Multipotent adult progenitor cells (MAPCs) are bone marrow-derived nonhematopoietic stem cells with a broad differentiation potential and extensive expansion capacity. A comparative study between human mesenchymal stem cells (hMSCs) and human MAPCs (hMAPCs) has shown that hMAPCs have clearly distinct phenotypical and functional characteristics from hMSCs. In particular, hMAPCs express lower levels of MHC class I than hMSCs and cannot only differentiate into typical mesenchymal cell types but can also differentiate in vitro and in vivo into functional endothelial cells. The use of hMSCs as cellular immunomodulatory stem cell products gained much interest since their immunomodulatory capacities in vitro became evident over the last decade. Currently, the clinical grade stem cell product of hMAPCs is already used in clinical trials to prevent graft-versus-host disease (GVHD), as well as for the treatment of acute myocardial infarct, ischemic stroke, and Crohn's disease. Therefore, we studied the immune phenotype, immunogenicity, and immunosuppressive effect of hMAPCs in vitro. We demonstrated that hMAPCs are nonimmunogenic for T-cell proliferation and cytokine production. In addition, hMAPCs exert strong immunosuppressive effects on T-cell alloreactivity and on T-cell proliferation induced by mitogens and recall antigens. This immunomodulatory effect was not MHC restricted, which makes off-the-shelf use promising. The immunosuppressive effect of hMAPCs is partially mediated via soluble factors and dependent on indoleamine 2,3-dioxygenase (IDO) activity. At last, we isolated hMAPCs, the clinical grade stem cell product of hMAPCs, named MultiStem, and hMSCs from one single donor and observed that both the immunogenicity and the immunosuppressive capacities of all three stem cell products are comparable in vitro. In conclusion, hMAPCs have potent immunomodulatory properties in vitro and can serve as a valuable cell source for the clinical use of immunomodulatory cellular stem cell product.

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Tumor growth coincides with an accumulation of myeloid-derived suppressor cells (MDSCs), which exert immune suppression and which consist of two main subpopulations, known as monocytic (MO) CD11bEur. J. Immunol. 2013Immunol. . 43: 2930Immunol. -2942 Immunomodulation 2931 IntroductionThe discovery of tumor-specific antigenic peptides recognized by CD8 + T cells has laid the foundation for immunotherapeutic approaches aimed at maximizing cytotoxic T lymphocyte (CTL) mediated eradication of cancer cells. The optimization of such therapies requires a thorough knowledge of the mechanisms regulating CTL induction and activity and of the countermeasures taken by tumors to avoid destruction. Naive CD8 + T cells constantly sample APCs in the secondary lymphoid organs. As a function of this activity, naive T cells express low levels of CD44 and high levels of the homing receptor CD62L, ensuring entry into LNs [1]. Upon activation at these sites, a series of events is initiated that dramatically alters the molecular repertoire of CD8 + T cells, enabling these cells to proliferate, migrate, and acquire effector functions [2]. Importantly, distinct features of CTL activation are obtained in different phases of the activation process and are not necessarily interdependent [3,4]. Thus, a brief DC-T-cell encounter is sufficient to upregulate the early activation markers CD44 and CD69, but longer stable contacts are needed to initiate IL-2 and IFN-γ secretion and the abundant expression of activation markers (including CD25, generating a high-affinity trimeric IL-2R , and IRF-1 −/− C57BL/6 mice bearing similarly sized EG7-OVA tumors. These MDSCs were added in various amounts to OVA-stimulated (250 μg/mL) OT-1 splenocytes and proliferation was measured after 42 h. The percentage suppression induced by various MDSC-SPC ratios is shown as individual replicates, measured via 3 H-thymidine incorporation. Each symbol represents an individual replicate, bars represent means, and data are pooled from two to six independent experiments. *p < 0.05, **p < 0.01, ***p < 0.001; ns, not significant, Mann-Whitney test.be reverted by the iNOS inhibitor L-NG-monomethyl arginine (L-NMMA) ( Fig. 2A(ii)), corroborating the existence of parallel IRF-1/iNOS-dependent and -independent suppressive pathways. This conclusion is strengthened by the partial reduction in suppressive capacity by WT MO-MDSCs upon L-NMMA addition ( Fig. 2A(ii)), and the fact that the NO-donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) could never decrease T-cell proliferation to the same extent as MO-MDSCs despite comparable NO concentrations in the culture ( Fig. 2A(i) and (ii)).Conversely, IFN-γR −/− , STAT-1 −/− , and IRF-1 −/− PMN-MDSCs displayed an NO-independent suppressive capacity, which was moderately, but significantly, lower than WT cells (Fig. 1B and 2B(ii)). Again, IFN-γ −/− PMN-MDSC-mediated suppression was not hampered (data not shown). The relatively minor importance of IFN-γ is not due to a lack of IFN-γ responsiveness, since IFN-γ treatment of PMN-MDSCs u...

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GvHD is an important complication of allogeneic hematopoietic SCT. Parent-in-F1 models are frequently used to study GvHD immunobiology; the characteristics of parent-in-F1 GvHD vary between strain combinations and induction protocols. Here, we observed that a high-dose challenge of non-irradiated B6DBA2F1 and B6SJLF1 recipients with C57BL/6 splenocytes left the majority of recipients clinically healthy, while inducing progressive high-grade donor T-cell chimerism. We investigated this previously undescribed pattern of parent-in-F1 T-cell alloreactivity and studied the effect of serial parental splenocyte infusions on epithelial and lymphohematopoietic tissues. The majority of recipients of 4 weekly splenocyte infusions showed long-term survival with gradual establishment of high-grade donor chimerism and without any signs of epithelial-tissue GvHD. A minority of recipients showed BM failure type of GvHD and, respectively, graft rejection. Moreover, long-term F1 chimeras showed protracted pancytopenia, and in peripheral lymphoid tissues severe lymphopenia and near-complete eradication of APCs and dysfunction in antigen-presenting capacity in remaining APC. Hematopoiesis and lymphoid tissue composition recovered only after multilineage donor chimerism had established. In conclusion, we report on a novel type of parent-in-F1 hybrid GvHD, where a cumulative high dose of C57BL/6 parental splenocytes in non-irradiated F1 mice induces subclinical but severe hematolymphoid-tissue GvHD, causing prolonged immuno-incompetence.

To date, myeloid-derived suppressor cells (MDSC) have been best studied in cancer, where they represent an escape mechanism for immune surveillance. MDSC are now also gaining interest in the context of transplantation. Suppressive CD11b þ myeloid progenitor cells have been reported to expand endogenously during BM chimerism induction in mice; in particular, in irradiated MHC-matched BM chimeras and in parent-in-F1 BM chimeras. Myeloid cell expansion coincided with a time frame where donor lymphocyte infusion (DLI) therapy-mediated GVL effects without GVHD. Hypothesizing that regulatory myeloid cells may have a role in regulating post-transplant T-cell alloreactivity, we performed a detailed phenotypic and functional characterization of these cells in the parent-in-F1 C57BL/6-[C57BL/6xDBA2] model. We found that transiently expanding CD11b þ myeloid progenitor cells comprise the two phenotypically and functionally distinct mononuclear and polymorphonuclear MDSC subsets that were recently described in tumor-bearing mice. Both MDSC subsets suppressed in vitro and in vivo alloreactive T-cell proliferation. Also, both the subsets mediated enhanced in vitro suppression when harvested from chimeras, given a prior in vivo challenge with non-tolerant donor T cells, indicating that allo-activated T cells can activate MDSC in vivo. This study provides the basis to investigate the---potentially beneficial---role of expanding MDSC in influencing the risk of GVHD during chimerism induction.

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