Saturday, May 5, 2018

IgA protects resident commensal microbiota against competitors

This week journal Science published new study from Sarkis Mazmanian lab at Caltech describing role of IgA in providing strain-specific competitive advantage to certain resident commensal microbiota. 

His lab has been studying immunobiology of Bacteroides fragilis (B. fragilis), a gut commensal. In initial series of experiments they have compared germ-free mice mono-colonized with either wild-type B. fragilis or its mutant variants such as, Δccf, shown to modify biosynthesis of its capsular polysaccharides. They noticed that in co-housing experiments wild-type B. fragilis from one mouse could out compete mutant variant in another mouse in a horizontal transfer assay.   




Since B. fragilis polysaccharides are known to interact with host's immune system, the authors wanted to find out whether host's immune system influenced co-housing experiments. Not surprisingly, the authors found that mutant B. fragilis did not efficiently bind IgA (induced by wild-type B. fragilis) and that it in turn induced IgA repertoire that bound wild-type B. fragilis even less effectively, suggesting some kind of association between IgA and missing antigens on mutant B. fragilis.  




To verify these observations, the authors compared co-housing experiments between germ-free IgA+ and IgA-KO mice (or treated with B cell depleting antibody) mono-colonized with wild-type B. fragilis. Indeed and surprisingly this time, wild-type B. fragilis resident in IgA KO mice were easily overtaken by wild-type B. fragilis from mono-colonized wild-type mice. These results suggested that in absence of IgA wild-type B. fragilis has lost competitive advantage against wild-type B. fragilis resident in IgA+ mice.




What could these results mean in biological context: it appears that certain resident commensal microbiota benefit from interacting with IgA. The authors proposed that "during health, IgA fosters mucosal colonization of microbiota with beneficial properties....while disease states may induce (or be caused by) IgA responses to pathogens or pathobionts that disrupt healthy microbiome equilibria." This is an interpretation that does not provide clear mechanistic explanation as to how IgA response could make such discrimination at the level of antigens between which microbes to keep and which ones to eject from the host. 


posted by David Usharauli

Saturday, April 28, 2018

SUPRA CAR T cell system provides more of cosmetic rather than a real advance

This week journal Cell published a new study from scientists at Boston University describing in their own words "a split, universal, and programmable (SUPRA) CAR system" that supposed to provide several advantages over conventional CAR-T cell system. I reviewed and present here my conclusions on this paper.   

The rationale behind SUPRA CAR T cell design was to develop flexible, "plug-and-play" system to fine tune CAR T cells' activity against tumors without need to redesign it over again. SUPRA consists of two modules: signaling zipCAR construct is artificially expressed by T cells on their surface and soluble zipFv construct expressing tumor antigen specific scFv portion which is injected into system. "Zipper" portions of zipCAR/scFv constructs could interact with each other and by injecting different variants of zipper one can modulate strength of interaction.



What advantage(s) SUPRA CAR T cell design provide? 

1st advantage the authors showed could be to tune signal strength of original SUPRA zipCAR T cells interaction with tumor specific zipFv construct by injecting competitive zipFv constructs that have different affinity to zipCAR module and thus modulate tumor specific zipFv action (to prevent cytokine storm).




2nd advantage is thought to be use of the same zipCAR T cells and inject two different zipFv constructs specific for two different tumor antigens (to prevent tumor escape).




3rd advantage the authors suggested would be to deploy decoy zipFv that could inhibit tumor specific zipFv activity only when decoy scFv interacts with non-tumor specific antigens and thus limiting non-target effects in different tissues (to prevent off-target tissue damage).





Later in the paper the authors went on to present series of experiments that showed comparison of effectiveness of SUPRA CAR T cell construct vs. conventional CAR T cells against two different tumor models in vivo.




On the surface all these experiments look quite impressive. However, close analysis of data shows that advantages are more of cosmetic in nature rather than real ones. First, none of those above mentioned three advantages were actually shown for tumor models in vivo (for some reason the authors did not show how injection of competitive low-affinity zipFv construct could affect tumor protection experiments in vivo or whether double antigen expressing tumors could be efficiently eradicated). Moreover, in vitro experiments showing decoy effect was done in manner that is incompatible for in vivo experiments (one cannot wash away decoy zipFv in vivo before introducing tumor specific zipFv construct and it is likely that free floating decoy zipFv construct could inhibit tumor specific zipFv activity even in absence of decoy tissue antigen).

posted by David Usharauli


Saturday, April 14, 2018

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+ SWHEL B cells)  but only one group also harbored a specific antigen detected by these transgenic SWHEL B cells and expressed "as as an integral membrane protein, mHEL3X, encoded by a transgene with a ubiquitin promoter".



As expected SWHEL 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, HEL3X, 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 SWHEL 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