Embryonic Stem Cells Cultured in Serum-Free Medium Acquire Bovine Apolipoprotein B-100 from Feeder Cell Layers and Serum Replacement Medium

Fetal bovine serum-free culture of endothelial progenitor cells-progress and challenges

Two decades after the first report on endothelial progenitor cells (EPC), their key role in postnatal vasculogenesis and vascular repair is well established. The therapeutic potential of EPC and their growing use in clinical trials calls for the development of more robust, reproducible, and safer methods for the in vitro expansion and maintenance of these cells. Despite many limitations associated with its usage, fetal bovine serum (FBS) is still widely applied as a cell culture supplement. Although different approaches aiming at establishing FBS-free culture have been developed for many cell types, adequate solutions for endothelial cells, and for EPC in particular, are still scarce, possibly due to the multiple challenges that have to be faced when culturing these cells. In this review, we provide a brief overview on the therapeutic relevance of EPC and critically analyse the available literature on FBS-free endothelial cell culture methods, including xeno-free, serum-free, and chemically defined systems.

Technical note: The role of circulating low-density lipoprotein levels as a phenotypic marker for Holstein cholesterol deficiency in dairy cattle

With the recent discovery of a Holstein cholesterol deficiency (HCD) haplotype, the USDA has labeled many dairy animals as HCD carriers based on haplotype and pedigree analysis. We set out to investigate the effect of HCD status on various cholesterol transport molecules, namely low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol, and triglycerides in both males and females. A genome-wide association study was also conducted to narrow down the genomic region correlated with varying LDL-C levels. In the study, 34 HCD carrier animals showed significantly lower cholesterol and LDL-C levels compared with their 34 closely related, non-HCD controls. The genome-wide association study based on 73 animals using 56,198 SNP markers revealed an association with chromosome 11 in the region of 66,218,925 to 66,946,746 bp. We also tested the effect of HCD status on sperm quality traits using fresh ejaculates and frozen-thawed semen samples, but did not find any discriminating effects. Our study has demonstrated the use of LDL-C as a key phenotypic marker for determining HCD status in dairy cattle and this is the first study that clearly shows a cause-effect relationship of the HCD haplotype on circulating LDL-C.

No evidence for αGal epitope transfer from media containing FCS onto human endothelial cells in culture

BACKGROUND

Current clinical applications of cell therapies and tissue engineered (TE) constructs aim to generate non-immunogenic cells in the best-case scenario of autologous origin. As the cells are cultured, it is theoretically possible that immunoreactive molecules present in xenogenic cell culture media components, such as fetal calf serum (FCS), are transmitted in the culturing process. This problem has propelled the search for xeno-free culture media; however, in vitro culturing of many cell types, especially TE constructs which consist of several cell types, still relies to a great extent on FCS. In this study, we investigated the degree to which xenoantigens are transmitted to human endothelial cells (EC) cultured in medium containing FCS.

METHODS

Human EC were isolated from pulmonary artery fragments and atrial appendage tissue samples by enzymatic digestion followed by magnetic-activated cell separation (MACS) utilizing CD31 antibodies. The cells were cultured in EGM-2 medium containing 10% FCS for several passages. Griffonia Simplicifolia Lectin I - Isolectin B4 (GSL I-B4) was used to detect cell surface-bound αGal epitopes either microscopically or flow cytometrically. Antibody binding to cells exposed to human sera prepared from healthy blood donors was investigated to detect surface-located xenoantigens. An antibody-dependent cytotoxicity assay was conducted with heat-inactivated human serum supplemented with rabbit complement and analyzed by flow cytometry after staining for living and dead cells (LIVE/DEAD assay kit). In all experiments, cells cultured in EGM-2 supplemented with 10% human serum (HS) served as controls.

RESULTS

Human EC were isolated and cultured successfully for ≥6 passages. GSL I-B4 staining showed no difference between human EC cultured in FCS and in HS. In contrast to porcine EC which showed strong staining with GSL I-B4 and binding of preformed human serum antibodies, human EC cultured in FCS media did not bind human antibodies from high titer anti-αGal and anti-Neu5GC antibody serum. Along these lines, the antibody-dependent cytotoxicity assay showed that human EC cultures independent of FCS or HS usage were not affected, whereas about 40% of porcine EC did not survive.

CONCLUSION

Despite culturing cells in an environment containing xenoantigens, we were unable to demonstrate the translocation of xenogenic epitopes onto the surface of human EC or find an increased sensitivity in preformed human xenoantibody-dependent complement activity. Therefore, our results suggest that the use of human cells for TE or cell therapy grown in cell culture systems complemented with FCS does not necessarily lead to an acute rejection reaction upon implantation.

Glycosaminoglycan-binding hydrogels enable mechanical control of human pluripotent stem cell self-renewal

Reaping the promise of human embryonic stem (hES) cells hinges on effective defined culture conditions. Efforts to identify chemically defined environments for hES cell propagation would benefit from understanding the relevant functional properties of the substratum. Biological materials are often employed as substrata, but their complexity obscures a molecular level analysis of their relevant attributes. Because the properties of hydrogels can be tuned and altered systematically, these materials can reveal the impact of substratum features on cell fate decisions. By tailoring the peptide displayed to cells and the substrate mechanical properties, a hydrogel was generated that binds hES cell surface glycosaminoglycans (GAGs) and functions robustly in a defined culture medium to support long-term hES cell self-renewal. A key attribute of the successful GAG-binding hydrogels is their stiffness. Only stiff substrates maintain hES cell proliferation and pluripotency. These findings indicate that cells can respond to mechanical information transmitted via GAG engagement. Additionally, we found that the stiff matrices afforded activation of the paralogous proteins YAP/TAZ, which are transcriptional coactivators implicated in mechanosensing and hES cell pluripotency. These results indicate that the substratum mechanics can be tuned to activate specific pathways linked to pluripotency. Because several different hES and induced pluripotent stem cell lines respond similarly, we conclude that stiff substrata are more effective for the long-term propagation of human pluripotent stem cells.

An ECM-based culture system for the generation and maintenance of xeno-free human iPS cells

Pluripotent stem cells (PSCs) including induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs) have emerged as a promising source for treating incurable diseases. Problems that urgently need to be resolved before the clinical application include avoiding potential xenopathogenic transmission and immune rejection that may be caused by the exposure of PSCs to animal-derived products. In addition, an efficient feeder cell-free culture condition would be required for reducing batch-to-batch variation and facilitating scale-up. Therefore, establishing an efficient xeno-free and extracelluar matrix-based culture system is a prerequisite for the clinical application of PSCs. In this study, by blocking protein kinase C and histone deacetylase activities, we formulated a medium that, in combination with vitronectin as an extracellular matrix, not only allows the long-term culture of hESCs and iPSCs but also efficiently generates xeno-free iPSCs. This xeno-free and feeder cell-free culture system would facilitate the clinical applications of both iPSC- and ESC-based cell therapies in the future.

Culture conditions affect cardiac differentiation potential of human pluripotent stem cells

Human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), are capable of differentiating into any cell type in the human body and thus can be used in studies of early human development, as cell models for different diseases and eventually also in regenerative medicine applications. Since the first derivation of hESCs in 1998, a variety of culture conditions have been described for the undifferentiated growth of hPSCs. In this study, we cultured both hESCs and hiPSCs in three different culture conditions: on mouse embryonic fibroblast (MEF) and SNL feeder cell layers together with conventional stem cell culture medium containing knockout serum replacement and basic fibroblast growth factor (bFGF), as well as on a Matrigel matrix in mTeSR1 medium. hPSC lines were subjected to cardiac differentiation in mouse visceral endodermal-like (END-2) co-cultures and the cardiac differentiation efficiency was determined by counting both the beating areas and Troponin T positive cells, as well as studying the expression of OCT-3/4, mesodermal Brachyury T and NKX2.5 and endodermal SOX-17 at various time points during END-2 differentiation by q-RT-PCR analysis. The most efficient cardiac differentiation was observed with hPSCs cultured on MEF or SNL feeder cell layers in stem cell culture medium and the least efficient cardiac differentiation was observed on a Matrigel matrix in mTeSR1 medium. Further, hPSCs cultured on a Matrigel matrix in mTeSR1 medium were found to be more committed to neural lineage than hPSCs cultured on MEF or SNL feeder cell layers. In conclusion, culture conditions have a major impact on the propensity of the hPSCs to differentiate into a cardiac lineage.

Xeno-free plant-derived hydrolysate-based freezing of human embryonic stem cells

Human embryonic stem cells (hESCs) are one of the most interesting cell types for tissue engineering and cell therapy. The large-scale banking of hESCs for research and future clinical application requires economic, defined, and xeno-free cryopreservation protocols. In this study, the possibility to substitute knockout serum replacement (KO-SR) in the cryopreservation process with vegetal and synthetic hydrolysates was investigated. To our knowledge, the use of hydrolysates in hESC cryopreservation has not been yet explored. Initially, 3 different hydrolysates (Ultrapep Soy, Hypep 4601 and EX-CELL(®) CD Hydrolysate Fusion) were tested in the cryopreservation solution. A concentration of 8 mg/mL EX-CELL CD Hydrolysate Fusion in the cryopreservation solution leads to the highest recovery ratio; thus, this solution was selected for additional cryopreservation experiments. To ensure reproducibility of the results, 3 hESC lines (HS181, H9, and BG01) were used. The hESCs were collected prefreezing by application of collagenase IV and cell dissociation solution. Experiments showed that it was feasible to substitute the KO-SR in both the cryopreservation solution as the thawing solution. The obtained recovery ratios were comparable to those obtained with KO-SR (no statistical significant difference; Student's t-test, P<0.05). Further optimization protocols showed a doubling of the obtained recovery ratio after addition of Rock-inhibitor Y-27632 post-thawing. The expansion profile and pluripotency analysis revealed no changes in normal hESC behavior. We conclude that the application of vegetal or synthetic hydrolysates is suitable for xeno-free hESC cryopreservation.

An Effective and Safe Supplement for Stem Cells Expansion Ex Vivo: Cord Blood Serum

Mesenchymal stem cells (MSCs) are potential and optimal stem cells in clinical cell therapy, and fetal bovine serum (FBS) is widely used for expansion of MSCs, despite the risks of infectious disease transmission and immunological reaction of the xenogenic origin. This study was designed to compare human four blood group cord blood serum (CBS) with FBS in culturing human placenta-derived mesenchymal stem cells (hPDMSCs), which were derived from four blood group donors. The expansion medium included: 10% FBS, 10% A-CBS, 10% B-CBS, 10% O-CBS, and 10% AB-CBS. Cumulative population doubling, generation time, fold expansion rates and differentiation capacity, cell cycle, and immunophenotype were also assessed. The results showed that fold expansion rate and cumulative population doubling of hPDMSCs significantly increased during long-term MSC expansion in CBS medium, but the generation time decreased compared with FBS. CBS might be an effective supplement for stem cells expand rapidly ex vivo. Cell cycle and differentiation assays showed that most of the hPDMSCs expanding in the presence of CBS were in stationary phase, which was the characteristic of stem cells, and they retained their ability to differentiate into chondrogenic and endothelial cells. By comparing these four blood groups of CBS, we found that there was no significant difference among different blood groups in culturing hPDMSCs, which were isolated from different blood group donors. So CBS may be an optimal replacement to avoid the risks of FBS application in expansion of stem cell for clinical cell therapy and tissue engineering.

Challenges for the Therapeutic use of Pluripotent Stem Derived Cells

Human embryonic stem cells (hESC) and induced pluripotent stem cells (hiPSC) are an attractive cell source for regenerative medicine. These cells can be expanded to vast numbers and can be differentiated to many desired pluripotent stem cells (PSC) derived therapeutic cells. Cell replacement bears promises, but also challenges. The introduction of exogenous cells in a recipient must address several different topics; its safety, the exclusion of tumor formation, the immunological response and possible rejection, the cells cleanliness and their biological quality, and quantity representing the functionality of the PSC derived therapeutic cells. Tumor formation requires the removal of any PSC remaining after differentiation. Immunological rejection can be addressed with immunomodulation of the cells and the recipient. Cleanliness can be optimized using good manufacturing practice quality systems. At last, the functionality of the cells must be tested in in vitro and in animal models. After addressing these challenges, precise strategies are developed to monitor the status of the cells at different times and in case of undesired results, corresponding counteracting strategies must exist before any clinical attempt.

Derivation of multipotent nestin(+)/CD271 (-)/STRO-1 (-) mesenchymal-like precursors from human embryonic stem cells in chemically defined conditions

The successful establishment of stem cell-based therapies requires multipotent, immunocompatible stem cells, highly efficient strategies for direct differentiation, and most importantly, optimal culture conditions for large-scale expansion of such cell populations. Other than adult tissues, human embryonic stem cells (hESCs) represent another infinitely expansible source for mesenchymal stem cell (MSC) derivation. Here, we reproducibly derived a population of Nestin(+)/CD271(-)/STRO-1(-) mesenchymal-like precursors from hESCs (hESC-MPs) in chemically defined conditions, without requiring any serum or serum replacement of animal origin, based on a Y-27632-assisted monolayer culture system. These cells showed slim fibroblastic morphology, and satisfied the criteria of MSCs including self-renewal, the expression of multiple MSC-specific markers and the ability to differentiate into osteoblasts, adipocytes and chondrocytes. Compared with previously reported hESC-derived MSCs, our hESC-MPs were more multipotent, and could differentiate into representative derivatives of all three embryonic germ layers including mature smooth muscle cells, cardiomyocytes, functional hepatocytes and neural cells expressing various neurotransmitter phenotypes, making them an attractive cell source for regenerative medicine.

Large-scale production of murine embryonic stem cell-derived osteoblasts and chondrocytes on microcarriers in serum-free media

The generation of tissue-engineered constructs from stem cells for the treatment of musculoskeletal diseases may have immense impact in regenerative medicine, but there are difficulties associated with stem cell culture and differentiation, including the use of serum. Here we present serum-free protocols for the successful production of murine embryonic stem cell (mESC) derived osteoblasts and chondrocytes on CultiSpher S macroporous microcarriers in stirred suspension bioreactors. Various inoculum forms and agitation rates were investigated. Produced osteogenic cells were implanted ectopically into SCID mice and orthotopically into a murine burr-hole fracture model. Osterix, osteocalcin and collagen type I were upregulated in osteogenic cultures, while aggrecan and collagen type II were upregulated in chondrogenic cultures. Histological analysis using alizarin red S, von Kossa and alcian blue staining confirmed the presence of osteoblasts and chondrocytes, respectively in cultured microcarriers and excised tissue. Finally, implantation of derived cells into a mouse fracture model revealed cellular integration without any tumor formation. Overall, microcarriers may provide a supportive scaffold for ESC expansion and differentiation in a serum-free bioprocess for in vivo implantation. These findings lay the groundwork for the development of clinical therapies for musculoskeletal injuries and diseases using hESCs and iPS cells.

Apolipoprotein B binding domains: evidence that they are cell-penetrating peptides that efficiently deliver antigenic peptide for cross-presentation of cytotoxic T cells

Low-density lipoproteins (LDLs) are a good source of cholesterol, which is important in cellular homeostasis and production of steroids. Apolipoprotein B-100 (ApoB-100), the sole protein component of LDL, is known to bind to cell surface LDL receptor (LDLR) or cell surface-bound proteoglycans and to be internalized into cells. We found that APCs, consisting of macrophages and dendritic cells, upregulate LDLR on culture in vitro without obvious stimulation. In contrast, T cell populations only upregulate LDLR on activation. Thus, we strategized that tagging immunogens to ApoB-100 might be a useful means to target Ag to APCs. We generated fusion proteins consisting of receptor binding sites in ApoB-100, coupled to OVA peptide (ApoB-OVA), as Ag delivery vehicles and demonstrated that this novel delivery method successfully cross-presented OVA peptides in eliciting CTL responses. Surprisingly, internalization of ApoB-OVA peptide occurred via cell surface proteoglycans rather than LDLRs, consistent with evidence that structural elements of ApoB-100 indicate it to have cell-penetrating peptide properties. Finally, we used this strategy to assess therapeutic vaccination in a tumor setting. OVA-expressing EL-4 tumors grew progressively in mice immunized with ApoB-100 alone but regressed in mice immunized with ApoB-OVA fusion protein, coinciding with development of OVA-specific CTLs. Thus, to our knowledge, this is the first article to describe the cell-penetrating properties of a conserved human origin cell penetrating peptide that may be harnessed as a novel vaccination strategy as well as a therapeutics delivery device.

Human-induced pluripotent stem cells produced under xeno-free conditions

Induced pluripotent stem cells (iPSCs) have radically advanced the field of regenerative medicine by making possible the production of patient-specific pluripotent stem cells from adult individuals. While cell differentiation protocols have been successfully developed, and animal models of human disease have proved that these cells have the potential to treat human diseases and conditions produced as a consequence of aging, degeneration, injury, and birth defects, logistical issues still remain unsolved and hamper the possibility of testing these cells in human clinical trials. Among them is the widely spread use of animal products for the generation and culture of iPSCs. We report here a xeno-free iPSC generation system that addresses all the steps of iPSCs production including the isolation and culture of adult skin fibroblasts, and iPSCs generation, expansion, and maintenance. iPSCs generated with a polycistronic lentiviral vector under xeno-free conditions displayed markers of pluripotency and gave rise to embryoid bodies (EBs) displaying indicators of the 3 primary germ layers. Xeno-free iPSCs injected into nude mice produced classic teratomas, and teratoma explants cultured under conditions favoring fibroblastic cells gave rise to cells morphologically indistinguishable from input cells. Protocols here described will facilitate the implementation of new cellular therapies for preclinical and clinical studies, potentially reducing the regulatory burden without compromising the differentiation potential of the cells.

Defined and serum-free media support undifferentiated human embryonic stem cell growth

Four commercially available serum-free and defined culture media tested on 2 human embryonic stem cell (hESC) lines were all found to support undifferentiated growth for >10 continuous passages. For hESC cultured with defined StemPro and mTeSR1 media, the cells were maintained feeder-free on culture dishes coated with extracellular matrices (ECMs) with no requirement of feeder-conditioned media (CM). For xeno-free serum replacer (XSR), HEScGRO, and KnockOut media, mitotically inactivated human foreskin feeders (hFFs) were required for hESC growth. Under the different media conditions, cells continued to exhibit alkaline phosphatase activity and expressed undifferentiated hESC markers Oct-4, stage-specific embryonic antigens 4 (SSEA-4), and Tra-1-60. In addition, hESC maintained the expression of podocalyxin-like protein-1 (PODXL), an antigen recently reported in another study to be present in undifferentiated hESC. The cytotoxic antibody mAb 84 binds via PODXL expressed on hESC surface and kills >90% of hESC within 45 min of incubation. When these cells were spontaneously differentiated to form embryoid bodies, derivatives representing the 3 germ layers were obtained. Injection of hESC into animal models resulted in teratomas and the formation of tissue types indicative of ectodermal, endodermal, and mesodermal lineages were observed. Our data also suggested that StemPro and mTeSR1 media were more optimal for hESC proliferation compared to cells grown on CM because the growth rate of hESC increased by 30%-40%, higher split ratio was thus required for weekly passaging. This is advantageous for the large-scale cultivation of hESC required in clinical applications.

Twisting immune responses for allogeneic stem cell therapy

Stem cell-derived tissues and organs have the potential to change modern clinical science. However, rejection of allogeneic grafts by the host's immune system is an issue which needs to be addressed before embryonic stem cell-derived cells or tissues can be used as medicines. Mismatches in human leukocyte class I antigens and minor histocompatibility antigens are the central factors that are responsible for various graft-versus-host diseases. Traditional strategies usually involve suppressing the whole immune systems with drugs. There are many side effects associated with these methods. Here, we discuss an emerging strategy for manipulating the central immune tolerance by naturally "introducing" donor antigens to a host so a recipient can acquire tolerance specifically to the donor cells or tissues. This strategy has two distinct stages. The first stage restores the thymic function of adult patients with sex steroid inhibitory drugs (LHRH-A), keratinocyte growth factor (KGF), interleukin 7 (IL-7) and FMS-like tyrosine kinase 3 (FLT3). The second stage introduces hematopoietic stem cells and their downstream progenitors to the restored thymus by direct injection. Hematopoietic stem cells are used to introduce donor antigens because they have priority access to the thymus. We also review several clinical cases to explain this new strategy.

Proteomic profiling of human embryonic stem cell-derived microvesicles reveals a risk of transfer of proteins of bovine and mouse origin

BACKGROUND AIMS

Microvesicles (MV) shed from the plasma membrane of eukaryotic cells, including human embryonic stem cells (hESC), contain proteins, lipids and RNA and serve as mediators of cell-to-cell communication. However, they may also contain immunogenic membrane domains and infectious particles acquired from xenogenic components of the culture milieu. Therefore, MV represent a potential risk for clinical application of cell therapy.

METHODS

We tested the ability of hESC and their most commonly used feeder cells, mouse embryonic fibroblasts (MEF), to produce MV. We found that hESC are potent producers of MV, whereas mitotically inactivated MEF do not produce any detectable MV. We therefore employed a combined proteomic approach to identify the molecules that constitute the major components of MV from hESC maintained in a standard culture setting with xenogenic feeder cells.

RESULTS

In purified MV fractions, we identified a total of 22 proteins, including five unique protein species that are known to be highly expressed in invasive cancers and participate in cellular activation, metastasis and inhibition of apoptosis. Moreover, we found that hESC-derived MV contained the immunogenic agents apolipoprotein and transferrin, a source of Neu5Gc, as well as mouse retroviral Gag protein.

CONCLUSIONS

These findings indicate that MV represent a mechanism by which hESC communicate; however, they also serve as potential carriers of immunogenic and pathogenic compounds acquired from environment. Our results highlight a potential danger regarding the use of hESC that have previously been exposed to animal proteins and cells.

Analysis of the distinct functions of growth factors and tissue culture substrates necessary for the long-term self-renewal of human embryonic stem cell lines

The role of individual supplements necessary for the self-renewal of human embryonic stem (hES) cells is poorly characterized, and furthermore we have found that previously reported feeder cell- and serum-free culture systems used for individual hES cell lines are unable to maintain HUES7 cells for more than one passage. We have therefore derived a feeder/serum-free culture system that can support the long-term (at least 10 passages) self-renewal of several euploid hES cell lines including MAN1, HUES7, and HUES1 with minimal spontaneous differentiation and without the need for manual propagation. This system contains fibroblast growth factor 2, activin A, neurotrophin 4, and the N2, B27 supplements together with a human fibronectin substrate. We demonstrate that these components exert distinct functions: both FGF2 and activin A were necessary to prevent differentiation of hES cells while NT4 promoted cell survival, FGF2 could not be substituted by IGFII, and the fibronectin substrate supported a rapid rate of hES culture expansion. Inhibition studies showed that β1 integrin-dependent attachment of hES cells to fibronectin was at least partially via the α5 subunit but independent of integrin αV.

Large scale production of stem cells and their derivatives

Stem cells have been envisioned to become an unlimited cell source for regenerative medicine. Notably, the interest in stem cells lies beyond direct therapeutic applications. They might also provide a previously unavailable source of valuable human cell types for screening platforms, which might facilitate the development of more efficient and safer drugs. The heterogeneity of stem cell types as well as the numerous areas of application suggests that differential processes are mandatory for their in vitro culture. Many of the envisioned applications would require the production of a high number of stem cells and their derivatives in scalable, well-defined and potentially clinical compliant manner under current good manufacturing practice (cGMP). In this review we provide an overview on recent strategies to develop bioprocesses for the expansion, differentiation and enrichment of stem cells and their progenies, presenting examples for adult and embryonic stem cells alike.