Dendritic cell inhibitory receptor 4 (DCIR4) is preferentially expressed on inflammatory and patrolling monocytes

Dendritic cell inhibitory receptor 4 (DCIR4, Clec4a1) is a lectin receptor and a member of mouse dendritic cell immunoreceptor family. Due to the lack of antibodies against DCIR4, expression of DCIR4 protein remains unknown. In this study, we established a specific monoclonal antibody against DCIR4 and investigated the expression of DCIR4 among immune cells. We found that DCIR4 was expressed on non-granulocytic subsets of CD11b⁺ cells in various immune organs including bone marrow, peripheral blood, spleen, skin-associated lymph nodes and mesenteric lymph nodes. DCIR4⁺ CD11b⁺ cells were dichotomized into DCIR4^High and DCIR4^Low cells distinguished by different levels of DCIR4 expression. By screening a panel of cell surface markers for expression, we found that in bone marrow, blood and spleen the DCIR4^Low and the DCIR4^High cells were Ly-6C⁺ CD43^Low CD11c^- inflammatory^- monocytes and Ly-6C^- CD43^HIgh CD11c⁺ patrolling monocytes, respectively. Using in vitro differentiation system, we also found that differentiation of Ly-6C⁺ monocytes into dendritic cells greatly diminished expression of DCIR4, while that into macrophages did not significantly affect DCIR4 expression. The establishment of the anti-DCIR4 antibody enables clearer definition of monocytes and provides a novel tool to investigate biology of monocytes and their progenies.