Page 1758 - Hematology_ Basic Principles and Practice ( PDFDrive )

1562 Part IX Cell-Based Therapies

low numbers of human MSCs and has been suggested to play a role inflammatory complex [MIC] A/B, UL binding protein 1, 2, 3),
in the blocking of monocyte differentiation into DCs. MSC-derived and Notch receptors (Jagged-1). Intriguingly, human MSCs do not
TGF-β was shown to play a role in the inhibition of natural killer upregulate co-stimulatory molecules (CD80, CD86) in response
(NK) cells in vitro and is the dominant cytokine that drives the to IFN-γ, and immune modulators such as TGF-β can markedly
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polarization of FoxP3 CD4 T cells to FoxP3 regulatory T (Treg) blunt MHC class II upregulation in response to inflammatory
cells. Accordingly, production of TGF-β by MSCs favors the induc- stimuli. IFN-γ–inducible IDO expression plays a major role in the
tion of Treg cells in vivo. PGE 2 is a metabolic product of arachidonic immunosuppressive properties of MSCs and defines an important
acid conversion by the enzyme COX-1 and COX-2. MSCs constitu- component of MSC immune plasticity. IDO catabolizes conversion
tively express COX-2 and secrete low levels of PGE 2. Expression of of tryptophan to kynurenine, which is an inhibitor of T-cell prolif-
COX-2 and secretion of PGE 2 are considerably increased when eration. Blocking IDO catabolic activity with 1-methyl tryptophan
encountering an inflammatory signal such as interferon (IFN)-γ, abolishes the suppressive activity of MSCs on T-cell proliferation in
TNF-α, or lipopolysaccharide (LPS). In addition to its role in the vitro. Another tryptophan-degrading enzyme, tryptophan 2,3 dioxy-
inflammatory reaction, PGE 2 is described as an important pathway genase (TDO), has a homeostatic housekeeping role predominantly
in T-cell immunosuppression mediated by MSCs. Human MSCs in the liver, and unlike IDO it does not respond to immunoactive
have also been shown to express an array of other potential immune signals. Human MSCs were shown to express TDO in the resting
suppressive effectors, including LIF, HLA-G, TNF-α–stimulated stage and IFN-γ stimulation does not upregulate its expression. This
gene/protein 6 (TSG-6), as well as soluble and cell surface-associated suggests the leading role of IFN-γ–inducible IDO in modulating
galectins, in particular galectin-1, galectin -3, and galectin -8. Expres- the tryptophan catabolic pathway and subsequent immunosuppres-
sion of several molecules known to play an important role in fetoma- sion by MSCs. Upregulation of IDO by IFN-γ can be augmented
ternal tolerance such as indoleamine dioxygenase (IDO) occurs in synergistically by other cytokines such as TNF-α, which are poor
human MSCs and these appear to be central in the suppressor inducers of IDO by themselves alone. TLR activators such as LPS
functionality of human culture-expanded MSCs. Exposure of MSCs and polyI:C have been shown to upregulate IDO through autocrine
to IFN-γ increases the expression of these factors that mediate inhibi- IFN-β signaling loop independent of IFN-γ. Interestingly, MSCs
tory action on T, B, and NK cell activities. MSCs can also affect with defective IFN-γ receptor 1 can still suppress T-cell prolifera-
differentiation of monocytes to IL-10-producing M2 macrophages tion. This suggests that other immunosuppressive mechanisms are
and dendritic cells, as well as their maturation, migration, and operative in synergy with IFN-γ–induced effects on MSCs to regulate
functions. 15 T-cell proliferation, such as: HLA-G5, PGE2, galectins, insulin-like
growth factor (IGF)-binding proteins, and TSG-6. The breadth
MESENCHYMAL STROMAL CELL IMMUNE PLASTICITY and effectiveness of an ongoing immune response are determined
by both T-cell proliferation and their effector function, as defined
IN RESPONSE TO INFLAMMATORY CUES by cytokine secretion and degranulation, respectively. Thus, IDO
expression by MSCs targets the proliferative response of T cells
In a resting state, cultured MSCs display immune homeostatic while PDL1/PDL2-PD-1 interaction likely regulates memory T-cell
features biased toward suppression and are further highly respon- function.
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sive to environmental inflammatory cues. In vivo experiments in Whereas it is known that most somatic cells express one or two
rodents and nonhuman primates have demonstrated that although TLRs, the exact expression profile of TLRs in MSCs is controversial.
a large portion of MSCs are trapped in the lung following systemic Protein expression of TLR1, TLR2, TLR3, TLR4, TLR5, TLR6,
intravenous administration, infused MSCs can migrate to several TLR7, and TLR9 has been reported in human and mouse MSCs;
organs such as the liver, spleen, BM, and kidneys. Their migra- however, in humans only TLR3 and TLR4 were expressed at levels
tory potential is associated with the expression of VCAM-1, which comparable to blood mononuclear cells. The response to TLR ligands
allows them to interact with endothelial cells. In addition, MSCs in MSCs appears quite different compared with macrophages. When
can express chemokine receptors (CCR1, CCR7, CCR9, CXCR4, activated, both cell types secrete chemokines, PGE 2 , and IL-6;
CXCR5, and CXCR6) that promote their migration to specific sites, however, only macrophages produce IL-10, TNF-α, and, following
such as the BM via SDF-1, or to inflamed tissue via other CC and IFN-γ priming, IL-12. In addition, factors secreted by TLR-activated
CXC chemokines. MSCs express multiple receptors for inflamma- MSCs, in particular PGE 2 , act in vitro and in vivo on surrounding
tory signals, such as receptors for chemokines, type I and II IFNs, macrophages, resulting in increased production of IL-10. It has been
IL-1, and TNF-α, as well as TLRs that bind to pathogen-associated shown that MSCs can reduce mortality in a mouse model of perito-
conserved motifs. These mediators are produced by fully activated nitis associated with septicemia and release of bacterial toxins in the
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immune cells and upregulate the expression of immunosuppres- circulation. In this study, MSCs injected in the systemic circulation
sive factors by MSCs, which in turn dampens the immune and/or of septic mice localized in the lung, where they were found sur-
inflammatory response. IFN-γ is a well-studied immunomodulatory rounded by macrophages. These macrophages were shown to produce
cytokine that robustly modulates the immunobiology of MSCs, and increased levels of the antiinflammatory cytokine IL-10 both in vivo
the in vivo suppressive properties of MSCs are likely dependent on and ex vivo in response to bacterial LPS compared with untreated
activation by IFN-γ secreted by activated lymphoid effector cells. septic mice. In vitro assays suggested that the suppressive effect of
The importance of IFN-γ for augmentation of the veto properties of MSCs on the macrophage inflammatory response to LPS was depen-
MSCs has been demonstrated in multiple key seminal observations: dent on the expression of the LPS receptor TLR4 by both cell types.
anti-IFN-γ receptor antibodies abrogate the suppressive properties of Results suggested that LPS or TNF-α–mediated activation of MSCs
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MSCs; IFN-γ receptor knock out of MSCs does not inhibit T cells ; upregulated the expression of cyclooxygenase 2 and PGE2 by MSCs,
and, IFN-γ licensing is crucial for MSCs to suppress T-cell effec- which in turn bind to the EP2 and EP4 receptors on macrophages,
tor functions. Accordingly, in vivo experiments in murine GVHD stimulating the production of IL-10.
models demonstrated that MSCs were not effective at controlling A fundamental discrepancy between the immune cell physiology
GVHD if mice were transplanted with T cells defective for IFN-γ of MSCs from different species is the relative importance of nitric
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production, and MSCs pretreated with IFN-γ were more potent oxide (NO) and IDO in murine and human MSCs. After inflam-
than nontreated MSCs at inhibiting GVHD. The downstream matory priming, human MSCs express extremely high levels of IDO
effects of IFN-γ activation on MSCs are protean. IFN-γ–primed and low levels of inducible nitric oxide synthase (iNOS), which is
MSCs robustly upregulate markers such as MHC I and II molecules, opposite to that seen with mouse MSCs. The in vitro functional
immune modulatory molecules (CD200, CD274/PD-L1/B7-H1), relevance of IDO bioactivity in human MSCs can be readily shown
cytokine/chemokine receptors (CXCR3, CXCR4, CXCR5, CCR7, by use of the specific inhibitor L-1 methyltryptophan (L-1MT),
CD119/IFN-γ receptor), adhesion molecules (CD54, CD106), which completely abolishes the inhibition of T-cell proliferation,
DNAM ligands (CD112, CD155), NKG2D ligands (macrophage whereas mouse MSCs are unaffected by L-1MT. Inhibition of NO