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

Genentech's Trojan horse destroys cocooned Staphylococcus aureus

Genentech did it again. It is truly amazing that since its inception in late 70s, Genentech, a bona fide biotech company, keeps its original spirit alive and encourages and supports R&D scientists to conduct and produce quality research befitting publication in top journals. I think in this regard Genentech does not have a single peer in whole of biotech industry.  

Just this week journal Nature published results from Genentech showing that it is possible to design anti-microbial antibody-antibiotic complexes capable of elimination of hard to reach intracellular microbes.

First, they showed that unlike freely floating extracellular S. aureus, intracellularly entrenched, hard to reach S. aureus are of clinical relevance. 

Indeed, in vitro experiments confirmed that intracellular S. aureus are resistance to conventional antibiotic application.

To overcome this resistance, Genentech's team designed antibody-antibiotic complex, called rifalogue, that was made of anti-S. aureus antibody and antibiotic rifampicin complex. The authors first showed that in vitro once rifalogue attached to S. aureus is internalized by cells rifampicin is released from complexes [in endosomes] and destroys S. aureus.

Finally, the authors confirmed effectiveness of this strategy against S. aureus in in vivo experiments as well.

In summary, these results (re)-opens the door for immunotherapy against infectious agents.

I would admit that for me data seems extremely clear-cut. This is a little bit concerning. I want to see these results confirmed in other labs. Another puzzling point is its mechanism of action. The model proposes that rifalogue works by engaging S. aureus with its Fab part and mammalian cells by its FcR. This suggests that rifalogue can't access or destroy S. aureus already cocooned within cells, but it could prevent freely floating S. aureus becoming intracellular. But regular rifampicin is capable of destroying freely floating S. aureus too. So this is a paradox for me [because results showed that rifalogue was still highly effective, even after delay of 24h, against "potentially" intracellular S. aureus]. 

David Usharauli

Fueling C. difficile growth

C. difficile is a spore-forming anaerobe, residing in the human gut. Usually it does not cause any pathology. However, antibiotic use (or over-use), or changes in gut motility can bring up the ugly side of this microbe. There are no vaccines or drugs available for the specific management of C. difficili infection. 

Curiously, in the past few years, several research studies indicated that C. difficile could be controlled by other gut microbes readily available in the healthy humans' feces.

If proven effective in humans, this will open a new chapter in medicine. Our physiology is heavily influenced by gut commensals and their role in human health and disease has just started to be unraveled.

In this respect, two new studies in journal Cell Host and Microbe provided additional clues about potential mechanisms of initiation of C. difficile disease. I will review both of them separately.

One study, led by Justin Sonnenburg at Stanford University of School of Medicine, has examined how fermentation end product, succinate, affect C. difficile growth pattern.

Initially, the authors observed that in germ-free mouse, C. difficile growth was accelerated in the presence of polysaccharide rich-diet and mouse gut flora commensal Bacteriodes Thetaiotaomicron, (Bt).

It is known Bt produces organic acid, succinate, as a fermentation end product. 

Using succinate-transporter mutant C. difficile, the authors confirmed that in vitro culture, WT C. difficile were able to utilize excess succinate for their growth enhancement.

In vivo experiments with succinate supplemented diet validated this observation.

Importantly, the authors showed that Bt-induced growth promotion was not observed with mutant C. difficile.  

To make their observation clinically relevant, the authors conducted the series of experiments with mice treated with antibiotics or gut hyper-motility (diarrhetic) drug.

As predicted, antibiotic treatment specifically increased succinate level.

Similarly, diarrhetic agent mimicked antibiotic's effect.

Finally, the authors showed that changes in gut motility fueled C. difficile growth.  

In summary, these results indicate that changes in gut flora or acceleration of gut motility could lead to specific increase in succinate level that is sensed and utilized by C. difficile for growth enhancement.

David Usharauli