Local immune response and bronchial reactivity in rats after capsaicin treatment

The interaction between the nervous system, immune system and bronchial reactivity was studied in rats by using the neurotoxin capsaicin. Rats were treated with capsaicin at 1-2 days of age or at adult age, before or after sensitization by subcutaneous injections with ovalbumin (OA). The levels of the neuropeptides neurokinin A and calcitonin gene-related peptide were decreased in the lung after capsaicin treatment, as determined with radioimmunoassay, whereas the levels of neuropeptide Y were unaffected. The levels of IgA, IgE and IgG in bronchial lavage were also affected by capsaicin treatment; however, the results were heterogeneous. Capsaicin treatment after sensitization reduced the bronchial reactivity to challenge with OA aerosol and serotonin iv. The results demonstrated that reduction of neuropeptide levels with capsaicin affected both bronchial reactivity and the levels of antibodies in bronchial lavage fluid. However, no correlation between these two parameters was seen, demonstrating the complexity of the system.     

Enhancement of the bronchial reactivity in immunized rats by neonatal treatment with capsaicin

Rats were injected with capsaicin at 1-2 days of age to abolish the content of substance P (SP) in nerve terminals. At 6 weeks of age the capsaicin-treated and control rats were sensitized daily for 1 or 2 subsequent weeks Monday through Friday with ovalbumin (OA). The OA was given without adjuvant as 300 ng subcutaneous (s.c.) injections in the neck region or as 1% aerosol for 30 min. The capsaicin-treated animals which were sensitized s.c. for 2 weeks reacted moderately with increased transpulmonary pressure (TPP) to airway challenge with OA, and strongly to intravenous (i.v.) challenge with OA or serotonin. The capsaicin-untreated animals, which were sensitized with OA, reacted weakly to the challenge. In the challenge. In the animals sensitized with aerosolized OA, slightly lower reactivity was seen compared with those sensitized s.c. Untreated and unsensitized control rats reacted only to serotonin challenge. No animal had any detectable serum or bronchial IgE antibodies. Aerosol-sensitized animals had IgG antibodies in both serum and bronchial lavage. Histologically, the animals treated with capsaicin in contrast to the untreated controls demonstrated a pronounced increase of lymphoid tissue around their bronchi. Their mast cell numbers were increased around vessels and in the pleura and their mucous cell numbers were increased in the epithelium of the bronchi and bronchioli. The sensitization did not add much to this histological picture.     

Antigen-Specific Down-Regulation of Bronchial Reactivity in Rats Sensitized Daily without Adjuvant

Daily sensitization with ovalbumin (OA) and dog serum albumin (DSA) without adjuvant was performed in rats for 2-week periods. When the antigen was administered subcutaneously (s.c.), antibody responses were induced, as assessed in serum and bronchial lavage, and strong increases in transpulmonary pressure (TPP) after intravenous (i.v.) challenge with antigen. Sensitization without adjuvant with antigen as aerosol for similar periods also evoked pronounced antibody formation, although only weak increases of TPP were seen after challenge. Animals sensitized s.c. with OA and simultaneously exposed to OA as aerosol exhibited suppression of the TPP increase after challenge, whereas the antibody responses were not affected to any great extent. In contrast, the increase of TPP after challenge in animals similarly sensitized s.c. with DSA were not suppressed by OA given simultaneously as aerosol or vice versa.     

Immune Stimulated Regional Inflammatory Responses Mediating Lung Reactivity in Rats

Daily sensitization of SPF BNxWi/Fu rats with ovalbumin (OA) in aerosol during 2-week periods with a 4-week interval resulted after 7 weeks in IgE, IgA and IgG antibodies in serum and bronchial fluid. After cultivation of the regional, axillary, brachial and mediastinal (ABM) lymph node cells, IgE antibodies were found in the culture supernatant. Such antibodies were not found in culture supernatants of spleen and inguinal lymph nodes. Regional formation of IgE antibodies was also noted in the ABM lymph node cell culture supernatants after subcutaneous (s.c.) injections of 100 ng OA in the neck region. When the injections were given in the tail-root region, the inguinal but not the ABM lymph nodes produced the IgE antibodies. The s.c. sensitization induced inconsistent and rather low IgG and no IgA antibody responses. The aerosol but not the s.c. sensitization induced accumulations of mononuclear cells and mucous cells in the lungs. Clinically, the rats sensitized s.c. in the neck region reacted to aerosol and intravenous (i.v.) challenge as early as 1 week after sensitization had started, whereas the animals sensitized in the tail-root regions reacted 7 and 8 weeks after repeated sensitization. The animals sensitized by aerosol showed only weak clinical reactivity after i.v. challenge.     

Antigen-Induced Bronchial Anaphylaxis in Actively Sensitized SD Rats

We studied the effects of anti-asthmatic and anti-inflammatory drugs on antigen-induced bronchial anaphylactic reactions (BAR) in Sprague Dawley (SD) rats immunized with ovalbumin (OA) and alum. The animals were treated locally (intratracheal instillation (i.t.) or by aerosol) with terbutaline (TERB), disodium cromoglycate (DSCG), theophylline (THEO), the xanthine derivative 3,7-dihydro-1,8-dimethyl-3-phenyl-1H-purine-2,6-dione (D4026), budesonide (BUD) or dexamethasone (DEX) at various times before intravenous (i.v.) challenge with OA. The BAR was elicited by giving a low dose of OA. When the response to that challenge had levelled an additional high dose of antigen was given. Previous work had shown that TERB, DSCG, and D4026 given systemically (i.v.) at a suboptimal dose, had a better inhibitory efficacy when the animals were challenged with a low antigen dose late (6 weeks) than when challenged early (2-3 weeks) after immunization. We show here that such a difference in efficacy is not recorded after local treatment. Moreover, each of the drugs inhibited BAR to the same extent after i.t. as after i.v. treatment. Potent drugs like TERB and D4026 seemed to show similar efficacy when given either i.t. or by aerosol, whereas less potent drugs like DSCG and THEO were less effective when given by aerosol. In a previous study, we showed that the inhibitory capacity of glucocorticoids (GCS) on the BAR did not vary with sensitization conditions of the rats, BUD and DEX showed the same inhibitory capacity after intraperitoneal (i.p.) treatment as after i.t. treatment. In the present study, DEX showed increased whereas BUD showed decreased inhibitory capacity when given by aerosol 18-24 h before challenge. The duration of the anti-anaphylactic activity after inhalation was longer for DEX than for BUD.     

Induction of humoral immunity and pulmonary mast cells in mice and rats after immunization with aerosolized antigen.

Rats (BN X Wistar) and mice (CBA/Ca) were immunized by exposure in 10-day periods to an aerosol of ovalbumin (OA). In rats this immunization resulted in IgE antibodies detectable at very low levels in bronchial washings, whereas IgG, IgA and IgM antibodies were recorded both in serum and in bronchial washings. In mice, exposure to aerosolized antigen resulted in specific IgE and IgG antibodies in serum. The levels of IgM antibodies were low and no IgA antibodies could be recorded with the enzyme-linked immunosorbent assay (ELISA). Histological examination of lung tissue from immunized rats and mice revealed increased numbers of cells with characteristics of both immature and mature mast cells. In addition, in the rats these cells were more closely located to the bronchi in immunized than in control animals. In the latter animals the mast cells were located around the blood vessels. Immature mast cells were located in the bronchiole-associated lymphatic tissue (BALT) which showed a marked proliferation in immunized animals. The findings indicate that sensitization via the airways provides possibilities to develop a model in rodents for studies of IgE-mediated allergy in the lung.     

Parainfluenza virus type-3 infection attenuates the respiratory effects of antigen challenge in sensitized guinea pigs

Respiratory viral infections not only exacerbate asthma symptoms but may also be important in the pathogenesis of the disease. We therefore explored the effects of respiratory viral infection on the respiratory response of sensitized guinea pigs to antigen challenge. Lung tissue obtained from uninfected guinea pigs sensitized to ovalbumin aerosol released histamine upon incubation with the antigenin vitro. After antigen challengein vivo, sensitized animals had significantly greater numbers of eosinophils in their bronchoalveolar lavage fluid than did nonsensitized animals and exhibited airway hyperresponsiveness to methacholine aerosol. When ovalbumin sensitization was initiated 7 days after inoculation with parainfluenza virus type-3 (PI-3), antigen challenge elicited little histamine release from infected lung tissuein vitro. Likewise, subsequent to antigen challengein vivo, animals failed to exhibit airway hyperresponsiveness or an increased eosinophil population in bronchoalveolar lavage fluid. Similar effects were observed when sensitization was begun 19 days after PI-3 virus inoculation. The mechanism(s) responsible for the attenuated responses to antigen in PI-3 infected animals are unknown but may involve virus-induced effects on immune cells.     

Mononuclear cells, mast cells and mucous cells as part of the delayed hypersensitivity response to aerosolized antigen in mice

Mice (BALB/c) exposed to picryl chloride (PiCl) on their shaved abdomen and untreated animals were after 7 days exposed to daily aerosolized trinitrophenylated dog serum albumin (TNP-DSA). Mice exposed to PiCl tended to respond earlier and more strongly with both delayed hypersensitivity (DH) and IgG antibodies in serum and bronchial washings than did mice exposed to aerosol only. Picryl chloride sensitization resulted in spontaneously proliferating axillary lymph node cells, which could not be further stimulated with antigen or mitogen. Histological examination of lung tissue of aerosol-sensitized animals revealed an increase in mononuclear cells and mast cells around bronchioli and mucous cells, particularly in those animals exposed for prolonged periods and sensitized with PiCl prior to aerosol. Sensitization of mice with aerosolized TNP-DSA administrated in two 2- and 1-week periods with a 4-week interval responded with DH and IgG antibody in a dose dependent fashion irrespective of presensitization with PiCl. In bronchial washings IgG antibodies were found particularly after two 2-week periods of exposure. The cells taken from the axillary or brachial lymph nodes showed spontaneous proliferation. Culture of the cells to achieve mast cell maturation resulted in no or very low numbers of mast cells in the lymph nodes.     

Pharmacological modulation of the antigen-induced expression of the low-affinity IgE receptor (Fc epsilon RII/CD23) on rat alveolar macrophages.

Brown-Norway (BN) rats were sensitized by 3 aerosol exposures to ovalbumin (OA; 10 mg/ml) at days 1, 3 and 14. At day 21, the rats were challenged with the antigen or vehicle by aerosol. Alveolar macrophages (AM) were obtained by bronchoalveolar lavage and the expression of Fc epsilon RII/CD23 was assessed by flow cytometry after staining with the BB10 monoclonal antibody. A maximum of 74% of the AM from sensitized and challenged BN rats expressed FC epsilon RII/CD23 24 h after OA exposure, compared to 12% of the cells from rats exposed to vehicle. Sprague-Dawley rats were passively sensitized by intravenous injection of 0.1 or 0.05 ml/kg mouse ascitic fluid containing dinitrophenyl (DNP)-specific monoclonal IgE (2682-1) and after 24 h exposed to an aerosol of 5 mg/ml of DNP-bovine serum albumin for 30 min. In this case also, antigen exposure induced the expression of Fc epsilon RII/CD23 on 75% AM, compared to 17% AM from saline-challenged rats. Such an induction of Fc epsilon RII/CD23 on AM was, however, not observed when the animals were challenged with either histamine, serotonin or acetylcholine by aerosol. The antigen-induced expression of Fc epsilon RII/CD23 on AM was inhibited upon treatment of the rats with ketotifen or beclomethasone. In addition, oral or aerosol administration of respectively BN 50730 or BN 52021 (two structurally unrelated platelet-activating factor antagonists), inhibited the antigen-induced Fc epsilon RII/CD23 expression on AM, indicating the participation of this lipid mediator in this process.     

Physiological and immunological effects of chronic antigen exposure in immunized guinea pigs

Antigenic tolerance was induced in previously sensitized guinea pigs by challenging with ovalbumin (OA) aerosol 1 h/day, 5 days/week for 6 weeks. Reactivity was assessed visually and by lung mechanics. Sera from tolerant and sensitized animals showed comparable titers of antigen-specific antibody by passive cutaneous anaphylaxis with 6-hour, 4-day (heated serum) and 7-day sensitizations. In vitro contractile responses of airway smooth muscle revealed comparable histamine responses in sensitized and tolerant guinea pigs but decreased OA sensitivity in smooth muscle from tolerant animals. Although lung histamine content was equivalent in the two groups, antigen-induced histamine release from chopped lung preparations was significantly less in tolerant animals at a low antigen concentration. We conclude that antigen-induced histamine release is impaired in tolerant animals.     

Studies of desensitization and cross-desensitization to immunologic and nonimmunologic stimuli that evoke contraction and histamine release in superfused guinea pig trachea.

This study examined the possibility that there is cross-desensitization between immunologic and nonimmunologic stimuli that evoke contraction and histamine release (HR) in the isolated guinea pig trachea. Compound 48/80 and D-tubocurarine were found to cause homologous and heterologous desensitization for both contraction and HR from superfused trachea. Specific antigen challenge of trachea obtained from animals sensitized with either IgG1 (ovalbumin [OA]) or IgE (oxazalone-human serum albumin [OX-HSA]) also resulted in homologous desensitization for both contraction and HR. However, in experiments with animals sensitized with both IgG1 and IgE antibodies, prechallenge with OA resulted in cross-desensitization to OX-HSA, whereas the reverse sequence was ineffective in eliciting this phenomenon. This may be related to the type of desensitization produced by each antigen (specific versus nonspecific) or to heterogeneity of mast cells in the tissue. Prechallenge of the trachea with compound 48/80 or D-tubocurarine failed to alter subsequent effects of antigen after active sensitization with OA or passive sensitization with either IgG1 or IgE antibodies. Small but statistically significant decreases in tracheal responses to D-tubocurarine were observed after antigen prechallenge to active both IgG1 and IgE antibodies. This is the first study to demonstrate a cross-desensitization between compound 48/80 and D-tubocurarine and the first to examine cross-desensitization with IgG1 and IgE antibodies in the guinea pig trachea. The overall conclusion is that there is no major overlap in the desensitization mechanisms between immunologic and nonimmunologic stimuli in the guinea pig trachea.     

Bronchial hyperresponsiveness and cellular infiltration in the lung of guinea-pigs sensitized and challenged by aerosol

We have studied the development of airway hyperresponsiveness and the pulmonary cell infiltration in a guinea-pig model in which both initial sensitization and subsequent exposure to the antigen were performed by aerosol. Enhanced bronchopulmonary response to aerosol administration of acetylcholine (ACh) and 5-hydroxytryptamine (5-HT) was observed 3-4 hr and 18-24 hr after antigen exposure of sensitized animals. In contrast, when ACh and 5-HT were administered intravenously 3-4 hr after the challenge, no significant alteration of the dose-response curves was observed. However, 18-24 hr after antigen challenge, a marked leftward shift of the dose-response curve was observed on intravenous injection of ACh or 5-HT. The increased bronchial reactivity to aerosolized ACh in sensitized and challenged guinea-pigs reached a maximum by days 2-4, was still significantly increased at day 5 and returned to the basal value by day 8. No further alteration of the dose-related bronchopulmonary response to aerosol or intravenous administration of ACh was recorded 24 hr after a second antigen challenge, performed 8 days after the initial one. The analysis of bronchoalveolar lavage fluids showed a significant increase in the number of polymorphonuclear neutrophils 3-4 hr after the exposure of sensitized animals to the antigen, which was also associated with a significant eosinophilia at 18-24 hr. Histological examination of lung specimens obtained from animals 3-4 hr following challenge demonstrated eosinophil infiltration in the peribronchial regions and bronchial walls, as well as within the epithelium. Furthermore, as compared to time 3-4 hr, less eosinophils in the peribronchial area and submucosa were counted 24 hr after antigen challenge. However, a role of eosinophil-derived products in the development of bronchial hyperresponsivenss in this experimental model remains to be established.     

Characterization of allergen-induced bronchial hyperresponsiveness and airway inflammation in actively sensitized brown-Norway rats.

Bronchial responsiveness to inhaled acetylcholine (ACh) and inflammatory cell recruitment in bronchoalveolar lavage fluid (BALF) were studied in inbred Brown-Norway rats actively sensitized to, and later exposed to, ovalbumin (OA). We examined animals 21 days after initial sensitization at 18 to 24 hours, or 5 days after a single challenge, or after the last of seven repeated exposures administered every 3 days. BALF was examined as an index of inflammatory changes within the lung. Animals repeatedly exposed to OA aerosols had an increased baseline lung resistance and a significant increase in bronchial responsiveness to inhaled ACh compared to control animals at both 18 to 24 hours and 5 days after the last OA exposure. Sensitized animals receiving a single OA aerosol also demonstrated bronchial hyperresponsiveness (BHR) to inhaled ACh (p less than 0.01) at 18 to 24 hours of a similar order as the multiple-exposed group. There was a significant increase in eosinophils, lymphocytes, and neutrophils in BALF at 18 to 24 hours but not at 5 days after single or multiple exposure to OA aerosol in the sensitized groups. Control animals demonstrated no changes in bronchial responsiveness, although a small but significant increase in inflammatory cells was observed compared to saline-only treated animals. There was a significant correlation between bronchial responsiveness and eosinophil counts in the BALF in the single allergen-exposed group (Rs = 0.68; p less than 0.05). We conclude that (1) BHR after allergen exposure in sensitized rats is associated with the presence of pulmonary inflammation but persists despite the regression of inflammatory cells in BALF after multiple OA exposures, and (2) this rat model has many characteristics of human allergen-induced BHR.     

Thy-1 positive tingible body macrophages (TBM) in mouse lymph nodes

This study was prompted by the observation that cells in mouse lymph nodes (LN) with cytological characteristics of tingible body macrophages (TBM) appeared to be Thy-1 positive. The objective of this study was to determine if these large cells were TBM and to conclusively demonstrate their reactivity for Thy-1. The cells were studied using monoclonal antibodies (MoAb) against Thy-1 and macrophage markers including F4/80 and Ia antigens at both light microscopic (LM) and electron microscopic (EM) levels. Immunocytochemical reactivity by TBM for Thy-1 antigen specific MoAb was demonstrated by LM in both in situ and in vitro LN preparations. Furthermore, ultrastructural examination of these germinal center cells in situ demonstrated that the Thy-1 reactivity visualized at the LM level was associated with ribosomes and endoplasmic reticulum as well as their plasma membrane. Similarly, these cells expressed intracytoplasmic and membrane reactivity for Ia antigens and also for the macrophage specific antigen F4/80. This indicates that the reactivity is due to active synthesis of the Thy-1 antigen and not attributable to reactivity of any phagocytosed Thy-1 positive cells. As defined by their germinal center location and morphological characteristics, these Thy-1 reactive macrophages were identified as TBM. Germinal center TBM thus represent a unique, vigorously phagocytic subset of mature macrophages which express both macrophage and thymocyte markers.     

Bronchial sensitization in guinea pigs following ingestion of ovalbumin

Local bronchial mucosal hypersensitivity following antigen feeding was studied in the guinea pig. Groups of 6 animals were fed 1% ovalbumin (OA) in tap water or tap water without antigen (control group) for different feeding periods (14, 28, 42, and 56 days). Inhalative provocations with increasing concentrations of OA (0.5-8% OA) were performed at the end of each feeding period followed by body plethysmographic measurement of airway obstruction. Specific bronchial hypersensitivity to inhaled OA was not found in the control group, whereas specific bronchial reactivity to OA, described as reactivity index, was significantly different from the control group after 14 (p less than 0.05), 28 (p less than 0.001) and 42 days (p less than 0.01) of feeding. No difference to the control group was found after 56 days of feeding. Anti-OA IgG total and IgG1 in serum and bronchoalveolar lavage fluid were increased in OA-fed animals reaching maximal concentrations at day 28 of the feeding period. We conclude that oral feeding of a 1% solution of OA can induce a transient state of local hypersensitivity to inhaled antigen in the guinea pig as manifested by bronchoconstriction on OA inhalation and increased concentrations of local and systemic specific antibodies. This period of local hypersensitivity is followed by tolerance.     

The effects of indoramin on histamine and antigen-induced changes in respiration in guinea-pigs

Indoramin, a drug which blocks alpha-adrenergie, histamine and serotonin receptors, was tested as a protective agent during challenge with bronchoconstrictor agents in guinea-pigs. In conscious guinea-pigs, the time of onset of respiratory distress during continuous administration of aerosolized solutions of histamine, serotonin or ovalbumin (with animals pre-sensitized to this antigen) was measured using a force-displacement transducer applied to the animal's back. This time interval for each guinea-pig was compared with and without indoramin pre-treatment. Indoramin was administered by intraperitoneal injection or by aerosol treatment. In anaesthetized animals under artificial respiration, respiratory distress was induced by intravenous injection of histamine and measured by the Konzett-Rössler technique. Indoramin treatment significantly protected guinea-pigs in both types of experiment from the effects of each challenging agent.     

Inhibition of specific IgE responses in mice by pre-exposure to inhaled antigen.

Exposure of mice to aerosolized ovalbumin (OA) once weekly for 5 min, or once weekly to 10 microgram OA in PBS intranasally, elicited transient IgE responses which declined by the seventh week. When these animals were challenged intraperitoneally (i.p.) with soluble or alum-precipitated OA, their subsequent IgE responses were markedly suppressed relative to controls. In contrast, i.p. challenge provoked hemaagglutinating antibody (HA) responses to OA in the same animals which were considerably more vigorous than in controls. Adoptive transfer experiments employing splenocytes from mice repeatedly exposed to OA via the respiratory tract revealed the presence of suppressor cells active against OA-specific IgE but not HA responses. Radiotracer studies employing 125I-OA, administered intranasally and by aerosol, indicated that much of the antigen rapidly became associated with the gut.     

Dendritic cells are associated with augmentation of antigen sensitization by influenza A virus infection in mice

To study the mechanisms responsible for enhanced sensitization of inhaled antigen in respiratory viral infections, we examined the contribution of dendritic cells (DC) and T lymphocytes to the development of inhalation sensitization during infection with influenza A virus in mice. BALB/c mice were sensitized by inhalation of ovalbumin (OA) from 3 to 7 days after the inoculation with influenza A virus, and were challenged with OA 3 weeks later. Airway responsiveness and serum OA-specific IgE were increased. The numbers of eosinophils and CD4(+) and CD8(+) T cells in bronchoalveolar lavage fluid were also increased. These changes were not observed in animals only sensitized with OA or only inoculated with the virus. In animals only inoculated with the virus, DC were immunohistochemically detected on the bronchial epithelium on days 2-5. With OA inhalation after virus inoculation, DC with high expression of MHC class II were retained for 5 weeks. These results show that influenza virus infection induces the migration of DC to the bronchial epithelium, and that simultaneous inhalation of antigen causes the loading of antigen-peptide / class II molecule complex on DC. Thus, the migration of DC in viral infection may play some role in the augmentation of antigen sensitization.     

Modulation of Forssman glycosphingolipid expression by murine macrophages: coinduction with class II MHC antigen by the lymphokines IL4 and IL6

In contrast to murine spleen M phi, resident peritoneal M phi from health mice express very little Forssman glycolipid antigen (Fo). The following experiments suggest that Fo expression by peritoneal M phi may be associated with inflammation. Balb/c and CBA/J mice were given inflammatory stimuli by i.p. injection of live BCG, thioglycollate (TG), Corynebacterium parvum (CP), proteose peptone (PP), or LPS. Control animals received pyrogen-free saline. Expression of Fo and Ia antigen by peritoneal M phi was determined by immunofluorescence after 4 d. Application of TG or CP led to an up to 30-fold increase in Fo+, Ia+ double positive M phi over that in control animals. LPS caused mainly an increase in the percentage of double-positive M phi, whereas no effects were seen in BCG or PP treated animals. To clarify the possible involvement of cytokines in this process and to identify these, the effects of LPS and various cytokines on in vitro induction of Fo and Ia expression were studied in further experiments. LPS, IL6, and IL4 caused induction of up to 15% Fo+ and Ia+ M phi after a 4 d culture period. M phi colony stimulating factor (M-CSF) from lung-conditioned medium was also moderately active. IL1, TNF, and IL2 had no influence, whereas IFN-gamma only induced Ia. For a successful in vitro induction of Fo and Ia, a prior priming of the mice with PP appeared mandatory. This suggests that only M phi of a certain developmental stage can acquire Fo under the influence of the appropriate cytokines. The data may provide the first evidence for cytokine-mediated modulation of a glycolipid antigen of known chemical structure.     

SO2-induced bronchopathy decreases airway sensitization with intratracheal ovalbumin in the rat

We investigated in this study the effect of SO2-induced bronchopathy on airway sensitization to ovalbumin in the rat. Sprague-Dawley rats were immunized with a single intratracheal injection of ovalbumin (OA) 100 micrograms in 0.1 ml PBS or 0.1 ml Bordetella pertussis (BP) heat-killed vaccine (6.5 X 10(9) cells X ml-1). The rats were primed immediately after SO2 exposure (60 h, 200 ppm; group I, n = 16) and three months after exposure was achieved (group II, n = 24), then compared to a control group exposed to air (group III, n = 30). Airway sensitization was evaluated by the in vitro contractile response to antigen challenge using paired tracheal rings. Specific IgE level was determined with PCA reactions. No significant difference was found in the maximal contractile responses to carbamyl choline within and between each group. Excepted in animals of group III, OA alone was not found able to sensitize the airways. When OA was used in association with BP, sensitization of the airways occurred, but this occurrence was found to depend upon a previous SO2 exposure: 73.3% in group III, 41.7% in group II and 25% in group I were sensitized. In addition, only five animals (BP + OA injected rats of group III) displayed a PCA positive reaction. It is concluded that: 1) the concomitant intratracheal injection of BP with OA increased the occurrence of specific airway sensitization, 2) a previous chronic exposure to SO2 decreased the specific tracheal smooth muscle sensitization to intratracheal ovalbumin. This decrease persisted, although slighter, when immunization was done three months after the exposure to SO2 was stopped.