Carbon Monoxide Suppresses Neointima Formation in Transplant Arteriosclerosis by Inhibiting Vascular Progenitor Cell Differentiation

Objective: Evidence indicates that bone marrow progenitor cells (BMPC) are a major contributor to neointima formation in transplant arteriosclerosis. HO-1 (heme oxygenase 1, Hmox1 ) and carbon monoxide (CO), a product of heme degradation by HO-1, ameliorate neointima formation by inhibiting proliferation of smooth muscle cells. We investigated the mechanism whereby HO-1 and CO modulate BMPC and mitigates neointima formation in transplant arteriosclerosis. Approach and Results: Using a murine model of aortic transplantation, bone marrow chimeric mice, and in vitro experiments, we report that CO does not inhibit mobilization of BMPC into the circulation or their homing to the vessel adventitia, but instead suppresses differentiation of BMPC into smooth muscle cells after they arrive in the adventitia. Specifically, the effect of CO on differentiation of BMPC into smooth muscle cell is mediated in part, by limiting PDGFR-β (platelet derived growth factor receptor-β) signaling. Hmox1 −/− BMPC exhibit a greater propensity to differentiate into smooth muscle cell in vitro, in part by regulating PDGFR-β + expression. Furthermore, wild-type mice transplanted with Hmox1 −/− bone marrow cells show augmented neointima formation after allografting versus control. CO exposure significantly ameliorated neointima formation, which remains more severe with Hmox1 −/− bone marrow cell versus air-treated mice receiving HO-1-expressing bone marrow cell, highlighting the importance of endogenous HO-1 in neointima formation. Conclusions: Host BMPC contribute to neointima formation in transplant arteriosclerosis and the protective effect afforded by HO-1/CO against neointima formation is mediated in part through the regulation of PDGFR-β expression. We propose that suppressing differentiation of BMPC is a major mechanism by which HO-1 and CO prevent neointima expansion after transplant.

Impact of Local Alloimmunity and Recipient Cells in Transplant Arteriosclerosis

Rationale: Transplant arteriosclerosis is the major limitation to long-term survival of solid organ transplantation. Although both immune and nonimmune cells have been suggested to contribute to this process, the complex cellular heterogeneity within the grafts, and the underlying mechanisms regulating the disease progression remain largely uncharacterized. Objective: We aimed to delineate the cellular heterogeneity within the allografts, and to explore possible mechanisms underlying this process. Methods and Results: Here, we reported the transcriptional profiling of 11 868 cells in a mouse model of transplant arteriosclerosis by single-cell RNA sequencing. Unbiased clustering analyses identified 21 cell clusters at different stages of diseases, and focused analysis revealed several previously unknown subpopulations enriched in the allografts. Interestingly, we found evidence of the local formation of tertiary lymphoid tissues and suggested a possible local modulation of alloimmune responses within the grafts. Intercellular communication analyses uncovered a potential role of several ligands and receptors, including Ccl21a and Cxcr3 , in regulating lymphatic endothelial cell-induced early chemotaxis and infiltration of immune cells. In vivo mouse experiments confirmed the therapeutic potential of CCL21 and CXCR3 neutralizing antibodies in transplant arteriosclerosis. Combinational use of genetic lineage tracing and single-cell techniques further indicate the infiltration of host-derived c-Kit + stem cells as heterogeneous populations in the allografts. Finally, we compared the immune response between mouse allograft and atherosclerosis models in single-cell RNA-seq analysis. By analyzing susceptibility genes of disease traits, we also identified several cell clusters expressing genes associated with disease risk. Conclusions: Our study provides a transcriptional and cellular landscape of transplant arteriosclerosis, which could be fundamental to understanding the initiation and progression of this disease. CCL21/CXCR3 was also identified as important regulators of immune response and may serve as potential therapeutic targets in disease treatment.

Targeting Heme Oxygenase-1 in the Arterial Response to Injury and Disease

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme into carbon monoxide (CO), iron, and biliverdin, which is rapidly metabolized to bilirubin. The activation of vascular smooth muscle cells (SMCs) plays a critical role in mediating the aberrant arterial response to injury and a number of vascular diseases. Pharmacological induction or gene transfer of HO-1 improves arterial remodeling in animal models of post-angioplasty restenosis, vascular access failure, atherosclerosis, transplant arteriosclerosis, vein grafting, and pulmonary arterial hypertension, whereas genetic loss of HO-1 exacerbates the remodeling response. The vasoprotection evoked by HO-1 is largely ascribed to the generation of CO and/or the bile pigments, biliverdin and bilirubin, which exert potent antioxidant and anti-inflammatory effects. In addition, these molecules inhibit vascular SMC proliferation, migration, apoptosis, and phenotypic switching. Several therapeutic strategies are currently being pursued that may allow for the targeting of HO-1 in arterial remodeling in various pathologies, including the use of gene delivery approaches, the development of novel inducers of the enzyme, and the administration of unique formulations of CO and bilirubin.