Autologous and allogeneic adult mesenchymal stem/stromal cells (MSCs) are increasingly being investigated for treating a wide range of clinical diseases. Allogeneic MSCs are especially attractive due to their potential to provide immediate care at the time of tissue injury or disease diagnosis. The prevailing dogma has been that allogeneic MSCs are immune privileged, but there have been very few studies that control for matched or mismatched major histocompatibility complex (MHC) molecule expression and that examine immunogenicity in vivo. Studies that control for MHC expression have reported both cell-mediated and humoral immune responses to MHC-mismatched MSCs. The clinical implications of immune responses to MHC-mismatched MSCs are still unknown. Pre-clinical and clinical studies that document the MHC haplotype of donors and recipients and measure immune responses following MSC treatment are necessary to answer this critical question. This review details what is currently known about the immunogenicity of allogeneic MSCs and suggests contemporary assays that could be utilized in future studies to appropriately identify and measure immune responses to MHC-mismatched MSCs.

Introduction: The horse is a valuable species to assess the effect of allogeneic mesenchymal stromal cells (MSCs) in regenerative treatments. No studies to date have examined recipient response to major histocompatibility complex (MHC)-mismatched equine MSCs. The purposes of this study were to immunophenotype MSCs from horses of known MHC haplotype and to compare the immunogenicity of MSCs with differing MHC class II expression. Methods: MSCs and peripheral blood leukocytes (PBLs) were obtained from Thoroughbred horses (n = 10) of known MHC haplotype (ELA-A2, -A3, and -A9 homozygotes). MSCs were cultured through P8; cells from each passage (P2 to P8) were cryopreserved until used. Immunophenotyping of MHC class I and II, CD44, CD29, CD90, LFA-1, and CD45RB was performed by using flow cytometry. Tri-lineage differentiation assays were performed to confirm MSC multipotency. Recombinant equine IFN-γ was used to stimulate MHC class II negative MSCs in culture, after which expression of MHC class II was re-examined. To assess the ability of MHC class II negative or positive MSCs to stimulate an immune response, modified one-way mixed leukocyte reactions (MLRs) were performed by using MHC-matched and mismatched responder PBLs and stimulator PBLs or MSCs. Proliferation of gated CFSE-labeled CD3+ responder T cells was evaluated via CFSE attenuation by using flow cytometry and reported as the number of cells in the proliferating T-cell gate. Results: MSCs varied widely in MHC class II expression despite being homogenous in terms of “stemness” marker expression and ability to undergo trilineage differentiation. Stimulation of MHC class II negative MSCs with IFN-γ resulted in markedly increased expression of MHC class II. MLR results revealed that MHC-mismatched MHC class II-positive MSCs caused significantly increased responder T-cell proliferation compared with MHC-mismatched MHC class II-negative and MHC-matched MSCs, and equivalent to that of the positive control of MHC-mismatched leukocytes. Conclusions: The results of this study suggest that MSCs should be confirmed as MHC class II negative before allogeneic application. Additionally, it must be considered that even MHC class II-negative MSCs could upregulate MHC class II expression if implanted into an area of active inflammation, as demonstrated with in vitro stimulation with IFN-γ.

Introduction: This study tested the hypothesis that Major Histocompatibility Complex (MHC) incompatible equine mesenchymal stromal cells (MSCs) would induce cytotoxic antibodies to donor MHC antigens in recipient horses after intradermal injection. No studies to date have explored recipient antibody responses to allogeneic donor MSC transplantation in the horse. This information is critical because the horse is a valuable species for assessing the safety and efficacy of MSC treatment prior to human clinical application. Methods: Six MHC heterozygote horses were identified as non-ELA-A2 haplotype by microsatellite typing and used as allogeneic MHC-mismatched MSC recipients. MHC homozygote horses of known ELA-A2 haplotype were used as MSC and peripheral blood leukocyte (PBL) donors. One MHC homozygote horse of the ELA-A2 haplotype was the recipient of ELA-A2 donor MSCs as an MHC-matched control. Donor MSCs, which were previously isolated and immunophenotyped, were thawed and culture expanded to achieve between 30x10⁶ and 50x10⁶ cells for intradermal injection into the recipient’s neck. Recipient serum was collected and tested for the presence of anti-donor antibodies prior to MSC injection and every 7 days after MSC injection for the duration of the 8-week study using the standard two-stage lymphocyte microcytotoxicity dye-exclusion test. In addition to anti-ELA-A2 antibodies, recipient serum was examined for the presence of cross-reactive antibodies including anti-ELA-A3 and anti-RBC antibodies. Results: All MHC-mismatched recipient horses produced anti-ELA-A2 antibodies following injection of ELA-A2 MSCs and developed a wheal at the injection site that persisted for the duration of the experiment. Anti-ELA-A2 antibody responses were varied both in terms of strength and timing. Four recipient horses had high-titered anti-ELA-A2 antibody responses resulting in greater than 80% donor PBL death in the microcytotoxicity assays and one of these horses also developed antibodies that cross-reacted when tested on lymphocyte targets from a horse with an unrelated MHC type. Conclusions: Allogeneic MSCs are capable of eliciting antibody responses in vivo that can be strong and also cross-reactive with MHC types other than that of the donor. Such responses could limit the effectiveness of repeated allogeneic MSC use in a single horse, and could also result in untoward inflammatory responses in recipients.

Summary: A system for identifying equine major histocompatibility complex (MHC) haplotypes was developed based on five polymorphic microsatellites located within the MHC region on ECA 20. Molecular signatures for 50 microsatellite haplotypes were recognized from typing 353 horses. Of these, 23 microsatellite haplotypes were associated with 12 established equine leucocyte antigen (ELA) haplotypes in Thoroughbreds and Standardbreds. Five ELA serotypes were associated with multiple microsatellite subhaplotypes, expanding the estimates of diversity in the equine MHC. The strong correlations between serological and microsatellite typing demonstrated a linkage to known MHC class I protein polymorphisms and validated this assay as a useful supplement to ELA serotyping, and in some applications, a feasible alternative method for MHC genotyping in horse families and in population studies.