Myeloid Dendritic Cells Contribute to Hematopoietic Homeostasis

Dendritic cells (DCs) are antigen-presenting cells that are distributed throughout the body, and their main function is thought to be immune-surveillance. There is considerable phenotypic and functional heterogeneity of dendritic cells that tracks, in part, with their tissue localization. Myeloid dendritic cells (mDC), also known as conventional dendritic cells, are DCs with a myeloid origin. A previous study showed that perivascular mDCs in the bone marrow provide signals that regulate the survival of mature B cells (Sapoznikov et al., Nat Immunol, 2008). Using Cx3cr1^gfp/+mice, we show that mDCs, defined as CX3CR1-GFP-bright, MHCII-bright cells, represent 0.2 ± 0.08% of bone marrow cells. They are localized to both venous sinusoids and arterioles in the bone marrow, placing them in the perivascular stem niche, along with CXCL12-abundant reticular (CAR) cells and Nestin-GFP⁺ cells. To assess the contribution of mDCs to the regulation of hematopoiesis, we used two independent mouse models to ablate mDCs: CD11c^DTR and Zbtb46^DTR mice. We previously reported that ablation of mDCs induces modest hematopoietic stem/progenitor (HSPC) mobilization. We show that mDC ablation also suppresses osteoblast function, with expression of osteocalcin mRNA (a marker of mature osteoblasts) decreasing 3.5-fold after mDC ablation (from 18.7 ± 9.9 to 5.3 ± 3.0; P < 0.05).

To our surprise, mDC ablation (in both models) was associated with a significant loss of bone marrow macrophages. Prior studies have shown that macrophage ablation results in a loss of mature osteoblasts and modest HPSC mobilization. Thus, it is not clear whether mDCs have an independent effect on HSPC trafficking and osteoblast function. To address this issue, we first asked whether the macrophage loss after mDC ablation was mediated in a non-cell autonomous fashion. Mixed bone marrow chimeras were established containing both Zbtb46^DTR and wild-type hematopoietic cells. Upon treatment with diphtheria toxin, we observed depletion of Zbtb46^DTR but not wild-type mDCs (as expected). In contrast, a similar decrease in both Zbtb46^DTR and wild-type macrophages was observed. These data show that the decrease in macrophages is an indirect consequence of mDC ablation and suggest that mDCs generate signals that contribute to macrophage retention and/or survival in the bone marrow. To further address the role of macrophages in this phenotype, we generated mice expressing Zbtb46-DTR alone, CD169-DTR alone (previously shown to ablate macrophages), or mice carrying both Zbtb46-DTR and CD169-DTR. As reported previously, ablation of macrophages induces a modest mobilization of Kit⁺ Sca1⁺ lineage^- (KSL) cells to the spleen (7.1 ± 3.2 x10⁴ versus 3.0 ± 1.2 x10⁴ for PBS treated mice; P =0.06). Ablation of mDCs also induces modest mobilization (9.1 ± 2.5 x10⁴ versus 5.8 ± 3.5 x10⁴ for PBS treated mice; P < 0.05). Preliminary analysis of double mutant mice (n = 4) suggest an additive effect of combined mDC and macrophage ablation on HSPC mobilization with 25.9 ± 18.4 x10⁴ KSL cells per spleen (P < 0.05 compared with macrophage alone ablation). Likewise, preliminary analysis suggests that the magnitude of osteoblast suppression (as measured by osteocalcin expression) is greater in double mutant mice. Collectively, these data suggest that bone marrow mDCs, in addition to a possible role in immune surveillance, contribute to blood homeostasis through multiple mechanisms. Specifically, mDCs appear to generate signals that are required for macrophage retention and/or survival in the bone marrow. mDCs also regulate HSPC trafficking and osteoblast function through a macrophage independent mechanism. Studies are underway to identify signals generated by mDCs that mediate these biological responses.