Replication of Human Sapovirus in Human-Induced Pluripotent Stem Cell-Derived Intestinal Epithelial Cells

Establishment of a Novel Caco-2-Based Cell Culture System for Human Sapovirus Propagation

Human sapovirus (HuSaV), first identified in the 1970s, is a significant cause of acute gastroenteritis, particularly in young children. Despite its clinical significance, research on HuSaV has been limited due to the absence of a reliable cell culture system. In 2020, a breakthrough study reported that HuSaV GI.1 and GII.3 strains could be cultured and serially propagated using HuTu80 cells in the presence of bile acids. However, in 2024, a subsequent study reported that effective replication in HuTu80 cells requires specialized cells that have undergone over 100 passages. In this study, we sought to identify an alternative cell culture system for HuSaV. HuSaV GI.1 can replicate and be serially propagated using Caco-2 cells under bile acid supplementation. Importantly, the Caco-2 cells were freshly sourced from the American Type Culture Collection, ensuring reproducibility for laboratories worldwide. Furthermore, Caco-2MC cells were established via single-cell cloning from in-house Caco-2/Cas9 cells with 91.5% HuSaV-susceptible. HuSaV strains GI.1, GI.2, GI.3, GII.1, GII.3, and GV.1 were successfully propagated using Caco-2MC cells, with RNA copy numbers increasing up to 4.4 log10-fold within 5 days post-infection. This efficient HuSaV cell culture system represents a significant advancement in HuSaV research.

Charting the Impact of Maternal Antibodies and Repeat Exposures on Sapovirus Immunity in Early Childhood From a Nicaraguan Birth Cohort

BACKGROUND

Sapovirus is an important cause of acute gastroenteritis in childhood. While vaccines against sapovirus may reduce gastroenteritis burden, a major challenge to their development is a lack of information about natural immunity.

METHODS

We measured sapovirus-specific IgG in serum collected between 2017 and 2020 of mothers soon after delivery and at 6 time points in Nicaraguan children until 3 years of age (n = 112 dyads), using virus-like particles representing 3 sapovirus genotypes (GI.1, GI.2, GV.1).

RESULTS

Of the 112 children, 16 (14.3%) experienced at least 1 sapovirus gastroenteritis episode, of which GI.1 was the most common genotype. Seroconversion to GI.1 and GI.2 was most common between 5 and 12 months of age, while seroconversion to GV.1 peaked at 18 to 24 months of age. All children who experienced sapovirus GI.1 gastroenteritis seroconverted and developed genotype-specific IgG. The impact of sapovirus exposure on population immunity was determined by antigenic cartography: newborns share their mothers' broadly binding IgG responses, which declined at 5 months of age and then increased as infants experienced natural sapovirus infections.

CONCLUSIONS

By tracking humoral immunity to sapovirus over the first 3 years of life, this study provides important insights for the design and timing of future pediatric sapovirus vaccines.

H and B Blood Antigens Are Essential for In Vitro Replication of GII.2 Human Norovirus

Background

Human norovirus (HuNoV) is a major cause of enteric infectious gastroenteritis and is classified into several genotypes based on its capsid protein amino acid sequence and nucleotide sequence of the polymerase gene. Among these, GII.4 is the major genotype worldwide. Epidemiological studies have highlighted the prevalence of GII.2. Although recent advances using human tissue- and induced pluripotent stem cell (iPSC)-derived intestinal epithelial cells (IECs) have enabled in vitro replication of multiple HuNoV genotypes, GII.2 HuNoV could replicate only in tissue-derived IECs and not in iPSC-derived IECs.

Methods

We investigated the factors influencing GII.2 HuNoV replication in IECs, focusing on histo-blood group antigens. We also assessed the immunogenicity of GII.2 virus-like particles (VLPs) and their ability to induce neutralizing antibodies. Antibody cross-reactivity was tested to determine whether GII.2 VLPs could neutralize other HuNoV genotypes, including GII.4, GII.3, GII.6, and GII.17.

Results

Our findings indicated that GII.2 HuNoV replication in vitro requires the presence of both H and B antigens. Moreover, GII.2 VLPs generated neutralizing antibodies effective against both GII.2 and GII.4 but not against GII.3, GII.6, or GII.17. Comparatively, GII.2 and GII.17 VLPs induced broader neutralizing responses than GII.4 VLPs.

Conclusions

The findings of this study suggests that GII.2 and GII.17 VLPs may be advantageous as HuNoV vaccine candidates because they elicit neutralizing antibodies against the predominant GII.4 genotype, which could be particularly beneficial for infants without prior HuNoV exposure. These insights will contribute to the development of effective HuNoV vaccines.

Propagating and banking genetically diverse human sapovirus strains using a human duodenal cell line: investigating antigenic differences between strains

There are four genogroups and 18 genotypes of human sapoviruses (HuSaVs) responsible for acute gastroenteritis. To comprehend their antigenic and virological differences, it is crucial to obtain viral stocks of the different strains. Previously, we utilized the human duodenum-derived cell line HuTu80, and glycocholate, a conjugated bile acid, to replicate and propagate GI.1, GI.2, and GII.3 HuSaVs (H. Takagi et al., Proc Natl Acad Sci U S A 117:32078-32085, 2020, https://10.1073/pnas.2007310117). First, we investigated the impact of HuTu80 passage number on HuSaV propagation. Second, we demonstrated that taurocholate improved the initial replication success rate and viral RNA levels in fecal specimens relative to glycocholate. By propagating 15 HuSaV genotypes (GI.1-7, GII.1-5, -8, and GV.1-2) and accomplishing preparation of viral stocks containing 1.0 × 109 to 3.4 × 1011 viral genomic copies/mL, we found that all strains required bile acids for replication, with GII.4 showing strict requirements for taurocholate. The deduced VP1 sequences of the viruses during the scale-up of serial passaged virus cultures were either identical or differed by only two amino acids from the original sequences in feces. In addition, we purified virions from nine strains of different genotypes and used them as immunogens for antiserum production. Enzyme-linked immunosorbent assays (ELISAs) using rabbit and guinea pig antisera for each of the 15 strains of different genotypes revealed distinct antigenicity among the propagating viruses across genogroups and differences between genotypes. Acquisition of biobanked viral resources and determination of key culture conditions will be valuable to gain insights into the common mechanisms of HuSaV infection.

IMPORTANCE

The control of human sapovirus, which causes acute gastroenteritis in individuals of all ages, is challenging because of its association with outbreaks similar to those caused by human norovirus. The establishment of conditions for efficient viral propagation of various viral strains is essential for understanding the infection mechanism and identifying potential control methods. In this study, two critical factors for human sapovirus propagation in a conventional human duodenal cell line were identified, and 15 strains of different genotypes that differed at the genetic and antigenic levels were isolated and used to prepare virus stocks. The preparation of virus stocks has not been successful for noroviruses, which belong to the same family as sapoviruses. Securing virus stocks of multiple human sapovirus strains represents a significant advance toward establishing a reliable experimental system that does not depend on limited virus-positive fecal material.

Strong evolutionary constraints against amino acid changes in the P2 subdomain of sapovirus GI.1 capsid protein VP1

Sapovirus (SaV) is a nonenveloped RNA virus that causes acute gastroenteritis in humans. Although SaV is a clinically important pathogen in children, an effective vaccine is currently unavailable. The capsid protein VP1 of SaVs forms the outer shell of the virion and is highly diverse, as often seen in the virion-surface proteins of RNA viruses, creating an obstacle for vaccine development. We here report a unique phenomenon pertaining to the variation of SaV VP1. Phylogenetic and information entropy analyses using full-length VP1 sequences from a public database consistently showed that the amino acid sequences of the VP1 protein have been highly conserved over more than 40 years in the major epidemic genotype GI.1 but not in GI.2. Structural modeling showed that even the VP1 P2 subdomain, which is arranged on the outermost shell of the virion and presumably exposed to anti-SaV antibodies, remained highly homogeneous in GI.1 but not in GI.2. These results suggest strong evolutionary constraints against amino acid changes in the P2 subdomain of the SaV GI.1 capsid and illustrate a hitherto unappreciated mechanism, i.e., preservation of the VP1 P2 subdomain, involved in SaV survival. Our findings could have important implications for the development of an anti-SaV vaccine.

Porcine Sapovirus Protease Controls the Innate Immune Response and Targets TBK1

Human sapoviruses (HuSaVs) and noroviruses are considered the leading cause of acute gastroenteritis worldwide. While extensive research has focused on noroviruses, our understanding of sapoviruses (SaVs) and their interactions with the host's immune response remains limited. HuSaVs have been challenging to propagate in vitro, making the porcine sapovirus (PSaV) Cowden strain a valuable model for studying SaV pathogenesis. In this study we show, for the first time, that PSaV Cowden strain has mechanisms to evade the host's innate immune response. The virus 3C-like protease (NS6) inhibits type I IFN production by targeting TBK1. Catalytically active NS6, both during ectopic expression and during PSaV infection, targets TBK1 which is then led for rapid degradation by the proteasome. Moreover, deletion of TBK1 from porcine cells led to an increase in PSaV titres, emphasizing its role in regulating PSaV infection. Additionally, we successfully established PSaV infection in IPEC-J2 cells, an enterocytic cell line originating from the jejunum of a neonatal piglet. Overall, this study provides novel insights into PSaV evasion strategies, opening the way for future investigations into SaV-host interactions, and enabling the use of a new cell line model for PSaV research.