Elsevier

Current Opinion in Virology

Volume 29, April 2018, Pages 79-86
Current Opinion in Virology

Human organoid cultures: transformative new tools for human virus studies

https://doi.org/10.1016/j.coviro.2018.04.001Get rights and content

Studies of human infectious diseases have been limited by the paucity of functional models that mimic normal human physiology and pathophysiology. Recent advances in the development of multicellular, physiologically active organotypic cultures produced from embryonic and pluripotent stem cells, as well as from stem cells isolated from biopsies and surgical specimens are allowing unprecedented new studies and discoveries about host–microbe interactions. Here, we summarize recent developments in the use of organoids for studying human viral pathogens, including intestinal infections with human rotavirus, norovirus, enteroviruses and adenoviruses (intestinal organoids and enteroids), neuronal infections with Zika virus (cerebral organoids) and respiratory infections with respiratory syncytial virus in (lung bud organoids). Biologic discovery of host-specific genetic and epigenetic factors affecting infection, and responses to infection that lead to disease are possible with the use of organoid cultures. Continued development to increase the complexity of these cultures by including components of the normal host tissue microenvironment such as immune cells, blood vessels and microbiome, will facilitate studies on human viral pathogenesis, and advance the development of platforms for pre-clinical evaluation of vaccines, antivirals and therapeutics.

Introduction

Studies of infectious agents have traditionally focused on in vitro culture systems using transformed cell lines and animal models. While these systems have enabled historic progress in the understanding of microbial pathogenesis and host–pathogen interactions, and facilitated the development of vaccines and therapeutics, the physiological relevance of these models for human pathogens and diseases can be limited. Routinely used cell lines are immortalized and cancer-derived and may not adequately reflect responses of normal human cells [1, 2, 3]. Many signaling pathways are altered in cancer cells and have profound effects on the core metabolism in cancer-derived lines [4]. Furthermore, cell lines are typically comprised of a single cell type and do not mimic the architecture, environmental complexity and functionality of tissues which are comprised of many different cell types. While some of these drawbacks are overcome in animal models, these have other restrictions for studying human infectious diseases. Many human pathogens display unique human specificity that precludes their study in animals while in other cases, animal models fail to reproduce human pathophysiology. Further, findings and effects seen in animal models do not always translate to humans, as has been observed in many different fields [5, 6, 7].

The development of in vitro organoid cultures offer remarkable new model systems to study infectious agents and disease pathogenesis [8]. While organotypic cultures comprised of three dimensional (3D) cell aggregates have been in existence for decades, the term organoids now largely refers to self-organizing, propagatable 3D cultures derived from stem cells that recapitulate the organization, functionality, and genetic signature of the specific tissue or organ and host from which they are derived. Organoids can be derived from embryonic and pluripotent stem cells (ESCs and PSCs, respectively) as well as stem cells isolated from specific human tissues. Through new understanding of defined developmental cues and growth factors, stem cells can be directed to grow ex vivo into organ-like structures. Unlike transformed cells lines and animal models, organoid cultures are multicellular, reflect the cellular heterogeneity of specific human organs and are physiologically active; thus, they are increasingly being validated as relevant models for studies of infectious disease, particularly of human-specific pathogens. Organoids have been established for multiple organs including the intestine, stomach, esophagus, liver, kidneys, lungs, brain, prostate, pancreas, retina and ovary [8]. Although the use of this new technology is in its infancy, paradigm shifting results and unexpected discoveries have been made with several human viruses (Figure 1) and select data are summarized below. Other recent articles address aspects of organoids for disease studies that are not covered in this review [8, 9, 10, 11, 12, 13].

Section snippets

Human intestinal organoids and enteroids

The human gastrointestinal tract is a complex organ with a polarized epithelial layer that contains different cell types including enterocytes, enteroendocrine cells, tuft cells, goblet cells, Paneth cells and stem cells. Distinct regions of the intestine (duodenum, jejunum and ileum, proximal and distal colon) perform unique functions and demonstrate segment-specificity in terms of transport, protein expression and interactions with pathogens [14, 15]. The breakthrough in intestinal organoid

Cerebral organoids

Given the complex organization of the developing cortex with different progenitor and neuronal layers, the generation of 3D brain organoids from human PSCs that mimic human brain development is a significant breakthrough in neuroscience [42]. Brain organoids have been used extensively in recent times for studies with Zika virus (ZIKV), a mosquito-borne Flavivirus. In 2016, the WHO declared a global health emergency due to the link between the ZIKV outbreak and the increased incidence of

Lung organoids

Lung bud organoids containing the mesoderm and pulmonary endoderm were generated from human PSCs and develop into branching airway and early alveolar structures after in vivo transplantation (xenotransplantation) under the mouse kidney capsule as well as in Matrigel 3D cultures [55••]. Resembling lung buds at the second trimester of gestation, these organoids can be infected with respiratory syncytial virus (RSV), the most common cause of bronchiolitis and pneumonia in children younger than 1

Challenges and future directions

While organoids have several advantages over transformed cell lines and animal models, key challenges remain to be addressed in order to improve their utility for studying viruses (Box 1). One limitation is that current organoid models are devoid of components of the normal host microenvironment such as immune cells and blood vessels. Co-culture models incorporating other cell types including endothelial cells and immune cells is an area of ongoing development [56, 57, 58]. In the case of

Conclusions

The development of organoids representing different sites of human infection is an exciting advance that can facilitate studies and new discoveries about host–virus interactions including molecular pathogenesis of viral infections, novel cell tropisms, new insights into host innate immune responses and characterizing interactions between pathogens (Figure 2). Importantly, human organoids/enteroids provide a functional and physiologically relevant model for pre-clinical evaluation of vaccines,

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The authors would like to acknowledge support from grants U18-TR000552, UH3-TR00003, U19-AI116497, R01-AI080656, R01-AI105101, P30-DK56338 from the National Institutes of Health and the Food Research Initiative Competitive grant 2011-68003-30395 from the US Department of Agriculture, National Institute of Food and Agriculture.

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