Ana Izcue

CTLA-4 promotes Foxp3 induction and regulatory T cell accumulation in the intestinal lamina propria

Thymic induction of CD4+Foxp3+ regulatory T (Treg) cells relies on CD28 costimulation and high-affinity T-cell receptor (TCR) signals, whereas Foxp3 (forkhead box P3) induction on activated peripheral CD4+ T cells is inhibited by these signals. Accordingly, the inhibitory molecule CTLA-4 (cytotoxic T-lymphocyte antigen 4) promoted, but was not essential for CD4+ T-cell Foxp3 induction in vitro. We show that CTLA-4-deficient cells are equivalent to wild-type cells in the thymic induction of Foxp3 and maintenance of Foxp3 populations in the spleen and mesenteric lymph nodes, but their accumulation in the colon, where Treg cells specific for commensal bacteria accumulate, is impaired. In a T cell–transfer model of colitis, the two known CTLA-4 ligands, B7-1 and B7-2, had largely redundant roles in inducing inflammation and promoting Treg cell function. However, B7-2 proved more efficient than B7-1 in inducing Foxp3 in vitro and in vivo. Our data reveal an unappreciated role for CTLA-4 in establishing the Foxp3+ compartment in the intestine.

Essential role of Mediator subunit Med1 in invariant natural killer T-cell development

CD1d-restricted invariant NKT (iNKT) cells are a unique lineage of T lymphocytes that regulate both innate and adaptive immunity. The Mediator complex forms the bridge between transcriptional activators and the general transcription machinery. Med1/TRAP220 (also called DRIP205) is a key component of Mediator that interacts with ligand-bound hormone receptors, such as the vitamin D receptor. Here, we show that T-cell–specific Med1 deficiency results in a specific block in iNKT cell development but the development of conventional αβ T cells remains grossly normal. The defect is cell-intrinsic and depends neither on apoptosis, cell-cycle control, nor on CD1d expression of CD4+CD8+ double-positive thymocytes. Surprisingly, ectopic expression of a Vα14-Jα18 T-cell receptor transgene completely rescues the defect caused by Med1 deficiency. At the molecular level, thymic iNKT cells in Med1−/− animals display reduced levels of IL-2Rβ and T-bet expression and could not complete terminal maturation. Thus, Med1 is essential for a complete intrathymic development of iNKT cells.

Eomesodermin Expression in CD4+ T Cells Restricts Peripheral Foxp3 Induction

CD4+ T cells polarize into effector Th subsets characterized by signature transcription factors and cytokines. Although T-bet drives Th1 responses and represses the alternative Th2, Th17, and Foxp3+ regulatory T cell fates, the role of the T-bet–related transcription factor eomesodermin (Eomes) in CD4+ T cells is less well understood. In this study, we analyze the expression and effects of Eomes in mouse CD4+ T lymphocytes. We find that Eomes is readily expressed in activated CD4+ Th1 T cells in vivo. Eomes+ CD4+ T cells accumulated in old mice, under lymphopenic conditions in a T cell transfer model of colitis, and upon oral Ag administration. However, despite its expression, genetic deletion of Eomes in CD4+ T cells did not impact on IFN-γ production nor increase Th2 or Th17 responses. In contrast, Eomes deficiency favored the accumulation of Foxp3+ cells in old mice, after in vivo differentiation of Eomes-deficient naive CD4+ T cells, and in response to oral Ag in a cell-intrinsic way. Enforced Eomes expression during in vitro regulatory T cell induction also reduced Foxp3 transcription. Likewise, bystander Eomes-deficient CD4+ T cells were more efficient at protecting from experimental autoimmune encephalitis compared with wild-type CD4+ T cells. This enhanced capacity of Eomes-deficient CD4+ T cells to inhibit EAE in trans was associated with an enhanced frequency of Foxp3+ cells. Our data identify a novel role for Eomes in CD4+ T cells and indicate that Eomes expression may act by limiting Foxp3 induction, which may contribute to the association of EOMES to susceptibility to multiple sclerosis.

Immunology: Malnutrition promotes rogue bacteria

Dietary lack of a single amino acid impairs intestinal immunity in mice, altering the guts microbial community and leaving it vulnerable to damage. The finding helps to explain how malnutrition favours gut inflammation. See Letter p.477 Malnutrition affects many millions of people in the developing world and remains a problem in wealthy nations, especially for disadvantaged groups. In many cases, it is the associated diarrhoea and intestinal inflammation that cause morbidity and death. A study published in this issue presents a molecular explanation for the increased susceptibility to intestinal inflammation in malnutrition. Angiotensin converting enzyme 2 (ACE2), which has a central role in blood-pressure regulation and has been implicated in diabetes, heart failure and viral infection, is shown to influence dietary amino-acid homeostasis, innate immunity, gut microbial ecology and susceptibility to colitis. Mice deficient in this enzyme show impaired tryptophan metabolism and develop colitis, which is alleviated by dietary tryptophan and its metabolite, nicotinamide. This surprising result explains nutritional effects that have been known for centuries and provides a molecular link between malnutrition and the intestinal microbiome.

Higher Sensitivity of Foxp3+ Treg Compared to Foxp3- Conventional T Cells to TCR-Independent Signals for CD69 Induction.

T lymphocytes elicit specific responses after recognizing cognate antigen. However, antigen-experienced T cells can also respond to non-cognate stimuli, such as cytokines. CD4+ Foxp3+ regulatory T cells (Treg) exhibit an antigen-experienced-like phenotype. Treg can regulate T cell responses in an antigen-specific or bystander way, and it is still unclear as to which extent they rely on T cell receptor (TCR) signals. The study of the antigen response of Treg has been hampered by the lack of downstream readouts for TCR stimuli. Here we assess the effects of TCR signals on the expression of a classical marker of early T cell activation, CD69. Although it can be induced following cytokine exposure, CD69 is commonly used as a readout for antigen response on T cells. We established that upon in vitro TCR stimulation CD69 induction on Foxp3+ Treg cells was more dependent on signaling via soluble factors than on TCR activation. By contrast, expression of the activation marker Nur77 was only induced after TCR stimulation. Our data suggest that Treg are more sensitive to TCR-independent signals than Foxp3- cells, which could contribute to their bystander activity.

KLRG1 impairs regulatory T-cell competitive fitness in the gut

Immune homeostasis requires the tight, tissue‐specific control of the different CD4+ Foxp3+ regulatory T (Treg) cell populations. The cadherin‐binding inhibitory receptor killer cell lectin‐like receptor G1 (KLRG1) is expressed by a subpopulation of Treg cells with GATA3+ effector phenotype. Although such Treg cells are important for the immune balance, especially in the gut, the role of KLRG1 in Treg cells has not been assessed. Using KLRG1 knockout mice, we found that KLRG1 deficiency does not affect Treg cell frequencies in spleen, mesenteric lymph nodes or intestine, or frequencies of GATA3+ Treg cells in the gut. KLRG1‐deficient Treg cells were also protective in a T‐cell transfer model of colitis. Hence, KLRG1 is not essential for the development or activity of the general Treg cell population. We then checked the effects of KLRG1 on Treg cell activation. In line with KLRG1s reported inhibitory activity, in vitro KLRG1 cross‐linking dampened the Treg cell T‐cell receptor response. Consistently, lack of KLRG1 on Treg cells conferred on them a competitive advantage in the gut, but not in lymphoid organs. Hence, although absence of KLRG1 is not enough to increase intestinal Treg cells in KLRG1 knockout mice, KLRG1 ligation reduces T‐cell receptor signals and the competitive fitness of individual Treg cells in the intestine.