the epithelia, polarized monolayers of epithelia express CEACAM1, CEACAM5, and CEACAM6 on the apical surface and at cell-cell junctions (49, 54). Bacterial binding to these receptors is sufficient to mediate apical adherence and transcellular transcytosis across the monolayers, allowing the bacteria to emerge in the subepithelial spaces (54). Bacterial binding to CEACAM1 also causes the direct suppression of immune cell activity due to the coinhibitory function of CEACAM1 (9). While these studies suggest an essential role for CEACAM binding in host colonization and persistence, the exquisite specificity of the protein-protein interactions mediated by each of these adhesins has so far prevented their contribution to infection from being addressed in vivo.We previously demonstrated that nontypeable H. influenzae (NTHI) effectively colonizes the upper respiratory tract (URT) of chinchillas and that when this rodent host is also coinfected with a common human URT virus, NTHI can induce culturepositive otitis media (or middle ear infection) via a process that closely reflects the disease course in human...