During the culture of non-myocyte cells (NMCs) isolated from neonatal or adult mouse hearts, round weakly adherent cells growing on the fibroblast and endothelial cell layer appeared.

The aim of this study was to identify the nature and function of these weakly adherent cardiac cells (WACs).

We first characterized WACs by flow cytometry analysis. These cells were CD45+ F4/80+ CD11b+ CD206+ CD64+ cells, demonstrating their appartenance to the M2 macrophage cell subset. This was confirmed by qPCR analysis. In vivo, in neonatal or adult unmanipulated hearts, cells with the same phenotype represent about 5 to 6% of the total non-myocyte cells.

We then tested the function of these cells in vitro. When cultured alone in adequate medium during 3 weeks, WACs were able to express mRNAs coding for cardiomyocyte specific markers: Nk×2.5 (×4), Troponin I (×19), beta (×4) and alpha (×9) Myosin Heavy Chain when compared to the WACs after isolation. mRNAs coding for Sca-1 (×5.5) and c-kit (×3.5) were also increased. At the protein level, some of these cells expressed CD31 or smooth muscle actin as well as Nk×2.5 or Troponin I.

WAC's supernantant inhibited neonatal NMC proliferation (−25%, P<0.001) but stimulated the proliferation of the endothelial cells (+34%, P<0.05) after 7 days of culture. The fibroblast proliferation was not affected by WAC's supernatant.

In conclusion, we identified in neonatal and adult non-myocyte cell cultures, macrophages belonging to the M2 subset able to modulate the proliferation of endothelial cells. Furthermore, these cells are able to express some cardiogenic markers. Their role in vivo in physiological as well as in pathophysiological conditions must be investigated.