Exploitation of Herpesvirus Immune Evasion Strategies to Modify the Immunogenicity of Human Mesenchymal Stem Cell Transplants

Abstract

BackgroundMesenchymal stem cells (MSCs) are multipotent cells residing in the connective tissue of many organs and holding great potential for tissue repair. In culture, human MSCs (hMSCs) are capable of extensive proliferation without showing chromosomal aberrations. Large numbers of hMSCs can thus be acquired from small samples of easily obtainable tissues like fat and bone marrow. MSCs can contribute to regeneration indirectly by secretion of cytokines or directly by differentiation into specialized cell types. The latter mechanism requires their long-term acceptance by the recipient. Although MSCs do not elicit immune responses in vitro, animal studies have revealed that allogeneic and xenogeneic MSCs are rejected.Methodology/Principal FindingsWe aim to overcome MSC immune rejection through permanent down-regulation of major histocompatibility complex (MHC) class I proteins on the surface of these MHC class II-negative cells through the use of viral immune evasion proteins. Transduction of hMSCs with a retroviral vector encoding the human cytomegalovirus US11 protein resulted in strong inhibition of MHC class I surface expression. When transplanted into immunocompetent mice, persistence of the US11-expressing and HLA-ABC-negative hMSCs at levels resembling those found in immunodeficient (i.e., NOD/SCID) mice could be attained provided that recipients' natural killer (NK) cells were depleted prior to cell transplantation.Conclusions/SignificanceOur findings demonstrate the potential utility of herpesviral immunoevasins to prevent rejection of xenogeneic MSCs. The observation that down-regulation of MHC class I surface expression renders hMSCs vulnerable to NK cell recognition and cytolysis implies that multiple viral immune evasion proteins are likely required to make hMSCs non-immunogenic and thereby universally transplantable.

Mesenchymal stem cells: immune evasive, not immune privileged

“…Expression of either US6 or US11 resulted in significant suppression of MHC class I surface expression, and facilitated hMSC evasion of recognition by cytotoxic lymphocytes. Interestingly, in contrast to de la Garza-Rodea’s report 93 , expression of US11 also resulted in protection from lysis in co-cultures with NK-92MI cells. However, the mechanism of hMSC evasion from NK cells was not fully elucidated.…”

“…For example, de la Garza-Rodea et al looked to viruses for inspiration to enable MSCs to evade immune detection. To permanently suppress MHC class I surface expression, they transfected hMSCs with viral immunoevasins from bovine herpes virus type 1, Epstein-Barr virus and human cytomegalovirus (HCMV) 93 . Of these, only the US11 protein from HCMV strongly suppressed hMSC expression of MHC class I.…”

“…Moreover, US11-MSCs locally injected into the ear of immunocompetent mice persisted for a period similar to that of hMSC injected into immunodeficient mice. However MSC immune evasion was achieved only after NK cell depletion, as the lack of MHC class I expression made hMSCs susceptible to ‘missing self ’ NK cell-mediated lysis 93 . Soland et al also infected hMSCs with immunoevasins from HCMV 94 (US2, US3, US6 or US11).…”

Modulation of Human Mesenchymal Stem Cell Immunogenicity through Forced Expression of Human Cytomegalovirus US Proteins

“…Also, the results presented here demonstrating that, at low NK-to-MSC ratios, the ability of NK to kill both transduced and untransduced MSC is diminished, are in agreement with others that showed this same effect, and that both IFN-gamma and IL-10 were involved in the mechanism of regulating NK function [65]. However, this mechanism does not explain the decrease in NK cytotoxicity towards MSC transduced with US 2,3 and 11 at ratios of 20∶1 and 10∶1, and does not concur with a study which demonstrated that down-regulation of MHC-I expression rendered the cells susceptible to NK cells when transplanted in a xenograft model [66]. However, since our results were obtained with in vitro assays and in the other study the results were obtained after in vivo transplantation, it is difficult to directly compare the seemingly conflicting results.…”

Pressure Ulcers: Description of a New Model and Use of Mesenchymal Stem Cells for Repair

“…In these studies, cells were also administered intradermally and the numbers of cells administered were of the same order of magnitude, the main differences with our experiments being that the MSCs were of syngeneic origin and that additional MSCs were directly applied onto the surface of the open wound. We used human MSCs, but that was not likely the cause of the difference as our recipients were immunodeficient NOD/SCID mice in which the reduction of the implanted cells over time [42] was similar to that noted by Wu et al [38]. …”