CD300c is an activating receptor expressed on human monocytes.

Human CD300 molecules comprise a family of receptors that regulate many immune cell processes. They are mostly expressed on myeloid cells, although expression of two members, CD300a and CD300c, has also been described on lymphocytes. However, due to the lack of specific antibodies that distinguish between these two receptors, it has been difficult to determine the expression pattern and function of CD300a and CD300c in primary cells. Here, we have identified a specific monoclonal antibody, clone TX45, that recognizes only CD300c and show that within freshly isolated blood leukocytes, monocytes are the only cells that express CD300c on the cell surface. In vitro differentiation experiments revealed that CD300c is differentially expressed on different monocyte-derived cells, including macrophages and dendritic cells. Furthermore, TLR ligands LPS and flagellin dynamically regulate the expression of CD300c. Cross-linking of this receptor with clone TX45 monoclonal antibody induced calcium mobilization, upregulation of the costimulatory molecule CD86 and the production of inflammatory cytokines. Importantly, LPS-mediated production of inflammatory cytokines by monocytes was further enhanced if CD300c was simultaneously engaged by the agonist antibody. Altogether, our results show that human CD300c is an activating receptor expressed on monocytes and that it has a potential role in inflammatory responses.