Regulatory T cells (Treg) can suppress undesired immune responses in allergic inflammation and autoimmunity. In hopes of enhancing Treg activity to expand the currently limited immunotherapeutic options for allergic airway disease, Wilson et al. (p. 6942 ) addressed the effects of treatment with IL-2, which is required for the maintenance of Treg populations. Unfortunately, rIL-2 administration exacerbated airway inflammation. Based on previous data that IL-2:anti-IL-2 mAb complexes could expand Treg, the authors next tried administering these complexes to allergen-sensitized mice and found that they reduced airway inflammation and airway hyperresponsiveness in multiple models of allergic inflammation. The IL-2:anti-IL-2 complexes could also ameliorate established airway disease, suggesting therapeutic potential. Both natural (CD4+CD25+Foxp3+) and inducible (CD4+CD25+IL-10+) Treg were expanded in the lung following treatment; of these, the inducible, IL-10-expressing Treg were found to be essential for suppression of airway inflammation, effector T cell proliferation, and lung pathology. This demonstration of in vivo induction of IL-10-producing Treg able to suppress allergic disease identifies a novel strategy that could be applied to potential therapeutic applications.

Regulatory T cells (Treg) have been implicated in the pathogenesis of type I diabetes (T1D), and it has been assumed that patients with T1D have defective Treg that fail to suppress autoreactive T cells. Although defects in suppression of effector T cells (Teff) have been observed in these individuals, it is not known whether these defects are intrinsic to the Treg or the Teff. Schneider et al. (p. 7350 ) sought to resolve this issue via in vitro analysis of Treg from individuals with T1D. Adaptive Treg (aTreg) generated from the peripheral blood of these individuals (T1D aTreg) showed impaired suppression of autologous Teff (T1D Teff) compared with control aTreg. Interestingly, the authors found no difference between the abilities of T1D aTreg and control aTreg to suppress Teff from control individuals. In contrast, T1D Teff were resistant to suppression by both control aTreg and natural Treg isolated from either control or diabetic individuals, suggesting that the defects previously attributed to T1D Treg might instead be intrinsic to T1D Teff. Further in vitro experiments supported this conclusion and demonstrated that the defect in T1D Teff was not transferable to other T cells. Although some individuals may also have defects in their Treg that contribute to diabetes development, these data suggest that the predominant defect in suppression in T1D resides in the effector T cell population.

Two papers in this issue examine the steroid hormone vitamin D and the mechanism of its involvement in innate immunity. Stimulation of monocytes via TLR2/1 has been shown to induce vitamin D-mediated antimicrobial activity by up-regulating the vitamin D receptor (VDR) and the 25D-1α-hydroxylase (CYP27b1) that converts inactive vitamin D (25D3) to its active form (1,25D3). VDR stimulation causes expression of the antimicrobial peptide cathelicidin, which is required for vitamin D-dependent antimicrobial activity. As stimulation of human monocytes via TLR2/1 also leads to their differentiation into macrophages, Krutzik et al. (p. 7115 ) assessed whether these two TLR2/1-mediated functions might be linked by IL-15 expression. Indeed, IL-15 was required for TLR2/1-induced, vitamin D-dependent antimicrobial activity and directly up-regulated cathelicidin, VDR, and CYP27b1, in addition to promoting macrophage differentiation. Macrophages differentiated with IL-15, but not with IL-4, converted 25D3 to 1,25D3, which went on to induce cathelicidin expression. These IL-15-differentiated macrophages could thus use 25D3 to induce antimicrobial activity against Mycobacterium tuberculosis. The data demonstrate that TLR2/1 stimulation induces IL-15, which can both induce macrophage differentiation and trigger the vitamin D-mediated antimicrobial response.

In the second article, it is noted that vitamin D has been shown to modulate innate immunity in many tissues. A number of innate immune molecules that are up-regulated by stimulation of the VDR are expressed in airway epithelial cells, leading Hansdottir et al. (p. 7090 ) to analyze the potential involvement of these cells in vitamin D-mediated host defense. Primary human respiratory epithelial cells were found to express CYP27b1 and to constitutively convert 25D3 to 1,25D3. This activation of vitamin D led to the expression of VDR-responsive genes, including those encoding cathelicidin and the TLR4 coreceptor CD14. Inhibition of vitamin D activation with itraconazole reduced the induction of these VDR-regulated genes, further supporting the activity of vitamin D in these cells. In contrast to published data on macrophages and other cell types, TLR2 ligands did not affect CYP27b1 expression or cathelicidin up-regulation in airway epithelial cells. However, synthetic viral dsRNA or live virus increased CYP27b1 expression, which enhanced vitamin D activation, and synergized with 25D3 to induce cathelicidin up-regulation. This observation that airway epithelial cells can endogenously activate vitamin D suggests a mechanism for the involvement of these cells in antiviral innate immunity. Taken together, the data presented in these two articles clarify mechanisms by which vitamin D may modulate both antibacterial and antiviral immunity.

Several models predict how double-positive (DP) thymocytes may differentiate into single-positive CD4+ or CD8+ T cells with appropriately MHC II- or MHC I-restricted TCRs. The stochastic/selection model proposes that DP thymocytes randomly choose to express a coreceptor during positive selection and then must be “rescued” by an appropriate signal through the TCR and coreceptor to survive and differentiate into mature CD4+ or CD8+ T cells. Adoro et al. (p. 6975 ) assessed the validity of this model by developing a novel “coreceptor rescue” strategy using a CD4 transgene (E8I-CD4) that was expressed specifically in thymocytes that had already undergone positive selection and had chosen to adopt the CD8 lineage. Previous coreceptor rescue strategies used earlier coreceptor expression, which could have affected the outcome of selection. Introduction of the E8I-CD4 transgene did not rescue any MHC II-restricted thymocytes that had incorrectly adopted the CD8 lineage fate. Additional experiments confirmed that CD4 coreceptors encoded by the E8I-CD4 transgene were signaling competent and could be expressed on MHC II-expressing CD8+ thymocytes. The failure of these transgenic CD4 coreceptors to reveal any incorrectly generated MHC II-expressing CD8+ thymocytes argues strongly that the CD4/CD8 lineage choice is neither stochastic nor significantly error prone.

Exposure to environmental factors including infections and heavy metals such as mercury can trigger autoimmunity through processes that are not well understood. To analyze the involvement of natural killer T (NKT) cells in mercury-induced autoimmunity, Vas et al. (p. 6779 ) studied the effects of two synthetic NKT cell ligands, PBS 57 and 4-deoxy α-GalCer, on the development of disease. Overall, NKT cell activation, which was induced by administration of these synthetic ligands or the NKT-ligand-bearing bacteria Sphingomonas capsulata, exacerbated mercury-induced autoimmunity. However, although the two synthetic ligands were predicted to stimulate NKT cells similarly, they had strikingly disparate effects in this system. PBS 57 could prevent the induction of tolerance to mercury-induced autoimmunity, but, interestingly, could also antagonize the TLR9-mediated breaking of established tolerance. In contrast, 4-deoxy α-GalCer did not affect tolerance induction but augmented the breaking of tolerance. Additional experiments suggested that these NKT cell ligands may have differentially modulated regulatory T cell populations or the Th1/Th2 cytokine balance. These data demonstrate that specific NKT cell ligands can regulate autoimmunity triggered by an environmental toxin and urge caution in the development of treatments for these complex syndromes.

Aging is known to impair adaptive immunity, but its effects on innate immunity are less clear. Plasmacytoid dendritic cells (pDCs) are important mediators of innate antiviral immunity and act by secreting high levels of type I IFNs upon TLR7 or TLR9 stimulation. Stout-Delgado et al. (p. 6747 ) assessed the effects of aging on pDC function and found that, compared with pDCs from young mice, pDCs from aged mice produced reduced IFN-α in response to TLR9 ligand administration or viral infection. Aged mice were less able to clear viral infections than were young mice, but adoptive transfer of young pDCs into aged hosts could restore viral clearance and IFN-α production. To address the mechanism of pDC dysfunction in aged mice, the authors analyzed signaling pathways and found that IRF7 and PI3K up-regulation were strongly impaired in response to either TLR9 or IFNα/β receptor stimulation in aged pDCs. Reactive oxygen species often accumulate with age and were found to be increased in pDCs from aged vs young mice. Reduction of this oxidative stress by calorie restriction led to a partial restoration of IFN-α production and IRF7 and PI3K up-regulation in pDCs from aged mice. Taken together, these data demonstrate defects in the pDCs of aged mice that may help explain the impairment in antiviral immunity observed in older individuals.

In NOD mice, NK cells show impaired activation and cytotoxic activity. Previous studies suggested that this defect was related to overexpression of the NKG2D ligand RAE-1, which caused down-regulation of NKG2D expression. Maier et al. (p. 7073 ) chose to test this hypothesis for the NOD NK cell defect through a series of genetic analyses. Initial analyses demonstrated that genes outside of the known diabetes susceptibility regions were responsible for the NK cell defect in NOD mice. Contradicting earlier studies, no correlation was found between strain-specific Raet1 alleles, which encode RAE-1, and NK cell cytotoxicity. The down-regulation of NKG2D that was previously reported on NOD NK cells in vitro was not observed in vivo, and levels of NKG2D expression did not correlate with NK cell activity. Further analysis indicated that the NOD NK cell defect is a complex trait controlled by multiple genes. For example, upon examining cytokine induction in the different mouse strains following injection with the TLR3 ligand poly(I:C), the authors found impaired up-regulation of IL-12p40 in the NOD mice. This study emphasizes the complexity of the regulation of NK cell activity in NOD mice and will be important for future investigation of the roles of these cells in diabetes.

Summaries written by Jennifer Hartt Meyers, Ph.D.