The Programmable Cell of Monocytic Origin (PCMO): A Potential Adult Stem/Progenitor Cell Source for the Generation of Islet Cells

<i>In vitro</i> production of myeloid-derived suppressor cells from peripheral blood monocytes

Myeloid-derived suppressor cells (MDSCs) are of interest as key regulators of the immune response for the development and improvement of cellular technologies in biomedicine. Enhancing the suppressive activity of these cells is important for developing therapies for autoimmune diseases and miscarriages, and their suppression may be useful in the treatment of cancer, since MDSCs are known to suppress antitumor immunity. However, there is a problem that prevents the active study of MDSCs, i.e., the difficulty in obtaining sufficient numbers of this cell population. Isolation of MDSCs in cancer patients poses an ethical challenge. Moreover, these MDSC may differ in subpopulation composition and suppressive activity due to individual factors. Researchers who generate human MDSC from bone marrow cells may also face similar problems. Therefore, finding a reliable and affordable source of these cells to facilitate the study of their functions is extremely important. Attempts to obtain human MDSCs in vitro have been ongoing for a long time. GM- CSF, IL-6, IL- 1β, IL-4, PGE2, LPS, M-CSF, IFNγ are described as factors that induce the ex vivo MDSC differentiation. However, despite multiple factors used, not all protocols are clearly reproducible, leading to generation of a sufficient number of cells in the target population. Previously, we had also developed a scheme for MDSC differentiation from CD11b+ cells derived from human peripheral blood, which made it possible to obtain a tangible but still insufficient percentage of cells to study functional activity. To increase the number of MDSCs in cultures, we developed a protocol aimed for differentiation of these cells from peripheral blood monocytes (CD14+ cells) previously transformed into PCMO (programmed cells of monocytic origin). The monocytes isolated by immunomagnetic separation were cultured in a de-differentiating medium (complete culture medium supplemented with M-CSF, IL-3 and β-mercaptoethanol) for one week. Later on, the medium was replaced by the addition of GM-CSF, being cultured for three days, followed by addition of LPS and IL-1β in order to induce suppressive activity. We have found that culturing CD14+ cells on a two-week schedule with prior creation of dedifferentiation conditions resulted in a slightly decreased percentage of viable cells in culture. However, there was a trend towards an increased ratio of MDSCs in culture (from an average of 34 to 40%) and an increase in their suppressive activity (arginase and IDO expression). The percentage of Arg+ cells increased by average of 10%, and IDO+ cells, by 16%. Moreover, the percentage of mature M-MDSCs was significantly (several-fold) higher when compared with differentiation protocol using CD11b+ cells. Hence, this method of MDSCs production enables us to increase the number of cells belonging to the conditionally “mature” monocyte subpopulation of MDSCs, as well as the percentage of functional suppressor cells in the population. The described scheme may be used to improve the quality of studies aimed at modulating MDSC functions in order to develop new therapeutic approaches.

Peripheral Blood Monocytes as Adult Stem Cells: Molecular Characterization and Improvements in Culture Conditions to Enhance Stem Cell Features and Proliferative Potential

Adult stem or programmable cells hold great promise in diseases in which damaged or nonfunctional cells need to be replaced. We have recently demonstrated that peripheral blood monocytes can be differentiated in vitro into cells resembling specialized cell types like hepatocytes and pancreatic beta cells. During phenotypic conversion, the monocytes downregulate monocyte/macrophage differentiation markers, being indicative of partial dedifferentiation, and are partially reprogrammed to acquire a state of plasticity along with expression of various markers of pluripotency and resumption of mitosis. Upregulation of stem cell markers and mitotic activity in the cultures was shown to be controlled by autocrine production/secretion of activin A and transforming growth factor-beta (TGF-β). These reprogrammed monocyte derivatives were termed "programmable cells of monocytic origin" (PCMO). Current efforts focus on establishing culture conditions that increase both the plasticity and proliferation potential of PCMO in order to be able to generate large amounts of blood-derived cells suitable for both autologous and allogeneic therapies.

Phenotypic and functional comparison of two distinct subsets of programmable cell of monocytic origin (PCMOs)-derived dendritic cells with conventional monocyte-derived dendritic cells

Dendritic cells (DCs) are professional antigen-presenting cells with the ability to induce primary T-cell responses. They are commonly produced by culturing monocytes in the presence of IL-4 and GM-CSF (cells produced in this manner are called conventional DCs). Here we report the generation of two functionally distinct subsets of DCs derived from programmable cells of monocytic origin (PCMOs) in the presence of IL-3 or tumor necrosis factor alpha (TNF-α). Monocytes were treated with macrophage colony-stimulating factor (M-CSF) and IL-3 for 6 days and then incubated with IL-4 and IL-3 (for IL-3 DCs) or with IL-4, GM-CSF and TNF-α (for TNF-α DCs) for 7 days. Monocytes were then loaded with tumor lysate (used as antigen), and poly (I∶C) was added. The maturation factors TNF-α and monocyte conditioned medium (MCM) were added on days 4 and 5, respectively. The phenotypes of the DCs generated were characterized by flow cytometry, and the cells' phagocytic activities were measured using FITC-conjugated latex bead uptake. T-cell proliferation and cytokine release were assayed using MTT and commercially available ELISA kits, respectively. We found that either IL-3DCs or TNF-α DCs induce T-cell proliferation and cytokine secretion; the cytokine release pattern showed reduced IL-12/IL-10 and IFN-γ/IL-4 ratios in both types of DCs and in DC-primed T-cell supernatant, respectively, which confirmed that the primed T cells were polarized toward aTh2-type immune response. We concluded that PCMOs are a new cell source that can develop into two functionally distinct DCs that both induce a Th2-type response in vitro. This modality can be used as a DC-based immunotherapy for autoimmune diseases.

Co-cultures of programmable cells of monocytic origin and mesenchymal stem cells do increase osteogenic differentiation

Impaired bone healing can occur with numerous pathologic conditions like trauma, osteoporosis, and infection. Therefore tissue-engineering strategies that aim to enhance osteogenic differentiation of stem cells in order to accelerate bone healing are a major goal of contemporary regenerative research. In this study we cultivated mesenchymal stem cells (MSC) together with the recently patented programmable cells of monocytic origin (PCMO) to test whether co-cultures promote an osteogenic differentiation process. PCMO have recently been shown to have pluripotent characteristics and do support the regeneration processes of liver and heart diseases. Quantitative real time PCR expression profiles of osteogenic marker genes such as alkaline phosphatase in co-cultures of PCMO and MSC showed that MSC differentiated into osteoblast-like cells more rapidly as compared to mono-cultures. Alkaline phosphatase expression and enzyme activity levels were highly increased in co-cultures compared to mono-cultures of MSC. Tests for mineralized matrix formation also indicated that PCMO have a positive effect on co-cultured MSC under osteogenic culture conditions. However, analysis of collagen 1A did not show enhanced expression. In summary, PCMO obviously have the ability to promote osteogenic differentiation of MSC in vitro while their own pluripotent potential is not sufficient to develop osteoblast-like characteristics themselves.

Characterization of myelomonocytoid progenitor cells with mesenchymal differentiation potential obtained by outgrowth from pancreas explants

Progenitor cells can be obtained by outgrowth from tissue explants during primary ex vivo tissue culture. We have isolated and characterized cells outgrown from neonatal mouse pancreatic explants. A relatively uniform population of cells showing a distinctive morphology emerged over time in culture. This population expressed monocyte/macrophage and hematopoietic markers (CD11b(+) and CD45(+)), and some stromal-related markers (CD44(+) and CD29(+)), but not mesenchymal stem cell (MSC)-defining markers (CD90(-) and CD105(-)) nor endothelial (CD31(-)) or stem cell-associated markers (CD133(-) and stem cell antigen-1; Sca-1(-)). Cells could be maintained in culture as a plastic-adherent monolayer in culture medium (MesenCult MSC) for more than 1 year. Cells spontaneously formed sphere clusters "pancreatospheres" which, however, were nonclonal. When cultured in appropriate media, cells differentiated into multiple mesenchymal lineages (fat, cartilage, and bone). Positive dithizone staining suggested that a subset of cells differentiated into insulin-producing cells. However, further studies are needed to characterize the endocrine potential of these cells. These findings indicate that a myelomonocytoid population from pancreatic explant outgrowths has mesenchymal differentiation potential. These results are in line with recent data onmonocyte-derivedmesenchymal progenitors (MOMPs).

microRNA-based cancer cell reprogramming technology

Epigenetic modifications play crucial roles in cancer initiation and development. Complete reprogramming can be achieved through the introduction of defined biological factors such as Oct4, Sox2, Klf4, and cMyc into mouse and human fibroblasts. Introduction of these transcription factors resulted in the modification of malignant phenotype behavior. Recent studies have shown that human and mouse somatic cells can be reprogrammed to become induced pluripotent stem cells using forced expression of microRNAs, which completely eliminates the need for ectopic protein expression. Considering the usefulness of RNA molecules, microRNA-based reprogramming technology may have future applications in regenerative and cancer medicine.