Studies on the density, distribution, and surface phenotype of intraepithelial class II major histocompatibility complex antigen (Ia)-bearing dendritic cells (DC) in the conducting airways

Allergen-induced changes in bone-marrow progenitor and airway dendritic cells in sensitized rats

Eosinophilic airway inflammation is orchestrated by T-helper (Th)-2 lymphocytes. We have previously demonstrated that dendritic cells (DC) are essential for the presentation of antigen to these Th2 cells leading to airway inflammation. Here, we have examined the presence of DC in the lungs, the kinetics of appearance, and the possible involvement of the bone-marrow progenitor for DC in a rat model of ovalbumin (OVA)-induced airway inflammation. Sensitized rats were exposed to 0, 1, 3, or 7 consecutive daily OVA aerosols. Control rats were sham sensitized and/or exposed to phosphate-buffered saline (PBS), and bronchoalveolar lavage (BAL) was performed 24 h after the last challenge. DC were identified in BAL fluid as low-density, low-autofluorescence, CD3(-), CD45RA-, OX62(+), OX6(+) cells that had long surface extensions and strong costimulatory activity. Low but detectable amounts of BAL DC were seen in sensitized, unexposed animals. After three OVA exposures, the inflammatory infiltrate consisted of CD4(+)-activated T cells, eosinophils, and monocytes. The number of BAL DC was significantly increased in OVA-sensitized/OVA-exposed animals compared with sham-sensitized or PBS-exposed animals. The kinetics of DC increase closely parallelled those in other inflammatory cells. Bone-marrow cells taken from the OVA-sensitized and -exposed group were grown in the DC growth factor granulocyte macrophage colony-stimulating factor for 6 d and the yield of OX62(+)OX6(+) DC was 60% higher compared with PBS-exposed or sham-sensitized animals. We conclude that allergen exposition in sensitized rats increases the number of DC in the airways and the production of progenitors for DC in the bone marrow.

Airway inflammation in asthma and chronic obstructive pulmonary disease with special emphasis on the antigen-presenting dendritic cell: influence of treatment with fluticasone propionate

Asthma is a chronic inflammatory disorder of the airways characterized by variable airflow limitation and airway hyperresponsiveness. The type of inflammatory response in asthma is compatible with a major contribution of professional antigen-presenting cells. The airways in chronic obstructive pulmonary disease (COPD) are also markedly inflamed; however, the predominant types of inflammatory cells and the main anatomical site of the lesion appear to differ from those in asthma. COPD is characterized by reduced maximum expiratory flow and slow forced emptying of the lungs. Steroids are the most prominent medication used in the treatment of asthma and COPD; however, the beneficial effect of steroid treatment in COPD is subject of debate. We investigated the efficacy of fluticasone propionate (FP) treatment in atopic asthmatics and in COPD patients with bronchial hyperreactivity who smoke. The effect of the treatment on bronchial hyperreactivity and indices of the methacholine dose-response curve were analysed, as well as indices of inflammation of the airway mucosa with special emphasis on the antigen presenting dendritic cell. Treatment of allergic asthmatic patients resulted in improvement of lung function (FEV1), a decrease in bronchial hyperresponsiveness and a decrease of maximal airway narrowing. During the FP-treatment of COPD patients, FEV1 remained stable, while FEV1 deteriorated significantly in the placebo group. Therefore, steroid treatment may have a beneficial effect in COPD patients with bronchial hyperresponsiveness (BHR). Since immunohistochemical analysis of bronchial biopsy specimens from asthma and COPD patients show disease-specific aspects of inflammation, the anti-inflammatory effect of FP is obtained through modulation of different cell populations in asthma and COPD.

Regulation of Pulmonary T Cell Responses to Inhaled Antigen: Role in Th1- and Th2-Mediated Inflammation

DO11.10 transgenic mice, expressing an OVA-specific TCR, were used to study pulmonary T cell responses to inhaled Ags. Before OVA inhalation, the activation of lung parenchymal T cells elicited both strong proliferative responses and IL-2 production. However, following Ag inhalation the proliferative responses of the lung T cells, when restimulated in vitro with OVA323–339 peptide or immobilized anti-CD3, were severely attenuated and associated with a decrease in the level of production of IL-2 but not IFN-γ. Such immune regulation was tissue-specific, because T cell responses in the lymph nodes and spleens were normal. This dramatic aerosol-induced attenuation of parenchymal T cell proliferation was also observed in BALB/c mice immunized with OVA and in BALB/c mice following adoptive transfer of DO11.10 T cells bearing either a Th1 or Th2 phenotype. In mice that had received Th2 cells, the reduced proliferative responses were associated with a decrease in IL-2 expression but augmented IL-4 and IL-5 production. Invariably, the inhibition of proliferation was a consequence of the action of F4/80+ interstitial macrophages and did not involve alveolar macrophages or their products. These observations demonstrate that clonal expansion of T cells in the lung compartment is prevented following the onset of either Th1- or Th2-mediated inflammation. This form of immune regulation, which appears as a selective defect in IL-2-driven proliferation, may serve to prevent the development of chronic pulmonary lymphoproliferative responses.

A Polymorphism* in the 5' flanking region of the CD14 gene is associated with circulating soluble CD14 levels and with total serum immunoglobulin E

Total serum immunoglobulin (Ig)E levels are genetically regulated, but the mechanism of inheritance is not well understood. Cytokines produced by T-helper (Th)1 and Th2 lymphocytes control IgE synthesis. Bacterial antigens may favor the development of Th1 cells from naive CD4-positive T cells through a CD14-dependent pathway. CD14 is constitutively expressed on the surface of monocytes and macrophages, and is also present in serum in a soluble form (sCD14). The CD14 gene maps to chromosome 5q31.1, a candidate region for loci regulating total serum IgE. We hypothesized that genetic variants in the CD14 gene could influence Th-cell differentiation and thus total serum IgE. We identified a C-to-T transition at base pair -159 from the major transcription start site (CD14/-159). Among 481 children recruited from a general population sample, frequency of allele C was 51.4%. TT homozygotes had significantly higher sCD14 levels than did carriers of both the CC and CT genotypes (P = 0.01). TT homozygotes also had significantly lower levels of IgE than did carriers of the other two genotypes, but differences were significant only among children who were skin test-positive to local aeroallergens (P = 0.004). There was no association between CD14/-159 and either interleukin (IL)-4 or interferon (IFN)-gamma responses by peripheral blood mononuclear cells. However, IFN-gamma and IL-4 responses were positively and negatively correlated, respectively, with serum sCD14 levels. We conclude that CD14/-159 plays a significant role in regulating serum sCD14 levels and total serum IgE levels.

Tissue macrophages associated with angiogenesis in chronic airway inflammation in rats

Angiogenesis is a feature of chronic inflammation produced by Mycoplasma pulmonis infection of the respiratory tract. The mechanism of this angiogenesis is unknown, but cellular growth factors and matrix remodeling proteases produced by inflammatory cells are likely to be involved. The goal of this study was to determine the relationship between changes in the number, shape, and distribution of ED2-immunoreactive macrophages and the development of angiogenesis in the tracheal mucosa of Wistar rats after M. pulmonis infection. In pathogen-free rats, ED2-positive cells were scattered in the airway mucosa (261 +/- 42 cells/mm2 of surface, mean +/- SE). Most cells were irregularly shaped and had moderate ED2 immunoreactivity. No lymphoid tissue was present. The number of ED2-positive cells increased rapidly after infection, was 120% above baseline at 1 wk, and remained significantly increased throughout the 4-wk study (P < 0.05). Angiogenesis was first detected at 2 wk, and at 3 wk the vessel length density was nearly 8-fold the pathogen-free value. At 3 and 4 wk, focal sites of angiogenesis coincided with discrete clusters of round, strongly immunoreactive ED2-positive cells (1,340 +/- 124 cells/mm2) in polyp-like collections of mucosal lymphoid tissue. The close association of distinctive ED2-positive cells with angiogenic blood vessels suggests a relationship between a subset of tissue macrophages and the angiogenesis associated with M. pulmonis infection. The time course of the changes indicates that the initial influx of ED2-positive macrophages precedes the angiogenesis, and the rounding of the cells parallels the growth of new vessels.

Dendritic cells: a link between innate and adaptive immunity

Dendritic cells (DC) constitute a unique system of cells able to induce primary immune responses. As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology, that shows the complexity of this cell system. Although our understanding of DC biology is still in its infancy, we are now in a position to use DC-based immunotherapy protocols to treat cancer and infectious diseases.

Immunobiology of dendritic cells in the respiratory tract: steady-state and inflammatory sentinels?

A network of dendritic cells present in the epithelial lining of the respiratory tract function as sentinel cells which are able to detect the presence of foreign antigenic material and to process these antigens in such a way that they can be transported to local lymph nodes and presented to naive T cells. We will discuss the immunobiology of this network of cells as found in the rat.

Resting respiratory tract dendritic cells preferentially stimulate T helper cell type 2 (Th2) responses and require obligatory cytokine signals for induction of Th1 immunity

Consistent with their role in host defense, mature dendritic cells (DCs) from central lymphoid organs preferentially prime for T helper cell type 1 (Th1)-polarized immunity. However, the "default" T helper response at mucosal surfaces demonstrates Th2 polarity, which is reflected in the cytokine profiles of activated T cells from mucosal lymph nodes. This study on rat respiratory tract DCs (RTDCs) provides an explanation for this paradox. We demonstrate that freshly isolated RTDCs are functionally immature as defined in vitro, being surface major histocompatibility complex (MHC) II lo, endocytosishi, and mixed lymphocyte reactionlo, and these cells produce mRNA encoding interleukin (IL)-10. After ovalbumin (OVA)-pulsing and adoptive transfer, freshly isolated RTDCs preferentially stimulated Th2-dependent OVA-specific immunoglobulin (Ig)G1 responses, and antigen-stimulated splenocytes from recipient animals produced IL-4 in vitro. However, preculture with granulocyte/macrophage colony stimulating factor increased their in vivo IgG priming capacity by 2-3 logs, inducing production of both Th1- and Th2-dependent IgG subclasses and high levels of IFN-gamma by antigen-stimulated splenocytes. Associated phenotypic changes included upregulation of surface MHC II and B7 expression and IL-12 p35 mRNA, and downregulation of endocytosis, MHC II processing- associated genes, and IL-10 mRNA expression. Full expression of IL-12 p40 required additional signals, such as tumor necrosis factor alpha or CD40 ligand. These results suggest that the observed Th2 polarity of the resting mucosal immune system may be an inherent property of the resident DC population, and furthermore that mobilization of Th1 immunity relies absolutely on the provision of appropriate microenvironmental costimuli.

Dendritic cells: migratory cells that are attractive

Dendritic cells (DC) are professional antigen presenting cells, playing an important role in the initiation of T- and T cell dependent immune responses. DC are highly mobile cells and the sequential migration of DC in and out of tissues is accompanied by phenotypical as well as functional changes instrumental to their function as sentinels of the immune system. Herein, we will review recent progress in understanding the origin of DC, their migratory behaviour and their capacity to attract and interact with lymphocytes, with emphasis on the chemokine system.

Glucocorticoid-induced apoptosis of dendritic cells in the rat tracheal mucosa

Dendritic cells are antigen-presenting cells that constitutively express high levels of major histocompatibility complex class II (Ia) antigen on their plasma membrane. Previous studies have shown that the number of dendritic cells in the rat airway mucosa decreases rapidly after glucocorticoid treatment. We sought to determine whether apoptosis contributes to this steroid-induced cell decrease. Dendritic cells in tracheal whole mounts were revealed by immunoperoxidase staining using the OX-6 (anti-Ia) monoclonal antibody. In untreated rats, a dense network of Ia-immunoreactive (Ia+) cells with highly branched cytoplasmic processes was observed just beneath the tracheal epithelium (1,405 +/- 140 cells/mm2 mucosa; mean +/- SEM, n = 6). In rats treated with dexamethasone (10 mg/kg, intraperitoneally), four distinct changes in dendritic cell morphology were evident 4 to 8 h after injection: (1) appearance of large Ia+ granules in cytoplasmic processes, (2) narrowing of cytoplasmic processes, (3) loss of Ia immunoreactivity from the cell surface, and (4) fragmentation of cells into small Ia+ bodies. These changes accompanied a 56% decrease in the number of Ia+ cells over 8 h. The contribution of apoptosis to this decrease in Ia+ cells was determined by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) of nucleosomal DNA fragments in histologic sections. The number of TUNEL+ bodies increased from a control value of 174 +/- 47 bodies/mm2 mucosa to 2,108 +/- 294 bodies/mm2 mucosa at 4 h and 936 +/- 343 bodies/ mm2 mucosa at 8 h (n = 4 rats per time point). The location of TUNEL+ bodies closely corresponded to that of Ia+ cells stained in adjacent histologic sections. We conclude that apoptosis contributes to the rapid decrease in airway dendritic cells after glucocorticoid treatment.

Dendritic Cell Heterogeneity In Vivo: Two Functionally Different Dendritic Cell Populations in Rat Intestinal Lymph Can Be Distinguished by CD4 Expression

DC derived from rat pseudo-afferent lymph (L-DC) vary in CD4, CD11b/c, Thy1, and OX41 expression. CD4 and OX41 are expressed by the same subpopulation (50–60%) of L-DC. CD4+/OX41+ L-DC express short fine processes and low nonspecific esterase, whereas CD4− DC/OX41− express long pseudopodia, high nonspecific esterase, and many cytoplasmic inclusions. These differences are stable in culture. Both populations express similar amounts of MHC class II, ICAM-1, CD11b/c and OX62. Most CD4−/OX41− L-DC are strongly positive for B7, but CD4+ L-DC express less B7, and some may be negative. Both populations express invariant chain, but both the absolute numbers and levels of expression were higher for CD4− DC. Surprisingly, CD4+ L-DC are more potent APC than CD4− cells in MLRs, for sensitized T cells in vitro and for naive T cells in vivo. Cultured CD4+/OX41+ DC can still process and present native Ag. Cultured CD4−/OX41− cells cannot present native Ag but can stimulate strong MLRs. CD4− DC invariant chain expression decreases in culture, whereas expression by CD4+ DC is stable for 48 h. CD4+ and CD4− L-DC have similar turnover times in vivo, suggesting that one population is not the precursor of the other. Thus, two separate DC populations that differ functionally and phenotypically migrate from intestine to mesenteric nodes. This may reflect distinct DC lineages or differentiation modulated by different microenvironmental stimuli.

Adhesion, Transendothelial Migration, and Reverse Transmigration of In Vitro Cultured Dendritic Cells

Dendritic cells (DC) are migratory cells which exhibit complex trafficking properties in vivo, involving interaction with vascular and lymphatic endothelium and extracellular matrix (ECM). The underlying mechanisms involved in these processes are still ill defined. In the present study we have investigated the ability of DC to interact in vitro with human vascular endothelial cells (EC) and ECM. DC were differentiated from monocytes by in vitro exposure to granulocyte-macrophage colony-stimulating factor and interleukin-13 for 7 days. In adhesion assays a considerable proportion of DC bound to resting EC monolayers: (17% ± 4%, mean ± SE of eight experiments). Adhesion to tumor necrosis factor (TNF)-activated EC was increased to 29% ± 5% (n = 8). Binding to resting EC was strongly inhibited by anti-CD11a and CD11b, but not by CD11c monoclonal antibodies (MoAbs); on TNF-activated EC, anti–VLA-4 in concert with anti-CD18 inhibited adhesion by more than 70%. Binding to a natural ECM, derived from cultured EC, or to purified fibronectin was high: 52% ± 6% (n = 8) involved VLA-4 and VLA-5 integrins. In a transmigration assay, 10% ± 2% (n = 6) of input cells were able to cross the EC monolayer. Unlike adhesion, transendothelial migration was significantly reduced by anti-CD31 MoAb. The amount of DC transmigrated through a monolayer of EC was increased twofold to threefold by a defined set of C-C chemokines including RANTES, MIP1α, MIP5, and, to a lesser extent, by MIP1β and MCP-3. Most importantly, in view of the trafficking pattern of these cells, a significant proportion of DC (13% ± 4% of input cells seeded) was able to migrate across the endothelial basement membrane and, subsequently, across the endothelial barrier (reverse transmigration). The adhesion molecules and chemoattractants characterized herein are likely to underlie the complex trafficking of DC in vivo.

Programming for responsiveness to environmental antigens that trigger allergic respiratory disease in adulthood is initiated during the perinatal period

Allergy to airborne environmental antigens (allergens) is a major cause of asthma in children and adults. This review argues that the development of allergen-specific immunologic memory of the type that predisposes to allergy development is the end result of a T-cell selection process operative during infancy, which is triggered via encounters between the immature immune system and incoming airborne allergens from the environment. In normal individuals this process leads to the development of allergen-specific T-memory cells that secure the T helper (Th)-1 pattern of cytokines, which actively suppress the growth of their allergy-inducing Th-2 cytokine-secreting counterparts. However, these protective allergen-reactive Th-1 memory cells fail to develop in some individuals, permitting the subsequent proliferation of allergen-specific Th-2 cells that can trigger allergic reactions. Recent evidence suggests that genetic predisposition to allergy may be due in part to hyperactivity of control mechanisms operative in utero and which normally protect the fetoplacental unit against the toxic effect of Th-1 cytokines.

Transplacental Priming of the Human Immune System to Environmental Allergens: Universal Skewing of Initial T Cell Responses Toward the Th2 Cytokine Profile

The expression of Th2-skewed immunity against soluble protein Ags present in the normal environment is recognized as the primary cause of allergic inflammation in atopics. In contrast, nonallergic normal individuals display low level Th1-skewed immunity against the same Ags (“allergens”), which is perceived as conferring protection against Th2-dependent allergic sensitization. The type of T cell memory that develops against these Ags is currently believed to be the result of complex interactions between environmental and genetic susceptibility factors, which occur postnatally when the naive immune system directly confronts the outside environment. The results of the present study challenge this general concept. We demonstrate here for the first time that Th2-skewed responses to common environmental allergens, comprising IL-4, IL-5, IL-6, IL-9, and IL-13, are present in virtually all newborn infants and are dominated by high level production of IL-10. Moreover, these responses are demonstrable within 24 h of culture initiation, arguing against a significant contribution from covert in vitro T cell priming and/or differentiation. These findings imply that the key etiologic factor in atopic disease may not be the initial acquisition of allergen-specific Th2-skewed immunity per se, but instead may be the efficiency of immune deviation mechanisms, which in normal (nonatopic) individuals redirect these fetal immune responses toward the Th1 cytokine phenotype.

Bacterial lipopolysaccharide contamination of commercial collagen preparations may mediate dendritic cell maturation in culture

Dendritic cells (DC) are potent antigen presenting cells, which are responsible for the initiation of naive T and T-dependent immune responses. The present studies were based upon recent reports that commercial collagen I preparations induce the maturation of human DC in vitro. We show that human blood monocyte-derived (GM-CSF and IL-4 cultured) DC pulsed on collagen I-coated plates undergo a dose-dependent increase in stimulatory capacity in oxidative mitogenesis assays. This is accompanied by the upregulation of costimulatory molecules (CD40, CD80, CD86), CD25, ICAM-1 and the DC-specific marker CD83. The maturation effect is more potent than TNF-alpha, which is a known mediator of DC function. However, bacterial lipopolysaccharide (LPS), a powerful inducer of DC maturation, was found to be present at very high levels in one commercial collagen solution that was tested. The effect of LPS upon DC maturation was similar to culture with collagen. Furthermore, a different collagen I preparation with low levels of LPS contamination was less effective at inducing DC maturation, while spiking the collagen solution with LPS prior to plastic coating equalised these effects. Finally, human monocyte-derived DC were found not to express typical collagen receptors VLA-1, 2 and 3. We therefore propose that LPS contamination may at least partially explain reported collagen I induced DC maturation.

Dendritic Cells Are Required for the Development of Chronic Eosinophilic Airway Inflammation in Response to Inhaled Antigen in Sensitized Mice

Asthma is characterized by chronic eosinophilic inflammation of the airways, and allergen-specific Th2 lymphocytes are thought to play a major role in the development and maintenance of this type of inflammation in allergic asthma. It is generally accepted that airway dendritic cells (DC) are essential for stimulating naive T cells in a primary immune response to inhaled Ag and for the development of allergic sensitization. We have examined the role of airway DC in stimulating memory T cells in a secondary response to inhaled Ag and the subsequent development of chronic airway inflammation. In our mouse model of asthma, OVA aerosol challenge in OVA-sensitized mice leads to CD4-dependent peribronchial and perivascular eosinophilic inflammation, lung Th2 cytokine production, and systemic IgE production. We have used conditional depletion of airway DC by treatment of thymidine kinase-transgenic mice with the antiviral drug ganciclovir to deplete DC during the secondary exposure to OVA. In sensitized thymidine kinase-transgenic mice, a significant decrease in the number of bronchoalveolar CD4 and CD8 T lymphocytes and B lymphocytes was seen after ganciclovir treatment. In addition, Th2 cytokine-associated eosinophilic airway inflammation was almost completely suppressed. These studies demonstrate for the first time that the DC is essential for presenting inhaled Ag to previously primed Th2 cells in the lung, leading to chronic eosinophilic airway inflammation. Altering the function of airway DC may therefore be an important target for new anti-asthma therapy.

Regulation of immune responses at mucosal surfaces: allergic respiratory disease as a paradigm

The epithelial surfaces of the respiratory tract represent a fragile interface between the immune system and the outside environment. In order to maintain local homeostasis, the adaptive immune system must continuously discriminate between innocuous antigens which are ubiquitous in the atmosphere, and antigens associated with microbial pathogens. Any breakdown in this discrimination process can potentially lead to chronic inflammatory disease. The mechanisms employed by the mucosal immune system to maintain this delicate balance are many and varied, and a comprehensive understanding of how they collectively operate would provide novel insight into a wide variety of diseases. This discussion reviews recent progress in this area, focusing on allergic respiratory disease as a model for the study of immune regulation at mucosal surfaces.

The TH1/TH2 paradigm in allergy

Recent evidence has been accumulated to suggest that allergen-reactive type 2 helper T cells (Th2) play a triggering role in the activation and/or recruitment of IgE antibody-producing B cells, mast cells and eosinophils, i.e. the cellular triad involved in the allergic inflammation. Interleukin (IL)-4 production by a still unknown cell type (T cell subset, mast cell/basophil?) at the time of antigen presentation to the Th cell is critical for the development of Th2 cells. Other cytokines, such as IL-1 and IL-10, and hormones, such as calcitriol and progesterone, also play a favoring role. In contrast, cytokines such as interferon (IFN-alpha, IFN-gamma, IL-12 and transforming growth factor (TGF)-beta, and hormones, play a negative regulatory role on the development of Th2 cells. However, the mechanisms underlying the preferential activation by environmental allergens of Th2 cells in atopic individuals still remain obscure. Some gene products selectively expressed in Th2 cells or selectively controlling the expression of IL-4 have recently been described. Moreover, cytokines and other gene products that dampen the production of IL-4, as well as the development and/or the function of Th2 cells, have been identified. These findings allow us to suggest that the up-regulation of genes controlling IL-4 expression and/or abnormalities of regulatory mechanisms of Th2 development and/or function may be responsible for Th2 responses against common environmental allergens in atopic people. The new insights in the pathophysiology of T cell responses in atopic diseases provide exciting opportunities for the development of novel immunotherapeutic strategies. They include the induction of nonresponsiveness in allergen-specific Th2 cells by allergen peptides or redirection of allergen-specific Th2 responses by Th1-inducing cytokines, altered peptide ligands, allergens incorporated into recombinant microorganisms or bound to appropriate adjuvants, and plasmid DNA vaccination. In severe atopic patients, the possibility of nonallergen-specific immunotherapeutic regimens designed to target Th2 cells or Th2-dependent effector molecules, such as specific IL-4 transcription factors, IL-4, IL-5 and IgE, may also be suggested.

Blockade of CD49d (alpha4 integrin) on intrapulmonary but not circulating leukocytes inhibits airway inflammation and hyperresponsiveness in a mouse model of asthma

Immunized mice after inhalation of specific antigen have the following characteristic features of human asthma: airway eosinophilia, mucus and Th2 cytokine release, and hyperresponsiveness to methacholine. A model of late-phase allergic pulmonary inflammation in ovalbumin-sensitized mice was used to address the role of the alpha4 integrin (CD49d) in mediating the airway inflammation and hyperresponsiveness. Local, intrapulmonary blockade of CD49d by intranasal administration of CD49d mAb inhibited all signs of lung inflammation, IL-4 and IL-5 release, and hyperresponsiveness to methacholine. In contrast, CD49d blockade on circulating leukocytes by intraperitoneal CD49d mAb treatment only prevented the airway eosinophilia. In this asthma model, a CD49d-positive intrapulmonary leukocyte distinct from the eosinophil is the key effector cell of allergen-induced pulmonary inflammation and hyperresponsiveness.

Alveolar macrophages regulate the induction of primary cytotoxic T-lymphocyte responses during influenza virus infection

Virus-specific cytotoxic T lymphocytes (CTL) are thought to be responsible for the eradication of respiratory influenza virus infections by direct cytolysis of virus-infected epithelial cells. In this study, we provide evidence for a role for alveolar macrophages (AM) in the regulation of pulmonary virus-specific CTL responses. Prior to infection with influenza virus, AM were selectively eliminated in vivo with a liposome-mediated depletion technique, and virus-specific CTL activities of lung and mediastinal lymph node (MLN) cells were assayed ex vivo and compared with those for normal mice. AM depletion resulted in increased primary CTL responses and changed the kinetics of the CTL response. Flow cytometric analysis of lung and MLN cells showed that the percentage of CD8+ cells was not altered after AM depletion and that lung cells from AM-depleted mice had an increased capacity to lyse virus-infected cells. Upon restimulation in vitro, virus-specific CTL activity in lung cells of normal mice was similar to that in lung cells of AM-depleted mice. Furthermore, elimination of AM resulted in increased virus titers in the lung, but virus clearance as a function of time was not affected. Our results show that AM regulate virus-specific CTL responses during respiratory influenza virus infection by removing viral particles, by downregulating the priming and activity of CTL in MLN cells, and by inhibiting the expansion of virus-specific CTL in the lung.

Human airway epithelial cells stimulate T-lymphocyte lck and fyn tyrosine kinase

Previous studies have shown that human airway epithelial cells (AEC) can stimulate allogeneic peripheral blood T-lymphocyte (PBT) proliferation. We now sought to determine which AEC surface molecule/T-cell coreceptors are involved in this process. AEC-induced PBT proliferation was inhibited by 25 microM genestein or herbamycin A (0.9 and 1.8 microM), both tyrosine kinase inhibitors. Anti-phosphotyrosine immunoblots performed on PBT lysates after coculture with AEC demonstrated phosphorylation of 56kD and 60kD bands. To determine whether CD3 associated p59fyn, or CD4 and CD8 associated p56lck phosphotyrosine kinases (PTK) were involved, we assayed kinase activity in lymphocyte lysates immunoprecipitated with anti-p56lck and p59fyn mAbs. PBT cells or murine T-cell line transfectants expressing human CD4 (3G4) or human CD8alpha (3G8) were cocultured with AEC or A549, an alveolar-like cell line lacking class II Ag expression. After A549 or AEC coculture, p56lck activity in PB T-cells peaked at 2 min whereas p59fyn kinase activity continued to rise at 8 min. AEC (expressing class II Ags) stimulate PTK activity in both 3G8 and 3G4 cells. A549 stimulated p56lck in 3G8, but not in 3G4 cells. This activation of p56lck was not blocked by preincubation of A549 with anti-class I or anti-CD1d mAbs. An antibody generated in our laboratory, which recognizes an epithelial specific surface molecule (mAb L12) and which blocks AEC driven PBT proliferation, was shown to block PTK activity of peripheral blood T-cell lysates, though not of 3G8 lysates. These studies suggest that AEC are capable of stimulating CD4 and CD8 associated lck and CD3 associated fyn kinases through class II dependent and independent pathways.

Recruitment of circulating allergen-specific T lymphocytes to the lung on allergen challenge in asthma

BACKGROUND

In allergic subjects with asthma, the migration of CD4+ T cells to the lungs in the hours after allergen exposure may contribute to allergic inflammation in the target organ.

OBJECTIVE

We studied allergen-specific T cells from the peripheral blood and lungs of allergic subjects with asthma at baseline and after allergen challenge.

METHODS

In each patient, blood samples were taken 10 minutes before and 24 hours after the inhalation of a major sensitizing allergen. In vitro proliferation of peripheral blood CD4+ T cells specific for the same allergen used in the in vivo challenge was assessed. In one patient two Dermatophagoides pteronyssinus-specific T-cell clones (TCCs) were derived from peripheral blood, and their T-cell receptors were sequenced to determine their clonotypic determinants on the beta chains. The T-cell receptor determinants of the allergen-specific TCCs were sought in blood and bronchoalveolar lavage samples taken from this patient.

RESULTS

We found that allergen inhalation is followed by a decrement in the specific proliferation of peripheral CD4+ T cells to the same allergen used for bronchial provocation. In one patient the clonotypic determinants of two allergen-specific TCCs diminished in the peripheral blood, whereas they were simultaneously expanded in the lower respiratory tract.

CONCLUSION

Our data suggest that allergen-specific T cells are recruited from the peripheral blood to the bronchial lumen after allergen challenge.

Why are we not all allergic: basic mechanisms for tolerance development

Harmless antigens encountered on the mucosal surface are normally tolerated in the sense that they do not induce inflammatory immune responses. Oral tolerance is the type of mucosal immune regulation that prevents inflammatory reactions to food proteins. However, parasites and invasive microorganisms at the mucosal surfaces must be recognised and dealt with in a proper manner. The immune system does so by cross-regulating the response where it either produces IgA to exclude invasion, IgE to fight parasites, or IgG to destroy the invasive organisms. Allergy is an anti-parasitic reaction to a misinterpreted but harmless antigen. This lack of tolerance induction is influenced by genetic factors controlling the amount of interleukin (IL)-4 produced initially in the immune response. IL-4 directs B-cells to produce IgE, induces naive T-cells to become IL-4 producing T-helper cells (Th2 cells) and prevent other T-cells from entering into the IFN-γ-producing Th1 pathway. Long lasting Th2 clones lose their IL-12 responsiveness and can no longer be induced to produce IFN-γ thus they are locked in an allergy inducing Th2 phenotype. Environmental factors play on the genetic background and influence the outcome of the immune response. Mucosal tolerance depends on an intact mucosal surface, is influenced by the age of the subject, and can be manipulated through antigen dose and place of entry. Immune-manipulating therapy may be more successful in primary than in secondary prevention of allergy.

Atopic versus infectious diseases in childhood: a question of balance?

There is an increasing interest in the concept that respiratory tract infections during early childhood may in some circumstances confer protection against sensitisation to aeroallergens, via "bystander" stimulation of Th-1 associated immune functions in the regional lymph nodes draining the airway mucosa. We hypothesise below that this phenomenon may be but one component of a broader process operative during early postnatal life, in which generalised contact with the microbial environment plays an obligatory role in stimulating the functional maturation of the Th-1 arm of the immune response. We argue further that one of the most potent sources of such stimulation is provided by the normal commensal flora of the gastrointestinal tract, which is establish during early infancy.

Steroids inhibit uptake and/or processing but not presentation of antigen by airway dendritic cells

Recent studies from our laboratory indicate that local and (particularly) systemic steroids can modulate the traffic of dendritic cells (DC) through resting and inflamed airway epithelial tissues. The present report focuses upon the T-cell activating properties of DC, which are controlled by granulocyte-macrophage colony-stimulating factor (GM-CSF) signals, and in particular the question of whether the DC-stimulating effects of GM-CSF are susceptible to regulation by steroids. We present evidence that while dexamethasone inhibited GM-CSF-dependent uptake and/or processing of exogenous antigen by DC, it was ineffective in blocking the presentation of preprocessed self antigen to alloreactive T cells in a one-way mixed lymphocyte reaction (MLR). Associated GM-CSF-induced up-regulation of major histocompatability complex (MHC) class II and CTLA4 ligand expression by DC were also unaffected by dexamethasone phosphate (DX), reinforcing the view that the inhibitory effects of steroids on the T-cell activating functions of DC are restricted to steps upstream from presentation of processed antigen to the T-cell receptor (TCR). These findings have potentially important implications in relation to the use of topical steroids in the treatment of atopic asthma, a disease in which local T-cell activation in airway tissue is a key pathogenic factor, and which furthermore is characterized by intense production of GM-CSF within the airway epithelium.

Airway surfactant, a primary defense barrier: mechanical and immunological aspects

Epidemiologic studies have shown strong associations between mortality and morbidity from respiratory and cardiac causes and exposure to fine (PM10), but not coarse, particulates. A plausible mechanistic explanation for these associations is lacking. It has been shown that particles may be retained for an extended period of time in the airways, and that their clearance is inversely proportional to particle size. Such particles are localized in close association with the airway epithelium, and if they consist of low surface energy material, will be coated with an osmiophilic layer, consistent with surfactant. Particles are displaced into this position by surface and line tension forces exerted by the surfactant film at the air-aqueous interface. Particle displacement due to line tension is much greater for smaller particles in the micrometer range. The surface forces acting on the particles leave deep indentations on the epithelial cells. During the displacement process they may come into contact with airway macrophages in the mucous layer and/or dendritic cells situated in the airway epithelium. The smallest particles may even penetrate the mucosa to enter the interstitial compartment. In addition to altering the physical properties of particles, surfactant coatings reduce particle toxicity and enhance phagocytosis by opsonization. We speculate that surfactant acts as a primary defense barrier and plays a role in antigen presentation and elimination at the air-mucus interface of the airways.

Allergen recognition in the origin of asthma

Allergic respiratory diseases such as bronchial asthma are believed to result directly from the repeated local expression in airway tissues of T helper (Th) 2-polarized T cell immunity to inhaled allergens. Recent evidence suggests that these T cell responses are typically primed in utero and subsequently reshaped during postnatal allergen exposure via immune deviation, leading to the eventual emergence of stable allergen-specific T cell memory which is polarized towards the Th1 (normal) or Th2 (atopic) phenotype. The underlying Th1/Th2 switching process is influenced by a number of host and environmental factors that are poorly understood. Prominent amongst these are factors that affect the kinetics of maturation of immune competence during the early postnatal period. In particular, there is mounting evidence that the immunological milieu at the materno-fetal interface is naturally skewed towards the Th2 phenotype (possibly an evolutionary adaptation to protect the placenta against the toxic effects of Th1 cytokines). Furthermore, this bias appears to be preserved for varying periods into infancy, which may account for the presence of a high risk 'window' for allergic sensitization in early postnatal life. It is hypothesized that the principal impetus for postnatal development of a normal Th1/Th2 balance (and hence closure of the high risk sensitization window) is provided via contact with Th1-stimulatory commensal and pathogenic micro-organisms at the body's major mucosal surfaces.

Inflammatory infiltration of the upper airway epithelium during Sendai virus infection: involvement of epithelial dendritic cells

We undertook the present study to determine the nature of the cellular inflammatory response within the epithelial lining of the rat trachea during a Sendai virus infection. In particular, we aimed to investigate changes in the resident population of epithelial dendritic cells. Rats were infected with Sendai virus, and tracheal tissue was examined immunohistochemically at various times with a panel of cell-specific monoclonal antibodies. We found that Sendai virus infection was restricted to only the lumenal layer of epithelial cells and that virus nucleoprotein was present from days 2 to 5 postinfection. Starting around day 2 or 3, there was a large cellular influx consisting of macrophages, neutrophils, NK cells, and T cells; this coincided with expression of high levels of ICAM-1 on the basal (uninfected) layers of the epithelium. The T cells were mostly alphabeta T-cell receptor positive; however, a smaller influx of gammadelta T cells also took place. The number of resident dendritic cells increased markedly during infection, with numbers peaking around day 5 and remaining elevated 14 days later. The peak of the inflammatory response occurred on day 5 and declined thereafter, with the exception of dendritic cell and alphabeta T-cell numbers, which remained elevated. Starting around day 3, the tracheal epithelial cells expressed increasing levels of major histocompatibility complex class II antigen. This expression was maximal at day 5 and declined rapidly thereafter. In vitro culture of tracheal segments demonstrated that viral infection was not per se responsible for the upregulation of class II expression and that when cultured in the presence of gamma interferon, class II antigen was induced on epithelial cells.

Dendritic cells are recruited into the airway epithelium during the inflammatory response to a broad spectrum of stimuli

A key rate-limiting step in the adaptive immune response at peripheral challenge sites is the transmission of antigen signals to T cells in regional lymph nodes. Recent evidence suggests that specialized dendritic cells (DC) fulfill this surveillance function in the resting state, but their relatively slow turnover in most peripheral tissues brings into question their effectiveness in signaling the arrival of highly pathogenic sources of antigen which require immediate mobilization of the full range of host defenses for maintenance of homeostasis. However, the present report demonstrates that recruitment of a wave of DC into the respiratory tract mucosa is a universal feature of the acute cellular response to local challenge with bacterial, viral, and soluble protein antigens. Consistent with this finding, we also demonstrate that freshly isolated respiratory mucosal DC respond in vitro to a variety of CC chemokines as well as complementary cleavage products and N-formyl-methionyl-leucine-phenylalanine. This suggests that rapid amplification of specific antigen surveillance at peripheral challenge sites is an integral feature of the innate immune response at mucosal surfaces, and serves as an "early warning system" to alert the adaptive immune system to incoming pathogens.

Macrophages and allergic lung disease

Allergic lung diseases such as atopic asthma and extrinsic allergic alveolitis are now recognized as chronic inflammatory lung diseases promoted by dysregulation of T cell-mediated immune mechanisms. The basis of this regulation and the impact of the atopic status of these individuals on this chronic inflammatory disease have yet to be fully explained. The studies described in this paper reveal mechanisms of macrophage lymphocyte interaction in which evidence is presented that a balance of functionally distinct macrophage subsets needs to be maintained to regulate T cell reactivity in the lung. Similarly a balance within the T cell populations may influence and regulate the relative proportions of functionally distinct macrophages. Investigations of bronchoalveolar lavage and biopsy from patients with allergic lung disease have revealed a gross imbalance within the lung macrophage populations and an associated dysregulation in T cell stimulation. In vitro studies have revealed that imbalances in the macrophage populations may lead to changes in local level of cytokine production specifically TGF-beta which would then impact on the control of T cell populations. Conversely aberrant development of activated T cells with a TH2-like cytokine repertoire may influence the balance of macrophages. Our in vitro studies have revealed that macrophage phenotype and function can be modulated in vitro by contact with T cell-derived cytokines and that this change in phenotype is reflected in a change in function. These data support the hypothesis that components of the immune system normally associated with allergic reactions may be stimulated in the absence of any overt atopic reactivity in the individual concerned. Thus immediate type allergic reactions may represent a "super-imposed" burden [provocating factor] in atopic individuals but the underlying immunopathogenesis of these diseases may not be dependent on this state of immediate type hypersensitivity. It is concluded that the loss of balance within functional distinct macrophage populations within the lung may represent the fundamental problem in allergic lung disease. This possibility is discussed in the light of other work in this field.

Regulation of dendritic cell numbers and maturation by lipopolysaccharide in vivo

Dendritic cells (DC) are described as "nature's adjuvant," since they have the capacity to sensitize T cells in vivo upon first encounter with the antigen. The potent accessory properties of DC appear to develop sequentially. In particular, the ability to process antigens and to sensitize native T cells develops in sequence, a process termed "maturation" that is well described in vitro. Here, we obtain evidence for maturation in vivo in response to the bacterial product lipopolysaccharide (LPS). Before LPS treatment, many DC are found at the margin between the red and white pulp. These cells lack the M342 and DEC-205 markers, but process soluble proteins effectively. 6 h after LPS, DC with the M342 and DEC-205 markers are found in increased numbers in the T cell areas. These cells have a reduced capacity to process proteins, but show increases in the B7 costimulator and T cell stimulatory capacity. 48 h after LPS, the number of DC in the spleen is reduced markedly. We interpret these findings to mean that LPS can cause DC in the marginal zone to mature and to migrate into and then out of the T cell areas.

Granulocyte-macrophage colony-stimulating factor overrides the immunosuppressive function of corticosteroids on rat pulmonary dendritic cells

Pulmonary dendritic cells (DC) are present in extremely small numbers, but they are the most potent antigen-presenting cells in the lungs. Pure populations of DC can be isolated from the lung following collagen digestion, Percoll gradient centrifugation, removal of phagocytic cells and flow cytometric sorting for cells which exhibit high levels of surface major histocompatibility complex (MHC) class II molecules. Exogenous GM-CSF enhances this immunostimulatory capacity of the pulmonary DC. Soluble factors produced by type II airway epithelial cells and interstitial macrophages also enhance the immunostimulating capacity of pulmonary DC while alveolar macrophages suppress it. Thus, the function of DC may be regulated by locally produced cytokines. Corticosteroids are widely used as immunosuppressive agents in pharmacotherapy. While these agents are known to inhibit T cell proliferation and macrophage activation, their effects on DC are not known. We found that dexamethasone (Dex) pretreatment resulted in about a 50% reduction in the immunostimulatory capacity of rat pulmonary DC. This was associated with downregulation of MHC class II (Ia) expression. Dex-induced suppression of DC function could be restored with GM-CSF. We conclude that corticosteroids downregulate antigen-presenting capacity by direct suppression of pulmonary DC. This immunosuppressive effect of corticosteroids on DC may, however, be abrogated by exogenous GM-CSF. Corticosteroids and GM-CSF are therapeutic agents with potent direct immunomodulating effects on DC.

Sequestration of inhaled particulate antigens by lung phagocytes. A mechanism for the effective inhibition of pulmonary cell-mediated immunity

Dendritic cells (DCs) have emerged as the dominant antigen-presenting cells (APCs) of the lung, playing a vital role in the induction of cell-mediated immunity to inhaled antigens. We have previously demonstrated that an airway challenge with the soluble antigen hen egg lysozyme yields rapid acquisition of specific antigen-presenting cell activity by purified pulmonary DCs and a cell-mediated immune response in the lung upon secondary challenge. To examine how a particulate antigen leads to a cell-mediated response in vivo, graded concentrations of heat-killed Listeria (HKL) were injected intratracheally into Lewis rats. The bacteria were rapidly ingested by lung macrophages and polymorphonuclear leukocytes. The ability of purified pulmonary DCs pulsed in vivo by an airway challenge with HKL to subsequently stimulate HKL-specific responses ex vivo showed a threshold response, requiring a dose in excess of 10(9) organisms/rat. By contrast, all dosages of HKL yielded specific sensitization of lymphocytes in the draining bilar nodes. Pulmonary DCs purified from rats after a secondary in vivo airway challenge with HKL at day 14 were ineffective antigen-presenting cells except at high dosages of antigen. The generation of cell-mediated pulmonary inflammation paralleled the antigen-presenting cell activity of pulmonary DCs and was observed only at high antigen dosages. Hen egg lysozyme immobilized onto polystyrene beads and injected intratracheally yielded comparable results to those observed with HKL. We suggest that a pulmonary cellular immune response is generated to an inhaled particulate antigen when the protective phagocytic capacities of the lung are exceeded and antigen is able to interact directly with interstitial DCs. The diversion of particulate antigens by pulmonary phagocytes may help to limit undesirable pulmonary inflammation while allowing the generation of antigen-specific immune lymphocytes in vivo.

The dendritic cell lineage: a ubiquitous antigen-presenting organization

Dendritic cells are specialized antigen-presenting cells with two unique characteristics: the greatest stimulatory potential and the ability to stimulate naive T-lymphocytes. They originate from the bone marrow and reach their destination via hematogenous or lymphatic migration. Their phenotype is characterized by a high expression of major histocompatibility complex class II molecules and a high expression of adhesion molecules (CD25, CD54, CD58, CD72, and CD80). Pulmonary dendritic cells may be investigated by histologic examination, phenotype analysis, and function studies in a mixed lymphocyte reaction. Their isolation requires enzymatic digestion of lung tissue and subsequent steps of cell separation. The complexity of these manipulations makes it difficult to obtain large numbers of viable cells. A close anatomic relationship with alveolar macrophages underlines a functional interconnection: macrophages down-regulate the antigen-presenting function through release of tumor necrosis factor alpha. Dendritic cells most probably play a major role in lung diseases such as histiocytosis, primary and secondary cancers, and both acute and chronic lung graft rejection. Identification of the precise functional pathways might lead to therapeutic use of modulation of dendritic cell function.

The biology of airway dendritic cells

Recent work from our laboratory has identified a network of constitutively class II MHC (Ia) bearing dendritic cells (DC) within the epithelium of the conducting airways of laboratory animal species and in humans. The density of DC within the respiratory tract is highest in those areas exposed to greater amounts of inhaled antigen and further work has identified these DC as being critically important in controlling the induction of immune responses within the airways. The DC population in the airway epithelium is renewed every 48-72 h; this represents a more rapid turnover than DC in other tissues which are exposed to a smaller antigenic load. In addition to these results we will discuss other work which shows that airway DC are a very reactive population, comparable with neutrophils in their response to acute inflammatory stimuli and that their numbers and Ia content can be modulated following exposure to topical and systemic steroids. Finally we will discuss the development of these cells after birth and how this may influence the pathogenesis of immune regulated diseases such as asthma and allergic rhinitis.