Author(s): Tomoichiro Oka 1,2, Garrett T. Stoltzfus 1, Chelsea Zhu 1, Kwonil Jung 1, Qiuhong Wang 1,*, Linda J. Saif 1,*
Noroviruses (NoVs) and Sapoviruses (SaVs) are non-enveloped, single stranded, positive sense RNA viruses of the family Caliciviridae . Based on the complete capsid sequences, NoVs and SaVs are classified into at least five genogroups (GI, GII, GIII, GIV, and GV)  and 15 genogroups (GI-GXV), respectively , both of which are further divided into multiple genotypes [3,4]. NoVs have been detected from humans, swine, cattle, sheep, rodents, cats, dogs, and lions . SaVs have been detected from humans, pigs, mink, dogs, sea lions, bats, chimpanzees, rodents, and carnivores [2,6].
Human NoVs (HuNoVs) have been recognized for over four decades. A single genotype, genogroup II and genotype 4 (GII.4), is the leading cause of acute non-bacterial gastroenteritis in humans worldwide since around 2000 . Although human volunteer studies showed that histopathological changes occurred in the small intestine of HuNoV-infected individuals , the cell type(s) in which human HuNoVs replicate in immunocompetent individuals are still unknown. Recently, HuNoVs were reported to replicate in the intestinal enterocytes of immunocompromised transplant patients . Both HuNoV structural protein VP1 and non-structural (RNA polymerase and genome-linked viral protein VPg) antigens were detected in the intestinal epithelial cells, providing direct evidence of viral replication in these cells. Although HuNoV antigens were also detected in immune cells in the lamina propia, proof of definitive HuNoV replication in these immune cells was lacking because an epithelial cell marker was also detected simultaneously in those cells, suggesting that these immune cells contained HuNoV antigens acquired via phagocytosis of HuNoV-infected epithelial cells.
Efficient in vitro cell culture systems have been achieved for murine NoVs using murine macrophage cell lines (e.g., RAW264.7), primary macrophages and dendritic cells , and mouse B cell lines . Murine NoV RNA titers in the RAW264.7 cell supernatant increased > 6 log10 within 4 days post-inoculation . HuNoV RNA levels in feces in acute gastroenteritis were extremely high (up to 12 log10 genomic RNA copies/g or mL of stool) , despite their low infectious dose levels (approximately 1~3 x 103 genomic copies) . However, the reported HuNoV RNA increases in 3D cultures of differentiating Caco-2 or a derivative cell line CBBe2 ranged from 2-3 log10 based on qRT-PCR [15,16]. During our study and manuscript preparation, two new methods using human B cells or human intestinal enteroids have been reported for successful propagation of HuNoVs in vitro , but also with 2-3 log10 increased RNA levels based on qRT-PCR [11,17,18].
Human SaVs (HuSaVs) also have been recognized for over four decades and cause acute non-bacterial gastroenteritis in humans; however, their infection site(s) and the cell type(s) that are susceptible to HuSaVs in vivo remain unknown . A single genotype, genogroup I and genotype 2 (GI.2), was predominantly detected from acute non-bacterial gastroenteritis outbreaks throughout Japan in 2012 and 2013 . The reported propagation of HuSaV in African green monkey kidney cells and primary...