Gut IgA are naturally microbiota-reactive and polyreactive (cross-reactive)

Gut immune system naturally produces large quantities of IgA, an antibody isotype frequently found at mucosal surfaces. Since these IgA antibodies are found in mice in absence of immunization and infection they were dubbed natural and were thought to be specific for microbiota or food antigens. However a formal proof for such conclusions were lacking. 

This week journal Science published a new study from Bendelac's Lab to show that these naturally occurring IgA antibodies are present even in mice devoid of microbiota or food antigens.

In this study the authors analyzed specificity of IgA antibodies using single cell analysis. Interestingly, IgA bound to some but not to all microbiota species. 

Furthermore, most of gut IgA bound to all kind of microbiota-derived components showing a broad polyreactivity (cross-reactivity). Separate test using broadly-neutralizing antibody (bnAb) panel directed against influenza stalk region showed co-staining for microbiota coated by IgA. 

Interestingly, unlike other tissues, numbers of IgA+ plasma cells in small intestine were not reduced in germ-free mice.

Even more surprising, numbers of IgA+ plasma cells in small intestine were not reduced in germ-free mice fed antigen-free diet (amino acid diet).

These results suggest that 

(a) not all microbiota species are targeted by IgA that by itself requires further studies to understand why it is the case.

(b) natural, microbiota-reactive IgA in small intestine develop in absence of exogenous antigenic stimulation that suggests that such specificities are inherited and accumulate spontaneously. 

(c) selection of broadly neutralizing antibodies against viruses could be influenced by microbiota-derived antigens (polyreativity, cross-reactivity)  

posted by David Usharauli

Species-specific microbial attachment to host epithelial cells determines gut IL-17 induction

Gut microbes drive maturation of host's immune system. Not every microbe is able to do it, however. So what microbial qualities determines its impact on host?  

New paper in journal Cell has revealed a simple rule of thumb: microbes that could attach themselves to gut epithelial cells drove induction of local Th17 cells and IgA. Interestingly this microbial quality found to be species-specific. 

Initially, the authors showed that monocolonization of germ-free mice or rats with endogenous segmented filamentous bacteria  (mouse-SFB and rat-SFB) induced Th17 cells in a species-specific manner (mouse data are shown here ).   

Ex vivo stimulation of lamina propria cells with autoclaved fecal antigens (A/C) showed that IL-17 secretion and IgA production were correlated with species-specific access to SFB antigens in the gut.

Additional experiments with WT microbes or microbes lacking adhesion molecules confirmed that epithelial adhesion determined IL-17/IgA production.

In summary, these results indicate that direct adhesion to gut epithelial cells is necessary pre-condition for local immune response induction. Mechanistically, this adhesion by microbes induces production of serum amyloid A (SAA) that primed local T cells for Th17 phenotype.

David Usharauli

  

Gut IgA is mostly T-independent and mostly microbiota-centric

The most IgA is produced at mucosal surfaces in response to antigenic exposure. While IgA is considered to be part of adaptive immune system, controversy still persists regarding signals needed for its production.

New article in journal Immunity may put an end to this discussion, finally. The authors, led by Albert Bendelac (he was one of the discoverer of NKT cells), have used IgA-seq methodology and showed that gut IgA production is mostly T-independent and mostly specific for gut microbiota. 

Initially, the authors showed that proportion of gut microbiota both in small or large intestine are covered with IgA (there are IgA^- and IgA⁺).  

Next, the authors have used IgA-seq approach to phenotype microflora. This method consists of capturing of IgA from the feces followed by sequencing of antigens attached to IgA (in this case microbial 16S rRNA).

Interestingly, unlike IgA^- microflora, there was a clear overlap between IgA⁺ microflora both from large and small intestine, implying common origin.

Next, the authors elucidated the origin of gut IgA. First, both wild-type and T cell-deficient mice found to produce comparable gut microbiota-centric IgA.

Second, no difference was found between wild-type and T_FH-deficient mice (Bcl-6^ΔT) either.

However, the authors showed that few selected microflora members, such as SFB and Mucispirillium could elicit T-dependent IgA response.

Finally, the authors' experiments revealed that most of gut IgA was produced by subset of non-canonical B1 cells called B1b cells in a T-independent manner.

In summary, this study showed that (1) most of small intestine microflora is coated with IgA (2) these IgA are produced in absence of T cell help (3) these IgA is derived from B1b cells whose repertoire was broad and of comparable diversity to that of canonical, spleen B cells and (4) some members of microflora, SFB and Mucispirillium  did appear to induce IgA production via canonical T-dependent manner.

Why is this study relevant? First, it suggests that gut immune health is established in absence of T helper cells via B1b cells. Not much is known about B1b population. Is it possible that many gut immune pathologies (ulcerative colitis, celiac disease, crohn's disease) originate because of alteration in B1b cells?      

David Usharauli

Gut-associated IgA repertoire diversity is maintained independently of continuous gut flora exposure

Gut-associated immune system has a delicate task to maintain tolerance towards endogenous gut flora and food-associated antigens while at the same time to detect and mount protective immune response against invading pathogens.

Clonal size and repertoire diversity of gut-associated IgA pool are influenced by presence of flora antigens. However, little is known of the mechanisms that control gut IgA repertoire stability in response to antibiotic treatment or pathogen exposure.

This new paper in Nature Immunology tried to do exactly this. The authors serially sampled and sequenced individual mouse gut IgA pool longitudinally, after (a) antibiotic, (b) pathogen or (c) defined flora colonization, to determine changes in IgA repertoire diversity.

Initially, the authors showed that IgA repertoire diversity in germ-free mice exposed to various combination of flora (mono or poly-colonization) correlated with richness of colonized flora.

However, when GF mouse gut IgA pool was analyzed, before and after flora exposure, the authors noticed no major IgA repertoire change, implying flora-independent IgA repertoire maintenance (through it is not quite clear here how the authors differentiate IgA repertoire "diversity" versus repertoire "similarity").

Interestingly, the authors made similar observation in gut IgA pool from healthy volunteers exposed to antibiotics.

In addition, the article contains several other experiments, but I was not able to decipher their meaning or relevance to the concept, so I did not discuss them here. In general, paper is poorly written and lacks natural flow between experimental hypothesis and experimental results.

In summary, my interpretation of this paper is following: (a) gut IgA repertoire diversity is non-overlapping between individuals, including genetically identical mice, (b) gut IgA repertoire stability is minimally influenced by exposure to antibiotics or new flora (however if it is true and gut immune system maintains its integrity upon antibiotic exposure, then how can we account for frequent GI issues following antibiotic treatments?)

David Usharauli