Macrophages upon encountering pathogens, either phagocytose those successfully eliminating the spread of infections, or the pathogen evades the phagocytic killing and causes death of macrophages via various fates. In many scenarios when macrophages internalize a pathogen and undergoes epigenetic change and converts into pro-inflammatory sub type. Could these pro inflammatory phenotype macrophages infiltrate back into the bone marrow to influence other progenitor cells to differentiate into myeloid cells (myelopoesis) ?
- Macrophage Infiltration into Bone Marrow: Current research on whether circulating macrophages in the host can infiltrate back into the bone marrow is not very clear. However, there is experimental evidence suggesting that a portion of the macrophages in the bone marrow may originate from peripheral circulating monocytes. Some studies indicate that these monocytes can migrate into the bone marrow and differentiate into macrophages, thereby replenishing or replacing the resident macrophages in the marrow. However, other experiments suggest that the macrophages in the bone marrow may primarily maintain their numbers and functions through maturation in situ, without requiring a large influx of circulating monocytes.
- Neutrophil Re-entry into Bone Marrow: Other studies indicate that circulating neutrophils can re-enter the bone marrow for clearance or redistribution. However, these neutrophils not only enter the bone marrow but may also infiltrate other tissues, where they are cleared by resident macrophages in those tissues. This suggests that circulating cells have a certain degree of mobility, but their specific behavior still needs further research to clarify.
- Macrophages Inducing Hematopoiesis in Bone Marrow: Regarding whether these infiltrating macrophages can induce hematopoiesis in the bone marrow, current research shows that macrophages in the bone marrow can indeed regulate hematopoietic functions. Macrophages influence the hematopoiesis process in various ways, including secreting cytokines, regulating the bone marrow microenvironment, and promoting the differentiation of stem cells. However, whether these macrophages can directly induce hematopoiesis (i.e., myeloid hematopoiesis) is still unclear, and the specific mechanisms of action for these infiltrating macrophages remain to be further studied.
- Conclusion: In short, while circulating macrophages may potentially re-enter the bone marrow, whether they can directly induce myelopoiesis in the bone marrow is still inconclusive. More experiments are needed to verify this.
Seyfried AN, Maloney JM, MacNamara KC. Macrophages Orchestrate Hematopoietic Programs and Regulate HSC Function During Inflammatory Stress. Front Immunol. 2020 Jul 23;11:1499. doi: 10.3389/fimmu.2020.01499. PMID: 32849512; PMCID: PMC7396643.
Digesting the role of bone marrow macrophages on hematopoiesis
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