A subset of human monocytes express the TIE2 angiopoietin receptor. (A) Flow cytometry analysis of PB granulocytes (G), lymphocytes (L), and monocytes (M) identified on the basis of physical gating (dot plot on the left; gates indicated by dashed line) shows TIE2 expression in a subset of monocytes (open red line in the histogram plots on the right; filled line IgG isotype control). Percentages of marker-positive cells are indicated. (B) The TIE2⁺ PBMCs (red gate in left panels) are mostly CD14⁺ monocytes (dot plots on the right). Representative analysis of at least 16 performed on different donors. (C) The vast majority of TIE2⁺ PBMCs do not express the CEC/EPC markers AC133 or CD146. Rare TIE2⁺AC133⁺ and TIE2⁺CD146⁺ cells may represent EPCs and CECs, respectively. Similar findings were obtained on 2 different PBMC samples. (D) A small subset of TIE2⁺ cells are VEGFR-2⁺CD14⁺, likely representing previously described monocytes with endothelial-like functional capacity. (E) T-cell–depleted PBMCs were stained with FITC-conjugated anti-CD14, PC5-conjugated anti-CD16, and PE-conjugated anti-TIE2 or isotype control antibodies. Expression of CD14 and CD16 (dot plot on the left) identifies 2 distinct monocyte subsets (see text). The gated cell populations (stained in different colors) were analyzed for expression of TIE2 (top right dot plots) versus isotype control (bottom right dot plots). Note that the CD14^lowCD16⁺ fraction (resident monocytes; red dots) is highly enriched in TIE2⁺ cells, whereas the CD14^highCD16⁻ fraction (inflammatory monocytes; violet dots) contains few TIE2⁺ cells. CD14⁻ cells (blue and pink dots) mainly represent B and natural killer cells and are mostly TIE2⁻. Representative analysis of at least 6 experiments performed on different donors. (F) The 2 main monocyte subsets were sorted from PBMCs using the indicated gates (dot plot on the left) and reanalyzed for expression of CD14, CD16, and TIE2 (dot plots on the right) and after cytospin and May-Grünwald-Giemsa staining.