Antigen-specific Foxp3+ Tregs control tolerance to gut pathobiont H. hepaticus

Helicobacter hepaticus, H. hepaticus, is an opportunist commensal, a pathobiont, that causes gut inflammation in IL-10 deficient but not in WT mice. A New study in journal Nature from Dan Littman's lab tried to understand basis for such dual nature of host response to H. hepaticus.

The authors have generated transgenic T cells specific for H. hepaticus-unique protein, HH_1713. Next, the authors co-transferred H. hepaticus-specific T cells (HH7-2tg cells) alongside with segmented filamentous bacteria, SFB-specific T cells (7B8tg cells) into WT mice exposed to H. hepaticus. Interestingly, HH7-2tg cells mostly differentiated into RORγt+ Foxp3+ Tregs while 7B8tg cells mostly differentiated into Th17 cells, as observed in earlier studies.

However, similar transfer of H. hepaticus-specific T cells into IL-10KO mice exposed to H. hepaticus mostly yielded inflammatory T cells (Th17 and Th1 cells).

It appears that transcription factor c-MAF control differentiation of H. hepaticus-specific T cells into RORγt+ Foxp3+ Tregs and its deficiency recapitulates H. hepaticus-specific T cells differentiation into inflammatory T helper cells observed in IL-10KO hosts when exposed to H. hepaticus.

In summary, this study indicates that in WT mice harbor H. hepaticus specific Foxp3+ T cells that keep tolerance to H. hepaticus. These induced Foxp3+ T cells are probably generated (or expand) by chain reaction orchestrated by existed antigen-specific natural Tregs since induced Tregs don't develop efficiently when transferred into RAG KO hosts lacking natural Tregs. IL-10 or c-MAF deficiency could alter overall microbiota composition (unrelated to H. hepaticus) and drive shift from Treg program into inflammatory pathway.

posted by David Usharauli

Identification of prostate-antigen specific natural Tregs (in mice)

Foxp3+ Tregs are central player in maintaining tolerance to self and other environmental antigens. However, till to this date we know little of their antigen specificity. It is because unlike conventional CD4+ T cells, Tregs do not secrete [upon antigen recognition] any cytokine that uniquely identifies them. The best marker is still Foxp3 molecule, an intracellular transcription factor.   

So it is always interesting to see new study that could identify Treg epitope, such as this new paper in Immunity that provided evidence that in mice peptide spanning residues 646–658 of prostate-specific TRPM8 channel-associated factor 3 protein (Tcaf3) is a natural epitope for thymic MJ23 TCR transgenic Treg development.

The authors showed that development of MJ23+ Tregs from adoptively transferred MJ23+ thymocytes (un-differentiated T cells) were only supported in hosts expressing intact Tcaf3 (and not in Tcaf3 KO mice).   

Next, using sensitive tetramer based antigen-specific T cell detection, the authors showed that WT mice also harbored Tcaf3[646–658]-tetramer specific T cells that were enriched in Tregs compared to other antigen-specific T cells (2W1S). Interestingly, Aire-KO mice which do not efficiently express peripheral antigens in the thymus harbored reduced numbers of Tcaf3[646–658]-tetramer specific Tregs.

Finally, the authors showed that prostate tissue from Aire KO mice harbored significantly more Tcaf3[646–658]-tetramer specific Tregs compared to prostate tissue from normal mice. I found these particular results problematic because should not normal mice prostate supposed to contain Tregs to prevent autoimmunity? Or are Tregs keeping autoreactive T cells in check in draining lymph nodes? 

In summary, this study showed that in mice prostate-specific Tcaf3[646–658] epitope is a natural ligand that selects Tregs in a Aire-dependent manner.

posted by David Usharauli