a b s t r a c tExtracellular ATP is a danger signal released by dying and damaged cells, and it functions as an immunostimulatory signal that promotes inflammation. However, extracellular adenosine acts as an immunoregulatory signal that modulates the function of several cellular components of the adaptive and innate immune response. Consequently, the balance between ATP and adenosine concentration is crucial in immune homeostasis. CD39 and CD73 are two ectonucleotidases that cooperate in the generation of extracellular adenosine through ATP hydrolysis, thus tilting the balance towards immunosuppressive microenvironments. Extracellular adenosine can prevent activation, proliferation, cytokine production and cytotoxicity in T cells through the stimulation of the A2A receptor; however, recent evidence has shown that adenosine may also affect other processes in T-cell biology. In this review, we discuss evidence that supports a role of CD73 and CD39 ectonucleotidases in controlling naive T-cell homeostasis and memory cell survival through adenosine production. Finally, we propose a novel hypothesis of a possible role of these ectonucleotidases and autocrine adenosine signaling in controlling T-cell differentiation.

Vitamin A, a generic designation for an array of organic molecules that includes retinal, retinol and retinoic acid, is an essential nutrient needed in a wide array of aspects including the proper functioning of the visual system, maintenance of cell function and differentiation, epithelial surface integrity, erythrocyte production, reproduction, and normal immune function. Vitamin A deficiency is one of the most common micronutrient deficiencies worldwide and is associated with defects in adaptive immunity. Reports from epidemiological studies, clinical trials and experimental studies have clearly demonstrated that vitamin A plays a central role in immunity and that its deficiency is the cause of broad immune alterations including decreased humoral and cellular responses, inadequate immune regulation, weak response to vaccines and poor lymphoid organ development. In this review, we will examine the role of vitamin A in immunity and focus on several aspects of T cell biology such as T helper cell differentiation, function and homing, as well as lymphoid organ development. Further, we will provide an overview of the effects of vitamin A deficiency in the adaptive immune responses and how retinoic acid, through its effect on T cells can fine-tune the balance between tolerance and immunity.

CD4 + CD25 + Foxp3 + regulatory T (Treg) cells mediate immunological self-tolerance and suppress immune responses. Retinoic acid (RA), a natural metabolite of vitamin A, has been reported to enhance the differentiation of Treg cells in the presence of TGF-β. In this study, we show that the co-culture of naive T cells from C57BL/6 mice with allogeneic antigen-presenting cells (APCs) from BALB/c mice in the presence of TGF-β, RA, and IL-2 resulted in a striking enrichment of Foxp3 + T cells. These RA in vitro-induced regulatory T (RA-iTreg) cells did not secrete Th1-, Th2-, or Th17-related cytokines, showed a nonbiased homing potential, and expressed several cell surface molecules related to Treg-cell suppressive potential. Accordingly, these RA-iTreg cells suppressed T-cell proliferation and inhibited cytokine production by T cells in in vitro assays. Moreover, following adoptive transfer, RA-iTreg cells maintained Foxp3 expression and their suppressive capacity. Finally, RA-iTreg cells showed alloantigen-specific immunosuppressive capacity in a skin allograft model in immunodeficient mice. Altogether, these data indicate that functional and stable allogeneic-specific Treg cells may be generated using TGF-β, RA, and IL-2. Thus, RA-iTreg cells may have a potential use in the development of more effective cellular therapies in clinical transplantation. Keywords: Allogeneic regulatory T cells r Homing r Retinoic acid r Tolerance r TransplantationAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionRegulatory T (Treg) cells are responsible for inducing and maintaining peripheral tolerance [1]. Treg cells are classified into two Correspondence: Dr. Daniela Sauma e-mail: dsauma@u.uchile.cl major subpopulations: thymus-derived Treg cells, which are generated in the thymus and circulate in the periphery as functional mature Treg cells [2][3][4], and peripherally derived Treg cells, which are generated in the periphery from CD4 + CD25 − naive * These authors contributed equally to this work.C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2015. 45: 452-463 Immunomodulation 453T cells [5][6][7]. Treg cells target effector T cells and dendritic cells (DCs) by modulating their maturation and function through several mechanisms that suppress immune responses [8][9][10]; these include secretion of inhibitory cytokines (IL-10, IL-35, and TGF-β), granzyme/perforin-dependent mediated cytolysis, metabolic disruption [11][12][13][14][15], and the expression of LAG3 and CTLA4, which alter DC function [16,17]. In addition to the aforementioned mechanisms, Treg-cell suppressive capacity is dependent on Treg-cell migration and retention in the microenvironment where regulation is required. Differential expression patterns of chemokine receptors (such as CCR4, CCR5, and CCR9), integrins (α4β7), and selectins (CD62L) contribute to selective retention and trafficking of Treg cells and allow their appropriate localization during ...

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SummaryThe CD73 ectonucleotidase catalyses the hydrolysis of AMP to adenosine, an immunosuppressive molecule. Recent evidence has demonstrated that this ectonucleotidase is up-regulated in T helper type 17 cells when generated in the presence of transforming growth factor-b (TGF-b), and hence CD73 expression is related to the acquisition of immunosuppressive potential by these cells. TGF-b is also able to induce CD73 expression in CD8 + T cells but the function of this ectonucleotidase in CD8 + T cells is still unknown. Here, we show that Tc17 cells present high levels of the CD73 ectonucleotidase and produce adenosine; however, they do not suppress the proliferation of CD4 + T cells. Interestingly, we report that adenosine signalling through A2A receptor favours interleukin-17 production and the expression of stem cell-associated transcription factors such as tcf-7 and lef-1 but restrains the acquisition of Tc1-related effector molecules such as interferon-c and Granzyme B by Tc17 cells. Within the tumour microenvironment, CD73 is highly expressed in CD62L + CD127 + CD8 + T cells (memory T cells) and is down-regulated instrating that CD73 is expressed in memory/naive cells and is down-regulated during differentiation. These data reveal a novel function of CD73 ectonucleotidase in arresting CD8 + T-cell differentiation and support the idea that CD73-driven adenosine production by Tc17 cells may promote stem cell-like properties in Tc17 cells.