Immunogenicity of allogeneic mouse mesenchymal stem cells (MSC)

Adult mesenchymal stem cells (MSC) are multipotential cells which can differentiate into various cell types thus giving them utility in tissue engineering regenerative medicine and cell-based therapies. Use of allogeneic MSC offers the prospect of ―off-the-shelf‖ tissue engineered products and tailor-made ―designer‖ therapies with huge benefits to patients and industry. However, their immunological properties are poorly defined and this has hampered their potential clinical utility. Recent studies have suggested that allogeneic MSC are immunoprivileged in addition to possessing immunosuppressive properties. Interestingly, dermal fibroblasts (DF) have also been suggested to be functionally similar to MSC although their immunological properties are controversial.

This study sought to systematically investigate the immunomodulatory properties of allogeneic MSC, namely (i) immunosuppression and (ii) (immunogenicity of allogeneic MSC before and after differentiation using an allogeneic mouse model which employed two genetically distinct strains; Balb/c (H2 d) and C3H (H2 k) which are used as responder (recipient) and stimulator (donor) respectively. Immunosuppressive properties were investigated using adaptations of the one-way mixed lymphocyte reaction (MLR) while the immunogenicity was tested using the lymphocyte transformation assays (LTA). DF were similarly tested for comparison.

MSC and DF were successfully isolated from the bone marrow and abdominal skin respectively of Balb/c and C3H mice, expanded and characterised by flow cytometry. MSC expressed MHC I, Sca-1, CD29, CD44, CD90.2 and CD105 but not MHC II, CD11b, CD34, CD45, CD80 and CD86. In contrast to MSC, DF were negative for CD29, CD44 and CD105. Both MSC and DF successfully differentiated into adipocytes, chondrocytes and osteocytes in the tri-lineage test; the benchmark test for stem cells.

Various parameters including medium changes, cell viability following mitotic inactivation of stimulator cells, type and concentration of serum for medium supplementation, responder to stimulator cell ratio and total cell numbers were tested to determine appropriate conditions for carrying out the MLR and LTA.

With regard to the immunosuppressive properties, both syngeneic and allogeneic MSC significantly suppressed one-way MLR and two-way MLR. DF also exhibited similar suppressive potency. In LTA, allogeneic, but not syngeneic MSC stimulated Balb/c lymphocyte proliferation. Interestingly, both syngeneic and allogeneic DF failed to stimulate lymphocyte proliferation.

Following chondrogenic differentiation, both syngeneic and allogeneic MSC and DF suppressed one-way MLR, albeit with reduced potency. With regard to their immunogenicity, allogeneic MSC and DF, but not syngeneic MSC and DF significantly stimulated lymphocyte proliferation.

Therefore, it was concluded that both allogeneic MSC and DF possess immunosuppressive properties before and after differentiation. Undifferentiated and differentiated allogeneic MSC and differentiated DF however, may not be immunoprivileged. Thus the clinical utility of allogeneic MSC may be limited by their immunogenicity.