Crohn's disease (CD) is a common chronic inflammatory disease of the small and large intestines and is thought to result from an inappropriate response of the host immune system to intestinal microbes7,8. Recent studies have shown that both murine and human mesenchymal stem cells (MSCs) can suppress inflammation in mouse models of intestinal inflammation1,3,4,5. There are multiple ongoing clinical trials that use human MSCs derived from bone marrow or adipose tissue to treat patients with inflammatory bowel disease (IBD), which includes CD9. Two routes for MSC therapy have been used in these clinical trials: one involves the systemic infusion (i.e., intravenously) of MSCs for luminal IBD (including CD), and the other involves the localized application/injection of stem cells in the fistula tract of patients with perianal CD. In a recent meta-analysis of MSC therapy for IBD, systemic (i.e., intravenous) MSC therapy for luminal IBD (including CD) was efficacious in up to 40% (95% CI: 7 - 79%) of patients, whereas the efficacy was much higher, at 61% (95% CI: 36 - 85%), when the MSCs were injected locally into the diseased CD fistula9. A recent phase III multicenter randomized placebo controlled trial of allogeneic adipose stem cells injected directly into the perianal fistula of CD patients showed statistically significant clinical and radiological evidence of perianal fistulae healing, corroborating the findings of the meta-analysis10. The reasons for the low efficacy of MSC therapy given intravenously for luminal CD has been inadequately investigated, but one reason may be the inadequate homing of MSCs to the site of inflammation. Murine studies in models of colonic inflammation have demonstrated that only a small percentage of MSCs (1 - 5%) injected intravenously reach the inflamed colon; the remaining MSCs are filtered by the lungs (first-pass effect)1,2,5,11,12. Multiple murine research studies have therefore used the intraperitoneal route (i.p.) for MSC administration in animal models of colitis4. However, they have also demonstrated that only a small fraction of cells reach and engraft the colon and that the efficacy is related to the secretion of soluble paracrine factors, like tumor necrosis factor-inducible gene 6 protein (TSG-6)2. The MSC mechanism of immunosuppression and healing involves a multipronged approach that involves paracrine; cell proximity-independent factors, like TSG-6; and cell proximity-dependent factors, like programmed death-ligand 1 (PD-L1); or Jagged 1. Therefore, MSC localization to the site of inflammation may result in increased efficacy9,13. In fact, a recent study showed that MSCs directly implanted at the site of colonic injury resulted in healing by secreting angiogenesis-promoting vascular endothelial growth factor (VEGF). On the other hand, a minimal healing effect was noted after intravenous injection5. To increase their localization to the site of inflammation (i.e., the small intestine in SAMP mice), this ultrasound-guided intracardiac injection technique for MSC administration in the left ventricle was developed. Image-guided injection ensures an accurate injection, which leads to a higher rate of success and to decreased morbidity and mortality rates. Moreover, the injection of MSCs into the left ventricle delivers them to arterial circulation, where they can reach the inflamed small intestine before becoming trapped in the lungs.
In this study, human bone marrow-derived MSCs (hMSCs) were used for injection in the SAMP-1/YitFc (SAMP) murine model of CD14. SAMP is a well-characterized spontaneous murine model of chronic inflammation that develops small-intestinal inflammation with nearly 100% penetrance14. The inflammation develops in response to microflora in the absence of any chemical, immunological, or genetic manipulation and closely resembles human CD11. Sex- and age-matched inflammation-free AKR/J (AKR) mice, the parental control mice of SAMP, were used in this study.
The hMSCs were isolated and expanded in the laboratory from bone marrow (BM) samples obtained from normal, unidentified donors after informed consent using validated and previously published protocols15,16. After isolation and expansion, the MSC ability in osteogenic, adipogenic, and chondrogenic differentiation was evaluated in the laboratory by multiple assays15. The osteogenic functional assay was performed by implanting ceramic cubes of hydroxyapatite/tricalcium phosphate matrix containing hMSCs subcutaneously in immunocompromised CB17-Prkdc SCID mice17. The cube assay demonstrates osteogenesis and chondrogenesis potential and is considered the ultimate test for evaluating individual MSC preparations17. To visualize hMSCs in vivo after injection, lentivirus was used to transduce hMSCs with triple reporter gene construct that consists of firefly luciferase (fl), monomeric red fluorescent protein (mRFP), and herpes simplex viral thymidine kinase (ttk), driven by a modified myeloproliferative sarcoma virus (mnd) promoter18. The firefly luciferase in the triple reporter is an enzyme that coverts injected luciferin to oxyluciferin in hMSCs and produces photons/white light. This is detected by the sensitive charge-coupled device (CCD) camera (bioluminescence) in an in vivo optical imaging system, enabling the visualization of live hMSCs in mice. Bioluminescence imaging (BLI) is a sensitive technique that can be used serially for tracking cells and for ex vivo analysis. The use of a strong mnd promoter drives the continuous expression of the triple fusion reporter gene construct and allows for the imaging of injected hMSCs for more than 16 weeks19. The hMSCs are difficult to transduce and have a low transduction efficiency. Using an optimized protocol, the hMSCs transduction efficiency was improved and the transgene expression was enhanced18. Using mRFP expression (one of the triple reporter genes) on flow cytometry, the ability to transduce hMSCs with a high efficiency of up to 83% was demonstrated. Differentiation assays and in vivo cube assays demonstrated the ability of the transduced hMSCs to differentiate into chondrocytes, adipocytes, and osteocytes17.