**2.3 Provide new insights**

Data from cell lines have earlier shown that the small open reading frame upstream of the main polyprotein ORF which is also present in the 5'UTR genomic region in enteroviruses, cannot be utilized for the initiation of translation [35]. Lulla et al. had reported for the first time that the small protein encoded by this uORF is crucial for virus release in human intestinal organoids [36]. The viruses lacking this uORF are therefore attenuated in this model. Later on, other publications on intestinal organoids have reiterated that different enteroviruses infect different cell types and induce an antiviral response characteristic of a particular cell type [37, 38].

To assess the influence of host conditions such as age and comorbidities on the progress and severity of viral infections, cross-interactions between co-detected pathogens in a single host can be studied closely with organoids. This was never feasible with cell lines because different viruses are often not culturable on the

same cell line. For example, respiratory viruses are well-known for causing asthma and pathologies like cystic fibrosis or chronic obstructive pulmonary disease. HAE infection samples collected from healthy and asthmatic donors with rhinovirus have shown a unique airway epithelial structure with inflammatory signaling in asthmatic patients [39, 40].

## **2.4 Utilization in fighting the SARS-CoV-2 pandemic**

Multiple types of organoid models were used to study the detrimental effect of SARS-CoV-2 infection on human hosts and its potential therapeutic interventions [41]. To begin with, HAE cultures served as faithful models for the lungs where efficient replication occurred through the infection of ciliated cells in the airway [42]. Therapeutic investigations on organoid models showed the repurposed drug remdesivir and remdesivir–diltiazem to be functional in resisting further SARS-CoV-2 infection [43]. Lamers et al. had proved for the first time that the human gut epithelium is the second major replication site of the virus [44]. Combined with the novel insights from other organoid research groups, it was proved that the SARS-CoV-2 genome is detectable in feces even after the virus is absent from oropharyngeal swabs, which explains the outcome of intestinal infection and potential fecal transmission [45].

These findings were closely followed by the observation of increased efficiency to infect secondary tissue by the virus. In terms of relative importance, the next area of investigation using organoids has been establishing the neuro-invasive aspect of SARS-CoV-2 by using brain organoid models [46]. Epidemiological studies showed the direct contribution of SARS-CoV-2 infection to neurological complications like headaches, ischemic stroke, and encephalitis, including cranial nerve-related complications such as anosmia and hyposmia, and ageusia [47, 48]. Recently, Pellegrini et al. utilized choroid plexus organoids to demonstrate the potential viral tropism for choroid plexus epithelial cells that affect the epithelium [49]. Damage to this barrier is suspected as a possible entry route for the virus into the cerebrospinal fluid and the brain.

### **2.5 Extensive research in Zika virus pandemic**

Zika virus, a mosquito-borne flavivirus, is reportedly the causative agent for the infection known as ZIKV. Although adult victims show mild symptoms, newborns are marked with microcephaly, a condition in which infants are born with an abnormally small head. Being spread in over 70 countries and territories globally, [50] ZIKV is declared a global health emergency by WHO whereby microcephalic fetal tissues have shown traces of ZIKV in damaged fetal brains [51]. Due to accessibility challenges with live infected human fetal samples and postmortem tissues showing a diverse range of quality and genetic history, clinical examinations are replaced for good by brain organoid model studies. These focus on cellular tropism and pathogenesis mechanisms of ZIKV in controlled settings [52].

In 2016, the first study on brain organoid models was published by Tang et al., where they used monolayer cultures of forebrain-specific neural progenitor cell (NPCs) to model ZIKV infection during human brain development [53]. These were the initial results towards projecting that ZIKV more efficiently infects NPCs layers over human pluripotent stem cells (hPSCs) or immature neurons. Infection of cerebral organoids and human neurospheres with ZIKV and dengue virus 2 (DENV2) has proved that only ZIKV attenuates NPC growth, suggesting that the extreme aftereffect of ZIKV infection as an exceptional feature of the flavivirus family [54]. Later on, studies using brain organoids derived from hPSCs have also

#### *Organoid Technology and the COVID Pandemic DOI: http://dx.doi.org/10.5772/intechopen.98542*

led to a significant understanding of various other aspects of ZIKV infection on fetal brain development [52].

Due to the limited accessibility of organoid methodologies to virology research groups and the delay in the pace of commercialization of this technology, the majority of the published work so far has been a result of cross-functional collaborative efforts [55]. This challenge is closely followed by complications arising from heterogeneity inherent to the structural complexity and cell-type diversity in brain organoid models compared to simpler analogs such as neurospheres [56]. Moreover, the low-throughput nature of culturing and analyzing organoids creates a significant obstacle in drug screening which usually needs a high-throughput styled experimental protocol. We anticipate the evolution of more sophisticated brain organoids in the future that involves the co-culturing of endothelial cells or microglial cells to enhance the physiological relevance of modeling ZIKV infection during fetal human brain development.
