Swaying leukocyte traffic from the bone marrow

Systemically injected lipid–polymer nanoparticles delivering small interfering RNAs into bone-marrow endothelium modulate leukocyte trafficking from the bone marrow to the blood.

In humans, the bone marrow makes billions of new blood cells and immune cells every day. These cells originate from haematopoietic stem cells (HSCs) released from specialized bone-marrow niches. This process can be triggered by the suppression of stromal derived factor 1 (SDF-1) in the bone marrow or by its overexpression in blood1,2. In addition, SDF-1 suppression mobilizes bone-marrow-derived leukocytes, whose migration is also influenced by the secretion of monocyte chemoattractant protein 1 (MCP-1), which is typically released during bacterial infection and other inflammatory diseases3. The interactions of both chemokines with their respective receptors (CXCR4 for SDF-1, and CCR2 for MCP-1) have been implicated in inflammatory pathologies4, and could therefore be targeted for therapeutic interventions1,4. One strategy involves the use of small interfering RNA (siRNA) to inhibit genes, and of nanoparticles to protect the siRNA from degradation and to selectively deliver it to target tissue. However, bone is much less perfused than other organs; hence, delivering therapeutics effectively to the bone marrow via systemic administration is challenging and usually requires high doses (this is because much of it is retained, metabolized or cleared). Reporting in Nature Biomedical Engineering, Matthias Nahrendorf, Daniel Anderson and colleagues now describe the use of siRNA-encapsulating multilamellar lipid–polymer nanoparticles to silence the expression of Sdf1 and Mcp1 in bone-marrow endothelial cells (BMECs) in live mice. Their approach enables the release or retention of bone-marrow HSCs and leukocytes. In a mouse model of atherosclerosis, the researchers show that the systemic administration of nanoparticle-encapsulated MCP-1 siRNA effectively silences Mcp1, reducing inflammation and fibrosis5.

Nahrendorf and co-authors’ nanoparticles consist of a poly(ethylene glycol) (PEG) moiety connected to a lipid chain that secures the PEG onto the nanoparticle’s outer lipid membrane (Fig. 1a). On the basis of previous experience6, the authors hypothesized that siRNA delivery to the bone marrow could be enhanced by changing the architecture of the PEG chains on the nanoparticles. To this end, they varied the length and molar percentage of the PEG chains as well as the length of the lipids securing the PEG chains. Systemic administration of different nanoparticle formulations loaded with Tie2-silencing siRNA in mice revealed the best formulation for effectively silencing Tie2 in the bone marrow and selectively targeting BMECs. Nanoparticles are usually taken up by phagocytes7, which include most leukocytes; yet the authors’ best formulation was preferentially taken up by BMECs rather than by leukocytes in organs of the mononuclear phagocyte system.

Fig. 1: Nanoparticles for the delivery of siRNA to bone-marrow endothelium.

a, Systemically injected multilamellar lipid–polymer nanoparticles selectively targeting BMECs. b, In the bone-marrow niche, silencing the expression of Sdf1 causes the mobilization of HSCs and leukocytes; the release of Mcp1-silencing siRNA instead halts the mobilization of the stem cells, lowering the number of circulating monocytes.

In silico and in vitro screening identified siRNA sequences for silencing Sdf1 and Mcp1. The most potent siRNAs for silencing each gene were encapsulated into the top nanoparticle formulation. Systemic delivery of siSdf1-encapsulating nanoparticles into mice led to a decrease in Sdf1 expression and in SDF-1 protein levels in the explanted femurs. Because the inhibition of Sdf1 mobilizes HSCs and leukocytes into the blood circulation, Nahrendorf and co-authors investigated the prevalence of cellular migration via flow cytometry. They observed a decrease of sca1+ c-kit+ HSCs in the bone marrow and an increase in the same cell populations in blood, as well as the attenuation of common myeloid progenitors and granulocyte macrophage progenitors in the femur of the animals, which indicated the release of these progenitor cells into the blood (Fig. 1b). Transplantation of blood taken from siSdf1 into lethally irradiated mice (that is, mice with most leukocytes wiped out) showed, after eight weeks, that most of the leukocytes in the recipients’ blood originated from the siSdf1 mouse.

Nahrendorf and co-authors also investigated whether Mcp1 siRNA could suppress leukocyte trafficking, as this may have implications in exacerbating inflammation. In fact, the secretion of MCP-1 during inflammation triggers the release of bone-marrow monocytes from the niche. Nanoparticles delivering siMcp1 injected with bacterial lipopolysaccharide into mice to induce systemic inflammation in the animals showed a larger number of monocytes in the bone marrow than in the blood. The authors then investigated the utility of siMcp1 halting the release of bone-marrow monocytes in the treatment of myocardial infarction in mice. Infarcted mice injected with siMcp1-releasing nanoparticles experienced a reduction in the number of circulating monocytes (including subsets of Ly6Chigh inflammatory monocytes), which indicated a concomitant reduction in monocyte release from the bone marrow. Moreover, in transgenic mice with a natural propensity to develop atherosclerosis, systemically administered siMcp1 led to reduced fibrosis, fewer CD11b+ myeloid cells and fibroblast cells, and less collagen and scarring.

The disruption of the interaction between SDF-1 and CXCR4, and the increase of SDF-1 levels in blood, are known to induce the mobilization of HSCs from the bone marrow2,8. Also, the increased production of SDF-1 in the myocardium following infarction leads to the homing of stem cells into the heart, facilitating its regeneration9. Should Nahrendorf and colleagues’ siSdf1-mediated enrichment of HSCs in the blood be safe (which would largely depend on the degree of the targeting specificity of the siRNA-loaded nanoparticles, and on their clearance pathways and degradation), the approach might eventually also be useful for the treatment of patients with blood disorders who need HSC transplantation. The targeted nanoparticle delivery of siMcp1 might also be an alternative for the treatment of atherosclerosis, as CCR2 antagonists have shown little success in clinical trials, probably owing to the absence of suitable surrogate endpoints4.


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Correspondence to Patrick C. H. Hsieh.

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Prajnamitra, R.P., Chen, HC. & Hsieh, P.C.H. Swaying leukocyte traffic from the bone marrow. Nat Biomed Eng 4, 1026–1027 (2020). https://doi.org/10.1038/s41551-020-00647-z

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