Advancing mouse models for transplantation research

Mouse models have been instrumental in understanding mechanisms of transplant rejection and tolerance, but cross-study reproducibility and translation of experimental findings into effective clinical therapies are issues of concern. The Mouse Models in Transplantation symposium gathered scientists and physician-scientists involved in basic and clinical research in transplantation to discuss the strengths and limitations of mouse transplant models and strategies to enhance their utility. Participants recognized that increased procedure standardization, including the use of prespecified, defined endpoints, and statistical power analyses, would benefit the field. They also discussed the generation of new models that incorporate environmental and genetic variables affecting clinical outcomes as potentially important. If implemented, these strategies are expected to improve the reproducibility of mouse studies and increase their translation to clinical trials and, ideally, new Food and Drug Administration-approved drugs.

A tissue-intrinsic IL-33/EGF circuit promotes epithelial regeneration after intestinal injury

Intestinal stem cells (ISCs) maintain the epithelial lining of the intestines, but mechanisms regulating ISCs and their niche after damage remain poorly understood. Utilizing radiation injury to model intestinal pathology, we report here that the Interleukin-33 (IL-33)/ST2 axis, an immunomodulatory pathway monitored clinically as an intestinal injury biomarker, regulates intrinsic epithelial regeneration by inducing production of epidermal growth factor (EGF). Three-dimensional imaging and lineage-specific RiboTag induction within the stem cell compartment indicated that ISCs expressed IL-33 in response to radiation injury. Neighboring Paneth cells responded to IL-33 by augmenting production of EGF, which promoted ISC recovery and epithelial regeneration. These findings reveal an unknown pathway of niche regulation and crypt regeneration whereby the niche responds dynamically upon injury and the stem cells orchestrate regeneration by regulating their niche. This regenerative circuit also highlights the breadth of IL-33 activity beyond immunomodulation and the therapeutic potential of EGF administration for treatment of intestinal injury.

Tumor-Derived IL33 Promotes Tissue-Resident CD8+ T Cells and Is Required for Checkpoint Blockade Tumor Immunotherapy

Immune checkpoint blockade (ICB) immunotherapy has revolutionized cancer treatment by prolonging overall survival of patients with cancer. Despite advances in the clinical setting, the immune cellular network in the tumor microenvironment (TME) that mediates such therapy is not well understood. IL33 is highly expressed in normal epithelial cells but downregulated in tumor cells in advanced carcinoma. Here, we showed that IL33 was induced in tumor cells after treatment with ICB such as CTL antigen-4 (CTLA-4) and programmed death-1 (PD-1) mAbs. ST2 signaling in nontumor cells, particularly CD8+ T cells, was critical for the antitumor efficacy of ICB immunotherapy. We demonstrated that tumor-derived IL33 was crucial for the antitumor efficacy of checkpoint inhibitors. Mechanistically, IL33 increased the accumulation and effector function of tumor-resident CD103+CD8+ T cells, and CD103 expression on CD8+ T cells was required for the antitumor efficacy of IL33. In addition, IL33 also increased the numbers of CD103+ dendritic cells (DC) in the TME and CD103+ DC were required for the antitumor effect of IL33 and accumulation of tumor-infiltrating CD8+ T cells. Combination of IL33 with CTLA-4 and PD-1 ICB further prolonged survival of tumor-bearing mice. Our study established that the "danger signal" IL33 was crucial for mediating ICB cancer therapy by promoting tumor-resident adaptive immune responses.

Recipient IL-33 stimulates donor T cells to promote Type 1 alloimmunity and lethal acute GVHD (TRAN1P.931)

Graft-versus-host disease (GVHD) is a severe and often fatal complication of allogeneic (allo) hematopoietic cell transplantation (HCT). Total body irradiation (TBI) conditioning of the recipient and subsequent GVHD-associated alloimmune responses cause significant gut epithelial cell damage. As IL-33 is an epithelial cell-derived cytokine with pleiotropic functions, we investigated the impact of IL-33 on GVHD pathogenesis. Mice received TBI and alloHCT (allogeneic bone marrow (BM) +/- T cells). Where indicated, recipient mice, donor BM or T cells were deficient for IL-33 or its receptor, ST2. Survival, weight, and clinical score were monitored. Tissues and serum were harvested for IL-33 quantitation and analysis of alloimmune responses. We find that IL-33 expression is increased rapidly in CD45- cells of the gut after TBI, and sustained at least through day 14 in alloHCT recipients. Recipient, but not donor, IL-33 acting on ST2-expressing donor T cells fueled Type 1 responses and GVHD. The IL-33 antagonist ST2-Fc prevented GVHD and reduced Type 1 alloimmune responses, whereas IL-33 administration post-alloHCT (d+3 to d+7) accelerated GVHD lethality and Type 1 alloimmunity. These potent responses overcome regulatory functions of IL-33 to promote GVHD. In summary, our findings are the first to reveal a detrimental role for IL-33 underlying GVHD pathogenesis and establish the IL-33/ST2 axis as a targetable pathway to lessen the risk of GVHD following alloHCT.

Myeloid dendritic cell-specific mTORC2 deficiency enhances Th1 and Th17 cell responses and inhibits B16 melanoma growth (TUM6P.959)

The mechanistic/mammalian target of rapamycin (mTOR) is a key integrative kinase that functions in at least two independent complexes: mTOR complex (C)1 and mTORC2. While mTORC2 in T cells promotes Th2 differentiation, its immunologic function in dendritic cells (DC) is largely unknown. We defined the role of rictor, a key mTORC2 component, in myeloid DC and their ability to polarize allogeneic T cells. When stimulated with TLR ligands, bone marrow-derived DC generated from conditional Rictor knockouts displayed lower co-inhibitory B7-H1 (CD274) molecule expression, enhanced IL-12p70, IL-23, IL-6 and TNFα production and augmented allogeneic T cell stimulatory ability. Analysis of antigen-specific CD4+ T cells after activation by Rictor-/- DC in vivo revealed significant expansion of IFN-γ+ and IL-17+, but not IL-10+ or CD4+Foxp3+ T cells. Consistent with this pro-inflammatory effect, intratumoral injection of Rictor-/- DC markedly slowed B16 melanoma growth in B6 WT mice compared to control DC (p<0.05), promoting enhancement of CD4+IL-17+ and CD8+INFγ+ T cells in the spleens of Rictor-/- DC treated animals compared with controls. These data indicate that DC lacking mTORC2 activity exhibit an enhanced pro-inflammatory profile with ability to promote Th1/Th17 responses and slowed melanoma progression versus control DC. Therefore, targeting mTORC2 provides new insight into molecular regulation of DC function with therapeutic implications for cancer.

The iron sequestering protein Lipocalin 2 is critical to IL-33-exposed dendritic cell stimulation of Th2 responses and allergic airway disease (IRC7P.424)

IL-33 is an IL-1 cytokine with emerging pleotropic functions, but ascribed a prominent role in Type 2-mediated responses to pathogens due to its capacity to generate and support T helper type 2 (Th2) cells. Likewise, IL-33 and Th2 cell are viewed as dominant drivers of inflammatory lung disorders, including asthma. Previous studies have revealed that IL-33 stimulates the capacity of dendritic cells (DC) to initiate Th2 responses and, thus instigate allergic airway disease (AAD) in mice. However, the precise mechanisms by which IL-33 promotes DC-mediated Th2 polarization was not known. To answer this question, we preformed microarray analysis on DC after exposure to IL-33. This analysis identified multiple genes involved in iron homeostasis and metabolism upregulated by IL-33. The most dramatically increase gene in DC by IL-33 was Lipocalin 2 (Lcn2), an iron sequestering protein that facilitates DC iron uptake. After IL-33-exposure, DC deficient in Lcn2 displayed a significantly reduced capacity to generate Th2 responses, however, LPS-exposed DC lacking Lcn2 were fully capable of Th1 polarization. Identical finding were observed when DC iron uptake was blocked through chelation during their exposure to IL-33. In a DC-induced model of AAD, blocking DC iron uptake resulted in significantly decreased disease severity. These data define a novel role for IL-33-stimulated iron uptake by Lcn2 in the generation of Th2 responses.

Tumoral expression of IL-33 inhibits tumor growth and modifies the tumor microenvironment through CD8+ T and NK cells

Cancer immunotherapy has shown great promise as a new standard cancer therapeutic modality. However, the response rates are limited for current approach that depends on enhancing spontaneous antitumor immune responses. Therefore, increasing tumor immunogenicity by expressing appropriate cytokines should further improve the current immunotherapy. IL-33 is a member of the IL-1 family of cytokines and is released by necrotic epithelial cells or activated innate immune cells and is thus considered a "danger" signal. The role of IL-33 in promoting type 2 immune responses and tissue inflammation has been well established. However, whether IL-33 drives antitumor immune responses is controversial. Our previous work established that IL-33 promoted the function of CD8(+) T cells. In this study, we showed that the expression of IL-33 in two types of cancer cells potently inhibited tumor growth and metastasis. Mechanistically, IL-33 increased numbers and IFN-γ production by CD8(+) T and NK cells in tumor tissues, thereby inducing a tumor microenvironment favoring tumor eradication. Importantly, IL-33 greatly increased tumor Ag-specific CD8(+) T cells. Furthermore, both NK and CD8(+) T cells were required for the antitumor effect of IL-33. Moreover, depletion of regulatory T cells worked synergistically with IL-33 expression for tumor elimination. Our studies established "alarmin" IL-33 as a promising new cytokine for tumor immunotherapy through promoting cancer-eradicating type 1 immune responses.

Myeloid DC-specific mTORC2 deficiency enhances Th1 and Th17 immunologic responses (IRC5P.472)

The mechanistic/mammalian target of rapamycin (RAPA) (mTOR) is a key integrative kinase that functions in at least two independent complexes: RAPA-sensitive mTOR complex (C)1 and RAPA-insensitive mTORC2. While mTORC2 in T cells promotes Th2 and Th17 differentiation, its immunologic function in dendritic cells (DC) is largely unknown. We defined the role of rictor, a key mTORC2 component, in myeloid DC and their ability to polarize interacting allogeneic Th cells. When stimulated with TLR ligands, bone marrow-derived DC generated from conditional rictor KOs displayed lower co-inhibitory B7-H1 (CD274) molecule expression, enhanced IL-12p70, IL-23, IL-6 and TNFα production and augmented allogeneic T cell stimulatory ability. Analysis of allopeptide-specific CD4+ T cells after activation by rictor KO DC in vivo revealed significant expansion of IFN-γ+ and IL-17+, but not IL-10+ or CD4+Foxp3+ T cells. These data establish that mTORC2 activity restrains pro-inflammatory responses of conventional DC and their ability to differentiate Th1/Th17 cells.

Metabolic mechanisms underlying IL-33 mediated Th2 polarization and allergic airway disease (IRM7P.494)

IL-33 is a recent IL-1 cytokine with emerging pleotropic functions. Experimental studies have suggested that IL-33 supports Treg responses, drives CD8+ T cell IFN-γ production, and acts as a potent mediator of Th2 responses. The current study tests the hypothesis that IL-33 modulates mDC gene expression and pathway activities facilitating Th2 polarization. Using gene expression and efficiency analysis, we identified multiple genes involved in iron homeostasis and metabolism which were upregulated in mDC by IL-33. Desferrioxamine (DFO) was utilized to assess the impact of blocking mDC iron uptake during IL-33 stimulation on subsequent Th2 polarization and capacity to stimuli allergic airway disease (AAD). Generation of CD4+IL-5+ Th2 cells by IL-33-exposed DC was ablated by iron chelation, resulting in decreased IL-5 production and cellular proliferation. However, induced Th1 responses were unaffected in DFO-treated LPS-stimulated DC. Using an OVA/IL-33 mDC induced model of AAD, blocking DC iron uptake with DFO decreased several phenotypic hallmarks of the disease, including IgE (p<0.05) and IL-33 production in the lungs of animals. In conclusion, iron uptake is necessary for IL-33 generation of pro-inflammatory DC supporting Th2 polarization. Blocking iron uptake in a model of mDC induced AAD alters disease phenotype when compared to control animals. These data suggest that regulation of iron uptake may be a potential mechanism behind IL-33 induction of Th2 responses.

IL-33 stimulates dendritic cell secretion of IL-2 that promotes selective expansion of ST2+Foxp3+ regulatory T cells (IRC5P.460)

IL-33 is a pleiotropic IL-1 family cytokine that signals via ST2 and expands ST2+Foxp3+ regulatory T cells (Treg) in vivo. As ST2+ Treg show poor expansion by direct IL-33 stimulation, we sought to define mechanisms mediating their expansion. IL-2 signaling promotes ST2 expression on CD4+ T cells, and dendritic cells (DC) express ST2 (able to respond to IL-33), and are a potential source of IL-2. Thus, we examined if IL-33 mediates ST2+ Treg expansion by stimulating DC IL-2 production. CD11c+ wild type (WT) or IL-2 knockout (KO) bone marrow DC were exposed to IL-33 or LPS. DC phenotype was evaluated by multi-color flow cytometric analysis. The influence of IL-33 DC on T cell function was evaluated in MLR with CD4+ T cells, and T cell proliferation and phenotype were determined by flow analysis. Cytokines were quantitated by ELISA. We found that unlike LPS, IL-33 does not influence DC surface phenotype or induce pro-inflammatory cytokine production compared to controls. However, IL-33 induces a 5-fold increase IL-2 production by DC. In MLR, IL-33 DC selectively expand an activated subset of ST2+Foxp3+ Treg that are CD44hiICOShi. IL-33 DC-derived IL-2 is critical since IL-33-exposed IL-2 KO DC fail to expand ST2+ Treg. In summary, IL-33 licenses DC to selectively expand a subset of activated Treg through production of IL-2, in the absence of classical DC activation. These findings may be harnessed to aid the development of novel therapeutics aimed at promoting immune tolerance.

Conditional STAT3-deficiency augments Flt3 ligand-driven myeloid-derived suppressor cell expansion but limits their suppressor function (IRM7P.487)

Fms-like tyrosine kinase 3 ligand (Flt3L) is a potent hematopoietic growth factor that profoundly expands immunostimulatory dendritic cells (DC). Despite this, the influence of Flt3L on immunoregulatory myeloid-derived suppressor cells (MDSC) has not been described. Since DC and MDSC arise from common myeloid progenitors, and Flt3L-driven DC expansion is STAT3-dependent, we sought to precisely define the role of STAT3 in MDSC downstream of Flt3L signaling. We bred myeloid cell-specific STAT3-deficient mice (LysMCre x STAT3loxP) and administered Flt3L (10 μg/d ip, 10d). Splenic DC (CD11c+) and MDSC (Gr-1+) were isolated by magnetic bead selection. Stimulation and suppression of CD3+ T cell responses was assayed in MLR. As expected, STAT3-deficiency prevented Flt3L-mediated CD11c+ DC expansion. However, we observed an increase in MDSC (CD11b+Gr1+) frequency and absolute numbers in STAT3-deficient mice administered Flt3L. Interestingly, Flt3L-expanded MDSC from STAT3-deficient mice exhibited reduced suppressive capacity against CD3+ T cells in MLR compared to controls. These data identify a previously unappreciated STAT3-independent pathway for MDSC expansion, but reinforce the need for STAT3 signaling for their full suppressive capacity. Likewise, we establish that STAT3 has reciprocal effects on suppressive MDSC and immunostimulatory DC expansion. Together, these findings enhance understanding of the immunomodulatory properties of Flt3L.

Enhanced pro-inflammatory profile and Th17 alloresponse by targeting mTORC2 in DC (P2138)

Mammalian target of rapamycin (mTOR) is an important integrative kinase that regulates immune cell function. mTOR functions in two independent complexes: mTOR complex (mTORC) 1 and 2. The immunosuppressant Rapamycin inhibits mTORC1 but not mTORC2. Our aim was to study the immune regulatory role and underlying mechanistic properties of mTORC2 in DC. We generated a conditional mTORC2 knockout, targeting Rictor. Bone marrow cells were obtained from KO and WT B6 controls, and DC propagated in the presence of IL-4 and GM-CSF, with or without LPS. The phenotype, cytokine production, STAT3 activation and ability of DC to induce allogeneic T cell proliferation were evaluated, and also the responder T cell phenotype and cytokine production. DC differentiation was not affected, while DC yield was slightly reduced in mTORC2-deficient BM cultures compared with WT controls. DC lacking mTORC2 activity displayed similar levels of MHC and co-stimulatory molecules, but diminished B7-H1 expression, increased levels of IL-12p40, diminished STAT3 activation and higher T cell stimulatory ability following LPS stimulation, compared with WT. Staining of allogeneic T cells stimulated by Rictor-KO DC revealed more CD4+IL-17+ cells than those stimulated with WT DC. These novel data indicate that DC lacking mTORC2 activity exhibit an enhanced pro-inflammatory profile with ability to promote Th17 responses. Thus, targeting mTORC2 provides new insight into molecular regulation of DC function.

Rapamycin-resistant mTORC1 restrains dendritic cell B7-H1 expression that requires IL-1β to enhance regulatory T cell induction (P1349)

Introduction: The mammalian Target of Rapamycin (mTOR) is a central regulator of dendritic cell (DC) function that performs the catalytic activity of mTOR complex (mTORC)1 and 2. mTORC2 functions independently from mTORC1 and is resistant to inhibition by rapamycin (RAPA); however, mTORC1 has both RAPA-sensitive and -resistant outputs. Our goal was to ascertain the role of RAPA-resistant mTOR in DC. Methods: WT C57BL/6 or B7-H1-/- bone marrow-derived DC were generated with the addition of RAPA or ATP-competitive mTOR inhibitor, which blocks all mTOR signaling. DC lacking rictor, an mTORC2-specific subunit, were generated from conditional rictor KO mice. DC induction of regulatory T cells (Treg) was determined in MLR, using BALB/c CD4+CD25- T cell responders. Results and Conclusion: RAPA and mTORC2 deletion reduced DC B7-H1 expression, but ATP-competitive mTOR inhibitors enhanced B7-H1 expression. Augmented B7-H1 expression was blocked by STAT3 inhibition and correlated with reduced expression of the STAT3 negative regulator, SOCS3. DC exposed to ATP-competitive mTOR inhibitors increased Treg induction, which was dependent on DC B7-H1. IL-1β neutralization additionally reduced Treg induction by B7-H1-/- ATP-competitive mTOR inhibitor-exposed DC, suggesting that IL-1β and B7-H1 act additively to promote Treg induction by these DC. These findings establish a RAPA-resistant mTORC1 pathway that acts through SOCS3 and STAT3 to regulate DC B7-H1 expression and Treg induction.

Flt3 ligand expands and activates myeloid-derived suppressor cells in a STAT3-independent manner (P1074)

Introduction: Fms-like tyrosine kinase 3 ligand (Flt3L) promotes dendritic cell (DC) expansion; however, its influence on myeloid-derived suppressor cells (MDSC), an immature population of myeloid cells that suppresses T cell immunity, has not been studied. Methods: BALB/c mice were treated with Flt3L (10 μg/d i.p., 10 d) in the presence or absence of STAT3 inhibitor S31-201 (5 mg/kg/d). Splenic DC and MDSC were isolated by CD11c+ and Gr1+ magnetic bead selection, respectively. Stimulation and suppression of CD3+ T cell responses were assayed by MLR. Syngeneic MDSC were adoptively transferred (5x106) to BALB/c recipients 1 d before C57BL/6 heart transplantation and survival monitored. Results and Conclusions: Flt3L increased MDSC (CD11b+Gr1+) frequency and absolute numbers. MDSC from Flt3L-treated mice potently suppressed CD3+ T cell proliferation greater than control MDSC from naïve splenocytes. Conversely, Flt3L-expanded DC stimulated increased proliferation of allogeneic T cells compared to those from naïve control mice. While STAT3 is considered crucial for MDSC expansion and activation, STAT3 inhibition reduced Flt3L-mediated DC, but not MDSC, expansion. STAT3 inhibition augmented MDSC expansion by Flt3L, without affecting their suppressive capacity. Flt3L-expanded MDSC, but not control MDSC, demonstrated in vivo suppressive activity by prolonging fully MHC-mismatched heart transplant survival. Together, these data identify a novel immunomodulatory function of Flt3L.

CD11c+ dendritic cells are required for IL-33-mediated expansion of ST2+Foxp3+ regulatory T cells in vivo (P1075)

IL-33 is an IL-1 cytokine that signals via ST2, which is expressed on T cells and myeloid cells. Although IL-33 promotes Th2 responses, its administration potently expands CD4+Foxp3+ regulatory T cells (Treg). We examined if IL-33 expands murine Treg directly or indirectly through its impact on CD11c+ DC. The ability of IL-33 to facilitate CD3/CD28-stimulated proliferation of wild-type (WT) or ST2-/- Treg was compared to IL-2. The impact of IL-33 on the capacity of DC to expand Treg was defined in vitro on bone marrow (BM)-generated DC from WT or ST2-/- mice and in vivo using CD11c-DTR BM chimeras administered diphtheria toxin (DT) to deplete CD11c+ cells during IL-33 treatment. The capacity of IL-33-expanded Treg from Foxp3-RFP reporter mice to suppress effector T cells was assessed. We found that IL-33 directly expands Treg in vitro, including an ST2+ subset absent from IL-2-treated cultures. IL-33-exposed DC generate ST2+ Treg from naïve T cells, and is dependent on ST2 expressed by DC. IL-33 failed to expand Treg in vivo, especially ST2+ Treg, in the absence of CD11c+ cells. In conclusion, IL-33 promotes the expansion of suppressive Foxp3+ Treg, including an ST2+ subset in vitro and in vivo. This results from IL-33 activity directly on Treg, but more significantly, indirectly through its impact on CD11c+ cells. These findings have important implications for further characterization of ST2+ Treg and the development of novel therapeutics aimed at promoting immune tolerance.

mTORC2 negatively regulates DC PD-L1 and IL-10 through SOCS3 and STAT3 (172.34)

The mammalian Target of Rapamycin (mTOR) is a central regulator of dendritic cells (DC) that functions in two independent complexes, mTOR complex (mTORC) 1 and 2. Rapamycin (RAPA) is a selective inhibitor of mTORC1, whereas novel ATP-competitive mTORC1/2 inhibitors (Torin1) inhibit both complexes. The immunoregulatory function of RAPA-resistant mTORC2 in DC is unknown. C57BL/6, PD-L1-/-, IL-6-/-, IL-10-/-, IL-12/23p40-/-, Ebi3-/-, interferon regulatory factor (IRF)1-/-, or FoxO1/3/4-/- bone marrow cells were cultured in GM-CSF and IL-4 to generate CD11c+ DC. RAPA or Torin1 was added on d2, and LPS added on d7. DC phenotype and signaling was assessed (d8) by flow cytometry and immunoblot, respectively. While RAPA reduced PD-L1, mTORC1/2 inhibition selectively upregulated PD-L1 expression on DC in a STAT3-dependent manner in unstimulated and LPS-stimulated DC. Whereas RAPA reduced IL-10 secretion, mTORC1/2 inhibition augmented IL-10 secretion following LPS stimulation. The PD-L1 regulator, IRF1, and FoxO, a STAT3 regulator, were not required for PD-L1 upregulation. PD-L1 upregulation on mTORC1/2-inhibited DC did not require autocrine IL-6, IL-10, IL-12/23, or IL-27. SOCS3, a negative regulator of STAT3, was reduced dramatically in mTORC1/2-inhibited, but not RAPA-exposed DC. mTORC2 negatively regulates STAT3 in DC through SOCS3. Blockade of mTORC2 signaling enhances PD-L1 expression and IL-10 secretion. These novel findings establish mTORC2 as a key regulator of DC function.

IL-33 directly supports the proliferation of suppressive, naturally-occurring regulatory T cells expressing the IL-33 receptor, ST2 (161.1)

Overview: Both IL-33 and its receptor, ST2, have been ascribed T helper type-2 (Th2) response promoting capacities. However, IL-33 increases splenic CD4+ forkhead box P3 (Foxp3)+ regulatory T cells (Treg) and post-transplant IL-33 monotherapy mediates a Treg-dependent prolongation of experimental cardiac allograft survival. As such, we addressed whether IL-33 directly targets Treg to support their expansion or suppressive function. Methods: The ability of recombinant IL-33 to facilitate anti-CD3/CD28-stimulated proliferation of BALB/c St2+/+ or St2-/- CD4+ CD25+ T cells was compared to IL-2. Following stimulation, Treg phenotype was defined by flow cytometry and their capacity to suppress naive T cell proliferation determined in bead-based suppression assays. Results: Following 3-4 days of culture, both cytokines promoted significant Treg proliferation vs. CD3/CD28 stimulation alone. However, IL-33 facilitated the expansion of a population ST2+ Foxp3+ cells that were absent from IL-2-treated cultures. Both IL-2 and IL-33-exposed Treg cultures exhibited suppressive activity. Conclusions: We identify the previously unreported capacity of IL-33 for direct expansion of Treg. Also, our data reveal that ST2, in addition to being expressed on Th2 cells, is found on an IL-33-expanded subset of suppressive Treg. These findings have significant implications for the therapeutic use of IL-33 to expand Treg, -cells critically important to tolerance and organ allograft survival.

Histone deacetylase inhibition facilitates GM-CSF-mediated expansion of myeloid-derived suppressor cells in vitro and in vivo (111.10)

Recent studies have demonstrated anti-inflammatory and tolerance-promoting properties of chromatin-modifying histone deacetylase inhibitors (HDACi) through their ability to modulate dendritic cells and regulatory T cells. However, the influence of these agents on rare, immunoregulatory myeloid-derived suppressor cells (MDSC) has not been examined. C57BL/6 (B6) bone marrow (BM) cells were cultured with GM-CSF and the HDACi trichostatin A (TSA) for 5d. Cell lineage was identified by flow cytometry and putative MDSC purified by anti-Gr1 immunomagnetic bead selection. For in vivo experiments, mice received hydrodynamic infusion of GM-CSF or control enhanced green fluorescent protein plasmid then TSA for 5d. Splenic MDSC were isolated by Gr1 positive selection, and suppressive function was tested in allogeneic mixed leukocyte cultures. Exposure of GM-CSF-stimulated BM cells to TSA led to expansion of suppressive MDSC (CD11b+Gr1+) that correlated with increased numbers of hematopoietic stem and progenitor cells (HSPC; Lin-Sca-1+c-Kit+). TSA enhanced mobilization of splenic HSPC in vivo following GM-CSF administration and increased CD11b+Gr1+ cells in the BM and spleen. Increased numbers of Gr1+ cells that suppressed effector T cell proliferation were isolated from spleens of TSA-treated mice given GM-CSF. In conclusion, HDACi enhance GM-CSF-mediated expansion of MDSC in vitro and in vivo, suggesting a novel mechanism of the anti-inflammatory action of HDACi.

The multidrug resistance transporter ABCG2 (breast cancer resistance protein 1) effluxes Hoechst 33342 and is overexpressed in hematopoietic stem cells

The human ATP-binding cassette superfamily G (White) member 2 (ABCG2) gene and its murine homologue breast cancer resistance protein 1 (Bcrp1) are recently described ATP-binding cassette transporters associated with drug resistance in tumor cell lines, including the MCF-7 cell line, selected for its resistance to mitoxantrone (MCF-7/MitoR). Infection of MCF-7 cells with the retroviral vector containing ABCG2 cDNA (G1-ABCG2) resulted in cells (MCF-7/ABCG2) that were resistant to mitoxantrone at levels similar to those observed in MCF-7/MitoR cells. Previous studies have shown that pluripotent hematopoietic stem cells overexpress the multidrug-resistant transport (MDR1) gene and efflux rhodamine, a substrate for the MDR1 transporter. Other studies have identified a primitive hematopoietic stem cell population, or side population (SP) cells, which are identified by their efflux of the fluorescent dye, Hoechst 33342. In an attempt to identify the transport genes responsible for this phenotype, we examined the uptake of Hoechst 33342 into MCF-7, MCF-7/MitoR, and MCF-7 cells infected with a retroviral vector expressing the ABCG2 gene (MCF-7/ABCG2). MCF-7/MitoR cells as well as MCF-7/ABCG2 cells demonstrated lower levels of Hoechst 33342 uptake compared with the parental MCF-7 cells. We also examined the level of the mouse Bcrp1 RNA in SP cells and non-SP cells isolated from mouse hematopoietic cells. Mouse SP cells expressed relatively high levels of Bcrp1 mRNA relative to non-SP cells. These results suggest that Hoechst 33342 is a substrate for the ABCG2 transporter and that ABCG2/Bcrp1 expression may serve as a marker for hematopoietic stem cells in hematopoietic cells.