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(+) 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 dearer definition of monocytes and provides a novel tool to investigate biology of monocytes and their progenies. (C) 2016 Elsevier Inc. All rights reserved.