The Meningococcal Adhesin NhhA Provokes Proinflammatory Responses in Macrophages via Toll-Like Receptor 4-Dependent and -Independent Pathways

Activation of macrophages by Toll-like receptors (TLRs) and functionally related proteins is essential for host defense and innate immunity. TLRs recognize a wide variety of pathogen-associated molecules. Here, we demonstrate that the meningococcal outer membrane protein NhhA has immunostimulatory functions and triggers release of proinflammatory cytokines from macrophages. NhhA-induced cytokine release was found to proceed via two distinct pathways in RAW 264.7 macrophages. Interleukin-6 (IL-6) secretion was dependent on activation of TLR4 and required the TLR signaling adaptor protein MyD88. In contrast, release of tumor necrosis factor (TNF) was TLR4 and MyD88 independent. Both pathways involved NF-κB-dependent gene regulation. Using a PCR-based screen, we could identify additional targets of NhhA-dependent gene activation such as the cytokines and growth factors IL-1α, IL-1β, granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF). In human monocyte-derived macrophages, G-CSF, GM-CSF, and IL-6 were found to be major targets of NhhA-dependent gene regulation. NhhA induced transcription of IL-6 and G-CSF mRNA via TLR4-dependent pathways, whereas GM-CSF transcription was induced via TLR4-independent pathways. These data provide new insights into the role of NhhA in host-pathogen interaction.