IBD converts tolerant antigens into immunogenic

Inflammatory bowel disease (IBD) is a pathological condition wherein body's immune cells wrongly attack its own or commensal microbiota-derived antigens that initiates a vicious cycles of permanent inflammation.

 

However, it is still not clear whether immune system attacks "new" antigens from microbiota or simply it loses adaptive tolerance to "old" ones. 

 

New study in Science Immunology tried to answer it to the extent it was possible to do in mouse model.

 

First, the authors generated IBD condition in mice by treating them with DSS + anti-IL10R. Keep in mind this is highly artificial model. Then, they transferred naïve T cells from previously established transgenic T cell lines specific for unknown commensal antigens that were known to drive Treg phenotype. Naïve T cells transferred into control WT mice generated Tregs while the naïve T cells transferred into IBD-conditioned mice preferably developed into effector T cells.

 

 

 

When analyzed for antigen specificity, the authors found that transgenic naïve T cells were reactive to antigens derived from Helicobacter species that have expanded during IBD-conditioning.

 

 

 

In vivo studies also confirmed that Helicobacter species could induce Treg generation from naïve transgenic T cells in "normal" condition.

 

 

 

Interestingly, transfer of T cells specific for other microbiota species that also underwent expansion during IBD-conditioning did not produce T cell expansion.

 

 

 

Finally, transfer of Treg-TCR transgenic naïve T cells into RAG-KO mice produced IBD when co-injected with Helicobacter species.

 

 

 

What these data indicate? In my view the authors made one correct and one wrong interpretation. First, they were correct to conclude that T cell response to IBD could be directed to "old" microbiota antigens rather than "new" never before seen microbiota-derived antigens. So basically in IBD we are losing tolerance rather than gaining immunity to microbiota antigens.

 

However, they made wrong conclusion that naïve T cells are converted into Tregs in vivo based on context (normal versus IBD). In their study loss of Treg generation is inhibited either during IBD-conditioning or in RAG KO hosts which could argue alternatively that such outcome has to do with failure of naïve T cells to interact with existing Tregs specific for the same or similar antigens in these scenarios (IBD or RAG-KO).

 

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

       

How Foxp3+ Tregs and microbiota work together to control immune system

Check out our new paper in PeerJ Preprints that unlocks the mystery of how Foxp3+ regulatory T cells work that enables proper immune functioning.

Usharauli D, Kamala T. (2017) An identical mechanism governs self-nonself discrimination and effector class regulation. PeerJ  Preprints 5:e3081v1  https://doi.org/10.7287/peerj.preprints.3081v1

Prevailing immunological dogma dictates self-nonself discrimination, meaning to respond or not, and effector class regulation, meaning choosing the most effective response, are two separate decisions the immune system makes when faced with a new antigen. Representing a cardinal departure from the past, our model instead predicts both self-nonself discrimination and effector class regulation are in fact one and the same process controlled by Foxp3+ regulatory T cells (Tregs) whose antigen-specific repertoire is entirely maintained by commensal microbiota-derived cross-reactive antigens.

 

 

posted by David Usharauli

Role of Th1 transcription factor Tbet in Foxp3+ Tregs' functionality

Sasha Rudensky's lab continues to publish high profile papers in Foxp3+ Treg biology though besides the complicated genetic cell labeling approaches it delivers little of real science. 

This time, the authors followed the fate of Tbet+ Tregs. Tbet is a canonical Th1 transcription factor and its expression in Foxp3+ Tregs is puzzling. In first set of experiments the authors analyzed Tbet expression in mice with "Tbx21tdTomato-T2A-creERT2 knock-in allele combined with the R26Y recombination and Foxp3Thy1.1 reporters". In these mice any cell that express Tbet (Tbx21) will be (1) labelled red (tdTomato), will up-regulate (2) ERT2 (estrogen receptor) and if Treg, it will express Thy1.1 marker in addition. Interestingly, the authors observed that in steady-state "RFP+ Treg cells comprised between 30–70% of CD44hiCD62Llo effector Treg cells in lymphoid organs and non-lymphoid tissues;". 

To understand the stability of any given Tbet+ Treg cell the authors injected these mice with estrogen receptor activator tamoxifen. Tamoxifen will activate cre molecule that will liberate R26Y (yellow color) and cells become double positive (red+yellow). Interestingly, for any given moment only around 1% of  Tbet+ Tregs are labeled with YFP even though as mentioned earlier between 20-70% of Tregs supposed to express estrogen receptor any given moment (see above). It means very low "Tbx21tdTomato-T2A-creERT2 knock-in" gene functionality. In any case, tracing of YFP+ Tregs showed that they were quite stable population over period of 7 months, at least.

When these mice were treated with tamoxifen and then exposed to L. monocytogenes (Lm) infection, YFP+ Treg population did not expand while RFP+ Tregs did, suggesting that conversion rather than expansion plays role in Tbet+ Treg generation (assuming YFP+ and RFP+ Tregs are equal in all regards, especially for TCR specificity). 

In general, Foxp3YFP-cre Tbx21fl/fl mice lacking Tbet in all Tregs are indistinguishable from littermate controls. However, the authors found that Tbx21RFP-creFoxp3fl mice lacking Foxp3 only in Tbet+ [Tregs] did show evidence of clinical pathology and heightened Th1 effector differentiation. However, interpretation of this experiment is difficult because as already mention above large proportion of Tregs in these paper do naturally express Tbet and will undergo loss of Foxp3 following Tbet expression. Pathology could be result of a simple number reduction of functional Tregs independent of Tbet functionality.   

Interestingly, reverse experiment in Foxp3fl-DTRTbx21RFP-creERT2 mice lacking Tbet-negative Tregs only but retaining Tbet+ Tregs showed normal Th1 effector differentiation but heightened Th2 and Th17 differentiation implying that Tbet+ Tregs can only regulate Th1 effector differentiation. Though the authors did not provide any data whether Foxp3fl-DTRTbx21RFP-creERT2 mice have any clinical pathology similar to Tbx21RFP-creFoxp3fl mice.

In summary, this study suggests that Tbet+ Tregs might have unique regulatory capacity and specialization though it is not clear mechanistically how Tbet+ Tregs can specifically regulate Th1 effector cells. The role of TCR specificity is relevant but unknown at this stage.

posted by David Usharauli