Prevalence of inhibitory LPS within gut microflora could underlie hygiene hypothesis

This week prestigious journal Cell published seminal microbiome study that could explain hygiene hypothesis that suggests that "microbial cleanliness" in developed countries predisposes individuals to immunopathologies such as allergy, multiple sclerosis and other autoimmune diseases. 

This study found that gut microflora in babies from Finland and Estonia, but not from Russian Karelia, are enriched in gut bacteria with inhibitory LPS that failed to properly "educate" newborns innate immune system thus predisposing them to higher incidence of immunopathologies.

The first finding was that microbiota in babies from Finland, but not from Russian Karelia, were enriched in Bacteroides. Now this fact by itself isn't too surprising. 

The second finding was that lipid A component of LPS from these Bacteroides [unlike lipid A component of LPS from E. coli] species were non-stimulatory in TLR4 assay. In fact, molecular analysis showed that Bacteroides harbored tetra- and penta-acylated lipid A structures, as opposed to the hexa-acylated lipid A seen in E. coli. Such structural modification converted Bacteroides lipid A into totally non-stimulatory ligand.

In fact, Bacteroides derived lipid A was inhibitory when combined with stimulatory, E.coli derived lipid A [LPS tolerance assay, where primary exposure to E. coli LPS makes responding cell refractory to secondary exposure].

Finally, third finding was that presence of non-stimulatory lipid A had immunological consequences since it failed to reduce incidence of diabetes in NOD mice [whereas lipid A from E. coli could, as expected].  

In summary, this important study has provided one of the first definite molecular evidence underlying hygiene hypothesis and suggested the path for its prevention.

David Usharauli

     

Dirt exposure could prevent asthma development in mouse

Allergic or exaggerated response to mostly innocuous antigens is a Terra Incognita in our understanding of how immune system really works. First question that comes to mind is why is there even such molecule as IgE that causes nothing but allergic reaction when combined with antigen? A hypothesis proposed by immunologist, Ruslan Medzhitov, suggests that the evolutionary role of IgE was to defend the body against toxins. But IgG could do that too? 

Another hypothesis commonly referred as hygiene hypothesis explains that allergic responses are, in fact, out-of-control type II immune reactions (highly skewed Th2-IgE-IL-13 axis) due to absence of "normal" education of immune system by reduced exposure to infections or infectious materials (bacteria, dirt, dust, fungi, dander, etc.) during early life (that supposed to induce Th1-IFNγ axis balancing Th2 response).

The new paper in journal Science is another example for such immune "balancing" act in support of hygiene hypothesis. Here, the authors reported that pre-exposure to low dose of LPS (endotoxin, a part of gram-negative bacteria) could raise the threshold for lung airway epithelial sensitivity to house dust mite (HDM) extract via modulation of NF-kb activity via up-regulation of A20 (Ubiquitin-editing protein).  

First, the authors showed that compared to control, mice pre-exposure to LPS before HDM application were protected from developing features of airway hypersensitivity (the most important figure is 1D).

Since LPS pre-exposure induced A20 up-regulation in lung epithelial cells, the authors tested mice with lung epithelial specific deletion of A20 (tnfaip3 KO). Indeed, beneficial effect of LPS exposure was abolished in these A20 deficient mice (tnfaip3 ^EC-KO).

Finally, the authors speculated that asthmatic individuals may be deficient for A20 in their lung epithelial cells. However, for some reason, contrary to the authors hypothesis, lung cells from severe asthmatics expressed more A20 molecules compared to lung epithelial cells from mild asthmatics (see figure below). The authors did not discuss this contradiction.

There are several weaknesses in this article. For one, most data are presented as bars rather than scatter plot with individual data points. 2nd, for some unknown reason, the authors decided not to show lung hypersensitivity assay with LPS pre-exposure from mutant mice, as it was done with wild-type mice in Fig. 1D. This assay is clinically the most relevant. This is a Science paper, to remind you.

So, in summary, while I do think that exposure to dirt during early life is relevant for human asthma development, based on results from this paper I am not convinced that expression of A20 in lung epithelial cells play a role here.  

David Usharauli