Organoid is stem cell derived multicellular self-organizing organ (Fig. 1) [10]. The major characteristic of organoid is mimicking the organization, functionality, and genetic signature of the specific tissue or organ in vivo. These features provide organoids with a potential to be used as a model to study infectious pathogens in terms of characteristics of the pathogens as well as host-pathogen interaction. Up to date, there are several organoids have been established such as intestine, stomach, esophagus, liver, kidneys, lungs, brain, prostate, pancreas, retina, and ovary [11]. Intestinal organoid is most well-known organoid type which is established with the successful culture of murine epithelial organoids from Lgr5⁺ intestinal stem cells in 2009, followed by the development of organoids from human pluripotent stem cells (PSCs) and biopsy samples [1213]. There are two-types of intestinal organoid: human intestinal organoid (HIO) and human intestinal enteroid (HIE). HIO is derived from PSCs that are epithelial cultures associated with mesenchyme. HIE is human tissue-derived organoids that are epithelial only cultures derived stem cells isolated from biopsies or surgical tissues. Using HIO model, rotavirus tropism, replication in enterocytes as well as mesenchymal cells, was identified [14]. Under HIE model, critical component for norovirus replication, bile 9a, was found [15]. Other organoids are also intensely used as infection models to investigate virus studies (cerebral organoid for Zika virus and lung organoid for respiratory syncytial virus) [161718].

Based on the in vivo mimicking features of organoids, there are some trials to use organoid as virus production system. Finkbeiner et al. [14] generated HIO system using embryonic stem cell line (WA09) and tested whether this system is suitable to cultivate human rotavirus. The generated HIO system successfully produced viral RNA and viroplasms after infection with clinically isolate samples collected from stool. Importantly, rotavirus replicated approximately 10 times higher in HIO system compared to conventional culture method using monkey epithelial cell line. Ettayebi et al. [15] tested HIE system as a cultivation system for human norovirus. In HIE system, human norovirus replicated time-dependent manner between 1- and 24-hour post infection (hpi). Genomic equivalent of viral progeny increased up to 1.5 to 2.5 log10 at 96 hpi compared to 1 hpi, suggesting that stem-cell derived intestinal organoid can be applicable to cultivate previously non-cultivatable human norovirus. This HIE system also suitable to assess virus neutralization. Although not actual organoid, three-dimensional (3-D) culture of Huh-7 showed the potential to cultivate HCV [19]. 3-D rotating wall vessel-culture method made Huh-7 cells form complex, multilayered, 3-D aggregates which is organoid-like phenotype. The cultured 3-D Huh-7 cells expressed high levels of metabolic-associate molecules (cytochrome P450s and UDP-glucuronosyltransferase), hepatocyte specific transcription factor (hepatocyte transcription factor), markers for re-localization of tight junction (Claudin-1, ZO-1, and Occludin-1), cell adhesion (E-cadherin and β-catenin), and polarity (CD26), and HCV receptor (CD81 and Scavenger receptor class B type 1). 3-D Huh-7 aggregates also showed localization of tight junction markers to apicalateral and/or basolateral planes which are consistent with localization pattern in primary hepatocytes and not observed in 2-D Huh-7. Most important, 3-D Huh-7 system was suitable for HCV infection and the production of HCV was exponentially increased, suggesting the potential of this new culture system for efficient HCV production. As described above, initial studies using organoids have been focused on the improving cultivation efficiency of currently uncultivable viruses. Given that the organoids are derived from human cells, this viral cultivation system does not need an adaptation process of viruses, indicating that there is no or relatively low potential to induce genetic mutation in the viral genome. In addition, there is no potential risk of adverse effect by host cell-derived non-human protein, which can be another merit of organoid system as a cultivation system of vaccine strains.

Although the potential of organoid in terms of virus producing efficiency was not verified in other various kind of virus, the initial trials suggested the possibility that organoid-based viral vaccine production can be more widely used in more broad ranges of viruses. To use organoid as a productive system for viral vaccine strain, there are some hurdles that remain to be solved (Table 1). The source of organoid and the generated organoid should be standardized to secure the quality of the system. In addition, the virus producing scheme under organoid system should be optimized to produce virus sufficiently without induction of genetic mutations in virus. And the cultivation and maintaining cost for organoid system also should be reduced to the degree comparable to that of conventional virus culture method. Technical advances to solve these issues will bring us more various and effective vaccines for viral infection.