Fragility of FOXP3+ Treg phenotype

This week I am reviewing two papers dealing with different aspects of FOXP3+ Treg biology. Both papers were published in journal Cell. These are predominantly mouse works so no need to get too excited. 

1st paper was published by Dario Vignali's group (department of Immunology, University of Pittsburgh School of Medicine). They have analyzed role of Neuropilin-1 (Nrp1) in Treg phenotype in tumor context. Neuropilin-1 is a marker of thymic Tregs though this notion is not universally accepted. Interestingly, in mouse tumor model, mouse that expressed Nrp1 on half of its Tregs rejected tumor with the same vigor as mouse expressing Nrp1 on all of its Tregs. 

Furthermore, in the context of tumor challenge, Treg-specific Nrp1 deficiency drastically increased IFN-γ production in both KO and WT Tregs.

It appeared that WT Tregs required sensing of IFN-γ derived from Nrp1KO Tregs to acquire "Nrp1KO-like" phenotype. 

Finally, the authors showed that therapeutic effect of checkpoint inhibitor anti-PD1 antibody against tumor required IFN-γ sensing by Tregs.

In summary, this study showed that Nrp1 deficiency makes Tregs fragile by converting them into IFN-γ producer cells which in turn affect WT Tregs phenotype as well. The reason the authors are using term fragility rather than simply instability has to do, they claim, with the difference in FOXP3 expression between fragile and unstable Tregs. 

In a separate study, the authors showed that Treg depletion inhibits hair regrowth (after depilation). 

However, it is not clear why Tregs should be involved in such physiological process when any other innate cells could do the same.   

David Usharauli

Some anergic T cells are precursors for Foxp3+ Tregs

T cell anergy is defined as T cell non-responsiveness to [secondary] antigen challenge. This concept has originated  in 1980s to explain T cell behavior following primary encounter with antigen (signal 1) in absence of co-stimulation (signal 2). Later, anergy concept was expanded to include T cell behavior following repetitive antigen encounter, similar to what today we would call T cell exhaustion.    

Generally speaking, anergic T cells are different from canonical thymic Foxp3⁺ Tregs. Anergic T cells do not express Foxp3 marker. While there are several surface and transcription factors that could "identify" anergic T cells, classical test for anergic T cells is to run several functional cytokine secretion assay and if memery/effector T cells do not produce cytokines, especially IL-2, and do not express Foxp3, you can call it anergic T cells. It is primitive but its all we have right now.  

In the past scientists thought that anergic T cells were necessary to maintain peripheral tolerance against self-antigens. It was believed that anergic T cells would compete for antigen access with self-reactive naive T cells (recent thymic emigrants) and would not allow their activation. This model lost its appeal once Foxp3⁺ T cells were re-discovered and re-defined. So, presently, functional significance of anergic T cells is in a kind of limbo.

So, it was interesting to see new paper in Nature Immunology that revisited the role of anergic T cells. In this study the authors tried to make sense of anergic T cells by postulating that some portion of anergic T cells represent precursors for classical Foxp3⁺ T regs.

First, the authors showed that among Foxp3-negative T cells, FR4^hiCD73^hi markers define anergic T cells (by low IL-2 production) that develop following antigen encounter in absence of co-stimulation (for example, pregnant females harbor pregnancy-associated anergic T cells specific for antigen [2W1S] expressed by male fetus). Interestingly, majority of those Foxp3-negative anergic T cells express neuropilin-1 (Nrp-1), a marker for thymus Tregs.

Next, the authors showed that in functional assays, FR4^hiCD73^hi anergic T cells behave very similar to classical Foxp3⁺ T cells.

Furthermore, when polyclonal Foxp3^-FR4^hiCD73^hi anergic T cells were transferred into lymphopenic host, anergic T cells gave rise to classical Foxp3⁺ T cells (compared to naive or effector T cells).

Importantly, if those newly formed Foxp3⁺ T cells [derived from anergic T cells] were selectively deleted it led to systemic autoimmune disease, implying functional role for such daughter Foxp3⁺ T cells.

Finally, the authors showed that new Foxp3⁺ T cells formed from anergic T cells could prevent arthritis development when co-transferred with cartilage antigen-specific KRN T cells (that themselves do not become anergic or develop into Foxp3⁺ T cells).  

In summary, this study showed that some anergic T cells (especially Nrp1+ subpopulation) could give rise to peripheral Foxp3⁺ T cells. However, not all T cells can produce anergic T cell that could serve as a precursors for Foxp3⁺ T cells (for example KRN T cells or 5C.C7 CD4 T cells). It remains seen what role, if any, anergic T cells play in un-manipulated host. So far we lack tools to selectively deplete anergic T cells in un-manipulated host to test their real-world biological significance. Also, since anergic T cells resemble thymic Foxp3⁺ T cells in Npr1 expression, it will be interesting to found out whether anergy induction is a part of thymic Foxp3⁺ T cell development program, in general.

David Usharauli