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       

Sunday, April 1, 2018

Chronic systemic inflammation in Lupus could be driven by bacterial antigen mimicry to human autoantigen Ro60

Earlier I discussed a new study in journal Science from Martin Kriegel's lab at Yale University School of Medicine that showed how translocation of commensal bacterial species E. gallinarum could amplify autoimmune phenotype in Lupus prone mouse model. It appears that his lab had another paper under review that was published this week in Science Translation Medicine, a sister publication run by Science. In this study the authors tried to show that auto-reactivity to auto-antigen Ro60 frequently observed in lupus susceptible patients could potentially be driven and sustained by commensal microbial species [turned pathobionts] expressing Ro60-like molecules.

The authors showed that there are substantial overlap between T cell epitopes in human Ro60 and Ro60 molecule from bacterial species such as Propionibacterium propionicum (P. prop) and Bacteroides thetaiotaomicron (B. theta)

Memory T cells freshly sorted from anti-Ro60 reactive SLE (lupus) patients responded to P. prop and B. theta lysates.   

In summary, this study suggests that commensal bacterial species turned pathobionts could initiate and sustain lupus phenotype in susceptible individuals. This is not a definitive study. First, it is obvious that commensals per se cannot induce lupus but only in susceptible individuals (almost every individual carries these commensals). What exactly constitutes this susceptibility in humans to lupus [or any other autoimmune diseases] is a black box presently. Moreover, their "analysis revealed no significantly different bacterial OTUs in the fecal, oral, or skin microbiomes between anti-Ro60–positive and anti-Ro60–negative subjects". However since resolution of currently available microbiome analytical tools are quite low it is still possible that at bacterial species or strain level there maybe significant differences between healthy vs. lupus and Ro60-positive vs. Ro60-negative populations.

posted by David Usharauli

Wednesday, March 21, 2018

Live, not dead bacteria, augments human antibody response via TLR8 - T follicular helper cell axis

Immune system protects against pathogens. In natural settings, live pathogens cause diseases. So, it is very intuitive to think that immune system could have developed a specialized way to detect pathogen's "live signature". Few years back, Julie Magarian Blander's lab (former postdoctoral scientist in Ruslan Medzhitov' lab) published study showing that mRNA from live bacteria served as "live signature" they called vita-PAMP

Now, new paper in nature immunology from Leif Sander's lab (a former postdoctoral scientist in her lab and the first author of initial study) showed that in humans TLR8 may serve as a detector of vita-PAMP mRNA from live bacteria and augment antibody response. It is very nice study done primarily on ex vivo/in vitro human cells cultures (interestingly, Blander's lab also published this month new study about vita-PAMPs in mice in journal Immunity. But it is very messy and bloated study. Basically, in this case pupil outdid his [former] master).

Here the authors showed that live, replication-defective E. coli strain but not heat-inactivated dead one, could induce differentiation of human follicular helper T cells (TFH cells).

These TFH cells were functionally active inducing antibody-secreting plasmablast generation from B cells.

Live bacteria, not dead one, specifically induced IL-12p70 generation from human monocytes (unlike mouse, differentiation of human TFH cells requires IL-12p70).

Indeed, antibody blockade confirmed a major role of IL-12p70 in generation of IL-21-producing human TFH cells.

Stimulation of human monocytes with various TLR agonists showed that engagement of TLR8 (single-strand RNA sensor) with its agonist ligand (CL075 or R848) were responsible for vita-PAMP effect on IL-12p70 production.

Similar vita-PAMP effect of TLR8 signaling were seen for human IL-21-producing TFH cell generation.

In summary, this study indicates that unlike LPS-derivative MPLA or CpG fortified vaccines, inclusion of TLR8 agonists, such as CL075, could augment antibody responses. While study is well done we need to keep in mind that it is produced by members of the 'same initial' group (Blander and Sander) who first reported vita-PAMP effect. We don't have analogous studies from other labs who can independently confirm these observations

posted by David Usharauli