The emerging paradigm that MSCs are immune privileged has fostered the use of “off-the-shelf” allogeneic MSC-based therapies in human clinical trials. However, this approach ignores studies in experimental animals wherein transplantation of MSCs across MHC boundaries elicits measurable allo-immune responses. To determine if MSCs are hypo-immunogeneic, we characterized the immune response in rhesus macaques following intracranial administration of allogeneic vs. autologous MSCs. This analysis revealed unambiguous evidence of productive allo-recognition based on expansion of NK, B and T cell subsets in peripheral blood and detection of allo-specific antibodies in animals administered allogeneic but not autologous MSCs. Moreover, the degree of MHC class I and II mismatch between the MSC donor and recipient significantly influenced the magnitude and nature of the allo-immune response. Consistent with these findings, real-time PCR analysis of brain tissue from female recipients administered varying doses of male, allogeneic MSCs revealed a significant inverse correlation between MSC engraftment levels and cell dose. Changes in post-transplant neutrophil and lymphocyte counts also correlated with dose and were predictive of overall MSC engraftment levels. However, secondary antigen challenge failed to elicit a measurable immune response in allogeneic recipients. Finally, extensive behavior testing of animals revealed no main effect of cell dose on motor skills, social development, or temperament. Collectively, these data indicate that allogeneic MSCs are weakly immunogenic when transplanted across MHC boundaries in rhesus macaques and this negatively impacts durable engraftment levels. Therefore the use of unrelated donor MSCs should be carefully evaluated in human patients.

Objective-Mesenchymal stem cells (MSCs) possess potent immuno-modulatory activity but whether they evade immune surveillance in an allogeneic transplant setting remains controversial. Herein we evaluated whether administration of major histocompatibility (MHC) class I mismatched MSCs induce an immune response in rhesus macaques.Methods-MSCs from a male donor were injected intra-cranially at two different doses into eight immuno-competent female infant rhesus macaques. Blood cell counts and circulating levels of lymphocyte subpopulations were quantified prior to surgery and at 10, 30, and 90-180 days postsurgery by flow cytometry. Immuno-reactivity of recipient PBMNCs to donor MSCs was evaluated in vitro and allo-antibody production in vivo was determined by ELISA and flow cytometry.Results-MSC transplantation induced transient but significant increases in circulating white blood cells, lymphocytes, and neutrophils in most transplant recipients but not sham-operated control animals. Flow cytometric analysis revealed a strong correlation between expansion of CD8 +ve , CD16 +ve , and CD8 +ve /CD16 +ve lymphocyte subpopulations in peripheral blood, the dose of administered MSCs, and degree of antigenic mismatch between donor and recipient. MSC-specific allo-antibodies were also detected in several transplant recipients. However, PBMNCs harvested from transplant recipients post-surgery exhibited no lytic activity against donor MSCs in vitro upon re-challenge.Conclusions-MSCs induced an allo-graft response in rhesus macaques that involved principally CD8 +ve , CD16 +ve , and CD8 +ve /CD16 +ve lymphocyte subpopulations and was cell dose and haplotype dependent. This study demonstrates that MSCs are weakly immunogenic in vivo when transplanted across MHC class I barriers.