The origin of FOXP3-expressing CD4⁺ regulatory T cells: thymus or periphery

Abstract

Naturally arising CD4⁺ regulatory T cells, which engage in the maintenance of immunologic self-tolerance, specifically express FOXP3, which encodes a transcription-repressor protein. Genetic defects in FOXP3 cause IPEX, an X-linked autoimmune/inflammatory syndrome. With FOXP3 as a specific marker for regulatory CD4⁺ T cells in humans, it is now possible to determine their origin and developmental pathway .

The immune system discriminates between self and non-self, maintaining immunologic self-tolerance (i.e., unresponsiveness to self-constituents). It is known that potentially hazardous self-reactive T and B cells are clonally deleted at immature stages of their development or inactivated upon encounter with self-antigens in the periphery. There is now accumulating evidence that, in addition to these passive mechanisms of self-tolerance, a population of CD4⁺ T cells, called regulatory T cells (T_R cells), engage in the maintenance of peripheral self-tolerance by actively suppressing the activation and expansion of self-reactive T cells (1–3). The majority, if not all, of such naturally occurring CD4⁺ T_R cells constitutively express CD25 (IL-2 receptor α chain) in the physiologic state. Indeed, removal of CD25⁺CD4⁺ T cells, which constitute 5–10% of CD4⁺ T cells in rodents and humans, leads to spontaneous development of various autoimmune diseases in otherwise normal mice (4). The removal of CD25⁺CD4⁺ T_R cells also triggers excessive or misdirected immune responses to microbial antigens, causing immunopathology, such as inflammatory bowel disease (IBD), due to hyper-reaction of the remaining T cells to commensal bacteria in the intestine (3).

FOXP3: master control gene for the development and function of natural CD4⁺ T_R cells

There is now evidence not only for the presence of CD25⁺CD4⁺ T_R cells in humans but also for their essential roles in controlling autoimmunity, immunopathology, and allergy in human diseases (5). This is best illustrated by IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome), a rare monogenic disease of male children that is accompanied by autoimmune disease (such as type 1 diabetes), IBD, and severe allergy similar to those produced in mice by depletion of CD25⁺CD4⁺ T_R cells (6). The causative gene, FOXP3 (Foxp3 in mice), which encodes a transcription repressor (7–10), is specifically expressed in CD25⁺CD4⁺ T cells in the thymus and periphery (11–13). Forced expression of the Foxp3 gene can convert murine naive T cells to T_R cells that phenotypically and functionally resemble naturally arising CD25⁺CD4⁺ T_R cells (11, 12). Furthermore, inoculation of CD25⁺CD4⁺ T cells prepared from normal mice can prevent autoimmune disease in Foxp3-defective mice (12). These findings collectively indicate that FOXP3 is a master control gene for the development and function of natural CD25⁺CD4⁺ T_R cells.

The origin and the developmental pathway of FOXP3-expressing T_R cells

The discovery of FOXP3/Foxp3 as a specific and stable marker for natural T_R cells now makes it possible to determine the origin and the developmental pathway of T_R cells in humans, as reported by Walker et al. in this issue of the JCI (14). It has been shown, mainly in rodents, that the normal thymus continuously produces CD25⁺CD4⁺ T_R cells as a functionally mature T cell subpopulation that recognizes a broad repertoire of self- and non-self antigens, and that abrogation of the thymic production of T_R cells leads to the development of autoimmune disease (1–3). Walker et al. (14) show that CD25⁺CD4⁺ T cells in the peripheral blood lymphocytes express FOXP3 and are capable of suppressing the activation and expansion of other T cells in vitro, as shown in rodents (11–13). Furthermore, they show that, in contrast with murine Foxp3 expression, activation of CD25^–CD4⁺ T cells by T cell receptor (TCR) stimulation induces FOXP3 expression, and that FOXP3-expressing T cells derived from CD25^–CD4⁺ T cells are equally as suppressive as natural CD25⁺CD4⁺ T_R cells (Figure 1) (14). This interesting finding suggests two possibilities regarding the origin of CD25⁺CD4⁺ T_R cells. One is that naive T cells can differentiate to CD25⁺CD4⁺ T_R cells upon TCR stimulation, in a manner similar to that in which the expression of the transcription factors T-bet and GATA-3 instruct naive T cells to differentiate to Th1 and Th2 cells, respectively (15, 16). Another possibility is that some of the functionally mature T_R cells produced by the thymus are CD25^– or lose CD25 expression with retention of their suppressive function, as has been shown in rodents (17–19). Such CD25^– T_R cells may become CD25⁺ upon activation, especially when other T cells respond to antigen stimulation, and IL-2 secreted by them may trigger the expansion of T_R cells. Given the specific expression of FOXP3 in T_R cells whether they are of thymic or peripheral origin, it remains to be determined whether other T cells with regulatory functions, such as IL-10–secreting Tr1 or TGF-β–secreting Th3 cells, may also express FOXP3 (20).

Figure 1.

The normal thymus produces FOXP3-expressing CD25⁺CD4⁺ T_R cells. Some of the naive CD25^–CD4⁺ T cells may also differentiate to FOXP3-expressing CD25⁺CD4⁺ T_R cells in the periphery. These T_R cells suppress the activation and expansion of self-reactive T cells that may cause autoimmune disease. Genetic defects of FOXP3 cause IPEX due to developmental or functional defects of T_R cells. Adapted with permission from Nature Immunology (21).

Besides self-tolerance and autoimmunity, evidence is now accumulating that natural CD4⁺ T_R cells actively engage in negative control of a broad spectrum of immune responses to quasi-self or non-self antigens, as in tumor immunity, organ transplantation, allergy, and microbial immunity (1–3). With FOXP3 as a useful tool for investigating T_R cells, further characterization of their developmental pathways will facilitate better control of pathologic as well as physiologic immune responses by expansion or reduction of T_R cell populations.