Outer membrane protein (Omp)A is highly represented and conserved in the Enterobacteriaceae family. Using a recombinant OmpA from Klebsiella pneumoniae (P40), we have analyzed the interaction between OmpA and macrophages. We report that Alexa488-labeled P40 binds (at 4 degrees C) to murine and human macrophages in a dose-dependent manner and is rapidly internalized (at 37 degrees C). No binding or internalization of the Alexa488-labeled glycophorin A control protein is observed under the same conditions. Furthermore, P40 up-regulates the production of IL-1beta, IL-8, IL-10, IL-12, and TNF-alpha by human macrophages and of NO by the RAW 264.7 murine macrophage cell line. P40 also synergizes with IFN-gamma and suboptimal concentrations of LPS to up-regulate the production of these mediators. In conclusion, P40 binds to and activates macrophages. These data suggest that recognition of OmpA by macrophages may be an initiating event in the antibacterial host response.