DCIR3 and DCIR4 are co-expressed on inflammatory and patrolling monocytes

Dendritic cell inhibitory receptor 3 (DCIR3) is a member of dendritic immuno-receptor family, of which protein expression has been unknown. We established a specific monoclonal antibody against mouse DCIR3 and investigated the expression of DCIR3 on immune cells of various immune organs. We found that DCIR3 was expressed on monocytes, but not on eosinophils and neutrophils. We also found the existence of a dichotomy in the levels of the expression of DCIR3 on monocytes in bone marrow, blood and spleen. Further investigation of the expression of several cell surface markers on DCIR3^High cells and DCIR3^Low cells revealed that DCIR3^High cells were Ly-6C^- CD43^High CD11c⁺ CD80⁺ NK1.1⁺ patrolling monocytes and that DCIR3^Low cells were Ly-6C⁺ CD43^Low CD11c^- CD80^- NK1.1- inflammatory monocytes. These results and our previous finding that DCIR4 is expressed at high level in patrolling monocytes and at a low level in inflammatory monocytes (Kameda et al., 2016) suggest that DCIR3 and DCIR4 are simultaneously expressed on monocytes. Indeed, DCIR4⁺ CD11b⁺ monocytes from various immune organs expressed DCIR3. We also found that DCIR1 was expressed on DCIR4^Low inflammatory monocytes but not on DCIR4^HIgh patrolling monocytes. The anti- DCIR3 antibody established in this study, together with the previously established anti-DCIR1 and anti-DCIR4 antibodies, would be a valuable tool to investigate biology and pathophysiology of monocytes.