Abstract
CD300C is highly homologous with an inhibitory receptor CD300A in an immunoglobulin-like domain among the human CD300 family of paired immune receptors. To clarify the precise expression and function of CD300C, we generated antibodies discriminating between CD300A and CD300C, which recognized a unique epitope involving amino acid residues CD300A(F56-L57) and CD300C(L63-R64). Notably, CD300C was highly expressed in human monocytes and mast cells. Cross-linking of CD300C by its specific antibody caused cytokine/chemokine production of human monocytes and mast cells. Fc receptor γ was indispensable for both efficient surface expression and activating functions of CD300C. To identify a ligand for CD300A or CD300C, we used reporter cell lines expressing a chimera receptor harboring extracellular CD300A or CD300C and intracellular CD3ζ, in which its unknown ligand induced GFP expression. Our results indicated that phosphatidylethanolamine (PE) among the lipids tested and apoptotic cells were possible ligands for both CD300C and CD300A. PE and apoptotic cells more strongly induced GFP expression in the reporter cells through binding to extracellular CD300A as compared with CD300C. Differential recognition of PE by extracellular CD300A and CD300C depended on different amino acid residues CD300A(F56-L57) and CD300C(L63-R64). Interestingly, GFP expression induced by extracellular CD300C-PE binding in the reporter cells was dampened by co-expression of full-length CD300A, indicating the predominance of CD300A over CD300C in PE recognition/signaling. PE consistently failed to stimulate cytokine production in monocytes expressing CD300C with CD300A. In conclusion, specific engagement of CD300C led to Fc receptor γ-dependent activation of mast cells and monocytes.
Highlights
Human CD300C is not fully characterized because of the unavailability of its specific antibody
CD300C was highly expressed in human monocytes and mast cells
The results showed that CD300C was more highly expressed in peripheral blood (PB) than in other tissues (Fig. 1A)
Summary
Human CD300C is not fully characterized because of the unavailability of its specific antibody. Results: Stimulation with a specific CD300C antibody activates human monocytes and mast cells that express high levels of CD300C. To identify a ligand for CD300A or CD300C, we used reporter cell lines expressing a chimera receptor harboring extracellular CD300A or CD300C and intracellular CD3, in which its unknown ligand induced GFP expression. We have recently identified ceramide as a physiological ligand for mouse CD300F/LMIR3, using reporter cells in which the interaction between an extracellular domain of LMIR3 and its ligand induced GFP expression driven by NFAT (nuclear factor of activated T cells) [30]. Our results indicate that specific engagement of CD300C by an unknown ligand, but not co-engagement of CD300C with CD300A, induces an FcR␥-dependent activation of human mast cells and monocytes
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