Due to their beneficial effects in an array of diseases, Mesenchymal Stromal Cells (MSCs) have been the focus of intense preclinical research and clinical implementation for decades. MSCs have multilineage differentiation capacity, support hematopoiesis, secrete pro-regenerative factors and exert immunoregulatory functions promoting homeostasis and the resolution of injury/inflammation. The main effects of MSCs include modulation of immune cells (macrophages, neutrophils, and lymphocytes), secretion of antimicrobial peptides, and transfer of mitochondria (Mt) to injured cells. These actions can be enhanced by priming (i.e., licensing) MSCs prior to exposure to deleterious microenvironments. Preclinical evidence suggests that MSCs can exert therapeutic effects in a variety of pathological states, including cardiac, respiratory, hepatic, renal, and neurological diseases. One of the key emerging beneficial actions of MSCs is the improvement of mitochondrial functions in the injured tissues by enhancing mitochondrial quality control (MQC). Recent advances in the understanding of cellular MQC, including mitochondrial biogenesis, mitophagy, fission, and fusion, helped uncover how MSCs enhance these processes. Specifically, MSCs have been suggested to regulate peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1α)-dependent biogenesis, Parkin-dependent mitophagy, and Mitofusins (Mfn1/2) or Dynamin Related Protein-1 (Drp1)-mediated fission/fusion. In addition, previous studies also verified mitochondrial transfer from MSCs through tunneling nanotubes and via microvesicular transport. Combined, these effects improve mitochondrial functions, thereby contributing to the resolution of injury and inflammation. Thus, uncovering how MSCs affect MQC opens new therapeutic avenues for organ injury, and the transplantation of MSC-derived mitochondria to injured tissues might represent an attractive new therapeutic approach.

Lung macrophages (Mφs) are essential for pulmonary innate immunity and host defense due to their dynamic polarization and phenotype shifts. Mesenchymal stromal cells (MSCs) have secretory, immunomodulatory, and tissue-reparative properties and have shown promise in acute and chronic inflammatory lung diseases and in COVID-19. Many beneficial effects of MSCs are mediated through their interaction with resident alveolar and pulmonary interstitial Mφs. Bidirectional MSC-Mφ communication is achieved through direct contact, soluble factor secretion/activation, and organelle transfer. The lung microenvironment facilitates MSC secretion of factors that result in Mφ polarization towards an immunosuppressive M2-like phenotype for the restoration of tissue homeostasis. M2-like Mφ in turn can affect the MSC immune regulatory function in MSC engraftment and tissue reparatory effects. This review article highlights the mechanisms of crosstalk between MSCs and Mφs and the potential role of their interaction in lung repair in inflammatory lung diseases.

Objective: To investigate the hepatoprotective effects of mitochondrial transplantation (MTx) in a murine liver ischemia/reperfusion (I/R) model. Background: Sequential liver ischemia, followed by reperfusion (I/R), is a pathophysiological process underlying hepatocellular injury in a number of clinical contexts, such as hemorrhagic shock/resuscitation, major elective liver surgery, and organ transplantation. A unifying pathogenic consequence of I/R is mitochondrial dysfunction. Restoration of mitochondria through transplantation (MTx) has emerged as a potential therapeutic in I/R. However, its role in liver I/R and its mechanisms of action remain poorly defined. Methods: We investigated the hepatoprotective effects of MTx in an in vivo mouse model of liver I/R and used in vivo imaging and various knockout and transgenic mouse models to determine the mechanism of protection. Results: We found that I/R-induced hepatocellular injury was prevented by MTx, as measured by plasma ALT, AST, and liver histology. In addition, I/R-induced pro-inflammatory cytokine release (IL-6, TNFα) was dampened by MTx, and anti-inflammatory IL-10 was enhanced. Moreover, MTx lowered neutrophil infiltration into both the liver sinusoids and lung bronchoalveolar lavage fluid, suggesting a local and distant reduction in inflammation. Using in vivo intravital imaging, we found that I/R-subjected Kupffer cells (KCs), rapidly sequestered transplanted mitochondria, and acidified mitochondria within lysosomal compartments. To specifically interrogate the role of KCs, we depleted KCs using the diphtheria toxin-inducible Clec4f/iDTR transgenic mouse, then induced I/R, and discovered that KCs are necessary for the beneficial effects of MTx. Finally, we induced I/R in the complement receptor of the immunoglobulin (CRIg) superfamily knockout mice and found that CRIg was required for mitochondria capture by KCs and mitochondria-mediated hepatoprotection. Conclusions: In this study, we demonstrated that CRIg-dependent capture of mitochondria by I/R-subjected KCs is a hepatoprotective mechanism in vivo. These data progress knowledge on the mechanisms of MTx and open new avenues for clinical translation.

Diabetic patients with peripheral arterial disease (PAD) often suffer from poor clinical outcomes such as limb-loss. Fatty acid binding protein 4 (FABP4) is mainly expressed by adipocytes and is known to play a significant role in the development of atherosclerosis. In this study, we sought to investigate whether FABP4 is associated with PAD in patients with type 2 diabetes mellitus (DM). FABP4 plasma levels were studied in 119 diabetic patients with PAD (DM-PAD) and 49 diabetic patients without PAD (DM-noPAD) presenting to St. Michael’s Hospital between October 2017 and September 2018. Levels of FABP4 in DM-PAD patients (23.34 ± 15.27 ng/mL) were found to be over two-fold higher than the levels in DM-noPAD patients (10.3 ± 7.59 ng/mL). Regression analysis demonstrated a significant association between FABP4 levels and DM-PAD after adjusting for age, sex, prior history of coronary arterial disease and white blood cells count (OR, 2.77; 95% CI, 1.81–4.31; p-value = 0.001). Relative to DM-noPAD controls, plasma FABP4 levels in DM-PAD patients were noted to be inversely correlated with the ankle brachial index (ABI; r= −0.374, p-value < 0.001). The diagnostic ability of FABP4 was investigated using receiver operator curves (ROC) and area under the curve (AUC) analysis. FABP4 had an AUC of 0.79, which improved to 0.86 after adjusting for age, sex and prior history of coronary arterial disease. This raises a possibility of utilizing FABP4 as a biomarker for diagnosing PAD in diabetic patients.

Hemorrhagic shock-resuscitation (HSR) following trauma contributes to organ dysfunction by causing ischemia-reperfusion injury (IRI). We previously showed that 'remote ischemic preconditioning' (RIPC) exerted multi-organ protection from IRI. Maintenance of mitochondrial quality by clearance of dysfunctional mitochondria via mitophagy is vital in restoring organ integrity. We hypothesized that parkin-dependent mitophagy played a role in RIPC-induced hepatoprotection following HSR. The hepatoprotective effect of RIPC in a murine model of HSR-IRI was investigated in wild type and parkin-/- animals. Mice were subjected to HSR ± RIPC and blood and organs were collected, followed by cytokine ELISAs, histology, qPCR, Western blots, and transmission electron microscopy. HSR increased hepatocellular injury, as measured by plasma ALT and liver necrosis, while antecedent RIPC prevented this injury; in parkin^-/- mice, RIPC failed to exert hepatoprotection. The ability of RIPC to lessen HSR-induced rises in plasma IL-6 and TNFα, was lost in parkin^-/- mice. While RIPC alone did not induce mitophagy, the application of RIPC prior to HSR caused a synergistic increase in mitophagy, this increase was not observed in parkin^-/- mice. RIPC induced shifts in mitochondrial morphology favoring mitophagy in WT but not in parkin^-/- animals. RIPC was hepatoprotective in WT mice following HSR but not in parkin^-/- mice. Loss of protection in parkin^-/- mice corresponded with the failure of RIPC plus HSR to upregulate the mitophagic process. Improving mitochondrial quality by modulating mitophagy, may prove to be an attractive therapeutic target in disease processes caused by IRI.