| Antigen presenting cells (APC) express a wide range of surface pattern recognition receptors (PRR) that involve in pathogen recognition, phagocytosis, antigen presentation, and induction of adaptive immunity. It is known that vaccine adjuvants may act as conserved microbial ligands, and function via initiation and manipulation of innate immune responses through certain PRRs. Meanwhile, signaling pathways mediated via various PRRs may contribute differently to the pathogenicity of exposure to microbial ligands and bioactive particulates.; Class A scavenger receptor (SR-A) and Toll-like receptor 4 (TLR-4) are two types of PRRs expressed on the cell surface of APCs. The present study utilized human and murine macrophage cell lines to investigate signaling mechanism and potential ligands, including lipopolysaccharide (LPS), silica, and airborne particulate matter PM1648, for the two PRRs. Biological potency of synthetic lipid A compounds was evaluated with an in vitro human cell assay system, and the length of compound secondary fatty acyl chains was found to play a critical role in compound immunological potency. With biological inhibition assays using monoclonal antibodies, we demonstrated that lipid A compounds were signaling through TLR-4. With direct binding assay and binding inhibition studies, we indicated a two-step binding mechanism of LPS to TLR-4 receptor. A possible cross-reactivity between TLR-4 and Fc receptor CD64 was also investigated.; It is clear that PRRs may function individually or cooperatively to direct adaptive immune responses following innate ligand recognition. Therefore, the possible cooperative mechanism between TLR-4 and SR-A for up-regulating murine APC activity in response to LPS, silica, and PM1648 was investigated. Using blocking antibodies and receptor deficient murine macrophages, we demonstrated that silica uses both TLR-4 and SR-A for signaling in order to activate macrophages efficiently, while LPS and PM1648 use TLR-4 for macrophage activation. APCs stimulated with silica sent signals other than soluble cytokines to T cells for activating host immune responses. SR-A appeared to be involved in negative regulation of cell activation by LPS and PM1648, as well as in mediating endotoxin and particulate direct binding and clearance. |
