Macrophage/monocyte-specific deletion of Ras homolog gene family member A (RhoA) downregulates fractalkine receptor and inhibits chronic rejection of mouse cardiac allografts

Background The cellular and molecular mechanisms of chronic rejection of transplanted organs remain obscure; however, macrophages are known to play a critical role in the injury and repair of allografts. Among multiple factors influencing macrophage infiltration to allografts, the fractalkine chemokine (C-X3-C motif) ligand 1(CX3CL1)/chemokine (C-X3-C motif) receptor 1 (CX3CR1) signaling pathway and actin cytoskeleton, which is regulated by a small guanosine-5׳-triphosphatase Ras homolog gene family member A (RhoA), are of the utmost importance. To define the role of macrophage/RhoA pathway involvement in chronic rejection, we generated mice with monocyte/macrophage-specific deletion of RhoA. Methods Hearts from BALB/c (H-2d) donors were transplanted into RhoA^flox/flox (no Cre) and heterozygous Lyz2^Cre+/–RhoA^flox/flox recipients treated with cytotoxic T-lymphocyte–associated protein 4 immunoglobulin to inhibit early T-cell response. Allografts were assessed for chronic rejection and monocyte/macrophage functions. Results The deletion of RhoA inhibited macrophage infiltration, neointimal hyperplasia of vasculature, and abrogated chronic rejection of the allografts. The RhoA deletion downregulated G protein–coupled fractalkine receptor CX3CR1, which activates the RhoA pathway and controls monocyte/macrophage trafficking into the vascular endothelium. This in turn promotes, through overproliferation and differentiation of smooth muscle cells in the arterial walls, neointimal hyperplasia. Conclusions Our finding of codependence of chronic rejection on monocyte/macrophage CX3CR1/CX3CL1 and RhoA signaling pathways may lead to the development of novel anti-chronic rejection therapies.