CD16+ monocytes with smooth muscle cell characteristics are reduced in human renal chronic transplant dysfunction

Abstract

In chronic transplant dysfunction (CTD), persistent (allo)immune-mediated inflammation eventually leads to tissue remodeling including neointima formation in intragraft arteries. We previously showed that recipient-derived neointimal α-SMA+ smooth muscle-like cells are present in human renal allografts with CTD. Human PBMC contain myeloid cells capable of differentiating into α-SMA+ cells in vitro; the phenotype of the ancestral subset is as yet unknown. This study aimed to investigate whether monocyte subsets contain cells with smooth muscle-like cell differentiation capacity and whether CTD in renal transplant recipients is associated with a shift in these monocyte subsets. To accomplish this goal, monocyte subsets from healthy controls were sorted based on CD14 and CD16 expression to investigate gene expression levels of mesenchymal markers α-SMA and SM22α. CD14+/CD16++ monocytes displayed increased α-SMA and SM22α mRNA expression compared with CD14++/CD16− monocytes, suggesting increased differentiation potential toward smooth muscle-like cells. Flow cytometry revealed that in non-CTD transplant recipients the percentage of CD14+/CD16++ monocytes was reduced, with an even further reduction in patients with CTD. To determine a potential correlation between CD14+/CD16++ monocytes and α-SMA+ cell outgrowth potential in vitro, PBMC of healthy controls and transplant recipients with and without CTD were cultured under fibrotic culture conditions, and indeed a significant correlation (p=0.0002, r=0.62) was observed. Finally, double staining for α-SMA and CD16 revealed presence of α-SMA+CD16+ cells in kidney explants from CTD patients, albeit at very low numbers.Our data represent evidence that, compared to CD14++CD16− monocytes, CD14+CD16++ monocytes have an increased expression of smooth muscle cell-associated genes. This monocyte subpopulation is reduced in renal transplant patients with CTD, possibly due to selective migration into the allograft.