Immune-privileged tissues formed from immunologically cloaked mouse embryonic stem cells survive long term in allogeneic hosts

The immunogenicity of transplanted allogeneic cells and tissues is a major hurdle to the advancement of cell therapies. Here we show that the overexpression of eight immunomodulatory transgenes (Pdl1, Cd200, Cd47, H2-M3, Fasl, Serpinb9, Ccl21 and Mfge8) in mouse embryonic stem cells (mESCs) is sufficient to immunologically 'cloak' the cells as well as tissues derived from them, allowing their survival for months in outbred and allogeneic inbred recipients. Overexpression of the human orthologues of these genes in human ESCs abolished the activation of allogeneic human peripheral blood mononuclear cells and their inflammatory responses. Moreover, by using the previously reported FailSafe transgene system, which transcriptionally links a gene essential for cell division with an inducible and cell-proliferation-dependent kill switch, we generated cloaked tissues from mESCs that served as immune-privileged subcutaneous sites that protected uncloaked allogeneic and xenogeneic cells from rejection in immune-competent hosts. The combination of cloaking and FailSafe technologies may allow for the generation of safe and allogeneically accepted cell lines and off-the-shelf cell products.

Characterization of embryonic stem cell-differentiated fibroblasts as mesenchymal stem cells with robust expansion capacity and attenuated innate immunity

BACKGROUND

Mesenchymal stem cells (MSCs) isolated from adult tissues (Ad-MSCs) have shown great promise for use in regenerative medicine. However, their poor in vitro expansion capacity and tissue scarcity have been major limitations. In this study, we demonstrate that mouse embryonic stem cells (mESCs) can differentiate into cells with MSC properties.

METHODS

Using previously established methods that characterize Ad-MSCs, we analyzed mESC-differentiated fibroblasts (mESC-FBs), including plastic adherence, clonogenic growth, MSC marker expression, tri-lineage differentiation potential, and the capacity to express immunomodulators.

RESULTS

Although previously characterized as mESC-differentiated fibroblasts (mESC-FBs), these cells exhibit major properties of Ad-MSCs. However, mESC-FBs also display unique features inherited from ESCs, including robust expansion capacity, senescence resistance, and attenuated innate immunity. In particular, mESC-FBs are insensitive to bacterial endotoxin (lipopolysaccharide, LPS) and do not express LPS-induced inflammatory molecules, in contrast to bone marrow (BM)-MSCs. We further demonstrate that mESC-FBs are resistant to the cytotoxicity associated with inflammatory cytokines, bacterial endotoxins (LPS and heat-killed bacteria), and macrophage-mediated inflammation.

CONCLUSIONS

While it remains to be determined how the unique properties of mESC-FBs will affect their immunoregulatory activity under an in vivo condition, our findings demonstrate that ESCs could be used as an alternative source to generate a new class of ESC-MSCs with unique features potentially useful in regenerative medicine.

Identification of embryonic precursor cells that differentiate into thymic epithelial cells expressing autoimmune regulator

Medullary thymic epithelial cells (mTECs) expressing autoimmune regulator (Aire) are critical for preventing the onset of autoimmunity. However, the differentiation program of Aire-expressing mTECs (Aire(+) mTECs) is unclear. Here, we describe novel embryonic precursors of Aire(+) mTECs. We found the candidate precursors of Aire(+) mTECs (pMECs) by monitoring the expression of receptor activator of nuclear factor-κB (RANK), which is required for Aire(+) mTEC differentiation. pMECs unexpectedly expressed cortical TEC molecules in addition to the mTEC markers UEA-1 ligand and RANK and differentiated into mTECs in reaggregation thymic organ culture. Introduction of pMECs in the embryonic thymus permitted long-term maintenance of Aire(+) mTECs and efficiently suppressed the onset of autoimmunity induced by Aire(+) mTEC deficiency. Mechanistically, pMECs differentiated into Aire(+) mTECs by tumor necrosis factor receptor-associated factor 6-dependent RANK signaling. Moreover, nonclassical nuclear factor-κB activation triggered by RANK and lymphotoxin-β receptor signaling promoted pMEC induction from progenitors exhibiting lower RANK expression and higher CD24 expression. Thus, our findings identified two novel stages in the differentiation program of Aire(+) mTECs.

IFIT1 Differentially Interferes with Translation and Replication of Alphavirus Genomes and Promotes Induction of Type I Interferon

Alphaviruses are a group of widely distributed human and animal pathogens. It is well established that their replication is sensitive to type I IFN treatment, but the mechanism of IFN inhibitory function remains poorly understood. Using a new experimental system, we demonstrate that in the presence of IFN-β, activation of interferon-stimulated genes (ISGs) does not interfere with either attachment of alphavirus virions to the cells, or their entry and nucleocapsid disassembly. However, it strongly affects translation of the virion-delivered virus-specific RNAs. One of the ISG products, IFIT1 protein, plays a major role in this translation block, although an IFIT1-independent mechanism is also involved. The 5'UTRs of the alphavirus genomes were found to differ significantly in their ability to drive translation in the presence of increased concentration of IFIT1. Prior studies have shown that adaptation of naturally circulating alphaviruses to replication in tissue culture results in accumulation of mutations in the 5'UTR, which increase the efficiency of the promoter located in the 5'end of the genome. Here, we show that these mutations also decrease resistance of viral RNA to IFIT1-induced translation inhibition. In the presence of higher levels of IFIT1, alphaviruses with wt 5'UTRs became potent inducers of type I IFN, suggesting a new mechanism of type I IFN induction. We applied this knowledge of IFIT1 interaction with alphaviruses to develop new attenuated variants of Venezuelan equine encephalitis and chikungunya viruses that are more sensitive to the antiviral effects of IFIT1, and thus could serve as novel vaccine candidates.