CCR7 provides localized access to IL-2 and defines homeostatically distinct regulatory T cell subsets

CD44^loCD62L^hi regulatory T cells preferentially access IL-2 in T cell zones due to expression of CCR7

Immune tolerance and activation depend on precise control over the number and function of immunosuppressive Foxp3⁺ regulatory T (T reg) cells, and the importance of IL-2 in maintaining tolerance and preventing autoimmunity is clear. However, the homeostatic requirement for IL-2 among specific populations of peripheral T reg cells remains poorly understood. We show that IL-2 selectively maintains a population of quiescent CD44^loCD62L^hi T reg cells that gain access to paracrine IL-2 produced in the T cell zones of secondary lymphoid tissues due to their expression of the chemokine receptor CCR7. In contrast, CD44^hiCD62L^loCCR7^lo T reg cells that populate nonlymphoid tissues do not access IL-2–prevalent regions in vivo and are insensitive to IL-2 blockade; instead, their maintenance depends on continued signaling through the co-stimulatory receptor ICOS (inducible co-stimulator). Thus, we define a fundamental homeostatic subdivision in T reg cell populations based on their localization and provide an integrated framework for understanding how T reg cell abundance and function are controlled by unique signals in different tissue environments.

MicroRNA-155 is required for effector CD8+ T cell responses to virus infection and cancer

MicroRNAs (miRNAs) regulate the function of several immune cells, but their role in promoting CD8(+) T cell immunity remains unknown. Here we report that miRNA-155 is required for CD8(+) T cell responses to both virus and cancer. In the absence of miRNA-155, accumulation of effector CD8(+) T cells was severely reduced during acute and chronic viral infections and control of virus replication was impaired. Similarly, Mir155(-/-) CD8(+) T cells were ineffective at controlling tumor growth, whereas miRNA-155 overexpression enhanced the antitumor response. miRNA-155 deficiency resulted in accumulation of suppressor of cytokine signaling-1 (SOCS-1) causing defective cytokine signaling through STAT5. Consistently, enforced expression of SOCS-1 in CD8(+) T cells phenocopied the miRNA-155 deficiency, whereas SOCS-1 silencing augmented tumor destruction. These findings identify miRNA-155 and its target SOCS-1 as key regulators of effector CD8(+) T cells that can be modulated to potentiate immunotherapies for infectious diseases and cancer.

Clara cells drive eosinophil accumulation in allergic asthma

Development of allergic asthma is a complex process involving immune, neuronal and tissue cells. In the lung, Clara cells represent a major part of the "immunomodulatory barrier" of the airway epithelium. To understand the contribution of these cells to the inflammatory outcome of asthma, disease development was assessed using an adjuvant-free ovalbumin model. Mice were sensitised with subcutaneous injections of 10 μg endotoxin-free ovalbumin in conjunction with naphthalene-induced Clara cell depletion. Clara epithelial cell depletion in the lung strongly reduced eosinophil influx, which correlated with decreased eotaxin levels and, moreover, diminished the T-helper cell type 2 inflammatory response, including interleukin (IL)-4, IL-5 and IL-13. In contrast, airway hyperresponsiveness was increased. Further investigation revealed Clara cells as the principal source of eotaxin in the lung. These findings are the first to show that Clara airway epithelial cells substantially contribute to the infiltration of eotaxin-responsive CCR3+ immune cells and augment the allergic immune response in the lung. The present study identifies Clara cells as a potential therapeutic target in inflammatory lung diseases such as allergic asthma.

Antagonism of TIM-1 blocks the development of disease in a humanized mouse model of allergic asthma

Studies in mice and humans have revealed that the T cell, immunoglobulin, mucin (TIM) genes are associated with several atopic diseases. TIM-1 is a type I membrane protein that is expressed on T cells upon stimulation and has been shown to modulate their activation. In addition to a recently described interaction with dendritic cells, TIM-1 has also been identified as a phosphatidylserine recognition molecule, and several protein ligands have been proposed. Our understanding of its activity is complicated by the possibility that TIM-1 possesses multiple and diverse binding partners. In order to delineate the function of TIM-1, we generated monoclonal antibodies directed to a cleft formed within the IgV domain of TIM-1. We have shown here that antibodies that bind to this defined cleft antagonize TIM-1 binding to specific ligands and cells. Notably, these antibodies exhibited therapeutic activity in a humanized SCID model of experimental asthma, ameliorating inflammation, and airway hyperresponsiveness. Further experiments demonstrated that the effects of the TIM-1-specific antibodies were mediated via suppression of Th2 cell proliferation and cytokine production. These results demonstrate that modulation of the TIM-1 pathway can critically influence activated T cells in a humanized disease model, suggesting that TIM-1 antagonists may provide potent therapeutic benefit in asthma and other immune-mediated disorders.

Langerhans cells suppress contact hypersensitivity responses via cognate CD4 interaction and langerhans cell-derived IL-10

Mice lacking epidermal Langerhans cells (LC) develop exaggerated contact-hypersensitivity (CHS) responses due to the absence of LC during sensitization/initiation. Examination of T cell responses reveals that the absence of LC leads to increased numbers of hapten-specific CD4 and CD8 T cells but does not alter cytokine expression or development of T regulatory cells. CHS responses and Ag-specific T cells are increased in mice in which MHC class II is ablated specifically in LC suggesting that direct cognate interaction between LC and CD4 cells is required for suppression. LC-derived IL-10 is also required for optimal inhibition of CHS. Both LC-derived IL-10-mediated suppression and full LC activation require LC expression of MHC class II. These data support a model in which cognate interaction of LC with CD4 T cells enables LC to inhibit expansion of Ag-specific responses via elaboration of IL-10.

Cutting edge: instructive role of peripheral tissue cells in the imprinting of T cell homing receptor patterns

Tissue-specific homing of effector and memory T cells to skin and small intestine requires the imprinting of specific combinations of adhesion molecules and chemokine receptors by dendritic cells in the draining lymph nodes. In this study, we demonstrate that CD8(+) T cells activated by Ag-pulsed bone marrow-derived dendritic cells were induced to express the small intestine homing receptors alpha(4)beta(7) integrin and chemokine receptor CCR9 in coculture with small intestinal epithelial cells. In contrast, in coculture with dermal fibroblasts the skin-homing receptor E-selectin ligand was induced. Interestingly, the imprinting of gut homing receptors on anti-CD3/anti-CD28 stimulated T cells was induced by soluble factors produced by small intestinal epithelial cells. Retinoic acid was identified as a crucial factor. These findings show that peripheral tissue cells directly produce homing receptor imprinting factors and suggest that dendritic cells can acquire their imprinting potential already in the peripheral tissue of origin.

Toll-like receptor and IL-12 signaling control susceptibility to contact hypersensitivity

Allergic contact hypersensitivity (CHS) is a T cell–mediated inflammatory skin disease. Interleukin (IL)-12 is considered to be important in the generation of the allergen-specific T cell response. Loss of IL-12 function in IL-12Rβ2–deficient mice, however, did not ameliorate the allergic immune response, suggesting alternate IL-12–independent pathways in the induction of CHS. Because exposure to contact allergens always takes place in the presence of microbial skin flora, we investigated the potential role of Toll-like receptors (TLRs) in the induction of CHS. Using mice deficient in TLR4, the receptor for bacterial lipopolysaccharide (LPS), IL-12 receptor (R) β2, or both, we show that the concomitant absence of TLR4 and IL-12Rβ2, but not the absence of TLR4 or IL-12Rβ2 alone, prevented DC-mediated sensitization, generation of effector T cells, and the subsequent CHS response to 2,4,6-trinitro-1-chlorobenzene (TNCB), oxazolone, and fluorescein isothiocyanate. Introduction of the TLR4 transgene into the TLR4/IL-12Rβ2 mutant restored the CHS inducibility, showing a requirement for TLR4 in IL-12–independent CHS induction. Furthermore, the concomitant absence of TLR2 and TLR4 prevented the induction of CHS to TNCB in IL-12–competent mice. Finally, CHS was inducible in germ-free wild-type and IL-12Rβ2–deficient mice, but not in germ-free TLR4/IL-12Rβ2 double deficient mice, suggesting that the necessary TLR activation may proceed via endogenous ligands.

Administration of a synthetic TLR4 agonist protects mice from pneumonic tularemia

Francisella tularensis is a Gram-negative intracellular pathogen that causes the zoonosis tularemia. Because F. tularensis LPS causes weak TLR4 activation, we hypothesized that administration of a synthetic TLR4 agonist, aminoalkyl glucosaminide phosphate (AGP), would boost the innate immune system and compensate for reduced TLR4 stimulation. Intranasal administration of AGPs induced intrapulmonary production of proinflammatory cytokines and chemokines. Mice treated with AGPs before and after inhalation of Francisella novicida exhibited augmented cytokine and inflammatory responses to infection; reduced bacterial replication in lung, liver, and spleen; and increased survival, whereas all PBS-treated control mice died within 4 days of infection, all AGP-treated mice showed prolonged time-to-death, and 30-60% of AGP-treated mice survived. The protective effect of AGP was lost in mice lacking IFN-gamma. Long-term survivors developed specific Th1 splenocyte responses and specific Abs dominated by IgG2 isotypes. Survivors were fully protected from rechallenge with aerosolized F. novicida. Thus, preventive administration of AGP successfully modulated innate immune responses to aerosolized F. novicida, leading to protective immunity to pneumonic tularemia. This is the first report of the protective effect of a TLR ligand on resistance to F. novicida-induced pneumonic tularemia.

Foxp3+ regulatory T cells maintain immune homeostasis in the skin

Cutaneous immune responses must be tightly controlled to prevent unwanted inflammation in response to innocuous antigens, while maintaining the ability to combat skin-tropic pathogens. Foxp3⁺ regulatory T (T reg) cells are potent immune regulators and are found at high frequency in both human and mouse skin. Although T reg cells migrate to the skin and can dampen immune responses during experimentally induced inflammation or infection, the importance of cutaneous T reg cells for maintaining normal immune homeostasis in the skin has not been addressed. To selectively block T reg cell function in the skin, we restored the T reg cell compartment in Foxp3-deficient scurfy mice with cells whose ability to migrate to the skin was impaired because of targeted mutation of α-1,3-fucosyltransferase VII (Fut7). Although Fut7-deficient T reg cells were present at normal frequency and could function in all other tissues examined, these animals rapidly developed severe cutaneous inflammation. Thus, skin-resident T reg cell are essential for maintaining normal immune homeostasis at this site.

Dendritic cells govern induction and reprogramming of polarized tissue-selective homing receptor patterns of T cells: important roles for soluble factors and tissue microenvironments

Tissue-selective homing is established during naive T cell activation by the tissue microenvironment and tissue-specific dendritic cells (DC). The factors driving induction and maintenance of T cell homing patterns are still largely unknown. Here we show that soluble factors produced during the interaction of T cells with CD11c(+) DC isolated from skin- or small intestine-associated tissues differentially modulate expression of the corresponding tissue-selective homing receptors (E-selectin ligands and alpha4beta7 integrin/CCR9, respectively) on murine CD8(+) T cells. Injection of tissue-specific DC via different routes induces T cells with homing receptors characteristic of the corresponding local tissue microenvironment, independent of the origin of the DC. These data indicate an important role for signals delivered in trans. Moreover, DC can reprogram the homing receptor expression on T cells previously polarized in vitro for homing to skin or small intestine. Importantly, skin-homing memory T cells stimulated directly ex vivo can also be reprogrammed by intestinal DC to a gut-homing phenotype. Our results show that tissue-selective homing receptor expression on effector and memory T cells is governed by inductive as well as suppressive signals from both DC and tissue microenvironments.

Fas-Mediated Inhibition of CD4+ T Cell Priming Results in Dominance of Type 1 CD8+ T Cells in the Immune Response to the Contact Sensitizer Trinitrophenyl

One of the unusual properties of chemically reactive haptens is their capacity to simultaneously generate immunogenic determinants for hapten-specific CD8(+) and CD4(+) T cells. Surprisingly, however, a clear dominance of CD8(+) effector T cells is observed in murine contact hypersensitivity to various haptens and upon T cell priming with hapten-modified APCs in vitro. In this study we show that trinitrophenyl-specific CD8(+) T cells actively prevent CD4(+) T cell priming in vitro. This process requires cell-cell contact and is dependent on the expression of Fas on the CD4(+) T cells. Our results reveal an important Fas-dependent mechanism for the regulation of hapten-specific CD4(+) T cell responses by CD8(+) T cells, which causes the dominance of CD8(+) effector T cells and the active suppression of a CD4(+) T cell response. Moreover, our demonstration of reduced contact hypersensitivity to trinitrophenyl in the absence of Fas, but not of perforin and/or granzymes A and B, underlines the important role of Fas as a pathogenetic factor for contact hypersensitivity.

UVB-irradiated dendritic cells fail to tolerize murine CD8 naïve or effector T cells

UVB radiation has been shown to induce T cell tolerance most likely via modulation of the function of antigen-presenting cells like dendritic cells (DC), which are therefore of interest for vaccination therapy. Since little is known about the effects of UVB-irradiated dendritic cells (UVB-DC) on CD8(+) T cells, which are the dominant effectors in various allergic and autoimmune diseases, we have investigated the potential of low dose UVB (100-200 J per m(2)) irradiated bone marrow-derived dendritic cells to induce tolerance in murine CD8(+) T cells specific for the contact allergen trinitrophenyl (TNP) or for a viral peptide. In contrast to the previously reported successful tolerization of primed CD4(+) Th1 cells, neither naïve CD8(+) T cells nor CD8(+) Tc1 effector cells or established CD8(+) T cell clones could be tolerized by TNP-modified or peptide-pulsed UVB-DC in vitro or in vivo. We observed, however, a reduced capacity of UVB-DC to prime naïve CD8(+) T cells. Our data demonstrate an important difference in the susceptibility of CD4(+) and CD8(+) T cells for tolerance induction using low-dose UVB-irradiated DC and have implications for DC therapy of CD8(+) T cell-mediated diseases.

Dendritic cell immunization route determines CD8+ T cell trafficking to inflamed skin: role for tissue microenvironment and dendritic cells in establishment of T cell-homing subsets

The effector/memory T cell pool branches in homing subsets selectively trafficking to organs such as gut or skin. Little is known about the critical factors in the generation of skin-homing CD8+ T cells, although they are crucial effectors in skin-restricted immune responses such as contact hypersensitivity and melanoma defense. In this study, we show that intracutaneous, but not i.v. injection of bone marrow-derived dendritic cells induced skin-homing CD8+ T cells with up-regulated E-selectin ligand expression and effector function in contact hypersensitivity. The skin-homing potential and E-selectin ligand expression remained stable in memory phase without further Ag contact. In contrast, i.p. injection induced T cells expressing the gut-homing integrin alpha(4)beta(7). Although differential expression of these adhesion molecules was strictly associated with the immunization route, the postulated skin-homing marker CCR4 was transiently up-regulated in all conditions. Interestingly, dendritic cells from different tissues effectively induced the corresponding homing markers on T cells in vitro. Our results suggest a crucial role for the tissue microenvironment and dendritic cells in the instruction of T cells for tissue-selective homing and demonstrate that Langerhans cells are specialized to target T cells to inflamed skin.

Truncation of the human immunodeficiency virus-type-2 envelope glycoprotein allows efficient pseudotyping of murine leukemia virus retroviral vector particles

The incorporation of human immunodeficiency virus-type-2 (HIV-2) envelope glycoprotein into murine leukemia virus (MuLV) particles was studied in a transient transfection packaging cell system. We observed that wild-type HIV-2 envelope protein or a frameshift mutant with 187 unrelated carboxyl-terminal residues did not allow the formation of infectious retroviral particles. In view of recent findings that an HIV-1 envelope protein variant with a shortened cytoplasmic domain was incorporated into MuLV particles, we constructed carboxyl-terminal truncations of the HIV-2 envelope protein. An envelope variant with 18 cytoplasmic amino acids formed only very few viral pseudotypes. The further removal of an additional 11 amino acids allowed the efficient pseudotyping of MuLV particles. As with the HIV-1 envelope protein, an HIV-2 envelope variant with 7 cytoplasmic amino acids was incorporated into functional MuLV particles. The pseudotyped vectors obtained are able to infect human CD4/CXCR4-expressing cells. Cell lines expressing human CD4 and other coreceptors could not be infected. This retroviral vector will prove useful for the study of HIV infection events mediated by the HIV-2 envelope glycoproteins, as well as for the targeting of CD4+ cells in the context of gene therapy of AIDS.