Gut-specific eosinophils suppress IL-17 production by antagonizing IL-1 signaling

Prior to discovery of Toll-like receptors (TLR) in late 90s, immunology was dominated by concepts involving adaptive immune cells. However, for past 15 years it has become evident that innate immune cells of "all shapes and forms that exist out there" are equal partners with T and B cells and frequently provide instructions and actually control the magnitude and directions of adaptive immune response. For example, development of successful, next generation vaccines would require incorporation of these new concepts. 

It seems innate immune system is even more complex and unpredictable than T or B cells are. For instance, this week Journal of Experimental Medicine published new study wherein the authors reported that eosinophils, a cell population usually implicated in allergy and anti-helminth response, suppressed Th17 cells by antagonizing IL-1 signaling.

First, the authors observed that Th17 cells in ΔdblGATA-1 mice [which lack eosinophil-lineage cells] were significantly increased in the small intestine, but not in the spleen or mesenteric LNs (MLNs).

In vitro generation of Th17 cells were inhibited by presence of small intestinal eosinophils. 

Functional profiling revealed that small intestinal eosinophils secreted high amount of IL-1 receptor antagonist (IL-1Ra), which competes with IL-1β for receptor binding (IL-1 signaling is involved in Th17 development).

Interestingly, eosinophils were the major producers of IL-1Ra in small intestine, since no IL-1Ra was detected in small intestinal tissue from ΔdblGATA-1 mice. Of note, eosinophils from blood or bone marrow produced little or no IL-1Ra. 

In addition, the authors showed that small intestinal eosinophils derived from MyD88-KO or Germ Free (GF) mice produced WT-level IL-1Ra, suggesting its independence from gut flora or major TLR signaling.

Role of IL-1Ra in eosinophil mediated Th17 suppression in small intestine was confirmed in experiments with eosinophils derived from IL-1Ra-KO mice.   

In summary, this study showed that small intestine harbors specialized eosinophils secreting high amount of IL-1Ra and suppressing steady-state development of Th17. 

David Usharauli

Sensing of RNA:DNA heteroduplexes by cGAS/STING pathway mirrors human autoimmune Aicardi–Goutières syndrome

Autoimmune diseases occur when bodies immune system continues to respond to self-antigens. Ordinarily, such [mis]directed responses are ascribed to adaptive immune system which possesses myriads of randomly generated receptors. So it is quite reasonable to assume that some of those receptor equipped clones (T or B cells) could go haywire and attack their own body's particular antigen or set of antigens. There are few autoimmune diseases known thus far that fall in this category (e.g. in Graves' disease, thyroid stimulating immunoglobulin [over]stimulates the same receptor that normally responds to only thyroid stimulating hormone). This is what is called lack or break of tolerance [mostly due to cross-reactivity]. 

However, much larger category of autoimmune diseases are due to genetic defects of different kind. These conditions harbor mutations in "housekeeping" molecules that are required for proper cellular homeostasis. Such mutations leads to immunopathologies that target broad spectrum of antigens or could even be restricted solely to the activation of innate immune system.    

For example, mutations in enzymes that ordinarily degrades self-DNA could lead to systemic immunopathologies due to activation of DNA sensing molecules cGAS or STING.

One such genetic mutation in ribonuclease (RNase) H2B (a enzyme that degrades cellular RNA:DNA heteroduplexes) leading to pathological cGAS/STING activation was recently described in the EMBO Journal.

This study revealed that mutation in RNase H2B that renders it less active led to up-regulation of interferon‐stimulated gene (ISG) transcripts. 

Follow-up experiments confirmed that interferon "signature" in RNase H2B KO cells was driven by cGAS (deletion via CRISPR/Cas9 genome editing) and STING activation (STING KO).

Exactly what type of self-DNA is detected that leads to cGAS and STING activation in cells with RNase H2B mutation is not yet clear. It could be (a) a cytosolic RNA:DNA heteroduplexes accumulated as a consequence of reduced RNA:DNA degradation, or it could be (b) a cytoplasmic DNA with embedded ribonucleotides that may accumulate due to impaired ribonucleotide excision repair (RER).

With the advance of human genomic testing, it is becoming evident that large portion of autoimmune diseases / immunopathologies may have a clear genetic basis and could be specifically targeted for therapeutic purpose.

David Usharauli

    

Microbiota-specific serum IgG protects against systemic bacterial dissemination

This week journal Immunity published a study from Gabriel Núñez lab describing the role of serum IgG in protecting the body against systemic dissemination of gut commensal bacteria.

Initially, the authors showed that WT mouse serum contained IgG specific for fecal bacteria and its production depended on (a) gut microflora and (b) T cells.

These data indicated that gut bacteria [or its parts] "circulate systemically in spite of intact intestinal barriers." Indeed, the presence of bacterial 16S rRNA gene was confirmed in the spleens, for example.

However, the authors showed that such "access" was not a "random" translocation. Only certain gut bacteria (for example, Enterobacteriaceae and Moraxellaceae, but not Porphyromonadaceae and Prevotellaceae) were able to disseminate internally.

Correspondingly, immunoblotting analysis of serum IgG against fecal bacterial antigens confirmed "selective" IgG response to gut flora gram-negative Escherichia coli (EC) or Klebsiella pneumoniae (KP), but not to gram-positive Enterococcus faecalis (EF) or Clostridium bifermentans (CB).

In parallel experiments, the authors showed that serum IgG from WT mice, but not from quasi-monoclonal (QM) mice which have limited B cell repertoire, could protect B cell deficient mice from gram-negative E.coli infection.

Since gram-negative bacteria are recognized by TLR4 the authors tested mice deficient TLR4 signaling pathway. Indeed, absence of TLR4 specifically on B cells selectively reduced serum IgG against fecal bacteria (single TLR4-KO mice showed the same effect, while single TLR2-KO had no effect).

Similar to B cell-deficient mice, mice with TLR4-deficient B cells were highly susceptible to E.coli infection and could be rescued only by IgG from WT mice, but not by IgG from quasi-monoclonal (QM) mice.  

Analysis of gram-negative WT or mutant bacteria showed that serum IgG was mostly directed against murein lipoprotein (MLP), a highly conserved outer membrane protein of ~7 kD expressed abundantly in gram-negative enterobacteria.

Finally, the authors showed that monoclonal anti-MLP IgG injection could protect B cell deficient mice from E.coli infection almost as efficiently as total WT IgG injection.

In summary, this study suggests that normal serum IgG protects the body in situations when gram-negative bacteria from gut gains access to systemic circulation (for example, in "leaky gut" or during a sepsis). Generation of such protective IgG requires presence of T cells and normal B cell receptor diversity.

It is not clear what role IgA plays in this process. The authors have not tested IgA-KO mice to determine its exact contribution in preventing gut flora translocation. No data are provided regarding presence and [amount] of gut bacteria in systemic circulation in different KO mice shown in this study either. This would have given some valuable information.

David Usharauli