Mesenchymal stem cells (MSCs) are currently defined as plastic-adherent cells with a fibroblast-like morphology that are capable of differentiating into bone, cartilage, and fat in vitro and that express a defined set of surface markers, which vary slightly by species [1, 2]. The origin of MSCs in vivo is controversial, but there is evidence to support that MSCs are a type of pericyte or adventitial cell [3, 4]. The multipotent properties of MSCs led to initial conclusions that these cells could be used clinically to repair or regenerate injured tissues [5], and animal studies supported that MSCs provided a therapeutic benefit [6]. However, MSCs have poor engraftment rates [7, 8] and there is little evidence to suggest that the primary function of MSCs is to differentiate into new tissue in vivo [9], questioning the relevance of differentiation to the therapeutic properties of MSCs when injected in a naive state. Tri-lineage differentiation assays may still be important in some cases for confirming that the cells used in studies are MSCs, since MSCs and fibroblasts have similar morphology and phenotype [10].

Secretion of paracrine factors is now recognized as the primary mechanism by which MSCs promote a regenerative environment conducive to healing with healthy tissue [11], although cell-to-cell contact has also been shown to be important under some conditions [12, 13]. MSCs home to sites of inflammation where they secrete a variety of soluble factors including growth factors, cytokines, and chemokines [14]. In-vivo studies have demonstrated that MSC therapy promotes angiogenesis and growth and differentiation of local progenitor cells, prevents fibrosis and apoptosis, attracts immune cells to the site of injury, and modulates immune responses [1417]. As engraftment appears to be unnecessary for the therapeutic effect, exogenous MSCs likely need to persist through the initial inflammatory phase and into the repair and remodeling phase of tissue healing to have a full therapeutic effect. Adult MSCs, which are obtained from the bone marrow, peripheral blood, or adipose tissue of patients, are currently being investigated in over 450 clinical trials to treat numerous diseases including musculoskeletal diseases, degenerative and traumatic neurological diseases, and immune-mediated diseases [18]. MSC therapy has been effective at treating several animal models of disease [19, 20] and shown success in human clinical trials [18]. The therapeutic benefits of MSC therapy demonstrated in preclinical trials has not translated to success in every human clinical trial, however, and the use of allogeneic versus autologous MSC therapy is one factor that may contribute to the differences in efficacy seen in some clinical trials [21, 22].

In-vitro expansion of MSCs prior to clinical use can take several weeks to obtain enough cells for administration, resulting in loss of stemness; the age and disease state of the patient can also negatively affect the quality of the cells [23, 24]. Adult allogeneic MSC therapy is particularly attractive as it allows for immediate treatment with quality cells at the time of injury or diagnosis. In early studies, researchers discovered that allogeneic MSCs were capable of inhibiting the proliferation of major histocompatibility complex (MHC)-mismatched lymphocytes in mixed leukocyte reactions (MLR) in vitro [25]. MSCs produce a variety of immunomodulatory cytokines including transforming growth factor-β1, indoleamine 2,3-dioxygenase, inducible nitric oxide synthase, and prostaglandin E2, which contribute to the ability of MSCs to modulate immune responses [14]. This discovery initially indicated that MSCs were “immunoprivileged” and were subsequently promoted as safe to use in allogeneic settings without concern for immune rejection [25].

Although allogeneic MSC therapy is generally regarded as safe [26], there have been several reports of adverse clinical events including increased synovial cellularity and total nucleated cell counts following intra-articular injection of allogeneic MSCs in equine models [27, 28]. Most studies do not characterize if allogeneic donor MSCs and recipients are MHC-matched or MHC-mismatched, nor do they investigate if the MSCs induce immune responses and are rejected. Furthermore, few studies have compared allogeneic versus autologous MSC therapy using cells of comparable quality to determine if there is a difference in efficacy for tissue healing or disease outcome and if those differences correlate or not with immune rejection of allogeneic MSCs. In order to fully understand the potential of allogeneic MSC therapy, further investigation into the immune responses towards allogeneic cells and if immune responses affect the therapeutic outcome of MSC therapy are warranted.

The purposes of this review are to outline what is currently understood about immune responses to adult allogeneic MSCs and to describe contemporary assays that could be utilized in future preclinical studies and clinical trials to appropriately identify and measure immune responses to allogeneic MSCs. By gaining a better understanding of how and under what circumstances a recipient immune system responds to allogeneic MSCs, researchers can develop strategies to improve allogeneic MSC efficacy and ensure safety.