Mesenchymal stem cells alleviate idiopathic pneumonia syndrome by facilitating M2 polarization via CCL2/CCR2 axis and further inducing formation of regulatory CCR2 + CD4 + T cells.

Background: Our previous study revealed that mesenchymal stem cells (MSCs) can secrete large amounts of the chemokine CCL2 under inflammatory conditions and alleviate idiopathic pneumonia syndrome (IPS) by promoting regulatory CCR2 + CD4 + T-cell formation through the CCL2‒CCR2 axis. Given the abundance of macrophages in lung tissue, how these macrophages are regulated by MSC-based prophylaxis via IPS and their interactions with T cells in lung tissue during allo-HSCT are still not fully understood.

Methods: An IPS mouse model was established, and MSC-based prophylaxis was administered. In vitro coculture systems and an IPS model were used to study the interactions among MSCs, macrophages and T cells.

Results: Prophylactic administration of MSCs induced M2 polarization and alleviated acute graft-versus-host disease (aGVHD) and lung injury in an IPS mouse model. In vitro coculture studies revealed that M2 polarization was induced by MSC-released CCL2 and that these M2 macrophages promoted the formation of regulatory CCR2 + CD4 + T cells. Blocking the CCL2-CCR2 interaction in vitro reversed MSC-induced M2 polarization and abolished the induction of CCR2 + CD4 + T-cell formation. Additionally, in vivo administration of a CCL2 or CCR2 antagonist in the IPS mouse model exacerbated aGVHD and lung injury, accompanied by a reduction in M2 macrophages and reduced formation of regulatory CCR2 + CD4 + T cells in lung tissue.

Conclusions: MSCs alleviate IPS by facilitating M2 polarization via the CCL2‒CCR2 axis and further inducing the formation of regulatory CCR2 + CD4 + T cells.