CXCR4 regulates homing of monocytes back to reservoirs. (A and B) Homing capacity (calculated as a ratio) of monocytes or indicated immune cells of WT and Lyz2^creCxcr4^fl (A) or Cxcr4^WHIM (B) into the BM or spleen of their respective partners in parabiotic mice. Monocyte, neutrophil and T cell numbers were quantified in blood, BM and spleen before calculation of the respective homing ratios to normalize for unequal exchange in circulating leukocytes (n = 4–5). ns, not significant; *, P < 0.05; **, P < 0.01 (Student’s t test). Results are representative of one out of two independent experiments. (C) Percentage of nonhost cells present in blood of WT and Lyz2^creCxcr4^fl mice (left), and WT and Cxcr4^WHIM parabiotic mice (right). Results are expressed as mean ± SD (n = 4–5) and representative of one out of two independent experiments. (D and E) Total number of donor (CD45.1⁺) monocytes with a Ly6C^hi (left) or Ly6C^lo (right) phenotype detected in blood (D) or BM (E) in recipient CD45.2⁺ mice that had been pretreated with or without AMD3100. Results are expressed as mean ± SD (n = 4) and representative of one out of three independent experiments. *, P < 0.05; **, P < 0.01; ***, P < 0.001 (two-way ANOVA).