Bacteria help norovius infect B cells: Stomach ache, nausea, diarrhea—many people know the sort of gastrointestinal havoc norovirus can wreak. Despite this, norovirus biology remains unclear, because human norovirus cannot be grown in culture. Jones et al. now report that with the help of bacteria, human norovirus can infect cultured B cells (see the Perspective by Robinson and Pfeiffer). To infect B cells, human norovirus required the presence of gut bacteria that expressed proteins involved in determining blood type. Mouse norovirus also infected B cells, and the treatment of mice with antibiotics protected them from norovirus infection. Science , this issue p. 755 ; see also p. 700

Noroviruses are understudied because these important enteric pathogens have not been cultured to date. We found that the norovirus murine norovirus 1 (MNV-1) infects macrophage-like cells in vivo and replicates in cultured primary dendritic cells and macrophages. MNV-1 growth was inhibited by the interferon-αβ receptor and STAT-1, and was associated with extensive rearrangements of intracellular membranes. An amino acid substitution in the capsid protein of serially passaged MNV-1 was associated with virulence attenuation in vivo. This is the first report of replication of a norovirus in cell culture. The capacity of MNV-1 to replicate in a STAT-1-regulated fashion and the unexpected tropism of a norovirus for cells of the hematopoietic lineage provide important insights into norovirus biology.

Norwalk-like caliciviruses (Noroviruses) cause over 90% of nonbacterial epidemic gastroenteritis worldwide, but the pathogenesis of norovirus infection is poorly understood because these viruses do not grow in cultured cells and there is no small animal model. Here, we report a previously unknown murine norovirus. Analysis of Murine Norovirus 1 infection revealed that signal transducer and activator of transcription 1–dependent innate immunity, but not T and B cell–dependent adaptive immunity, is essential for norovirus resistance. The identification of host molecules essential for murine norovirus resistance may provide targets for prevention or control of an important human disease.

Infection of cells with adeno-associated virus (AAV) type 2 (AAV-2) is mediated by binding to heparan sulfate proteoglycan and can be competed by heparin. Mutational analysis of AAV-2 capsid proteins showed that a group of basic amino acids (arginines 484, 487, 585, and 588 and lysine 532) contribute to heparin and HeLa cell binding. These amino acids are positioned in three clusters at the threefold spike region of the AAV-2 capsid. According to the recently resolved atomic structure for AAV-2, arginines 484 and 487 and lysine 532 on one site and arginines 585 and 588 on the other site belong to different capsid protein subunits. These data suggest that the formation of the heparin-binding motifs depends on the correct assembly of VP trimers or even of capsids. In contrast, arginine 475, which also strongly reduces heparin binding as well as viral infectivity upon mutation to alanine, is located inside the capsid structure at the border of adjacent VP subunits and most likely influences heparin binding indirectly by disturbing correct subunit assembly. Computer simulation of heparin docking to the AAV-2 capsid suggests that heparin associates with the three basic clusters along a channel-like cavity flanked by the basic amino acids. With few exceptions, mutant infectivities correlated with their heparin- and cell-binding properties. The tissue distribution in mice of recombinant AAV-2 mutated in R484 and R585 indicated markedly reduced infection of the liver, compared to infection with wild-type recombinant AAV, but continued infection of the heart. These results suggest that although heparin binding influences the infectivity of AAV-2, it seems not to be necessary.

The unique region of the VP1 protein of parvoviruses was proposed to contain a parvoviral phospholipase A2 (pvPLA2) motif. Here, PLA2 activity is shown in the unique region of adeno-associated virus type 2 (AAV-2) VP1 when expressed as an isolated domain in bacteria. Mutations in this region of the capsid protein strongly reduced the infectivity of mutant virions in comparison to wild-type AAV-2. This correlated with effects on the activity of PLA2. The mutations had no influence on capsid assembly, packaging of viral genomes into particles or binding to and entry into HeLa cells. However, a delayed onset and reduced amount of early gene expression, as measured by Rep immunofluorescence, was observed. These results suggest that pvPLA2 activity is required for a step following perinuclear accumulation of virions but prior to early gene expression.