Guidance on the assessment of biocompatibility of biomaterials: Fundamentals and testing considerations

BACKGROUND

Assessing the biocompatibility of materials is crucial for ensuring the safety and well-being of patients by preventing undesirable, toxic, immune, or allergic reactions, and ensuring that materials remain functional over time without triggering adverse reactions. To ensure a comprehensive assessment, planning tests that carefully consider the intended application and potential exposure scenarios for selecting relevant assays, cell types, and testing parameters is essential. Moreover, characterizing the composition and properties of biomaterials allows for a more accurate understanding of test outcomes and the identification of factors contributing to cytotoxicity. Precise reporting of methodology and results facilitates research reproducibility and understanding of the findings by the scientific community, regulatory agencies, healthcare providers, and the general public.

AIMS

This article aims to provide an overview of the key concepts associated with evaluating the biocompatibility of biomaterials while also offering practical guidance on cellular principles, testing methodologies, and biological assays that can support in the planning, execution, and reporting of biocompatibility testing.

Culture Models to Investigate Mechanisms of Milk Production and Blood-Milk Barrier in Mammary Epithelial Cells: a Review and a Protocol

Mammary epithelial cells (MECs) are the only cell type that produces milk during lactation. MECs also form less-permeable tight junctions (TJs) to prevent the leakage of milk and blood components through the paracellular pathway (blood-milk barrier). Multiple factors that include hormones, cytokines, nutrition, and temperature regulate milk production and TJ formation in MECs. Multiple intracellular signaling pathways that positively and negatively regulate milk production and TJ formation have been reported. However, their regulatory mechanisms have not been fully elucidated. In addition, unidentified components that regulate milk production in MECs likely exist in foods, for example plants. Culture models of functional MECs that recapitulate milk production and TJs are useful tools for their study. Such models enable the elimination of indirect effects via cells other than MECs and allows for more detailed experimental conditions. However, culture models of MECs with inappropriate functionality may result in unphysiological reactions that never occur in lactating mammary glands in vivo. Here, I briefly review the physiological functions of alveolar MECs during lactation in vivo and culture models of MECs that feature milk production and less-permeable TJs, together with a protocol for establishment of MEC culture with functional TJ barrier and milk production capability using cell culture inserts.

Harmonization and standardization of nucleus pulposus cell extraction and culture methods

Background

In vitro studies using nucleus pulposus (NP) cells are commonly used to investigate disc cell biology and pathogenesis, or to aid in the development of new therapies. However, lab-to-lab variability jeopardizes the much-needed progress in the field. Here, an international group of spine scientists collaborated to standardize extraction and expansion techniques for NP cells to reduce variability, improve comparability between labs and improve utilization of funding and resources.

Methods

The most commonly applied methods for NP cell extraction, expansion, and re-differentiation were identified using a questionnaire to research groups worldwide. NP cell extraction methods from rat, rabbit, pig, dog, cow, and human NP tissue were experimentally assessed. Expansion and re-differentiation media and techniques were also investigated.

Results

Recommended protocols are provided for extraction, expansion, and re-differentiation of NP cells from common species utilized for NP cell culture.

Conclusions

This international, multilab and multispecies study identified cell extraction methods for greater cell yield and fewer gene expression changes by applying species-specific pronase usage, 60-100 U/ml collagenase for shorter durations. Recommendations for NP cell expansion, passage number, and many factors driving successful cell culture in different species are also addressed to support harmonization, rigor, and cross-lab comparisons on NP cells worldwide.

CYSRT1: an antimicrobial epidermal protein that can interact with late cornified envelope (LCE) proteins

Late cornified envelope (LCE) proteins are small cationic epidermal proteins with antimicrobial properties, and the combined deletion of LCE3B and LCE3C genes is a risk factor for psoriasis that affects skin microbiome composition. In a yeast two-hybrid screen we identified cysteine-rich tail 1 protein (CYSRT1) as an interacting partner of members of all LCE groups except LCE6. These interactions were confirmed in a mammalian cell system by co-immunoprecipitation. CYSRT1 is a protein of unknown function that is specifically expressed in cutaneous and oral epithelia and spatially colocalizes with LCE proteins in the upper layers of the suprabasal epidermis. Constitutive CYSRT1 expression is present in fully differentiated epidermis and can be further induced in vivo by disruption of the skin barrier upon stratum corneum removal. Transcriptional regulation correlates to keratinocyte terminal differentiation but not to skin bacteria exposure. Similar to LCEs, CYSRT1 was found to have antibacterial activity against Pseudomonas aeruginosa. Comparative gene sequence analysis and protein amino acid alignment indicates that CYSRT1 is highly conserved among vertebrates and has putative antimicrobial activity. To summarize, we identified CYSRT1 in the outer skin layer, where it colocalizes with LCE proteins and contributes to the constitutive epidermal antimicrobial host defense repertoire.

Stable reconstructed human gingiva–microbe interaction model: Differential response to commensals and pathogens

Background

To investigate human oral health and disease, models are required which represent the interactions between the oral mucosa and microbiome. Our aim was to develop an organotypic model which maintains viability of both host and microbes for an extended period of time.

Methods

Reconstructed Human Gingiva (RHG) were cultured air-lifted with or without penicillin-streptomycin (PS) and topically exposed to Streptococcus gordonii (commensal) or Aggregatibacter actinomycetemcomitans (pathogen) for 72 hours in agar. RHG histology, viability and cytokines (ELISA), and bacterial viability (colony forming units) and location (FISH) were assessed.

Results

The low concentration of topically applied agar did not influence RHG viability. Topically applied bacteria in agar remained localized and viable for 72 hours and did not spill over to infect RHG culture medium. PS in RHG culture medium killed topically applied bacteria. Co-culture with living bacteria did not influence RHG viability (Ki67 expression, MTT assay) or histology (epithelium differentiation, Keratin10 expression). RHG exposed to S. gordonii (with or without PS) did not influence low level of IL-6, IL-8, CCL2, CCL5, CCL20 or CXCL1 secretion. However, all cytokines increased (except CCL2) when RHG were co-cultured with A. actinomycetemcomitans. The effect was significantly more in the presence of living, rather than dead, A. actinomycetemcomitans. Both bacteria resulted in increased expression of RHG antimicrobial peptides (AMPs) Elafin and HBD-2, with S. gordonii exposure resulting in the most Elafin secretion.

Conclusion

This technical advance enables living human oral host–microbe interactions to be investigated during a 72-hour period and shows differences in innate immunology triggered by S. gordonii and A. actinomycetemcomitans.

Generating iPSCs with a High-Efficient, Non-Invasive Method-An Improved Way to Cultivate Keratinocytes from Plucked Hair for Reprogramming

Various somatic cell types are suitable for induced pluripotency reprogramming, such as dermal fibroblasts, mesenchymal stem cells or hair keratinocytes. Harvesting primary epithelial keratinocytes from plucked human hair follicles (HFs) represents an easy and non-invasive alternative to a fibroblast culture from invasive skin biopsies. Nevertheless, to facilitate and simplify the process, which can be divided into three main steps (collecting, culturing and reprogramming), the whole procedure of generating hair keratinocytes has to be revised and upgraded continuously. In this study, we address advancements and approaches which improve the generation and handling of primary HF-derived keratinocytes tremendously, e.g., for iPSCs reprogramming. We not only evaluated different serum- and animal-origin-free media, but also supplements and coating solutions for an enhanced protocol. Here, we demonstrate the importance of speed and accuracy in the collecting step, as well as the choice of the right transportation medium. Our results lead to a more defined approach that further increases the reliability of downstream experiments and inter-laboratory reproducibility. These improvements will make it possible to obtain keratinocytes from plucked human hair for the generation of donor-specific iPSCs easier and more efficient than ever before, whilst preserving a non-invasive capability.

Carboxamide Derivatives Are Potential Therapeutic AHR Ligands for Restoring IL-4 Mediated Repression of Epidermal Differentiation Proteins

Atopic dermatitis (AD) is a common T-helper 2 (Th2) lymphocyte-mediated chronic inflammatory skin disease characterized by disturbed epidermal differentiation (e.g., filaggrin (FLG) expression) and diminished skin barrier function. Therapeutics targeting the aryl hydrocarbon receptor (AHR), such as coal tar and tapinarof, are effective in AD, yet new receptor ligands with improved potency or bioavailability are in demand to expand the AHR-targeting therapeutic arsenal. We found that carboxamide derivatives from laquinimod, tasquinimod, and roquinimex can activate AHR signaling at low nanomolar concentrations. Tasquinimod derivative (IMA-06504) and its prodrug (IMA-07101) provided full agonist activity and were most effective to induce FLG and other epidermal differentiation proteins, and counteracted IL-4 mediated repression of terminal differentiation. Partial agonist activity by other derivatives was less efficacious. The previously reported beneficial safety profile of these novel small molecules, and the herein reported therapeutic potential of specific carboxamide derivatives, provides a solid rationale for further preclinical assertation.

Evaluation of native and non‐native biomaterials for engineering human skin tissue

A variety of human skin models have been developed for applications in regenerative medicine and efficacy studies. Typically, these employ matrix molecules that are derived from non‐human sources along with human cells. Key limitations of such models include a lack of cellular and tissue microenvironment that is representative of human physiology for efficacy studies, as well as the potential for adverse immune responses to animal products for regenerative medicine applications. The use of recombinant extracellular matrix proteins to fabricate tissues can overcome these limitations. We evaluated animal‐ and non‐animal‐derived scaffold proteins and glycosaminoglycans for the design of biomaterials for skin reconstruction in vitro. Screening of proteins from the dermal‐epidermal junction (collagen IV, laminin 5, and fibronectin) demonstrated that certain protein combinations when used as substrates increase the proliferation and migration of keratinocytes compared to the control (no protein). In the investigation of the effect of components from the dermal layer (collagen types I and III, elastin, hyaluronic acid, and dermatan sulfate), the primary influence on the viability of fibroblasts was attributed to the source of type I collagen (rat tail, human, or bovine) used as scaffold. Furthermore, incorporation of dermatan sulfate in the dermal layer led to a reduction in the contraction of tissues compared to the control where the dermal scaffold was composed primarily of collagen type I. This work highlights the influence of the composition of biomaterials on the development of complex reconstructed skin models that are suitable for clinical translation and in vitro safety assessment.

COVID-19 Pandemic Is a Call to Search for Alternative Protein Sources as Food and Feed: A Review of Possibilities

The coronavirus disease 2019 (COVID-19) pandemic is a global health challenge with substantial adverse effects on the world economy. It is beyond any doubt that it is, again, a call-to-action to minimize the risk of future zoonoses caused by emerging human pathogens. The primary response to contain zoonotic diseases is to call for more strict regulations on wildlife trade and hunting. This is because the origins of coronaviruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), SARS-CoV, Middle East respiratory syndrome coronavirus (MERS-CoV), as well as other viral pathogens (e.g., Ebola, HIV) are traceable to wild animals. Although COVID-19 is not related to livestock animals, the pandemic increased general attention given to zoonotic viral infections—the risk of which can also be associated with livestock. Therefore, this paper discusses the potential transformation of industrial livestock farming and the production of animal products, particularly meat, to decrease the risks for transmission of novel human pathogens. Plant-based diets have a number of advantages, but it is unrealistic to consider them as the only solution offered to the problem. Therefore, a search for alternative protein sources in insect-based foods and cultured meat, important technologies enabling safer meat production. Although both of these strategies offer a number of potential advantages, they are also subject to the number of challenges that are discussed in this paper. Importantly, insect-based foods and cultured meat can provide additional benefits in the context of ecological footprint, an aspect important in light of predicted climate changes. Furthermore, cultured meat can be regarded as ethically superior and supports better food security. There is a need to further support the implementation and expansion of all three approaches discussed in this paper, plant-based diets, insect-based foods, and cultured meat, to decrease the epidemiological risks and ensure a sustainable future. Furthermore, cultured meat also offers a number of additional benefits in the context of environmental impact, ethical issues, and food security.

From Hair to iPSCs-A Guide on How to Reprogram Keratinocytes and Why

Keratinocytes, as a primary somatic cell source, offer exceptional advantages compared to fibroblasts, which are commonly used for reprogramming. Keratinocytes can beat fibroblasts in reprogramming efficiency and reprogramming time and, in addition, can be easily and non-invasively harvested from human hair roots. However, there is still much to know about acquiring keratinocytes and maintaining them in cell culture. In this article, we want to offer readers the profound knowledge that we have gained since our initial use of keratinocytes for reprogramming more than 10 years ago. Here, all hints and tricks, from plucking the hair roots to growing and maintaining keratinocytes, are described in detail. Additionally, an overview of the currently used reprogramming methods, viral and non-viral, is included, with a special focus on their applicability to keratinocytes. This overview is intended to provide a brief but comprehensive insight into the field of keratinocytes and their use for reprogramming into induced pluripotent stem cells (iPSCs). © 2020 The Authors.

Isolation and maintenance of Batrachochytrium salamandrivorans cultures

Discovered in 2013, the chytrid fungus Batrachochytrium salamandrivorans (Bsal) is an emerging amphibian pathogen that causes ulcerative skin lesions and multifocal erosion. A closely related pathogen, B. dendrobatidis (Bd), has devastated amphibian populations worldwide, suggesting that Bsal poses a significant threat to global salamander biodiversity. To expedite research into this emerging threat, we seek to standardize protocols across the field so that results of laboratory studies are reproducible and comparable. We have collated data and experience from multiple labs to standardize culturing practices of Bsal. Here we outline common culture practices including a medium for standardized Bsal growth, standard culturing protocols, and a method for isolating Bsal from infected tissue.

Know your enemy: Unexpected, pervasive and persistent viral and bacterial contamination of primary cell cultures

In biomedical research, cell culture contamination is one of the main culprits of experimental failure. Contamination sources and concomitant remedies are numerous and challenging to manage. We herein describe two cases of uncommon contamination of cell cultures that we encountered, and the successful determination and eradication strategies. The first case describes the infection with human adenovirus C that originated from pharyngeal tonsils used for isolation of primary tonsillar epithelial cells. It is known that viral contamination of in vitro cell cultures can occur symptomless and is therefore difficult to identify. The contamination was pervasive and persistent, as it was widely spread in flow cabinets and apparatus, and has caused a serious delay to our research projects and the inevitable loss of valuable (patient-derived) cell sources. Eradication was successful by formalin gas sterilization of the flow cabinet and elimination of all infected cell lines from our biobank after PCR-guided determination. Secondly, we encountered a spore-forming bacterium, namely Brevibacillus brevis, in our cell culture facility. This bacterium originated from contaminated tap water pipes and spread via regular aseptic culture techniques due to survival of the bacterial spores in 70% ethanol. B brevis overgrew the cultures within a few days after seeding of the primary cells. Chlorine solution effectively killed this spore-forming bacterium. Both cases of contamination were identified using DNA sequencing which enabled the deployment of targeted aseptic techniques for the elimination of the persistent contamination.

Organotypic 3D Skin Models: Human Epidermal Equivalent Cultures from Primary Keratinocytes and Immortalized Keratinocyte Cell Lines

The three-dimensional culturing of human keratinocytes at the air-liquid interface yields a fully stratified epidermis including a functional stratum corneum and thus enables the study on epidermal structure and function in the context of biomedical, toxicological and pharmaceutical sciences. Here we provide a step-by-step detailed protocol for the isolation of human primary keratinocytes and the development of human epidermal equivalents generated from primary keratinocytes or immortalized keratinocytes (N/TERT-1; N/TERT-2G), including widely accepted procedures for the analysis of barrier function, tissue morphology, cell proliferation, and gene expression.

Immortalized N/TERT keratinocytes as an alternative cell source in 3D human epidermal models

The strong societal urge to reduce the use of experimental animals, and the biological differences between rodent and human skin, have led to the development of alternative models for healthy and diseased human skin. However, the limited availability of primary keratinocytes to generate such models hampers large-scale implementation of skin models in biomedical, toxicological, and pharmaceutical research. Immortalized cell lines may overcome these issues, however, few immortalized human keratinocyte cell lines are available and most do not form a fully stratified epithelium. In this study we compared two immortalized keratinocyte cell lines (N/TERT1, N/TERT2G) to human primary keratinocytes based on epidermal differentiation, response to inflammatory mediators, and the development of normal and inflammatory human epidermal equivalents (HEEs). Stratum corneum permeability, epidermal morphology, and expression of epidermal differentiation and host defence genes and proteins in N/TERT-HEE cultures was similar to that of primary human keratinocytes. We successfully generated N/TERT-HEEs with psoriasis or atopic dermatitis features and validated these models for drug-screening purposes. We conclude that the N/TERT keratinocyte cell lines are useful substitutes for primary human keratinocytes thereby providing a biologically relevant, unlimited cell source for in vitro studies on epidermal biology, inflammatory skin disease pathogenesis and therapeutics.