Several types of APCs in the BM take up soluble protein from the blood. (A) 2PM images of in vivo uptake of blood-borne antigen by BM cells of a Cx3cr1^gfp/+ mouse at different time points. An hour after injection of 0.1 mg of OVA–Alexa 594 (yellow), the antigen had been taken up by many cell types, including green CX₃CR1-GFP⁺ cells (arrows, middle) exhibiting DC morphology (insets, middle). By 24 hours, the fluorescent signal (arrows, right) persisted only in flat cells that line blood vessels (red), consistent with endothelial or perivascular cells (insets, right). Data are from 1 representative experiment of 2. Bar represents 50 µm. (B) Flow cytometric analysis of uptake of blood-borne antigen by various cells in the BM. The BM was harvested 2 hours after intravenous injection of 0.1 mg OVA–Alexa 594 and compared with the BM of noninjected mice. Representative histograms depict the OVA–Alexa 594 signal in pDCs (CD11c^int mPDCA-1^hi), T cells (CD3^hi), B cells (CD19^hi), conventional DCs (cDCs) (CD11c^hi MHC-II⁺), macrophages (CX₃CR1-GFP^hi, CD11c^int, F4/80⁺, MHC-II⁺), Ly6C^lo monocytes (CX₃CR1-GFP^hi, CD11c^int, F4/80⁺, MHC-II⁻), and Ly6C^hi monocytes (CX₃CR1-GFP⁺, CD11c⁻). Data are from 1 representative experiment of 3. Except for T cells and pDCs, all examined cell types took up the antigen to some extent, with cDCs being the most efficient.