Recent data suggest that hypoxia and its principal molecular signature HIF-1 (hypoxiainducing factor-1) may tune down inflammation by dictating anti-inflammatory programs. We tested the effects of hypoxia and HIF-1a on the homeostasis of naturally occurring regulatory T cells (Treg) and their transcriptional activator Foxp3. Hypoxia induced a timedependent increase in HIF-1a in mouse and human T cells. Hypoxia upregulated the expression of Foxp3 in Jurkat T cells, human and murine mononuclear cells. The effects of hypoxia on Foxp3 expression were HIF-1a-dependent as they were abolished upon transfection with short-interfering RNAs for HIF-1a and promoted by HIF-1a overexpression. Hypoxia increased the potency of Treg, as hypoxic CD4 1 CD25 1 lymphocytes were more effective than normoxic cells in suppressing the proliferation of CD4 1 CD25 À effectors. In vivo expression of HIF-1a achieved by hydrodynamic injection of the respective naked DNA similarly induced an increase in Foxp3 expression and an increase in the number of functionally active Foxp3 1 CD4 1 CD25 1 Treg. Thus, hypoxia dictates an antiinflammatory program by driving expression of HIF-1a that acts to increase the number and suppressive properties of naturally occurring CD4 1 CD25 1 Treg.Key words: Foxp3 Á HIF-1 Á Hypoxia Á Inflammation Á Regulatory T cells IntroductionUnder hypoxia, hypoxia-inducible factor-1 (HIF-1) plays a key role in controlling glycolysis, angiogenesis, erythropoiesis and cell survival [1]. HIF-1 constitutes an oxygen-dependent a-subunit and a constitutively expressed b-subunit that interact through mutual basic helix-loop-helix domains [2]. HIF-1a oxygen-dependent domain undergoes site-specific proline hydroxylation by prolyl hydroxylases, which allows subsequent ubiquitination mediated by the von Hippel-Lindau protein [3][4][5]. In the absence of molecular oxygen, HIF-a hydroxylation is abrogated, leading to its cellular accumulation. HIF subunits heterodimerize, and activate a wide range of target genes by binding to hypoxia response elements.T-lymphocytes that comprise a principal effector component of the cellular immune response are exposed to hypoxia in target microenvironments in which they are assumed to dictate inflammatory programs. Target sites include tumors and healthy organs that confront T cells with gradients of low oxygen tension. Recent literature suggests that blunted HIF-1a expression results in a net proinflammatory program evident by increased production inflammatory cytokines upon TCR engagement [6]. Similar results were obtained upon knocking out HIF-1 selectively in T cells [7]. Additionally, HIF-1a was reported to play an inhibitory role in the regulation of T-cell receptor signal transduction by controlling intracellular calcium balance [8]. Collectively, these observations suggest that HIF-1 drives a Th2 favored response by downregulated Th1 programs [9][10][11]. Naturally occurring CD4 1 CD25 1 regulatory T cells (Treg) comprise a relatively newly characterized population that has evolved to tune do...

No abstract

Aims:To determine whether low-dose calcitriol attenuates atherosclerosis in apoE-null mice and, if so, through which predominant mechanism. Methods: Starting at the age of 6 weeks, mice received intraperitoneal injections of either 0.25 ng/g body weight of calcitriol or the vehicle, every other day for 8 weeks.Results: Calcitriol treatment resulted in 35% reduction of atherosclerosis at the aortic sinus, and in a significant decrease in blood pressure. These effects were possibly mediated by downregulation of the renin-angiotensin system (RAS), as there was a 64% decrease in the aortic level of renin mRNA. None of the other components of the RAS or the prorenin receptor were affected by treatment. Lowdose calcitriol treatment did not modify the plasma level of monocyte chemoattractant protein-1, interferon γ, interleukin-4 and interleukin-10, which were similar in control and treated mice. Likewise, there was no difference in the percentage of splenic Foxp3 ＋ regulatory T cells. Calcitriol treatment resulted in an unfavorable metabolic profile (glucose and lipids), as determined after a limited fast, a difference that disappeared after food was withheld for a longer time. Conclusions: At a relatively low dosage, calcitriol attenuates the development of atherosclerosis in apoE-null mice, most probably by down regulation of RAS, and not through immunomodulation; however, even at this low dose, calcitriol appears to elevate calcium and to have potentially adverse metabolic effects. Exploring the potential antiatherogenic effects of non-calcemic and safer analogues is therefore warranted.