IBD converts tolerant antigens into immunogenic

Inflammatory bowel disease (IBD) is a pathological condition wherein body's immune cells wrongly attack its own or commensal microbiota-derived antigens that initiates a vicious cycles of permanent inflammation.

 

However, it is still not clear whether immune system attacks "new" antigens from microbiota or simply it loses adaptive tolerance to "old" ones. 

 

New study in Science Immunology tried to answer it to the extent it was possible to do in mouse model.

 

First, the authors generated IBD condition in mice by treating them with DSS + anti-IL10R. Keep in mind this is highly artificial model. Then, they transferred naïve T cells from previously established transgenic T cell lines specific for unknown commensal antigens that were known to drive Treg phenotype. Naïve T cells transferred into control WT mice generated Tregs while the naïve T cells transferred into IBD-conditioned mice preferably developed into effector T cells.

 

 

 

When analyzed for antigen specificity, the authors found that transgenic naïve T cells were reactive to antigens derived from Helicobacter species that have expanded during IBD-conditioning.

 

 

 

In vivo studies also confirmed that Helicobacter species could induce Treg generation from naïve transgenic T cells in "normal" condition.

 

 

 

Interestingly, transfer of T cells specific for other microbiota species that also underwent expansion during IBD-conditioning did not produce T cell expansion.

 

 

 

Finally, transfer of Treg-TCR transgenic naïve T cells into RAG-KO mice produced IBD when co-injected with Helicobacter species.

 

 

 

What these data indicate? In my view the authors made one correct and one wrong interpretation. First, they were correct to conclude that T cell response to IBD could be directed to "old" microbiota antigens rather than "new" never before seen microbiota-derived antigens. So basically in IBD we are losing tolerance rather than gaining immunity to microbiota antigens.

 

However, they made wrong conclusion that naïve T cells are converted into Tregs in vivo based on context (normal versus IBD). In their study loss of Treg generation is inhibited either during IBD-conditioning or in RAG KO hosts which could argue alternatively that such outcome has to do with failure of naïve T cells to interact with existing Tregs specific for the same or similar antigens in these scenarios (IBD or RAG-KO).

 

posted by David Usharauli

 

 

        

Fusobacteria, a gut commensal, contributes to autoimmune type I diabetes in mice

Initiation of autoimmune disease is still an immunological mystery. Some forms of autoimmune diseases are results of genuine genetic defects in signaling molecules within immune system. Other forms show strong linkage to certain HLA haplotypes that present antigenic epitopes. More recently scientists focused on the role of gut commensals in autoimmune diseases.

A new study in Jounral of Experimental Medicine showed that cross-reactivity at the epitope level between gut commensal Fusobacteria-derived magnesium transporter and β islet-specific glucose-6-phosphatase catalytic subunit–related protein (IGRP) contributed in autoimmune diabetes development in IGRP-specific CD8 T cell transgenic, CD8+ TCR NY8.3 NOD mice.

Initially, the authors observed that unlike MyD88KO NOD mice, MyD88KO CD8+ TCR NY8.3 transgenic NOD mice showed accelerated diabetes development (though unlike the authors, I don't find this surprising).

Interestingly, when co-housed with WT NOD mice, MyD88KO TCR NY8.3 transgenic NOD mice showed enhanced protection against diabetes, suggesting dominant role of fecal bacteria present in WT NOD mice in providing this protection.

Since it is known that NOD mice susceptibility to diabetes is commensal-dependent, the authors sequenced fecal microbiome in MyD88KO NY8.3 NOD mice to determine its composition. Not surprising, certain families of commensals underwent changes on MyD88KO background.

When the authors compared the IGRP_206–214 peptide sequence against bacterial protein sequences in the nonredundant protein sequence database, they found several hits shared strong homology with IGRP_206–214 peptid, the native autoantigen detected by NY8.3 CD8+ T cells. One such peptide, W15944, was derived from L. goodfellowii, a member of the phylum Fusobacteria (gram-negative anaerobe), a human and NOD mouse oral commensal.

Indeed, W15944 stimulated NY8.3 CD8+ T cells could transfer diabetes in NOD mice.

Finally, introduction of L. goodfellowii into WT NY8.3 NOD mice accelerated diabetes development, further suggesting role of L. goodfellowii in this process (while in general, this paper is of high quality, it lacks some of the crucial experiments such as, for example, (1) introduction of L. goodfellowii into NY8.3 NOD mice on germ-free background and (2) experiments with L. goodfellowii lacking cross-reactive W15944 peptide).

In summary, the author showed that in this artificially constructed NOD mice, IGRP-specific CD8 T cells could be activated by commensal-derived cross-reactive peptide and initiate autoimmune destruction of  β cells (it is remains to be seen why Foxp3+ regulatory T cells are incapable of preventing such T cell attack).

David Usharauli

Friendly gut microbes could be our most dangerous enemies

Some studies are interesting and thought-provoking, and some studies are just studies. But this new paper in journal Science is a beauty. One rarely comes across to this type of research.

As many of you many know, we harbor large numbers of friendly microorganisms inside (gut, lung) or outside (skin). In many aspects, these commensal microbes share characteristics, like LPS (endotoxin), with their pathogenic siblings. However, since commensal microbes can live with us in peace, they probably had acquired some properties to make such co-habitation possible. 

Indeed, the authors, led by Andrew Goodman at the Yale University School of Medicine, found that unlike pathogenic microorganisms (E. coli, S. enterica, C. rodentium), major subsets of our commensal microbial world were highly resistant to several cationic anti-microbial peptides (AMP).

The authors determined that this increased resistance of commensals to AMP was relate to a single gene encoding enzyme, LpxF, responsible for removing phosphate group from LPS (component of microbial cell wall). 

In vivo experiments with mono-colonization of LpxF-deficient mutant human commensal microbe, B. thetaiotaomicron, confirmed that unlike LpxF-complemented B. thetaiotaomicron, B. thetaiotaomicron deficient for LpxF was easily displaced by wild-type B. thetaiotaomicron in presence of pathogenic C. rodentium infection or chemical, DSS-induced inflammation. Non-virulent tir mutant C. rodentium or commensal SFB had no impact on LxpF-deficient B. thetaiotaomicron population stability.

Similar results were observed when 14 member of human gut flora were transplanted into germ-free mice and exposed to C. rodentium infection, implying that LxpF played important role in population stability during gut inflammation.

Finally, gut microbes obtained directly from healthy humans displayed similar resistance to AMP.

In summary, these results suggest that human gut microflora acquired resistance to its own host's anti-microbial peptides thus providing additional mechanism responsible for peaceful co-existence and gut ecological stability. 

Of course, such resistance of gut commensals to AMP may pose problem when gut microbes invade inner systems, as during sepsis. So, this is a double-edge sword dilemma for host-microbe mutualism. 

The experiments that are missing, in my opinion: (1) Repeat of experiments with anti-microbial peptide deficient mice models to confirm that population stability is indeed related to resistance to AMPs; (2) It is not clear how presence of LxpF-deficient commensal microbe could affect the severity of inflammation induced by C. rodentium.

David Usharauli

Friends or Frenemies?

This is a second paper from Host Cell and Microbe describing how mouse gut residing commensal microbe's metabolic end-product, succinate, fuels the virulence of pathogenic Citrobacter rodentium (C. rodentium).

This study, led by Vanessa Sperandio at UT Southwestern Medical Center, Dallas, analysed virulence factor expression in mouse C. rodentium, in presence of mouse gut commensal Bacteroides thetaiotaomicron (Bt). The authors have used C. rodentium disease model that mimics E. coli infection in humans.

It appears that the authors' initial objectives were to study the mechanisms by which gut commensals were driving virulence factor expression in pathogenic microbes and only later diverted their attention towards the role of succinate metabolism in this process.

First, the authors showed that presence of Bt enhances expression of several virulence factors in E. coli and in an in vitro culture.  

Similar results were observed with C. rodentium  culture.

Interestingly, this enhancement was not observed in cra mutant E. coli that is incapable of sensing sugar fermentation products, like succinate (however, the authors did not show similar results with cra mutant C. rodentium).

In vivo experiments with either wild-type C. rodentium or mutant strains showed that reconstitution of antibiotic treated mice with Bt could enhance C. rodentium pathogenicity. The authors reported that severity of infection with cra mutant C. rodentium  was attenuated, though not abolished, it appears (implying that C. rodentium showed cra-succinate independent virulence).

Still, analysis of sugar fermentation products in the day 2 post infected mice cecum showed that there was selective increase in succinate in presence of Bt or C. rodentium.

Finally, addition of succinate to E. coli culture could enhance expression of virulence factors in the wild-type but not in a cra mutant E. coli strain (again, the authors did not show similar results with cra mutant C. rodentium).

In summary, this study and another one from Justin Sonnenburg's lab, highlighted the potential detrimental role of gut commensals in fueling the virulence of pathogenic microbes. We are thinking that the term gut commensal implies friendly microbe, however occasionally those friendly microbes could involuntarily provide support to pathogenic ones.

David Usharauli