Microbiota-generated butyrate works on Aire to amplify Treg numbers

A short but very interesting paper in Journal of Immunology caught my attention this week. In this study the scientists showed that butyrate, a short chain fatty acid derived from fiber fermentation by microbiota acts on GPR41 receptor in the thymus to increase Aire expression and amplify Foxp3+ Treg frequency (of note, compared to WT, GPR41-KO thymus already contains almost 5-fold less Tregs).  

Generally it is believed that microbiota works locally in the gut or other peripheral tissues to either convert naive T cells into Foxp3+ Tregs or expand existing Treg numbers. However no one yet managed to definitely show which pathway is functionally operational in vivo in physiological conditions. This study now could help to further narrow down biological mechanisms responsible for Treg biology. If microbiota-generated butyrate could work on thymus to increase Aire expression that in turn increases level of Thymus-derived Treg generation, then peripheral conversion pathway may play even less relevant role in physiological mechanisms of tolerance.  

The most obvious question after reading this article is why the authors did not test Aire KO mice to verify their conclusions.

posted by David Usharauli

   

Part of AIRE KO phenotype (APECED in humans) is dictated by gamma-delta T Cells

AIRE deficiency in mice (human equivalent of autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, APECED) leads to chronic tissue immunopathology. Due to role of AIRE in generation of thymic FOXP3+ T regulatory cells, it is believed that underlying cause for tissue pathology on AIRE KO background is a lack of Tregs specific for tissue specific antigens.

However, the new data in journal Immunity from the research group which pioneered the study of AIRE suggest that part of AIRE KO phenotype is dictated by changes in γδ T cells secreting cytokine IL-17.

The authors observed that while total γδ T cells numbers were not different WT and AIRE KO littermates, γδ T cells expressing IL-17 were up-regulated in AIRE KO mice.

Interestingly, AIRE KO mice also deficient for γδ T cells (double deficient mice) showed marked resistance to immunopathology to such tissues as eyes and lungs (though other target tissues were still susceptible).

At the cellular level, there was a significant increase in IL-23r-GFP+ γδ T cells (marker for IL-17+ γδ+ T cells) in retinal tissue in AIRE KO mice at early age, even before tissue pathology was apparent. Of note, the fact that IL-17+ γδ+ T cells are present in retinal tissue in WT mice as well points to its role in physiological processes, not just in pathology).

In summary, this study indicates that subset of γδ T cells could play a leading role in initiating certain tissue pathology on AIRE KO background.

It is not clear, however, how changes in γδ T cell compartment relates to changes in Treg compartment on AIRE KO mice. Also, the role of tissue microbiota and its changes in KOs should be considered to fully understand immunopathologies. In this paper the authors provided the answers to neither to these two relevant topics.

David Usharauli

Eye-specific autoimmunity in AIRE/Lyn double deficient animal

AIRE protein in the thymus directs expression of tissue-specific antigens that on one hand reduces number of auto-antigen specific T cell clones and on the other hand leads to generation of auto-antigen specific Foxp3+ regulatory T cells. However relationship between between central tolerance and its peripheral counterpart is not fully understood.

New study in Journal of Clinical Investigation examined relationship between hypomorphic AIRE function on Lyn^-/- background. The authors found that such combination selectively accentuated eye-specific autoimmunity.  

First, the authors observed that mice double deficient for AIRE^GW/+/Lyn^-/- functions (but not single deficient) developed spontaneous eye inflammation.

These AIRE^GW/+/Lyn^-/- mice showed expansion of eye-protein specific CD4 T cells and autoantibodies.

These auto-antigen specific T cells could infiltrate eye tissue causing inflammation.

The authors found that selective absence of Lyn in CD11c+ dendritic cells (Aire^GW/+CD11c-Cre Lyn^fl/fl) were sufficient to recapitulate this eye immunopathology.

Finally, the authors found that auto-antigen uptake by Lyn^-/- DCs in eye draining lymph nodes contributed to eye inflammation.

In summary, this study indicates that some of the clinically relevant autoimmune phenotypes could be linked to failure to establish both central and peripheral tolerance.

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

First-born thymic Foxp3+ regulatory T cells protect against tissue-selective AIRE-dependent autoimmune syndrome

Adaptive immune system should be able to distinguish between self and non-self antigens. This is a very fine process since at the molecular level both self and non-self antigens look the same. So how it is done?

New study in journal Science suggest one possible mechanism. The authors showed that in mice Foxp3+ regulatory T cells generated in the thymus during early life (between day 0 - 10) confer AIRE-dependent tissue selective tolerance to self-antigens. 

AIRE is a gene that controls tissue-specific protein expression in the thymus. The authors observed that conditionally T reg-depleted neonatal mice supplemented with T regs from AIRE KO donor, but not from WT donor, developed tissue-specific autoimmune syndrome.

Further experiments with adoptive transfer of T reg population tagged either during neonatal stage (day 0-10) or after weaning (day 35-45) revealed that only neonatally tagged T regs were enriched in Foxp3⁺ population able to prevent the development of AIRE-dependent tissue-selective autoimmune syndrome.

The results from this study suggest that the role and functionality among Foxp3⁺ T regs varies depending, at least, on time of their generation. It appears that first-born T regs generated during a neonatal stage provide specialized protection against development of autoimmune syndrome (though tagging per se did not specifically identify AIRE-driven Foxp3+ T regs in newborn mice in this study).

Interestingly, the authors reported that in NOD.Foxp3-DTR mice T reg depletion after weaning did not result in death of the experimental animals. This is in contrast to earlier reports from Rudensky's lab where adult Foxp3-DTR mice die after T reg depletion. Wonder what could have made such difference.   

David Usharauli