Last week Science magazine published a new article that literally took my breath away. This study is a classical example why is the scientific research so exciting.
In this new study, the research group led by Stephanie Karst at the University of Florida, studied Noroviruses target cell population.
Based on earlier studies on RAG KO and B cell KO mice (both lack B cells and show low NoVs titers), the authors speculated that B cell represent NoVs target population.
Initially, in vitro experiment confirmed that mouse NoVs could infect B cell lines (and macrophage cell line, RAW264.7 too) but not mouse epithelial cell line.
Of note, while mouse B cell line, WEHI, showed a high level of productive infection with NoVs (>90%), mouse B cell line, M12, showed a minimal level of productive infection with NoVs (~10%). However, both cell lines could amplify NoVs to the same level by day 3 post infection in vitro.
For some reason, the authors did not include primary mouse B cells for comparison in these series of experiments.
Next, the authors examined in vivo infectivity of mouse NoVs in wild-type or uMT mice. uMT mice lack B cells (but may have small number of IgA producing B cells). These experiments showed that uMT mice showed reduced viral load in tissues rich with B cells, like MLN. Interestingly, colon tissue showed comparable viral load in both control and B cell deficient host.
To further validate the hypothesis that NoVs infects B cells, the authors purified B cells from payer's patches from infected mice and run virus specific RT-PCR. The authors found that 1 out of 1000 B cells contained NoVs. However, in bulk cells from payer's patches, 1 out 100 cells contained NoVs (10-fold more). It seems other cell type(s), besides B cells, can be infected by NoVs more efficiently in vivo.
Next, the authors moved to human system. Unlike mouse B cell lines, human B cell line did not show productive infection with human NoVs. Surprisingly, when unfiltered sample from NoVs infected human stool was added to the human B cell line, the authors observed significant increase in viral load.
The transwell experiment with human epithelial cell line at the top and B cell line at the bottom, confirmed that B cell were required for NoVs amplification from unfiltered stool sample.
The authors speculate that human stool contained co-factor that facilitated B cell line infection by NoVs. Since it was known that human NoVs interact with histo-blood group antigen H, the authors tested the role of purified H antigen or H+ Enterobacter cloacae in in vitro NoVs infection model of human B cell line. As predicted, both purified H antigen and H+ Enterobacter cloacae, but not LPS or H- E. coli, could amplify NoVs infectivity.
It appears that H antigen improved human NoVs attachment to the target B cell line.
Finally, to verify the role of endogenous gut flora in mouse NoVs infectivity, the authors examined NoVs load in mice treated with broad spectrum antibiotics. Indeed, antibiotic treatment significantly reduced mouse NoVs infection. Interestingly, unlike uMT mice results, antibiotic treatment almost abolished NoVs infection of colon tissue.
In summary, this study expands our understanding as to how viruses interact with the host, in this case with human host. The observation that endogenous bacteria "helps" NoVs to infect the human host is an important step for better treatment of NoVs infection in humans.
As mentioned earlier, there are few results in this article that require further discussion. For example, experiments with primary B cells would have been more informative. In addition, uMT mice may still have small number of IgA secreting B cells, and may not be the ideal model to study the role of B cells in NoVs infection. Use of another B cell KO mice, JH-/-, or AID/μS double KO mice, would be more informative. And finally, it seems that colon tissue infectivity by NoVs was not influenced by absence of B cells. Why? Does density of H+ Enterobacter cloacae determines the role of B cells?