dy. Undiluted cell lysates from mock-infected cultures were used as negative controls to calculate a positivity cutoff value. Sample dilutions PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19770275 which had an OD value higher than the cutoff value were considered positive. 4 / 18 HAstV Delays Interferon Induction Measurement of transepithelial resistance of CaCo-2 cell monolayers CaCo-2 cells were differentiated for 21 days on 1.12-cm2 semipermeable tissue culture inserts, with cell culture medium renewal on alternate days. Transepithelial resistance was confirmed to be at least 1,000 Oxcm2 using an EVOM voltometer before infection, and was monitored every 412 hours during the course of infection. Results were presented as percentages of the insert’s TER reading versus 0 hpi. Statistical analysis Comparisons between means were performed using the student t-test using the IBM SPSS Statistics version 20 software. P values < 0.05 were considered statistically significant. Results HAstV infection induces a weak type I IFN response To examine whether HAstV infection induces an IFN response, CaCo-2 cells were infected at a MOI of 1 and RNA was extracted at 0, 3, 12 and 24 hpi. Mock-infected cells, cells transfected with polyI:C and cells treated with 1,000 U/ml of recombinant type I IFN were used as controls. Conventional RT-PCR was performed to detect viral RNA, IFN- mRNA, and ISG56 mRNA. GAPDH mRNA was amplified as a quality control for RNA. Results show that polyI:C Salvianic acid A chemical information transfection induced IFN- gene transcription as early as 3 hours post-transfection, while IFN- mRNA could not be detected in HAstV-infected cells before 24 hpi. A time course analysis of HAstV RNA synthesis was performed by qRT-PCR after infecting cells at 2 different MOIs. As expected, a 3-log increase in viral RNA titers was observed during the first 24 hours for PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19768759 both MOIs. Together, these results suggest that HAstV delays the onset of IFN induction sufficiently until a large amount of progeny particles are produced. Expression of ISG56 mRNA accumulated in cells treated with exogenous IFN and polyI:Ctransfected cells starting at 12 h. Detection of ISG56 mRNA in HAstV-infected cells at 24 hpi 
could indicate that IFN- had already been released to the extracellular media, although it has also been described that some viruses from diverse families can induce synthesis of ISG56 mRNA in the absence of both IFN production and/or virus replication. In order to analyze the magnitude of the IFN response, levels of IFN- mRNA were quantified by qRT-PCR, and subcellular localization of IRF3 was examined by immunofluorescence during the course of infection at a MOI of 1, which results in more than 90% of infected cells as measured by IF. IFN- mRNA levels induced by HAstV peaked at 48 hpi but were 3.2-fold lower than those induced by polyI:C transfection. Since transfection efficiency of CaCo-2 cells is low and polyI:C transfection only resulted in 6 0.7% of cells with nuclear IRF3 producing IFN- mRNA, when comparing the total levels of IFN- mRNA produced by polyI:Ctransfected cultures to the levels produced by infected cultures, it would seem that in these latter cultures, either each infected cell produced very low levels of IFN- mRNA or that IFN- mRNA would only be produced by a few number of infected cells. Co-staining of viral structural proteins and IRF3 showed that only 2 0.2% of HAstV-infected cells had nuclear IRF3 at 24 hpi. At 48 hpi, the percentage of cells with nuclear IRF3 increased up to 10.34 1.8 0%, but in any case was not clo