Identification of a distinct cluster of GDF15 macrophages induced by differentiation exhibiting anti-inflammatory activities.

Introduction: Macrophage-mediated inflammatory response may have crucial roles in the pathogenesis of a variety of human diseases. Growth differentiation factor 15 (GDF15) is a cytokine of the transforming growth factor-β superfamily, with potential anti-inflammatory activities. Previous studies observed in human lungs some macrophages which expressed a high level of GDF15.

Methods: In the present study, we employed multiple techniques, including immunofluorescence, flow cytometry, and single-cell RNA sequencing, in order to further clarify the identity of such GDF15 macrophages.

Results: We demonstrated that macrophages derived from human peripheral blood mononuclear cells and rat bone marrow mononuclear cells by differentiation with granulocyte-macrophage colony stimulating factor contained a minor population (~1%) of GDF15 cells. GDF15 macrophages did not exhibit a typical M1 or M2 phenotype, but had a unique molecular signature as revealed by single-cell RNA sequencing. Functionally, the derived GDF15 macrophages were associated with reduced responsiveness to pro-inflammatory activation; furthermore, these GDF15 macrophages could inhibit the pro-inflammatory functions of other macrophages via a paracrine mechanism. We further confirmed that GDF15 was a key mediator of the anti-inflammatory effects of GDF15 macrophage. Also, we provided evidence showing that GDF15 macrophages were present in other macrophage-residing human tissues in addition to the lungs. Further scRNA-seq analysis in rat lung macrophages confirmed the presence of a GDF15 sub-population. However, these data indicated that GDF15 macrophages in the body were not a uniform population based on their molecular signatures. More importantly, as compared to the derived GDF15 macrophage, whether the tissue resident GDF15 counterpart is also associated with anti-inflammatory functions remains to be determined. We cannot exclude the possibility that the priming/induction protocol used in our study has a determinant role in inducing the anti-inflammatory phenotype in the resulting GDF15 macrophage cells.

Conclusion: In summary, our results suggest that the GDF15 macrophage cells obtained by induction may represent a distinct cluster with intrinsic anti-inflammatory functions. The (patho)physiological importance of these cells warrants further investigation.