Gut microbes fuel effectiveness of anti-CTLA-4 cancer immunotherapy. Part I

Recently two back-to-back papers in Science Express received much media attention. These studies claim that gut microbial flora influence clinical effectiveness of checkpoint inhibitors approved for cancer immunotherapy. I am going to review and provide my analysis of these studies.

First paper I will review came from European team led by Laurence Zitvogel. Of note, her team had another tumor immunology paper just 1 week ago in Science. First notable thing about her publications is the number of names included as authors. Her lab had done this since I first heard about her studies in 2007. Either she is a good collaborator or everyone in her lab is get credit for everyone else's work as a matter of right [definitely good for career].

In this new paper, her team showed that effectiveness of anti-CTLA-4 antibody in cancer immunotherapy was greatly affected by presence of gut microbiota.

For example, they showed that inhibition of cancer growth by anti-CTLA-4 antibody injected in germ-free or antibiotic-treated mice was not as good as in control WT mice. 

Analysis of gut flora revealed that presence of specific set of microbes [B. fragilis, for example] were required to show anti-cancer effectiveness of anti-CTLA-4 antibody.

Now these result are quite preliminary and it is not clear how microbes are enhancing anti-CTLA-4 effectiveness. For example, some microbes can live within growing tumor tissue and anti-CTLA-4 therapy simply unleashes immune response against them destroying tumor tissue in the process. There is a possibility that microbes support presence of cross-reactive T cells required to target tumor antigens when they are unleashed by anti-CTLA-4 immunotherapy. In all, based on this paper we can't say how microbes help anti-CTLA-4 and how much this "help" is meaningful in clinical settings. Maybe second paper will provide some ideas in this regard. Stay tuned for next review. 

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

Fezl ascends to the thymic throne as Aire's equal and co-ruler

A thymus controls T cell development and central tolerance. It first selects newborn T cells capable of recognizing MHC molecules and then purges those T cells who interact with MHC + self peptide complexes too strongly. 

But what about self-antigens that are physiologically expressed in peripheral tissue, such as insulin (endocrine), neuronal antigens (CNS), testis and ovary antigens (reproductive)? Initially, it was assumed that mechanism called "'peripheral tolerance" was responsible for elimination of peripheral antigen-specific T cells. But later, it was shown that protein called Aire controlled expression some of the tissue-restricted antigens (TRAs) in the thymus thus facilitating central tolerance to those peripheral antigens. Interestingly, analysis of Aire KO thymus had shown that there were Aire-independent TRAs expressed in the thymus, implying the existence of yet unknown mechanism.

Now, new Cell paper revealed the identity of molecule responsible for Aire-independent TRAs expression. It turned out that thymic protein Fezf2, also known as Fezl, was responsible for expression of unique set of TRAs in the thymus independently of Aire.   

First, the authors found that Fezl was highly expressed in mTECs, thymic cells responsible for central tolerance.

Role of Fezl in tolerance was tested in nude mice lacking endogenous thymus. Nude mice receiving Fezl KO thymic transplantation developed peripheral tissue immunopathology [because TRAs-specific T cells were not deleted].

At the same time, the authors showed that Aire's expression was not altered by absence of Fezl.

Expression pattern of TRAs in thymus revealed that Aire and Fezl mostly controlled non-overlapping set of TRAs.

Experiments with thymic-specific Fezl deficiency showed peripheral lymphoid tissue enlargement and autoantibody development.

Finally, the authors found that unlike Aire, Fezl expression was controlled by lymphotoxin beta receptor (LtβR) pathway.

In summary, this truly breakthrough study revealed a new dimension for thymic TRAs expression. Mutations in Fezl could underlie some of the known forms of immunopathology.

David Usharauli