Saturday, June 16, 2018

First-born advantage of early produced CD8 T cells

During immune response to infection or vaccine antigen-specific T cells differentiate into various categories of effector/memory population. This is a stochastic process that follows some not yet fully understood "rules". New study in Cell suggests that one of those rules is a "date of birth" of individual CD8 T cells that participate in immune response. 

For this study the authors used genetically modified mice where T cells could be permanently color-labeled after injection of tamoxifen (CD4 promoter-driven tamoxifen-inducible CRE mice, CD4cre-ERT2). When compared to each other, CD8 T cells produced early on during neonatal stage (day 1-7) harbored high proportion of cells with innate-like phenotype (CD44+/CD122+) than those CD8 T cells produced after day 28.

Similar phenotype was maintained even in mice where both day 1 and day 28 CD8 T cells were produced in the same mouse (following neonatal RFP+ thymus transplantation into YFP+ adult mice and analyzed at indicated time period post transplantation).

Notably, early-born CD8 T cells were characterized by heightened sensitivity to innate cytokines such IL-18 and IL-12 and rapid initial response to cognate antigen (between day 1-7 post-infection).

The authors opined that various categories of effector/memory T cells observed in other studies should be re-interpreted in light of "layered" CD8 T cells wherein CD8 T cells of different "date of birth" are producing different effector/memory T cell population.

It is certainly interesting observation. But there are several missing opportunities in this study. First, due to their innate-like phenotype, the authors should have looked at the role of microbiota in imprinting these features of early-born CD8 T cells. Second, it is not clear and the authors did not examined if biologically, lack of early-born vs. adult-born CD8 T cells, would have modified the host's response to infection. Third, the authors did not analyze whether early-born versus adult-born CD8 T cells differed in their TCR profile (even using transgenic CD8 T cells on WT background is not proper control). Fourth, different effector/memory categories have been produced by injection of a single CD8 T cells so individual CD8 T cells can indeed produce diverse phenotype of effector/memory T cells. 

posted by David Usharauli

Saturday, June 2, 2018

Monocyte-derived cytokines IL-1 / IL-6 contribute to CAR-T cell-induced cytokine-release syndrome and neurotoxicity

This week Nature Medicine published two papers showing in mouse CAR-T model that cytokine-release syndrome (in both studies) and neurotoxicity (in one study only) were primarily driven by IL-1 and IL-6 cytokines released by monocytes following interaction with infused CAR-T cells. 

In the first study led by Michel Sadelain at Sloan Kettering Institute, New York, immunodeficient SCID-beige mice transplanted intra-peritoneally with human B cell tumor cell line and later infused with CD19 CAR-T cells develop cytokine-release syndrome (CRS) that were reversible by anti-IL-6 antibody,  injection.  

Injection of anti-IL-1 antibody, Anakinra, had similar protective effect against CRS-driven mortality without compromising anti-tumor effectiveness. 

However, due to some limitation of their mouse tumor model where CAR-T cells are of human origin and non-T cells such as monocytes are of mouse origin, the authors acknowledged that not all features of CAR-T cell toxicity could be reproduced here since "None of the reported pathologic findings indicative of neuropathology or associated with neurotoxicity (cortical laminar necrosis, hemorrhages, disseminated intravascular coagulation (DIC), gliosis or vasogenic, neurotoxic or interstitial edema) in human patients were observed in any of the mice examined in the present study".   

Fortunately, the second study is more extensive and fills much of holes of the first study. Here, the authors led by Attilio Bondanza at San Raffaele Hospital Scientific Institute, Milano, "transplanted human cord blood (CB) hematopoietic stem and progenitor cells (HSPCs) through intrahepatic injection into sublethally irradiated newborn NSG or triple transgenic NSG (SGM3) mice expressing human stem cell factor, granulocytemacrophage colony-stimulating factor (GM-CSF) and IL-3 and initially profiled lymphohematopoietic reconstitution." Basically, this mouse model, referred as newborn humanized SGM3 (nHuSGM3), had both T cell and non-T cell components of hematopoietic system mostly of human origin. In allogeneic tests, human T cells developed in nHuSGM3 mice showed expected functionality. 

Staining of cells in nHuSGM3 challenged with tumor and CAR-T cells that induced CRS showed that monocytes were producing IL-1 early on, followed by IL-6

Depletion of monocytes/macrophages with liposomal clodronate (LC) could rescue mice from CRS.

Importantly, while both anti-IL-1 and anti-IL-6 antibody injection could significantly reduce CRS, only anti-IL-1 antibody were able to reduce neurotoxicity in nHuSGM3 mice. Similar effect were seen with CD44v6 CAR-T cells as well.

In all, these two studies showed that in addition of anti-IL-6 injection, anti-IL-1 antibody therapy could significantly reduce complications of CAR-T cell immunotherapy in humans. They provided a strong evidence to suggest that monocytes were primarily responsible for CRS and neurotoxicity complications of CAR-T cell therapy. 

However, it is not clear why are monocytes getting activated after CAR-T cell infusion in the first place. There is interaction, at certain level, between CAR-T cells and monocytes, but is it antigen-specific via anti-CD19 CAR-T or endogenous TCR, or is it non-specific, is unknown presently. 

posted by David Usharauli 

Wednesday, May 23, 2018

Flagellin-specific T cells induce colitis by recognizing antigen other than flagellin

This is very interesting study from Journal of Immunology (JI). Here, researcher led by Timothy Hand at the University of Pittsburgh Medical School, showed that CBir1 transgenic T cells, thought to cause gut inflammation by recognizing flagellin expressing microbiota, were in fact specific for non-flagellin antigens

CBir1 transgenic mouse have been used for past 10 years to model human Crohn's disease in mice. CBir1 tetramer+ T cells recognize epitope from flagellin, antigen thought to be a target in Crohn's disease. Interestingly, all of those experiments were done using CBir1 T cells on WT background. This could be an issue because it has been known for some time now that transgenic T cells on WT background could use alternative Vα chain to form TCR with new specificity.

To avoid such limitation, here the authors generated CBir1 mouse on RAG KO background that only expressed transgenic Vα:Vβ chains. Surprisingly, unlike CBir1:WT T cells, CBir1:RagKO T cells when transferred in immunodeficient hosts did not induce colitis, and it was not because CBir1:RagKO T cells differentiate into Foxp3+ T cells. It appears that CBir1:RagKO T cells just did not see flagellin epitope in adoptive host.

In vitro tests showed that both CBir1:WT T cells and CBir1:RagKO T cells could respond to DCs pulsed with flagellin epitope [it would have been better and more relevant here to use DCs pulsed with gut flora component].

Other tests showed that CBir1:WT T cells in gut tissue could express alternative Vα chain to form a completely new TCR specificity together with transgenic Vβ chain such as against Ags derived from Helicobacter (HH1713 tetramer). 

In summary, it appears that CBir1 T cells initiate colitis by recognizing non-flagellin antigen from the gut flora through non-CBir1 TCR and only following gut inflammation and gut leakage do flagellin-specific CBir1 transgenic T cells get activated and participate in overall colitis.

So, what is missing from this study? One, it would have been relevant to transfer in vitro activated CBir1:RagKO T cells or activate them in vivo directly to see if then they could initiate colitis. Second, the authors could have tried monocolonization of germ-free mice to see the source of non-flagellin microbiota. Third, there is inconsistency between Fig. 3B and Fig. 6A with regard of proliferation of  CBir1:WT T cells in response to Vanc-treated samples (in vitro it did not proliferate but in vivo it did).

posted by David Usharauli

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       

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


Tuesday, March 20, 2018

IL-33 is a natural target of allergen proteases

IL-33 is a member of IL-1 family and requires proteolytic cleavage to form active form. It has already been linked to allergy manifestation. Now, new study in nature immunology expands on earlier observations to show that full-length non-active IL-33 (IL-33FL) is a natural target of group of allergens with proteases activity.

In general, many known allergens display protease activity such as from fungi, mites, pollens, insects. Co-incubation of IL-33FL with such allergen proteases in presence of innate lymphoid cells type II (ILC2) generated biologically-active smaller fragments and release of type II cytokines.

IL-33FL is an intra-nuclear proto-chemokine found in epithelial and endothelial cells. So, how allergens get access to it? It requires cell damage to release IL-33FL extra-cellularly. However, it is not clear if any allergen proteases can damage cells. At least one such allergen from fungus, Alternaria alternata (A. alternata) can damage cell and then cleave IL-33FL.

Similar effects were seen in vivo using IL-33KO mice. In these mice, recruitment of eosinophils, a readout for IL-33-driven allergic response, were only observed when injected with pre-incubated IL-33FL and A. alternata mixture.

In summary, these results suggest that allergen from A. alternata with protease activity can damage cells to release IL-33FL and cleave it into biologically-active shorter peptides. However, this study did not show that other allergens can deliver similar double punch. Also, it is not clear how this innate mechanism translates to adaptive immune system to generate allergen-specific T cell and antibody responses.

posted by David Usharauli

Thursday, March 15, 2018

Reduced CTLA-4 signaling predisposes to Th2 driven gastric tumorigenesis

Anti-CTLA4 antibodies such as Yervoy, has been used in clinical practice to treat solid tumors. It supposed to work either by augmenting and revitalizing effector T cells function directly or indirectly through temporal silencing of inhibitory Foxp3+ Treg population or both. However, new study from Journal of Experimental Medicine showed that at least in [genetically predisposed] mice reduced CTLA-4 signaling by itself could cause Th2 driven tumorigenic transformation of stomach epithelial tissue.

For this study the authors created transgenic CTLA4 shRNA knockdown (CTLA4KD) mice on the BALB/c × C57BL/6 (B6) mixed genetic background. This they did because it appears that BALB/c but not B6 mice were susceptible developing gastric tumors in this model. CTLA4KD mice showed gastric epithelial transformation by 20w of age. Similarly, month long treatment of newborn BALB/c mice with anti-CTLA4 antibody also led to gastric epithelial transformation.

This tumorigenic transformation was CD4 T cell dependent and effector T cells from CTLA4KD but not from WT mice could mediate it. It indicated that changes in effector T cell composition and functionality were driving de novo inflammatory tumorigenesis.

Interestingly, gastric epithelial transformation were happening even in germ-free CTLA4KD mice lacking microbiota. However, since these mice also harbor increased numbers of inflammatory T cells, in all subsets analyzed such as Th1, Th2, Th17, and independent of microbiota it could indicate that T cells could be responding to antigens from food or environment.

Finally, elimination of canonical T helper cytokines showed that surprisingly neither IFN-γ nor IL-17 but IL-4 deficiency could abolish gastric epithelial transformation under conditions of reduced CTLA-4 activity.

In summary, this study suggests that inherited or clinically-induced reduction of CTLA-4 signaling in predisposed individuals could paradoxically lead to inflammatory tumorigenesis driven by type II immunity.

posted by David Usharauli

Saturday, March 10, 2018

Translocation of a specific gut pathobiont, Enterococcus gallinarum, exacerbate autoimmune phenotype

A new study in journal Science suggests that a specific gut pathobiont, Enterococcus gallinarum, could exacerbate autoimmune phenotype in predisposed mouse strain. This autoimmune phenotype in mice are thought to represent mouse version of human systemic lupus erythematosus (SLE).

SLE is associated with genetic polymorphism linked to excessive signaling of RNA sensing Toll-like receptor 7 (TLR7) and type I interferons (IFNs). In the specific pathogen-free (NZW × BXSB)F1 hybrid mouse, responses to endogenous retrovirus glycoprotein 70 (ERV gp70) via TLR7 signaling leads to progressive autoimmune response by pathogenic anti-phospholipid [β2-glycoprotein I (β2GPI)] and anti–double-stranded DNA (dsDNA) antibodies.

The authors observed that certain antibiotic treatment significantly improved survival of (NZW × BXSB)F1 hybrid mice.

Further experiments showed that there was bacterial translocation from gut tissue into portal veins and livers in these mice that could be reduced by antibiotic treatment.

16S rRNA sequencing and species-specific PCR consistently revealed Enterococcus gallinarum (E. gallinarum) in the feces, small intestine and liver of (NZW× BXSB)F1 mice. Monocolonzation of germ-free mice with E. gallinarum (EG, here) revealed that it could specifically drive Th17 response, unlike E. faecalis or B. thetaiotaomicron.

Moreover, E. gallinarum could specifically drive  ERV gp70 expression in the liver cells,

and augment anti-nucleic acid antibody response.

Finally, the authors showed that liver tissues from human SLE patients harbored E. gallinarum.

In summary, this study proposes the following patho-mechanism of SLE: when residing in predisposed individuals E. gallinarum causes degradation of gut barrier function, then translocates internally, activates Th17 pathway and initiates "innate" autoimmune phenotype by activating expression of retroviral genes and amplifying endogenous nucleic acid detection system that breaks tolerance checkpoints and leads to auto-antibody formation, Ab-Ag complex deposition in tissues and inflammatory disease exaggeration. The authors proposed antibiotic treatment could provide relieves in certain SLE patients.

However, there are few unanswered questions in this study: first of all, it is clear that E. gallinarum does not induce autoimmunity by itself. Second, If Th17 activity is relevant for E. gallinarum action, then it would have been more valuable for the authors to compare E. gallinarum to segmented filamentous bacteria (SFB) a known inducer of Th17 response in the gut tissue.

posted by David Usharauli

Saturday, March 3, 2018

Specific microbiota species induce serum IgA that protects against sepsis

Some 10 years ago the scientists made observation that microbiota difference between different mouse colonies is responsible for selective TH17 expansion in the gut. Since then, field of immunology was flooded with numerous observations linking gut [and other tissue] microbiota to functional status of immune system.

One such study was recently published in journal Cell Host and Microbe. The researchers observed that serum IgA secreted by bone marrow residing plasma cells (BM PCs) were selectively enriched in mouse colony harboring members of Proteobacteria phylum. More importantly, these serum IgA protected mice during sepsis following gut damage.

Initially, the authors observed that their institute's B6 mouse colony (PENN-SPF) differed from commercial vendor B6 mice in their serum [but not small intestine] IgA status. Co-housing experiment indicated potential involvement of microbiota.

Indeed, 16S ribosomal gene sequencing showed enrichment of Proteobacteria phylum in local mouse colony (and also Deferribacteres).

Serum IgA bound microbiota and sequencing of serum IgA+/IgA- microbiota species confirmed selective enrichment of species within Proteobacteria phylum.

Development of microbiota-specific serum IgA were T cell-dependent.

Finally, serum IgA protected mice against sepsis following gut tissue damage and microbiota invasion (translocation).

In summary, serum IgA, but not intestinal IgA, is produced by bone marrow plasma cells in response to selective microbiota species, mostly from Proteobacteria phylum in mice. These serum IgA could bind microbiota, it developed in a T cell-dependent manner and protected host during gut flora invasion (translocation) in condition such as sepsis. However, it is not clear why serum IgG [in IgA KO mice] could not protect against sepsis in this study since one previous study already showed that serum IgG protected against gram negative bacteria such as E. coli. It is possible that serum IgA and IgG play non-redundant functions by protected against different microbial species.

posted by David Usharauli