B cell activation by antigen promotes desegregation of IgM and IgD islands

There are two models of B cell activation. In one model, B cell activation is promoted by clustering of individual IgM molecules. Second model, however, argues that B cell activation is promoted by de-clustering of IgM "islands". This second model is mainly based on research by Micheal Reth's lab.

This new paper from Reth's group published in Science Signaling provided additional evidence for molecular architecture of B cell receptors that support desegregation model of B cell activation. Here, the authors showed that upon antigen engagement both IgM and IgD islands undergo dissociation allowing receptor meetup.

To study B cell receptor architecture on resting or activated B cells, the authors have used 3 different but complementary approaches: (1) two-color direct stochastic optical reconstruction microscopy (dSTORM), (2) two marker transmission electron microscopy (TEM) and (2) proximity ligation assay (PLA). 

First, the authors showed that on resting B cells, both IgM and IgD are segregated into islands containing on average 30 IgM and 48 IgD molecules, respectively.

Next, the authors showed that upon activation, both IgM and IgD islands show reduction of number of receptors per islands, implying that individual IgM or IgD receptors are leaving the islands.  

Interestingly, the authors showed that both IgM and IgD islands could independently respond to the antigen.  

In addition, detection of IgM and IgD (co)-localisation by TEM after antigen exposure confirmed receptors meetup (receptors were detected by secondary antibodies conjugated to two different size gold particles).  

In summary, this study supports the hypothesis that upon B cell activation IgM and IgD receptors leave "home islands" and move close to each other, thus facilitating sharing of downstream signaling molecules. This study is a "biochemical" study that does not incorporate "biological" significance of BCR dissociation, however.   

David Usharauli

α-IL23, not α-IL17 antibody blockade, shows protection in IBD model

Previous studies indicated that cytokines IL-23 and IL-17 act in concert to induce and perpetuate autoimmune inflammation. This is certainly true for psoriatic skin inflammation.

However, two new back-to-back studies in journal Immunity provided evidence that when it comes to intestine inflammation, role of IL-23 and IL-17 goes in opposite direction. Here, blockade of IL-17 exacerbate while blockade of IL-23 inhibits intestine pathology and gut wall permeability.

Both studies came from research groups working in biotech/biopharma (Daniel Cua's group at Merck Research Laboratories and Jennifer Towne's group at Amgen [presently at Janssen]). Both studies reached similar conclusions. 

Here, the authors treated colitis-prone mice with blocking antibodies specific for IL-23 or IL-17 cytokine family. Surprisingly, both α-IL17A or α-IL17RA treatment worsened gut pathology, while α-IL23 (p40 or p19 subunits) antibody showed protection.

Unlike α-IL23, α-IL17RA antibody treatment was associated with increased gut wall permeability (serum sCD14 and LBP), implying IL-17 role in gut health.

Similar increased gut leakage (with FITC-dextran) was observed by Merck's team in chemical irritant DSS-induced GI inflammation model in IL-17KO mice (though Amgen's team did not observe it in DSS mouse model with α-IL17 treatment. It could be that α-IL17 antibody blockade did not fully inhibit IL-17 action as it could be expected in IL-17KO mice).

Additional experiments showed that γδ T cells were the main producers of gut IL-17 in DSS model and that γδ T cell KO mice showed the same increased GI tract permeability as IL-17KO mice.

Finally, Merck's team showed that (a) γδ T cell-derived IL-17 production in the gut were mostly IL-23 independent and (b) IL-23rKO mice were protected against worsening GI wall pathology.

In overall, these two studies suggest that local gut tissue associated IL-23 independent but γδ T cell-derived IL-17 production plays a protective role in gut permeability. Basically, this means that α-IL23 blockade, but not α-IL17 pathway inhibition, would most likely provide benefits to patients suffering from GI tract idiopathic inflammation.

David Usharauli

Sialic acid decorated multi-valent nanoparticles prevent sepsis mortality in mice

Sepsis is an uncontrolled tissue-immune [hyper]response that leads to the organ failure and frequently patients' death. At present, there is no specific treatment for sepsis, just supporting care. Obviously, sepsis represents important unmet medical condition.

New study in Science Translational Medicine provided evidence that sialic acid coated nanoparticles could prevent sepsis-induced death in mice model of acute sepsis.  

This study is based on prior evidence that binding of sialic acid (type of sugar) to its receptors called Siglecs on macrophages or neutrophils could inhibit inflammatory pathways. Here, the authors showed that unlike bi-valent anti-Siglec antibodies, administration of multi-valent sialic acid coated nanoparticles (in a form of α2,8 sacharide linkage), at both T= 0h or T= +2h, prevented endotoxin induced animal death (i.p. injection).

Similar protection was observed in anesthetized mice undergoing cecal ligation and puncture (CLP) procedure (here too, α2,8-NANO was administered i.p.).

Additionally, intra-lung administration of α2,8-NANO showed significant (but not as impressive) protection against lung inflammation (a secondary complication following cecal ligation and puncture procedure).

The authors showed that protection provided by α2,8-NANO was IL-10 dependent in endotoxin injection model (though, for some reason, the authors had used 1mg α2,8-NANO here, not 2mg α2,8-NANO, as in other figures). Also, application of exogenous IL-10 in IL-10KO mice would have provided more clear role of IL-10 here. 

Finally, the authors tried to show that α2,8-NANO application could be useful for human condition as well. They showed that α2,8-NANO could reduce inflammatory cytokine secretion from primary monocytes stimulated with endotoxin in vitro (but for some reason the authors decided to show "processed", not raw data, implying that data were either not consistent or showed too much variability).

In summary, this study suggests that sialic acid decorated multi-valent nanoparticles may play beneficial role in sepsis management (I want to note here that I am against conducting such distressing experiments on lab animals as are i.p. injection of endotoxin or CLP. We must find better alternative models).

As mention earlier, this study comes with 2 major drawbacks (that make this study not easily "translational"):

(1) while animal studies, shown here, were quite impressive, primary human cells showed less sensitivity towards α2,8-NANO application.
(2) even in animal models, α2,8-NANO was applied through i.p. injection. This route of injection is completely non-applicable for human use and I am puzzled that labs still continuing using it. Why not just use injection via i.v. route?

David Usharauli