Intrinsic requirement of SIRPα for cDC homeostasis in the spleen. (A-B) B6-Ly5.1 mice were lethally irradiated and then reconstituted with 5 × 10⁶ BM cells from WT or SIRPα MT mice (A). Conversely, WT or SIRPα MT mice were lethally irradiated and reconstituted with 5 × 10⁶ BM cells from B6-Ly5.1 mice (B). Next, 6-8 weeks after transplantation, splenocytes from each chimera were incubated with a biotin-conjugated mAb to CD4. The cells were also stained with an FITC-conjugated mAb to CD8, PE-conjugated streptavidin, a PE-Cy7–conjugated mAb to Ly5.1, an APC-conjugated mAb to CD11c, an APC-Cy7–conjugated mAb to B220, and PI. The expression of CD11c and B220 on PI-negative cells or that of CD4 and CD8 on cDCs was analyzed by 6-color flow cytometry (left panels). The relative numbers of cDCs and of CD8⁺, CD4⁺, or DN cDCs are expressed as a percentage of all viable splenocytes or of cDCs, respectively, on each plot. The percentages of cDCs as well as of CD8⁺, CD4⁺, or DN cDCs among total splenocytes were also determined (right panels); data are means ± SE for a total of 7-9 mice in 3 independent experiments. **P < .01 (Student t test). (C) B6-Ly5.1 mice were lethally irradiated and then reconstituted with an equal mixture of WT (Ly5.1⁺) and SIRPα MT (Ly5.1⁻) BM cells. Then, 6-8 weeks after transplantation, splenocytes from the mixed chimeras were stained and analyzed as in (A). The expression of CD4 and CD8 on Ly5.1⁺ cDCs or Ly5.1⁻ cDCs was analyzed by 6-color flow cytometry (left panel). The relative numbers of CD8⁺, CD4⁺, or DN cDCs are expressed as a percentage of Ly5.1⁺ cDCs or Ly5.1⁻ cDCs on each plot. The ratios of the percentages of MT BM (Ly5.1⁻)–derived CD8⁺ cDCs, CD4⁺ cDCs, DN cDCs, or B cells (defined as CD11c⁻B220⁺ cells) among total Ly5.1⁻ splenocytes to those of the corresponding cell types derived from WT BM (Ly5.1⁺) cells were also calculated (right panel). Data in the right panel are means ± SE for 3 mice per group and are representative of 2 independent experiments. **P < .01 (one-way analysis of variance, followed by Bonferroni test).