Daniel Campbell

Phenotypical and functional specialization of FOXP3 + regulatory T cells

Forkhead box P3 (FOXP3)+ regulatory T (TReg) cells prevent autoimmune disease, maintain immune homeostasis and modulate immune responses during infection. To accomplish these tasks, TReg cell activity is precisely controlled, and this requires TReg cells to alter their migratory, functional and homeostatic properties in response to specific cues in the immune environment. We review progress in understanding the diversity of TReg cells, TReg cell function in different anatomical and inflammatory settings, and the influence of the immune environment on TReg cell activity. We also consider how these factors affect immune-mediated disease in the contexts of infection, autoimmunity, cancer and transplantation.

FOXP3 modifies the phenotypic and functional properties of regulatory T cells

In the periphery, tolerance to self antigens is mainly mediated by the CD4+CD25+FOXP3+ subset of regulatory T cells, which can suppress the activity of autoreactive T cells that have escaped deletion in the thymus. The essential role of the transcription factor FOXP3 (forkhead box P3) in the development and function of these regulatory T cells has been well documented. It is also clear that regulatory T cells and effector T cells respond differently to T-cell receptor stimulation. In this Opinion article, we propose that these differences in responses are mediated by FOXP3, and are manifested by alterations in biochemical signalling pathways, patterns of gene expression and the appearance of cell-surface homing receptors.

Control of Regulatory T Cell Migration, Function, and Homeostasis

Foxp3+ regulatory T cells (Tregs) are essential for preventing autoimmunity and uncontrolled inflammation, and they modulate immune responses during infection and the development of cancer. Accomplishing these tasks requires the widespread distribution of Tregs in both lymphoid and nonlymphoid tissues, and the selective recruitment of Tregs to different tissue sites has emerged as a key checkpoint that controls tissue inflammation in autoimmunity, infection, and cancer development, as well as in the context of allograft acceptance or rejection. Additionally, Tregs are functionally diverse, and it has become clear that some of this diversity segregates with Treg localization to particular tissue sites. In this article, I review the progress in understanding the mechanisms of Treg trafficking and discuss factors controlling their homeostatic maintenance and function in distinct tissue sites.

Continual exit of human cutaneous resident memory CD4 T cells that seed distant skin sites

Tissue-resident memory T cells (TRM) persist locally in non-lymphoid tissues, providing front-line defense against recurring insults. However, strict tissue compartmentalization of memory may pose certain disadvantages for a large barrier organ like the skin, and the long-term migratory behaviour of human TRM and their contribution to the memory pool have not been fully elucidated. TRM at barrier surfaces are defined in part by expression of the markers CD103 and/or CD69 which function to retain TRM in epithelial tissues. Here, we found that CD4+CD69+CD103+ TRM in human skin can downregulate CD69, exit the tissue and be identified as a phenotypically unique population in the circulation of healthy individuals. These circulating TRM produce the cytokines IL-22 and IL-13, and express genes consistent with a role in host-defense and tissue-repair responses. RNA- and TCR-sequencing demonstrated that CD103+ TRM in the blood are transcriptionally and clonally related to CD69+CD103+ TRM in the skin. Furthermore, using a skin xenograft model, we confirmed that human cutaneous CD103+ TRM can exit the skin, enter the circulation, and recirculate to secondary human skin sites where they re-assume a TRM phenotype. Thus, although as a population CD4+ TRM in the skin are largely sessile, recirculation of cutaneous CD4+CD103+ TRM does occur in the steady state in humans, and these recirculating (rc)TRM cells can promote the spread of this functionally specialized T cell population throughout the skin.T cell memory is compartmentalized into circulating and tissue-resident cell populations. While circulating memory T cells continually patrol the body via the blood and lymphatics, tissue-resident memory T cells (TRM) persist locally in non-lymphoid tissues, providing a front-line of defense against recurring insults. TRM are abundant at barrier surfaces such as the intestines, lungs, and skin, and are defined by expression of the markers CD103 and/or CD69 which function to retain TRM in these epithelial tissues. However, we found that CD4+CD69+CD103+ TRM in human skin can downregulate CD69, exit the tissue and be identified as a phenotypically unique population in the circulation. Functionally, these mobilized (m)TRM produce the cytokines IL-22 and IL-13, and RNA sequencing identifies a set of signature genes shared by CD103+ TRM cells in blood and skin consistent with a role in host-defense and tissue-repair responses. Studies in skin-humanized mice given full-thickness skin grafts show that after exit from the skin, CD103+ mTRM can migrate to and seed distant tissue sites. Thus, contrary to current models, CD4+CD103+ TRM in the skin can be mobilized to the circulation, promoting the spread of a functionally specialized T cell population throughout the skin, a large barrier organ, and allowing for isolation and study of these cells from the blood, a widely accessible tissue source.Tissue-resident memory T cells (TRM) persist locally in non-lymphoid tissues where they provide front-line defense against recurring insults. TRM at barrier surfaces express the markers CD103 and/or CD69 which function to retain them in epithelial tissues. In humans, neither the long-term migratory behavior of TRM nor their ability to re-enter the circulation and potentially migrate to distant tissue sites have been investigated. Using tissue explant cultures, we found that CD4+CD69+CD103+ TRM in human skin can downregulate CD69 and exit the tissue. Additionally, we identified a skin-tropic CD4+CD69−CD103+ population in human lymph and blood that is transcriptionally, functionally and clonally related to the CD4+CD69+CD103+ TRM population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4+CD103+ TRM population can re-enter circulation, and migrate to secondary human skin sites where they re-assume a TRM phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4+ T cell memory in humans. One Sentence Summary Human CD4+CD103+ cutaneous resident memory T cells are found in the circulation of healthy subjects, and these cells can seed distant skin sites.

Dynamic expression of Id3 defines the stepwise differentiation of tissue-resident regulatory T cells

Foxp3+ regulatory T (TR) cells are phenotypically and functionally diverse, and broadly distributed in lymphoid and non-lymphoid tissues. However, the molecular pathways guiding the differentiation of tissue-resident TR populations have not been well defined. By regulating E-protein function, Id3 controls the differentiation of CD8+ effector T cells and is essential for TR maintenance and function. We show that dynamic expression of Id3 helps define three distinct TR populations, Id3+CD62LhiCD44l° central (C)TR, Id3+CD62LloCD44hi effector (e)TR and Id3- eTR. Adoptive transfer experiments and transcriptome analysis support a stepwise model of differentiation from Id3+ cTR to Id3+ eTR to Id3- eTR. Furthermore, Id3- eTR have high expression of functional inhibitory markers and a transcriptional signature of tissue-resident TR. Accordingly, Id3- eTR are highly enriched in non-lymphoid organs, but virtually absent from blood and lymph. Thus, we propose that tissue-resident TR develop in a multi-step process associated with Id3 downregulation.