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 T_H17 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

MHC class II epitope presentation modulates microbiota and protects against autoimmunity

It is not clear how exactly microbiota modulates host's immune system. Evidence are largely based on empirical observations and nonspecific factors secreted by microbiota. Presently very little is known if microbiota-immune system interface is also controlled at the level of antigen-specific adaptive immune system.    

 

New study from Diane Mathis lab published in PNAS suggests it may be the case.

 

Her lab studies human type I diabetes (T1D) mouse model known as NOD. NOD mice lack MHC II allele, Eα. In this study they used NOD mice expressing Eα, referred as Eα16/NOD. In mating experiments, they noticed when Eα was expressed by female but not by male parent, baby mice with NOD genotype showed significant protection from developing T1D, suggesting protection was transmitted vertically from Eα16/NOD mother to NOD pups. Interestingly, this protection was lost when pregnant mothers (dams) were treated with antibiotics pointing towards role of microbiota.

Experiments with germ-free sterile mice confirmed this observation.

In summary, this study showed that MHC class II [epitope] presentation modulates composition of microbiota in such a way to harbor species protective against T1D. Again, the authors were unable to specifically pinpoint any specific mechanism of protection, though they reported increase in Foxp3+ Treg numbers in Eα16/NOD mice compared to NOD (but found no difference in microbiota bound to IgA between mouse strains). It is likely that epitope presentation at the level of adaptive CD4 T cells contributed to development of protective environment.

posted by David


    

 

Experimental malaria vaccine shows 55% efficacy in small trial

This week Nature Medicine published results from small trial involving controlled human malaria infection (CHMI) and experimental malaria vaccine developed by Sanaria (Sanaria Inc., Rockville, Maryland, USA). This vaccine based on attenuated Plasmodium falciparum sporozoite (abbreviated as PfSPZ vaccine) showed 55% efficacy at 1 year post-immunization.

Currently, only malaria vaccine available on market, RTS,S, has efficacy of ~ 22%. In this new study, scientists tested different dosages and routes of immunization for new PfSPZ vaccine as follows:

The results of the controlled human malaria infection are shown below. Here, healthy malaria-naive volunteers were first vaccinated and then received malaria infection from actual mosquitoes bites that carry malaria clone 3D7. The best outcomes were achieved with groups 4 and 5 (i.v. administration, 4 doses of 2.7 × 10⁵ PfSPZ).

Next step was to understand immunological correlates of protection. However, this task was quite challenging, as it turned out. All vaccination protocols, with the exception of i.m. immunization, induced anti-malaria Ab or T cell responses. However, when vaccine trial participants were divided based on blood parasitemia following malaria challenge, it was observed that participants without parasitemia developed higher levels of anti-malaria serum Abs.

At the cellular level, however, the only marker that correlated with vaccine efficacy was frequency of unstimulated Vγ9+Vδ2+ γδ T cells which comprises ~75% of γδ T cells in blood. Actually, the frequency of Vγ9+Vδ2+ γδ T cells in pre-vaccinated individuals was the only marker that correlated with vaccine efficacy in challenge model.

In summary, this study showed that there is still room to improve malaria vaccine. Vaccine efficacy of 55% in 1-year followup is a significant progress when considering that In 2015 there were an estimated 214 million clinical cases of malaria in the world. Another outcome of this study is that fact that it is quite hard to find or define immune correlates of protection and this is especially true for parasitic infection such as malaria or dengue.

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