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.