Molecular mimicry to gut microbiota antigen protects against colitis but induces diabetes

Current issue of journal Cell has one very interesting but at the same time confusing research paper. In it, the authors proposed that

(a) diabetes susceptible mice strain, NOD, harbor CD8 T cells specific for microbiota antigen that cross-react with β cell antigen, IGRP, and

(b) such molecular mimicry prevents colitis but at the same time could induce diabetes.

First, the authors showed that MHC I alelle expressed in NOD mice (H2K^d) could bind IGRP_206-214 homologue derived from integrase family expressed by some gut Bacteroides species (BacIYL_36–44). 

At high dosage, such binding was functional in stimulating high affinity IGRP_206-214-specific T cells (17.4+ CD8 T cells).

Human T cells from PBMCs could apparently respond to it as well (though it is strange that it generated better stimulation index than Tetanus toxoid).

Then, the authors did the following experiment. They exposed IGRP^-/- 17.4+ TCR transgenic mice to chemical irritant (DSS) and observed that high affinity IGRP_206-214-specific T cells, 17.4+ CD8 T cells, but not low affinity ones (17.6+), could protect against colitis (I assume that they used IGRP^-/-mice to avoid diabetes development).

It appears that colitis protection depended on perforin expression by 17.4+ T cells. The authors speculated that 17.4+ CD8 T cells prevented colitis by eliminating dendritic cells laden with microbiota-derived antigen (BacIYL_36–44).

As a confirmation, the authors showed that germ-free TCR Tg NOD mice colonized with Bacteroides species expressing BacIYL_36–44 were protected against colitis.

Colitis protection was observed even in classical, adoptive naive CD4+ T cell transfer colitis model.

Interestingly, however, transfer of T cells from pre-diabetic NOD mice into germ-free NOD.scid mice colonized with Bacteroides species expressing BacIYL_36–44 did not accelerate diabetes development (here I assume DSS is required to accelerate T cells priming against IGRP by creating dysbiosis).

In summary, this study suggests the following scenario: diabetes-inducing CD8+ T cells cross-react with gut microbiota-derived antigen. When such microbiota-derived antigens become visible to T cells (during dysbiosis?) CD8+ T cells migrate to gut and eliminate dendritic cells laden with cross-reactive antigens. By eliminating DCs, other T cells are not able to induce inflammation in the gut, thus no colitis. However, the same beneficial CD8+ T cells later migrate to β cells, recognize similar looking antigen, IGRP, and mediate its destruction and diabetes.

Does such circuit makes any evolutionary sense? 

Update: Interestingly, other research group previously detected different set of gut microbiota antigens cross-reactive to IGRP_206-214. They used TCR NY8.3 transgenic NOD mice (that recognize the same IGRP epitope) and found that these CD8 T cells cross-reacted with IGRP_206–214 homologous peptide, W15944, derived from L. goodfellowii, a member of the phylum Fusobacteria (gram-negative anaerobe), a human and NOD mouse oral commensal. 

posted by David Usharauli   

Tolerance to insulin is maintained by Foxp3+ Tregs

A new study in Journal of Immunology suggests that tolerance to insulin is maintained by Foxp3+ Tregs rather than by deletion of insulin-reactive T cell clones. 

 

Here, the authors reconstituted mice with T cells on scid background transduced either with high (4-8) or low (12-4.1) affinity TCR specific for native insulin peptide (insulin epitope B:9–23). In addition, each of TCR construct were fused with either native insulin (INS) or modified insulin carrying super-affinity peptide (R22E). All mice expressing either INS or R22E but not irrelevant HEL were protected from developing diabetes.

The authors showed that while R22E deleted developing insulin-specific T cell clones in the thymus, native INS did not.

In fact, the authors showed that if the T cells also lacked Foxp3 molecule (scid-scurfy), then protection against diabetes was lost in mice exposed to native INS.

This study could be interpreted to show that with the exception of  epitopes which are able to delete (purge) cognate T cell clones in the thymus, tolerance to self in the periphery is maintained by thymic-derived Foxp3+ Tregs.

posted by David Usharauli 





    

It is really hard to replicate human autoimmune diseases in mice

A new study in PNAS highlights the challenges scientists face when trying to reproduce human diseases in mice.  

Here, the authors tried to reproduce human type I diabetes by creating humanized mice by transplantation of HLA-DQ8+ human fetal thymus and CD34+ stem cells into immunodeficient mice (to recreate human immune system in mouse) followed by transfer of autologous [hu-mice]-derived HLA-DQ8/insulin-B:9–23 specific TCR transduced human CD4+ T cells and followed by two successive low doses of streptozotocin (a chemical to damage islet β cells and release auto-antigen).

However, even these steps were not enough to induce diabetes in hu-mice. Only immunization with insulin B:9–23 peptide + adjuvant (HLA class II-restricted T-cell response to InsB:9–23 peptide is highly associated with T1D in humans) in addition to above mentioned "conditioning" were able to induce diabetes in hu-mice.

No one really knows how autoimmune diseases are initiated in humans and these study shows that it is really hard to "reproduce" it in mice. Of course, it is not known what factors could play the role of "streptozotocin" and "B:9–23 peptide immunization" in natural context in humans. 

posted by David Usharauli

Does TGF-β control T cell autoimmunity independent of Tregs?

When T cells attack body's own antigens its called autoimmunity. Each and every one with the adaptive immune system carry this potential. Mostly two mechanisms prevent autoimmunity: thymic deletion of overtly auto-reactive T cells (recessive tolerance) and Foxp3+ Tregs (dominant tolerance).

 

Within immune system, TGF-β plays important inhibitory role at T cell level. However, since TGF-β is involved in Treg biology, it is not clear if it has Treg-independent role in preventing autoimmunity.

 

New paper published in PNAS tried to answer this question.

 

The authors used OT-II RIP-mOva mice model (on RAG KO background) in which all CD4 T cells express OVA-specific T cell receptor and pancreas express OVA protein. These mice harbor OT-II Foxp3+ Tregs and they don't develop autoimmune diabetes.

To separate effect of Tregs versus TGF-β, the authors either compared TGF-βRII-KO mice vs. Foxp3KO (both on RAG1-KO OT-II RIP-mOva background) or adoptively transferred into RAG1-KO RIP-mOVA mice either Foxp3KO OT-II or OT-II T cells expressing TGF-βRII under the control of estrogen receptor. They noticed that OT-II T cell population lacking TGF-βRII but not Foxp3 could cause or accelerate autoimmune diabetes.

These two set of experiments are central for this paper. However, contrary to the authors' conclusions, these experiments do not fully answer Tregs versus TGF-β question. The main problem is that total TGF-βRII deficiency in all CD4 T cells affects both effector and Tregs (functionally at least if not number wise) while Foxp3 deficiency only affects Tregs. That is to say that if Tregs were TGF-βRII-sufficient and effector T cells TGF-βRII-deficient outcome could be different (WT Tregs might be able to stop effector OT-II cells). Another way to separate the role of Tregs versus TGF-β would be to specifically inactivate TGF-βRII in effector T cells leaving Tregs intact.

posted by David Usharauli


 

Absence of NOD2 signaling protects against chemical[STZ]-induced diabetes

Experimental models developed in "clean" laboratory mice rarely translates to human diseases. For example, NOD mice spontaneously develop Type 1 diabetes, as susceptible humans do, however pancreatic tissue pathology does not match. 

There is another model of T1D in mice called treptozotocin (STZ)-induced diabetes. STZ is toxic to insulin producing β cells in pancreas and drive T1D in a T cell-dependent manner. In new study published in Journal of Experimental Medicine the authors showed that treptozotocin induced T1D depended on gut flora derived muramyl dipeptide → NOD2 signaling.

The authors found that NOD2^-/- mice, but not NOD1^-/- or Rip2^-/- mice, were resistant to STZ effect.

Interestingly, similar protective effect against T1D were seen in WT mice treated with broad-spectrum antibiotics and STZ. However, addition of NOD2 ligand muramyl dipeptide reversed protection in [antibiotic + STZ] treated mice.

This study showed that NOD2 ligand, muramyl dipeptide derived from gut microbiota, contributes to T1D in STZ-treated mice.

David Usharauli