Immunoregulatory properties of I.A. mesenchymal stem cells are maintained in a xenogenic animal model

Purpose: Osteoarthritis is a chronic joint disease afflicting a significant portion of the world’s population with no currently available cure. Clinical trials using mesenchymal stem cell-based therapies have a positive effect on pain and quality of life in osteoarthritis but the mechanism of action remains unclear. This study aimed to identify the lymphocyte response to a xenogeneic human umbilical cord-derived MSC-based cell therapy in the face of osteoarthritis progression. Methods: A unilateral medial meniscal release model was employed in 14 sheep to create OA where n=7 sheep received I.A. cell therapy (OACT) in the operated knee and n=7 received saline as controls (OA). Another unoperated group (n=7) received cell therapy (CT Control). Three months later IA cell therapy consisting of 1.0× 107 Human SHC-MSCs* (Small Mesenchymal Stem Cells primed with Hypoxia and Calcium ions) was administered into the medial knee joint space. Synovial fluid was aspirated periodically for 13 weeks for flow cytometry analysis. At termination of the study, the stifle joints underwent macroscopic and histologic assessments using the OARSI histology scoring system for subchondral bone, cartilage and synovial membrane. Flow cytometry data was analyzed with a linear mixed effects model while pathology scores were analyzed by the Mann-Whitney U test and Wilcoxon signed rank test, with the Bonferroni correction for multiple comparisons. Results: There were no adverse reactions to the I.A. therapy. Cell therapy administration induced an influx of CD4+ leukocytes (Fig 1). Of these there was a significant increase in the proportion of CD4+CD25+ and CD4+CD25hi leukocytes; the CD4+CD25hi being subset that may be composed of regulatory T cells. There was no significant effect of the cell therapy treatment on the proportion of synovial fluid CD8+ cells or B cells (BAQ44A+), and B cells occurred in small numbers. Conclusions: Xenogeneic cell therapy did not trigger a classical host versus graft reaction leading to tissue injury. Cell therapy induced chemotaxis of CD4+ cells to the joint space but this was not associated with pathological change in tissues. Despite expression of activation markers (CD25+) these cells may regulate the immune response since CD4+cells can be sources of anti-inflammatory cytokines such as IL-4 and IL-10, as well as inhibiting the release of pro-inflammatory cytokines such as IL-1β or TNF-α. These findings support the notion that phenomena unrelated to long-term survival and engraftment of injected cells may have clear immunoregulatory effects.