Pulmonary immunity to Listeria is enhanced by elimination of alveolar macrophages

Macrophages Subvert Adaptive Immunity to Urinary Tract Infection

Urinary tract infection (UTI) is one of the most common bacterial infections with frequent recurrence being a major medical challenge. Development of effective therapies has been impeded by the lack of knowledge of events leading to adaptive immunity. Here, we establish conclusive evidence that an adaptive immune response is generated during UTI, yet this response does not establish sterilizing immunity. To investigate the underlying deficiency, we delineated the naïve bladder immune cell compartment, identifying resident macrophages as the most populous immune cell. To evaluate their impact on the establishment of adaptive immune responses following infection, we measured bacterial clearance in mice depleted of either circulating monocytes, which give rise to macrophages, or bladder resident macrophages. Surprisingly, mice depleted of resident macrophages, prior to primary infection, exhibited a nearly 2-log reduction in bacterial burden following secondary challenge compared to untreated animals. This increased bacterial clearance, in the context of a challenge infection, was dependent on lymphocytes. Macrophages were the predominant antigen presenting cell to acquire bacteria post-infection and in their absence, bacterial uptake by dendritic cells was increased almost 2-fold. These data suggest that bacterial uptake by tissue macrophages impedes development of adaptive immune responses during UTI, revealing a novel target for enhancing host responses to bacterial infection of the bladder.

Urinary tract infection is a common infection with a high propensity for recurrence. The majority of infections are caused by uropathogenic E. coli, a growing public health concern with increasing prevalence of antibiotic resistant strains. Finding therapeutic options that circumvent the need for antibiotics, while boosting patients’ immune response to infection is desirable to counteract further increases in antibiotic resistance and to provide long-lasting resistance to infection. Currently, little is known about how adaptive immune responses, which typically prevent recurrent infection in other organs, arise from the bladder during urinary tract infection. Here, we investigated the initial interactions between immune cell populations of the bladder and uropathogenic E. coli, finding that macrophages are the principal cell population to engulf bacteria. Interestingly, these same cells appear to inhibit the development of adaptive immunity to the bacteria, as their depletion, prior to primary infection, results in a stronger immune response during bacterial challenge. We found that in the absence of macrophages, dendritic cells, which are the most potent initiators of adaptive immunity, are able to take up more bacteria for presentation. Our study has revealed a mechanism in which specific immune cells may act in a manner detrimental to host immunity.

TLR Agonists as Modulators of the Innate Immune Response and Their Potential as Agents Against Infectious Disease

Immunotherapies that can either activate or suppress innate immune responses are being investigated as treatments against infectious diseases and the pathology they can cause. The objective of these therapies is to elicit protective immune responses thereby limiting the harm inflicted by the pathogen. The Toll-like receptor (TLR) signaling pathway plays critical roles in numerous host immune defenses and has been identified as an immunotherapeutic target against the consequences of infectious challenge. This review focuses on some of the recent advances being made in the development of TLR-ligands as potential prophylactic and/or therapeutic agents.

Opportunities and challenges of the pulmonary route for vaccination

INTRODUCTION

The respiratory tract is an attractive target for the delivery of vaccine antigens. Potential advantages of drug delivery by means of the pulmonary route include accessibility, non-invasiveness, ease of administration, and the possibility to reach an elaborate mucosal network of antigen-presenting cells.

AREAS COVERED

This review discusses current pulmonary vaccination strategies and their advantages and disadvantages.

EXPERT OPINION

To improve efficiency of vaccination and develop new strategies, a well-founded knowledge about composition and characterization of antigen-presenting cell populations throughout the respiratory tract is essential. In particular, respiratory tract dendritic cells, as key antigen-presenting cells in the lung, constitute an ideal target for vaccine delivery. Furthermore, particle size is a key factor when designing new inhalable vaccines, as size determines not only deposition in different respiratory tract compartments, but also how an antigen and its carrier will interact with lung tissue components and immune cells. An increased knowledge of different respiratory tract antigen-presenting cell populations and their interactions with other components of the immune system will enable new targeting strategies to improve the efficacy of pulmonary vaccination.

Recent advances into understanding some aspects of the structure and function of mammalian and avian lungs

Recent findings are reported about certain aspects of the structure and function of the mammalian and avian lungs that include (a) the architecture of the air capillaries (ACs) and the blood capillaries (BCs); (b) the pulmonary blood capillary circulatory dynamics; (c) the adaptive molecular, cellular, biochemical, compositional, and developmental characteristics of the surfactant system; (d) the mechanisms of the translocation of fine and ultrafine particles across the airway epithelial barrier; and (e) the particle-cell interactions in the pulmonary airways. In the lung of the Muscovy duck Cairina moschata, at least, the ACs are rotund structures that are interconnected by narrow cylindrical sections, while the BCs comprise segments that are almost as long as they are wide. In contrast to the mammalian pulmonary BCs, which are highly compliant, those of birds practically behave like rigid tubes. Diving pressure has been a very powerful directional selection force that has influenced phenotypic changes in surfactant composition and function in lungs of marine mammals. After nanosized particulates are deposited on the respiratory tract of healthy human subjects, some reach organs such as the brain with potentially serious health implications. Finally, in the mammalian lung, dendritic cells of the pulmonary airways are powerful agents in engulfing deposited particles, and in birds, macrophages and erythrocytes are ardent phagocytizing cellular agents. The morphology of the lung that allows it to perform different functions-including gas exchange, ventilation of the lung by being compliant, defense, and secretion of important pharmacological factors-is reflected in its "compromise design."

INFLAMMATORY CELL RECRUITMENT FOLLOWING THORACIC IRRADIATION

Ionizing radiation leads to a progressive injury in which a monocyte/macrophage-rich pneumonitis is followed by a chronic progressive fibrosis. In the present study, the role of macrophage/monocyte recruitment in the genesis of radiation-induced pulmonary fibrosis was examined. The objectives were threefold: (i) characterize the inflammatory cells recruited into the lung during the development of radiation-induced fibrosis; (ii) investigate changes in lung response following depletion of resident alveolar macrophages in vivo prior to radiation treatment; (iii) assess if inhalation of low levels of endotoxin would potentiate the radiation-initiated injury. One group of fibrosis-sensitive C57BL/6 mice was irradiated with a single dose of 15 Gy to the thorax. In a second group, resident inflammatory cells were depleted using clodronate, encapsulated into liposomes, 48 hours prior to irradiation with a single dose of 15 Gy to the thorax. Control animals were sham irradiated. All groups of animals then were examined 8, 16, or 24 weeks post irradiation. No difference in total cell numbers or cell differentials was observed between irradiated mice or those that were both liposome treated and irradiated at any time point. At 16 weeks, mice that received radiation showed a 5- to 6-fold increase in lymphocytes regardless of treatment as compared to control animals. At 24 weeks post irradiation, select groups were exposed to lipopolysaccharide (LPS) and examined 24 hours post inhalation. Lavageable protein was increased several fold in mice that received both radiation and LPS exposure as compared to 15 Gy or LPS exposure alone. These results demonstrate: (i) macrophages and lymphocytes are the predominately recruited cell types through 24 weeks post irradiation; (ii) recovery of inflammatory cells, regardless of prior macrophage depletion, were similar, suggesting that early responses are primarily driven by parenchymal cell injury; (iii) thoracic irradiation-induced injury can cause sensitization to a secondary stimulus that may result in injuries/responses not predicted by evaluating exposures individually.

Effects of depletion of neutrophils or macrophages on the inflammatory response induced by metalloelastase (MMP-12) in mice airways

Macrophage elastase (recombinant human matrix metalloproteinase-12, rhMMP-12), was instilled in mouse airways, inducing an early inflammatory response characterized by neutrophil recruitment and cytokine release in the bronchoalveolar lavage (BAL) fluids, followed by a delayed macrophage recruitment. We investigated the role played by alveolar macrophages and neutrophils in the delayed macrophage influx induced by rhMMP-12 (8 x 10(-3) U/mouse) in A/J mice. Mice depleted of circulating neutrophils, using a cytotoxic antibody, did not present an increase in neutrophil numbers in bronchoalveolar lavage fluids, 4 h and 24 h after rhMMP-12 instillation but the macrophage recruitment was not modified as compared to control mice at 7 days. Similar results were obtained using mice when the gene for neutrophil elastase was knocked out. Intranasal instillation of clodronate liposomes, 72 h prior to rhMMP-12 instillation, induced macrophage depletion which did not modify the macrophage recruitment at 7 days. Moreover, the stimulation of mouse macrophages by rhMMP-12 did not elicit the release of cytokines in culture supernatants. These results indicate that resident alveolar macrophages and recruited neutrophils do not play a role in the delayed macrophage recruitment induced by rhMMP-12.

The lung responds to zymosan in a unique manner independent of toll-like receptors, complement, and dectin-1

In vitro studies indicate that the inflammatory response to zymosan, a fungal wall preparation, is dependent on Toll-like receptor (TLR) 2, and that this response is enhanced by the dectin-1 receptor. Complement may also play an important role in this inflammatory response. However, the relevance of these molecules within the in vivo pulmonary environment remains unknown. To examine pulmonary in vivo inflammatory responses of the lung to zymosan, zymosan was administered by intratracheal aerosolization to C57BL/6, TLR2- TLR4-, MyD88-, and complement-deficient mice. Outcomes included bronchoalveolar fluid cell counts. We next examined effects of dectin-1 inhibition on response to zymosan in alveolar macrophages in vitro and in lungs of C57BL/6, TLR2-, and complement-deficient mice. Finally, the effect of alveolar macrophage depletion on in vivo pulmonary responses was assessed. Marked zymosan-induced neutrophil responses were unaltered in TLR2-deficient mice despite a TLR2-dependent response seen with synthetic TLR2 agonists. TLR4, MyD88, and complement activation were not required for the inflammatory response to zymosan. Although dectin-1 receptor inhibition blocked the inflammatory response of alveolar macrophages to zymosan in vitro, in vivo pulmonary leukocyte recruitment was not altered even in the absence of TLR2 or complement. Depletion of alveolar macrophages did not affect the response to zymosan. Neither complement, macrophages, nor TLR2, TLR4, MyD88, and/or dectin-1 receptors were involved in the pulmonary in vivo inflammatory response to zymosan.

Alveolar macrophages are necessary for the systemic inflammation of acute alveolar hypoxia

Alveolar hypoxia (Fi(O(2)) 0.10) rapidly produces inflammation in the microcirculation of skeletal muscle, brain, and mesentery of rats. Dissociation between tissue Po(2) values and inflammation, plus the observation that plasma from hypoxic rats activates mast cells and elicits inflammation in normoxic tissues, suggest that the response to hypoxia is initiated when mast cells are activated by an agent released from a distant site and carried by the circulation. These experiments tested the hypothesis that this agent originates in alveolar macrophages (AM). Male rats were depleted of AM by tracheal instillation of clodronate-containing liposomes. Four days after treatment, AM recovered by bronchoalveolar lavage were <10% of control. Control rats received buffer-containing liposomes. As expected, alveolar hypoxia (Fi(O(2)) 0.10) in control rats increased leukocyte-endothelial adherence, produced degranulation of perivascular mast cells, and increased fluorescent albumin extravasation in the cremaster microcirculation. None of these effects was seen when AM-depleted rats were exposed to hypoxia. Plasma obtained from control rats after 5 min of breathing 10% O(2) elicited inflammation when applied to normoxic cremasters. In contrast, normoxic cremasters did not develop inflammation after application of plasma from hypoxic AM-depleted rats. Supernatant from AM cultured in 10% O(2) produced increased leukocyte-endothelial adherence, vasoconstriction, and albumin extravasation when applied to normoxic cremasters. Normoxic AM supernatant did not produce any of these responses. The effects of hypoxic supernatant were attenuated by pretreatment of the cremaster with the mast cell stabilizer cromolyn. These data support the hypothesis that AM are the source of the agent that initiates hypoxia-induced systemic inflammation by activating mast cells.

Essential roles of IL-12 and dendritic cells but not IL-23 and macrophages in lupus-like diseases initiated by cell surface HSP gp96

Proinflammatory cytokine IL-23 but not IL-12 is critical for the pathogenesis of organ-specific autoimmune diseases including experimental autoimmune encephalitis and collagen-induced arthritis. The contribution by IL-23 in systemic autoimmune diseases such as lupus is undefined. We addressed this question in a murine lupus-like disease model, initiated by enforced cell-surface expression of an ER HSP gp96 in C57BL/6 background. We found a significant increase of p40 in the sera in these mice that preceded the onset of diseases. However, autoimmunity was abrogated in transgenic mice expressing membrane-bound gp96 reconstituted with p35-/- BM, but not with p19-/- BM. Moreover, we found that dendritic cells (DC) but not macrophages were the main producers of p40. To dissect the roles of DC further, we depleted DC using a diphtheria toxin-based inducible DC depletion system. We demonstrated that the integrity of DC was essential for autoimmunity. Our results thus revealed that IL-12 and DC are critical for the pathogenesis of lupus-like disease precipitated by cell surface gp96. This study further highlighted the significant biological differences between IL-12 and IL-23.

Francisella tularensis induces aberrant activation of pulmonary dendritic cells

Francisella tularensis is an obligate intracellular bacterium that induces severe, acute, often fatal disease when acquired by the respiratory route. Despite the seriousness of this pathogen, very little is understood about its interaction with key target cells in the airways and lungs (alveolar macrophages and airway dendritic cells (DC)) after inhalation. In this study we demonstrate replication of F. tularensis in primary DC. Early after infection, F. tularensis induced increased expression of MHC class II and CD86 on DC, but not macrophages. This was followed by depletion of DC from the airways and lungs. Despite logarithmic replication and phenotypic maturation of DC, F. tularensis failed to induce production of several key proinflammatory cytokines, including TNF-alpha and IL-6, from DC. However, F. tularensis infection did elicit production of the potent immunosuppressive cytokine, TGF-beta. Furthermore, F. tularensis actively suppressed the ability of DC to secrete cytokines in response to specific TLR agonists. Finally, we also found that infection of DC and macrophages in the lungs appears to actually increase the severity of pulmonary infection with F. tularensis. For example, depletion of airway DC and alveolar macrophages before infection resulted in significantly prolonged survival times. Together, these data suggest F. tularensis is able to selectively uncouple Ag-presenting functions from proinflammatory cytokine secretion by critical APCs in the lungs, which may serve to create a relatively immunosuppressive environment favorable to replication and dissemination of the organism.

Early interaction of Yersinia pestis with APCs in the lung

Despite the importance of pneumonic plague, little is known of the early pulmonary immune responses that occur following inhalation of Yersinia pestis. Therefore, we conducted studies to identify the early target cells for uptake of Y. pestis in the lungs following intratracheal or i.v. inoculation. Following intratracheal inoculation, Y. pestis was rapidly internalized primarily by a distinctive population of CD11c+DEC-205+CD11b- cells in the airways, whereas i.v. inoculation resulted in uptake primarily by CD11b+CD11c- macrophages and granulocytes in lung tissues. The airway cells internalized and were infected by Y. pestis, but did not support active replication of the organism. Intratracheal inoculation of Y. pestis resulted in rapid activation of airway CD11c+ cells, followed within 24 h by the selective disappearance of these cells from the airways and lungs and the accumulation of apoptotic CD11c+ cells in draining lymph nodes. When CD11c+ cells in the airways were depleted using liposomal clodronate before infection, this resulted in a significantly increased replication of Y. pestis in the lungs and dissemination to the spleen and draining lymph nodes. These findings suggest that CD11c+ cells in the airways play an important role in suppressing the initial replication and dissemination of inhaled Y. pestis, although these results will also require confirmation using fully virulent strains of Y. pestis. Depletion of these airway cells by Y. pestis may therefore be one strategy the organism uses to overcome pulmonary defenses following inhalation of the organism.

An innate immune system cell is a major determinant of species-related susceptibility differences to fungal pneumonia

Rats and mice are considered resistant and susceptible hosts, respectively, for experimental cryptococcosis. For both species, alveolar macrophages (AM) are central components of the host response to pulmonary Cryptococcus neoformans infection. We explored the role of AM in three strains of mice and three strains of rats during cryptococcal infection by comparing the outcome of infection after macrophage depletion using liposomal clodronate. AM depletion was associated with enhancement and amelioration of disease in rats and mice, respectively, as measured by lung fungal burden. The apparent protective role for AM in rats correlated with enhanced anti-cryptococcal activity as measured by phagocytic activity, oxidative burst, lysozyme secretion, and ability to limit intracellular growth of C. neoformans. Furthermore, rat AM were more resistant to lysis in association with intracellular infection. In summary, differences in AM function in rats and mice suggest an explanation for the species differences in susceptibility to C. neoformans based on the inherent efficacy of a central effector cell of the innate immune system.

Lung inflammation induced by endotoxin is enhanced in rats depleted of alveolar macrophages with aerosolized clodronate

Clodronate liposomes were given to rats via intratracheal inhalation to investigate the importance of alveolar macrophages (AMs) in inhaled endotoxin-induced lung injury. When AM depletion was maximal (87% to 90%), rats were exposed to lipopolysaccharide (LPS) or saline. Neither clodronate nor saline liposomes induced an influx of neutrophils (PMNs) into the lungs. However, depleted LPS-exposed rats had 5- to 8-fold higher numbers of lavage PMNs and greater lavage cell reactive oxygen species release compared to undepleted rats. Although AM depletion by itself did not significantly increase inflammatory cytokine expression in lung tissue, LPS-induced message levels for interleukin (IL)-1alpha, IL-1beta, IL-6, and tumor necrosis factor (TNF)-alpha were approximately 2-fold higher in AM-depleted rats compared to undepleted rats. These results indicate that cells other than AMs can recruit inflammatory cells into the lungs during acute LPS-induced injury and that AMs play an important suppressive role in the innate pulmonary inflammatory response.

Alveolar macrophages regulate neutrophil recruitment in endotoxin-induced lung injury

BACKGROUND

Alveolar macrophages play an important role during the development of acute inflammatory lung injury. In the present study, in vivo alveolar macrophage depletion was performed by intratracheal application of dichloromethylene diphosphonate-liposomes in order to study the role of these effector cells in the early endotoxin-induced lung injury.

METHODS

Lipopolysaccharide was applied intratracheally and the inflammatory reaction was assessed 4 hours later. Neutrophil accumulation and expression of inflammatory mediators were determined. To further analyze in vivo observations, in vitro experiments with alveolar epithelial cells and alveolar macrophages were performed.

RESULTS

A 320% increase of polymorphonuclear leukocytes in bronchoalveolar lavage fluid was observed in macrophage-depleted compared to macrophage-competent lipopolysaccharide-animals. This neutrophil recruitment was also confirmed in the interstitial space. Monocyte chemoattractant protein-1 concentration in bronchoalveolar lavage fluid was significantly increased in the absence of alveolar macrophages. This phenomenon was underlined by in vitro experiments with alveolar epithelial cells and alveolar macrophages. Neutralizing monocyte chemoattractant protein-1 in the airways diminished neutrophil accumulation.

CONCLUSION

These data suggest that alveolar macrophages play an important role in early endotoxin-induced lung injury. They prevent neutrophil influx by controlling monocyte chemoattractant protein-1 production through alveolar epithelial cells. Alveolar macrophages might therefore possess robust anti-inflammatory effects.

Expression of toll-like receptor 2 and 4 in lipopolysaccharide-induced lung injury in mouse

Pattern recognition receptors, which include the toll-like receptors (TLRs), are considered to play an important role in the response against lipopolysaccharide (LPS). In this study, we performed a reverse transcriptase/polymerase chain reaction (RT-PCR) study, Western analysis, immunohistochemical staining, and RT-PCR-amplified in situ hybridization of TLR2 and TLR4 in the case of LPS-induced lung injury. The expression of TLR2 and TLR4 increased in the lung rapidly after LPS inhalation and peaked at 24 h, followed by a gradual decrease. TLR2 and TLR4 expression was observed on the bronchial epithelium and tissue macrophages. In the early hours after inhalation of fluorescein-isothiocyanate (FITC)-labeled LPS, LPS was detected mainly on the bronchial epithelium and on a few of tissue macrophages. One day after inhalation, the LPS signals disappeared in the lungs of the mice, except for a few alveolar macrophages. The expression of TLR2, TLR4, and CD14 was coincident with the signals of FITC-labeled LPS. Instillation of liposome-encapsulated dichloromethylene diphosphonate induced a significant decrease in alveolar macrophages. In the macrophage-depleted mice, however, expression of TLR2 and TLR4 mRNA or protein was slightly suppressed in the lung after LPS inhalation. These data suggest that the bronchial epithelium and macrophages play crucial roles in LPS-induced lung injury through TLR2 and TLR4.

Accurate and simple discrimination of mouse pulmonary dendritic cell and macrophage populations by flow cytometry: methodology and new insights

BACKGROUND

The need to accurately discriminate dendritic cells (DCs) and macrophages (Mphs) in mouse lungs is critical given important biological differences. However, a validated flow cytometry-based method is still lacking, resulting in much confusion between both cell types.

METHODS

Single-cell suspensions freshly obtained from collagenase-digested lung tissue were stained with a CD11c-specific monoclonal antibody, detected using a PE-Cy5 or APC-conjugated secondary reagent. Cellular immunophenotype was simultaneously explored using a panel of PE-conjugated markers. The FL1 or FITC-detection channel was reserved for the assessment of autofluorescence.

RESULTS

CD11c-bright cells were heterogeneous and displayed a bimodal distribution with regard to autofluorescence (AF). CD11c+/low-AF cells were lineage-negative and showed features compatible with myeloid DCs. This was confirmed by morphology, potent T-cell stimulatory function in a mixed-leukocyte reaction, surface expression of MHCII and costimulatory molecules, and further immunophenotypical criteria, including the expression of Mac-1 and absence of CD8alpha. In contrast, CD11c+/high-AF cells displayed the features of pulmonary Mphs, including typical Mph morphology, very weak induction of T-cell proliferation, low to absent expression of MHCII and costimulatory molecules, and very low levels of Mac-1 as well as F4/80. We also show that only CD11c+/high-AF cells strongly expressed the macrophage marker MOMA-2, while interestingly Mac-3 was expressed at high levels by CD11c+/high-AF and low-AF alike.

CONCLUSIONS

This study shows that the combination of CD11c-expression and autofluorescence is necessary and sufficient to accurately separate DCs from macrophage subpopulations in mouse lungs.

A novel theory: biological processes mostly involve two types of mediators, namely general and specific mediators

A great number of papers have shown that free radicals as well as bioactive molecules can play a role of mediator in a wide spectrum of biological processes, but the biological actions and chemical reactivity of the free radicals are quite different from that of the bioactive molecules, and that a wide variety of bioactive molecules can be easily modified by free radicals due to having functional groups sensitive to redox, and the significance of the interaction between the free radicals and the bioactive molecules in biological processes has been confirmed by the results of some in vitro and in vivo studies. Based on these evidence, this article presented a novel theory about the mediators of biological processes. The essentials of the theory are: (a) mediators of biological processes can be classified into general and specific mediators; the general mediators include two types of free radicals, namely superoxide and nitric oxide; the specific mediators include a wide variety of bioactive molecules, such as specific enzymes, transcription factors, cytokines and eicosanoids; (b) a general mediator can modify almost any class of the biomolecules, and thus play a role of mediator in nearly every biological process via diverse mechanisms; a specific mediator always acts selectively on certain classes of the biomolecules, and may play a role of mediator in different biological processes via a same mechanism; (c) biological processes are mostly controlled by networks of their mediators, so the free radicals can regulate the last consequence of a biological process by modifying some types of the bioactive molecules, or in cooperation with these bioactive molecules; the biological actions of superoxide and nitric oxide may be synergistic or antagonistic. According to this theory, keeping the integrity of these networks and the balance between the free radicals and the bioactive molecules as well as the balance between the free radicals and the free radical scavengers would be of vital importance for physiological processes, and disturbance of these networks and balances would be a critical factor of pathological processes. Therefore, the investigators who want to get a deep and full understanding of the mechanism of a biological process should pay attention to the roles of both free radical and bioactive molecule species, and the free radical scavengers, which are used for health protection, such a vitamin E and carotenoid, should be taken in a suitable dosage.

Efficient depletion of alveolar macrophages using intratracheally inhaled aerosols of liposome-encapsulated clodronate

Rat alveolar macrophages (AMs) were depleted via intratracheal inhalation (ITIH) of clodronate-containing liposomes. AM depletion following ITIH delivery of clodronate liposomes was 33.2 +/- 14.2 on day 1, 88.1 +/- 6.2 on day 3, and 91.4 +/- 1.8 on day 4 relative to control rats given saline-containing liposomes. Almost all (approximately 99%) of the AMs remaining at the 3-day time point were peroxidase negative, suggesting that immature macrophages were not recruited from the circulation to replace those undergoing cell death on that day. Only 0.5% +/- 0.5% of bronchoalveolar lavage (BAL) cells were neutrophils at this time (normalized to controls). Whole-body inhalation did not induce as much AM depletion at 3 days (37.6% +/- 10.1%) and required larger amounts of liposome-encapsulated clodronate compared to ITIH. Intratracheal instillation (as opposed to inhalation) of clodronate liposomes produced a significant inflammatory response characterized by the influx of both polymorphonuclear neutrophils (PMNs) and macrophages. In subsequent pilot studies, the response to intratracheally instilled crystalline silica (75 microg) was found to be markedly reduced in rats depleted of AMs by the ITIH method. We conclude that ITIH of clodronate liposomes in rats is both efficient and useful for examining the role of AMs in pulmonary toxicology.

Alveolar macrophages, surfactant lipids, and surfactant protein B regulate the induction of immune responses via the airways

The influences of alveolar macrophages (AM) and pulmonary surfactant on the induction of immune responses via the airways were assessed. Mice were depleted of their AM by intratracheal instillation of multilamellar vesicles containing dichloromethylene-diphosphonate followed by intratracheal instillation of a T cell--dependent antigen, trinitrophenyl--keyhole limpet hemocyanin, in vesicles of various compositions. The primary immune response was determined in the spleen of these animals using an ELI-Spot assay. The secondary immune responses in the sera of the mice were assessed using enzyme-linked immunosorbent assays. An immune response was detected in animals depleted of their AM and intratracheally instilled with antigen in small unilamellar vesicles consisting of either phosphatidylcholine cholesterol or surfactant lipids. Incorporation of surfactant protein (SP)-B in the antigen vesicles enhanced the immune response, whereas SP-A or SP-C in the antigen vesicle did not have an effect. Strikingly, intratracheal instillation of SP-B containing antigen vesicles can induce an immunoglobulin M immune response in mice without depletion of AM. These results indicate that SP-B containing vesicles can enhance the induction of immune responses via the airways and further illustrate the important roles of both AM and pulmonary surfactant in the pulmonary immune system.

Human alveolar macrophages induce functional inactivation in antigen-specific CD4 T cells

BACKGROUND

Alveolar macrophages (AMCs) are the most abundant phagocytic cells in the lung, but they present antigen poorly to T cells.

OBJECTIVES

The objectives of our studies were to more clearly define the mechanisms by which AMCs present antigen to T cells and to determine whether AMCs actively inhibit T-cell activation.

METHODS

We studied purified human CD4 T cells and compared the capacity of allogeneic AMCs and peripheral blood monocytes to induce T-cell proliferation and cytokine production.

RESULTS

We previously demonstrated that human AMCs fail to upregulate expression of B7-1 and B7-2 on stimulation with IFN-gamma. We now demonstrate that AMCs actively induce T-cell unresponsiveness (functional inactivation) in an antigen-specific manner and reduce the capacity of CD4 T cells to respond on secondary stimulation. The induction of unresponsiveness was reversed by the addition of CD28 costimulation or IL-2. However, interruption of Fas/Fas ligand interactions or of B7/CTLA-4 interactions did not prevent unresponsiveness, indicating that neither CTLA-4 triggering nor Fas-induced apoptosis was involved in the induction of T-cell unresponsiveness.

CONCLUSIONS

These studies indicate that AMCs actively tolerize CD4 T cells in an antigen-specific fashion. We propose that AMCs mediate a form of immune privilege in the lungs that effectively limits immune responses in the pulmonary compartment but has little effect on systemic immunity.