Microbiota-derived peptide and autoimmune heart inflammation: a tale of missing data

Papers published in journal Science supposed to undergo thorough high-level vetting process. However, to err is human. Both reviewers and editors are humans and hence they frequently err, for the annoyance of scientists and for the joy of postdocs doing journal clubs.  

Here is an example of a paper that squeezed through the cracks of the Science vetting process. It claims that peptides derived from certain commensal microbiota species cross-react with heart muscle protein, MYH6, causing autoimmune heart inflammation. It has a great Figure 1 showing that MYH6-specific TCR transgenic mice on a germ-free background, lacking microbiota, is protected from heart autoimmunity.

Furthermore, they showed that the re-introduction of microbiota into germ-free makes these mice susceptible to heart inflammation similar to microbiota+ mice.

The authors then tried to identify the microbiota species that contribute to this inflammatory condition. An in silico search identified cross-reactive β-galactosidase (β-gal) mimic peptides in Bacteroides thetaiotaomicron (B. theta) and B. faecis with high similarity to MYH6.

The authors even introduced into germ-free TCR transgenic mice Bacteroides thetaiotaomicron (B. theta) lacking the β-gal. Up to now, it feels that the authors have checked all the boxes necessary for high-quality research. But then for some reason, they do not show survival data comparing Bacteroides thetaiotaomicron (B. theta) with and without the β-gal gene as in figure 1. They just showed how a lack of β-gal Bacteroides thetaiotaomicron modifies MYH6-T cells accumulation in the heart tissue.

So, why the authors don't show survival data of germ-free MYH6 TCR transgenic mice colonized with Bacteroides thetaiotaomicron -/+ β-gal gene? Isn't it the most important result for their hypothesis? Where were reviewers and editors looking?

posted by David Usharauli

IL-17 response to airborne fungi are driven by gut commensal pathobiont C. albicans

A new paper in @CellCellPress from Bacher/Scheffold lab showed that IL-17+ T cell response to airborne #fungus such as #Aspergillus fumigatus found in individuals with #asthma,#COPD, and CF,are in fact expanded by gut commensal pathobiont #Candida #albicans via #crossreactivity.

— David Usharauli (@3DiMMUNE) February 23, 2019

1. majority of IL-17+ #Tcells found in healthy donors are specific C. albicans. #Immunology #TH17 #fungi pic.twitter.com/3WITidrcCn

— David Usharauli (@3DiMMUNE) February 23, 2019

3. In fact, when C. albicans-specific T cells were depleted from #Aspergillus fumigatus-specific T cell pool IL-17 response was gone too suggesting that most if not all of IL-17+ T cells specific for A. fumigatus were indeed #crossreactive to C. albicans. pic.twitter.com/cZEu50Uwep

— David Usharauli (@3DiMMUNE) February 23, 2019

5. similar expansion of Aspergillus fumigatus-specific T cells producing IL-17 were observed in individuals with #crohnsdisease too. pic.twitter.com/L0VbmCcFj5

— David Usharauli (@3DiMMUNE) February 23, 2019

As both #fungi are naturally available in the environment any #immunopathology driven by excess of IL-17 response must be the result of some other changes in the host that makes it susceptible to pathology. what???

— David Usharauli (@3DiMMUNE) February 23, 2019

New auto[cross-reactive]-antigen, GDP-l-fucose synthase, identified in MS patients

Multiple sclerosis (MS) is a debilitating autoimmune disease affecting nervous system. Several auto-antigens have been traditionally suspected for MS pathology, mostly derived from neural tissues such myelin. However, more recently the list of relevant auto-antigens expanded to include such member as RAS guanyl releasing protein (RASGRP1–4) family. Similarly, a new paper in Science Translational Medicine presented evidence that yet another auto-antigen, GDP-l-fucose synthase, is a target antigen in a subset of MS patients.

The authors has used modified version of positional scanning epitope library to identify epitope derived from auto-antigen, GDP-L-fucose synthase, as a target epitope for CD4+ T cell clone, TCC21.1, derived from MS patients with DR15 haplotype. Peptides sequences matching GDP-l-fucose synthase was detected in brain tissue.

MS patients whose T cells displayed high response to peptides from GDP-L-fucose synthase in stimulation assay showed high responses to myelin proteins as well.

Curiously, almost all high responders carried DRB3*02:02 allele. 

Finally, the authors speculated that sequence similarity between human and microbiota-derived evolutionary conserved GDP-L-fucose synthase could be a factor that initiates MS pathology in these patients.

In summary, this study has a lot of nice data that support idea that GDP-L-fucose synthase is a  new auto-antigen that could be relevant in molecular diagnosis of MS pathology.

There are several questions about this study. GDP-L-fucose synthase expression is not restricted to brain tissue. So, it should be relevant to understand if other tissues were affected in those MS patients. Second, cross-reactivity per se is not sufficient to explain how and why autoimmune responses are being initiated. Microbiota expressing GDP-L-fucose synthase most likely reside in patients long before MS, maybe even since birth, and T cells are tolerant to them. So, what has to be changed, in a antigen-specific manner, to make T cells less tolerant to initiate specific autoimmune attack on nervous system and not a total autoimmunity targeting all available auto-antigens? 

posted by David Usharauli

Access to self antigens during germinal center reaction improves self/nonself discrimination against mimicry antigens

This week journal Science published short paper from Chis Goodnow's lab that raises very interesting question about biological significance for existence of anergic self-reactive B cells. Ordinarily, developing B cells when encountering self-antigens undergo deletion, receptor editing or physiological receptor signaling down-regulation that makes such 'anergic' B cells refractory to presence of normal level of self antigens. However, anergic B cells could be re-energized if challenged with high density self antigens or antigens sharing epitope similarity with self antigen.

Now, new study indicates that rather than developing into full blown auto-reactive immune response, anergic B cells when challenged with mimicry antigens mutates its receptors in a such a way, during process of hypermutation, as to achieve a high degree of discrimination between mimicry antigen and actual self antigen.   

The experimental set up itself is quite simple, only complex aspect was to analyze single cell B cell receptor mutation and their binding affinity recovered after antigen challenge. Two type of hosts were used here. Both groups harbor small numbers of self-reactive B cells (CD45.1+ SW_HEL B cells)  but only one group also harbored a specific antigen detected by these transgenic SW_HEL B cells and expressed "as as an integral membrane protein, mHEL^3X, encoded by a transgene with a ubiquitin promoter".

As expected SW_HEL B cells in double transgenic hosts were anergic with decreased surface immunoglobulin M (IgM) expression. However, these anergic B cells could be re-activated in germinal centers when challenged with Sheep red blood cells (SRBCs) covalently coupled with self antigen, HEL^3X, at high density.

Next set of experiments however showed very unusual results. When challenged with mimicry antigen DEL which slightly differs from self HEL antigen anergic B cell receptors in double transgenic hosts rapidly accumulated mutations that decreased binding affinity to self HEL antigen.

In fact, single cell BCR receptor analysis clearly showed that presence of self antigens dramatically enhanced anergic B cell receptor mutations that allowed up to 5,000-fold better discrimination capacity between self and mimicry antigen (pre vs. post comparison). This is based on assumption that starting affinity to self are the same for both normal and anergic SW_HEL B cells population. 

In summary, this study suggests that during germinal center reaction where B cell receptors undergo hypermutation, anergic B cell repertoire, in presence of self antigen, could be salvaged (redeemed) by accelerated accumulation of mutations that modifies their original specificity away from self antigens and allowing more fine discrimination between self and mimicry, cross-reactive nonself antigen. In this scenario, self antigens serve as negative-feedback templates that hypermutating receptors interacts repeatedly in real time to achieve minimal level of binding.

In my view such negative-feedback loop to B cells can only delivered by specialized cell type in germinal center that maintains, keeps memory of host's unadulterated "self antigen collection'' visible to B cells, a task somewhat similar to Foxp3+ Tregs. So, it is possible that new cell type need to be discovered that does it or it is also possible that the same Foxp3+ Tregs localized in germinal centers, referred as follicular Foxp3+ T regs, do it too. 

What are the global implication for such mechanism: It could explain why anergic B cells hang around and how their repertoire could be salvaged without compromising tolerance. The authors also puts forward another intriguing idea that commensal mcrobes and their antigens could serve as negative-feedback loop 'self' templates for anergic B cells that allows them to discriminate between self and mimicking nonself during immune response. 

posted by David Usharauli