INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

In allergic asthma, a close relationship has been demonstrated between IgE levels and the degree of AHR (1). Expression of IgE is thought to be controlled by both heavy-chain switching and the selection of IgE-committed cells. Some of the regulatory circuits for this process are mediated through the low affinity IgE receptor (CD23) either by stimulating B cells to produce more IgE or, when bound to IgE, by blocking its production (2). CD23 (FcRII) is a type II membrane glycoprotein expressed on various cell types, including mature B cells, follicular dendritic cells, T cells, subsets of CD4⁺ and CD8⁺ T-cells after activation and eosinophils (3). In addition to regulating IL-4-induced IgE production, CD23 has been implicated in cellular adhesion (4), antigen presentation (5), growth and differentiation of B and T cells (7), and rescue from apoptosis (9). The expression of CD23 is strikingly increased in allergic disorders (3, 10). Although it is possible that CD23 has an important role in IgE-mediated priming and activation of cells expressing this receptor, the relationship of CD23 expression to disease and the specific functions of CD23-positive cells is still unclear. Overproduction of IgE in response to common environmental antigens may be responsible for amplifying immune reactions which in turn lead to sustained inflammation and airway hyperresponsiveness (AHR). However, the in vivo relevance of many of these observations has recently been questioned by a series of studies on gene-targeted mice lacking expression of CD23, since these mice showed no other phenotypic difference, they had normal T and B cell development and demonstrated an increased production of IgE upon parasite infection (11).

While IgE appears to be involved in a network of immune and inflammatory events relating to immediate hypersensitivity reaction, the role of IgE in allergic inflammation is not clear. Using different approaches to sensitization and challenge, which show differences in IgE dependency for development of AHR, we investigated whether CD23 is functionally important to the inflammatory changes which develop following allergic sensitization leading to AHR. In studying IgE dependency for the development of AHR, we demonstrated that 10-day exposure to ovalbumin (OA) exclusively via the airways triggers an IgE-dependent development of tracheal smooth muscle hyperresponsiveness to electrical field stimulation (12, 13). We also utilized passive sensitization where the inflammatory changes induced following limited allergen challenges are mediated exclusively by OA-specific IgE (14). In contrast to 10-day OA exposure or passive sensitization, after systemic sensitization and repeated airway challenge the development of AHR has been shown to be IgE-independent (15), although these mice produce high levels of IgE. These alternative approaches to sensitization and challenge indicated that CD23 may not be essential for the development of allergen-induced AHR.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Mice

CD23 deficient (CD23^/) and wild type (CD23^+/+) mice were generated as previously described (5). The gene-targeted 129/Ola strain was crossed with C57 Bl/6, then the CD23 deficient F1 heterozygous mice were backcrossed to C57 Bl/6 mice twice. Those heterozygous mice were then intercrossed to produce homozygous wild-type and CD23 deficient mice. Confirmation for the functional disruption of the CD23 gene was performed by Southern Blot and Northern Blot analysis and has been published in detail in an earlier report (10). Briefly, genomic DNA was prepared from ES cells and adult tail tips. To ascertain targeting of the CD23 allele, 15 µg of DNA was digested with Sph I or Aat I and probed with a 0.4-kb BstEII/Xba I or a 0.6-kb BamHI/Xba I genomic DNA fragment. Incorporation of the single neomycin-resistance gene was further confirmed using a neo probe. For analysis of CD23 mRNA, total RNA was prepared from splenic B cells stimulated with LPS (10 µg/ml) in the presence of IL-4 (60 U/ml, Genzyme) for 36 h, electrophoresed, transferred to Hybond-N (Amersham) and hybridized with a CD23 cDNA fragment or a -actin probe. For analysis of CD23 expression we have performed FACScan analysis. The antibody used for identifying CD23 was a rat monoclonal antibody (B3B4) kindly provided by Dr. D. H. Conrad (Virginia Commonwealth University, Richmond, VA) (16). A detailed FACScan analysis of splenocytes and thymocytes as well as immunohistochemical analysis of germinal centers of spleen sections from homozygous CD23 deficient and wild-type mice were published by Fujiwara and coworkers (5).

Mice were housed in pathogen-free conditions and were maintained on an ovalbumin (OA)-free diet. Experiments were performed on age- and sex-matched groups between the age of 8-12 wk.

All experimental animals used in this study were under a protocol approved by the Institutional Animal Care and Use Committee of the National Jewish Medical and Research Center.

Sensitization and Airway Challenge

Mice were exposed to OA following three different protocols: (1) Mice were exposed for 10 consecutive days to an aerosol containing 1% OA using a DeVilbiss Aerosonic 5000 nebulizer (DeVilbiss Health Care Inc., Somerset, PA) as previously described (12); (2) Mice were actively immunized by intraperitoneal injection of 20 µg OA (Grade V; Sigma Chemical Co., St. Louis, MO) together with 20 mg alum (Inject Alum; Pierce, Rockford, IL) in 100 µl PBS (phosphate-buffered saline), or with PBS alone on Day 1 and Day 14. On Days 24, 25 and 26 mice received an aerosol challenge for 20 min with a 1% OA/PBS solution; (3) Mice were passively sensitized with OA-specific IgE as previously described (14). Briefly, mice were injected intravenously with 100 µl of hybridoma supernatant containing 200 µg of OA-specific IgE antibody on three consecutive days (Days 1, 2, 3) prior to four aerosol challenges with 1% OA on Days 5, 6, 7 and 8. As a control, mice received TNP-specific IgE (ATCC, Rockville, MD).

All mice were killed 48 h after their last OA exposure.

Electrical Field Stimulation of Trachea In Vitro

Airway responsiveness to electric field stimulation was determined 48 h after the last aerosol challenge of mice as described previously (12). Briefly, tracheas were removed and 0.5 cm long preparations were placed in Krebs-Henseleit solution suspended by triangular supports transducing the force of contractions. Electrical field stimulation with an increasing frequency from 0.540 Hz was applied and the contractions measured. Frequencies resulting in 50% of the maximal contractions (ES₅₀) were calculated from linear plots for each individual animal and were compared between the different groups.

In Vivo Measurement of Bronchial Responsiveness to Methacholine (MCh)

Bronchial responsiveness was assessed as a change in airway function after challenge with aerosolized MCh via the airways using a modification of methods previously described in rats (17) and in mice (12, 19). Mice were anesthetized with an intraperitoneal injection of pentobarbital sodium (70 to 90 mg/kg). A stainless steel 18 G tube was inserted as a tracheostomy cannula and was passed through a hole in the Plexiglass chamber containing the mouse. A four-way connector was attached to the tracheostomy tube, with two ports connected to the inspiratory and expiratory sides of a ventilator (Model 683; Harvard Apparatus, South Natwick, MA). Ventilation was achieved at 160 breaths per minute and a tidal volume of 0.15 ml with a positive end-expiratory pressure of 2-4 cm H₂O. The Plexiglass chamber was continuous with a 1.0-liter glass bottle filled with copper gauze to stabilize the volume signal for thermal drift.

Transrespiratory pressure was detected by a pressure transducer. Changes in lung volume were measured by detecting pressure changes in the plethysmographic chamber. Flow was measured by digital differentiation of the volume signal. Lung resistance (R_L) and dynamic compliance C_dyn) were continuously computed by a Macintosh computer software (Labview; National Instruments, Austin, TX) by fitting flow, volume, and pressure to an equation of motion.

The aerosolized bronchoconstrictor agents were administered through a bypass tubing via an ultrasonic nebulizer placed between the expiratory port of the ventilator and the four-way connector. Aerosolized agents were administered for 10 s with a tidal volume of 0.5 ml. After a dose of inhaled PBS was given, the subsequent values of R_L were used as baseline. Starting 3 min after saline exposure, increasing concentrations of MCh were given by inhalation (10 breaths), with the initial concentration set at 0.4 mg/ml. Increasing concentrations were given at 5-7 min intervals. Hyperinflations of twice the tidal volume were applied between each MCh concentration and performed by manually blocking the outflow of the ventilator in order to reverse any residual atelectasis and ensure a constant volume history prior to challenge. From twenty seconds up to three minutes after each aerosol challenge, the data of R_L and C_dyn were continuously collected and maximum values of R_L and minimum values of C_dyn were taken to express changes in murine airway function.

Serum Collection

Venous blood was collected from the tail vein before and at different time points during the sensitization period into serum separator tubes (Microtainer; Becton-Dickinson, Franklin Lakes, NJ). Serum samples were stored at 20° C pending analysis.

ELISA for Immunoglobulins

Serum antibody levels were determined as previously described (12). Briefly, ELISA plates (Dynatech, Chantilly, VA) were coated with OA (20 µg/ml NaHCO₃ buffer, pH 9.6) or with polyclonal goat anti-mouse IgE 3 µg/ml (The Binding Site Ltd., San Diego, CA) and incubated overnight at 4° C. Plates were blocked with 0.2% gelatin buffer (pH 8.2) for 2 h at 37° C. Serum was diluted 1:10. Standards containing OA-specific IgE and IgG were generated as described (20). For total immunoglobulins, commercial standards were used (Pharmingen, San Diego, CA). ELISA data were analyzed with the Microplate Manager software program for the Macintosh (Bio-Rad Labs, Richmond, VA).

Mononuclear Cell Culture for Proliferation Assay

Spleens were removed and placed in sterile PBS. Single-cell suspensions were prepared and mononuclear cells were purified by density gradient centrifugation (Lymphocyte Separation Medium; Organon Teknika, Durham, NC). Cells were washed, counted and resuspended in culture medium (RPMI 1640; GIBCO BRL, Gaithersburg, MD), containing heat-inactivated fetal calf serum (FCS 10%; Hyclone, Logan, UT), L-glutamine (2 mM), 2-mercaptoethanol (5 mM), HEPES buffer (15 mM), penicillin (100 U/ml), and streptomycin (100 µg/ml), all from GIBCO BRL. Cells were plated at 2 × 10⁶/ml in 96-well round bottom tissue culture plates in triplicate and incubated with medium alone, OA (100 ng/ml) or the combination of phorbol 12.13-dibutyrate (10 nM) and ionomycin (0.5 µM) for 48 h in a humidified atmosphere of 5% CO₂ at 37° C. Cell free supernates were harvested and stored at 20° C pending cytokine ELISA assays. Cell proliferation was assessed by uptake of (³H)-thymidine which was added to cell culture wells for the last 16 h of the incubation period. After incubation, cells were harvested onto a glass-fiber filter paper using a cell harvester apparatus and the incorporated radiolabel was counted using a -spectrometer. Results were expressed as mean counts of triplicate cultures.

BAL and Lung Digest Differential Cell Count

After measurement of lung function parameters, lungs were lavaged with 1 ml aliquots of 0.9% wt/vol. of sterile NaCl (room temperature) through a polyethylene syringe attached to the tracheal cannula. Lavage fluid was centrifuged (500 × g for 10 min at 4° C), and the cell pellet was resuspended in 0.5 ml of RPMI tissue culture medium.

Lung digestion was performed after exsanguination and perfusion of the lungs following the protocol previously described from our laboratory (14). Cells from BAL or lung digests were resuspended in RPMI and counted with a hemocytometer. Differential cell counts were made from cytospin preparations as described (14, 18). Cells were identified as macrophages, eosinophils, neutrophils and lymphocytes by standard morphology and at least 500 cells counted under ×400 magnification. The percentage and absolute numbers of each cell type were calculated.

Immunolabeling of Eosinophils

Immunocytochemistry was performed as described previously (20). Briefly, lung tissue was removed and fixed in 10% formalin solution. Four µm thick sections were cut, deparaffinized and treated with porcine trypsin for 30 min, 37° C. After washing 3 times, 10% goat serum was applied for 30 min at room temperature. Primary antibodies (rabbit polyclonal anti-mouse MBP, a gift of Dr. G. Gleich, Rochester, MN, and Dr. J. Lee, Scottsdale, AZ) were diluted in 3% goat serum and applied at 4° C overnight. Slides were then washed and stained with 1% Chromotrope 2R (Harlesco, Gibbstown, NJ) for 30 min at room temperature. After washing FITC-conjugated goat anti-rabbit secondary antibody was used and the slides were incubated for 1 h at room temperature in the dark. Coverslips were applied with Fluoromount. Slides were kept in 20° C until they were examined by a Zeiss microscope equipped with a fluorescein filter system under ×200 magnification. For counting, a computer software program was used (IP Lab Spectrum, Signal Analytics Co., Vienna, VA) and results were expressed as number of positive cells/unit area.

Data Analysis

Data were expressed as mean ± SEM. Nonparametric analysis of variance (Kruskal-Wallis method) was used to determine significant variance among the groups. If a significant variance was found, the Mann-Whitney U test was used to analyze the differences between individual groups. In the case of multiple comparisons, the Bonferroni correction was applied. A p value of < 0.05 was considered as significant. Data were analyzed with the MINITAB standard statistical package.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Kinetics of Immunoglobulin Production in Wild-Type and CD23-Deficient Mice

In mice sensitized (i.p.) and challenged with OA, serum samples were obtained on Days 0, 7, 14, 21, and 28. Immunoglobulin values were presented on a log scale throughout. Total IgG levels remained relatively constant (approximately 100 ng/ml) during this time period (Figure 1A). Total IgE levels plateaued at ~ 50 ng by Day 14 to Day 21 in both CD23^/ and CD23^+/+ groups (Figure 1B). There was no OA-specific IgG present in the samples before sensitization. After the first injection it rapidly increased with an additional increase after the booster on Day 14, and plateaued on Day 21. CD23^+/+ and CD23^/ mice showed identical changes. OA-specific IgE concentrations in the wild type mice reached a plateau by Day 14 and declined through Day 28 (Figure 2B). In the CD23^/ mice, OA-specific IgE reached significantly higher levels (p < 0.05, n = 12) on Days 14 and 21 when compared with CD23^+/+ mice (Figure 2B). Without sensitization, aerosol exposure to OA for 3 consecutive days did not affect serum IgE levels in either group (data not shown).

View larger version (10K): [in this window] [in a new window]   Figure 1.   Total immunoglobulin levels during the course of intraperitoneal sensitization. Mice were injected intraperitoneally with OA/alum in 100 µl PBS on Day 1 and Day 14 and received aerosol challenges with 1% OA/PBS solution on Days 25, 26, and 27. Mice were bled on Days 0, 7, 14, 21, and 28 and assessed for total IgG (A) and total IgE levels (B). Data represent the mean ± SEM from three experiments with n = 3, 4 mice in each experiment.

View larger version (14K): [in this window] [in a new window]   Figure 2.   Ovalbumin-specific immunoglobulin levels during the course of intraperitoneal sensitization and 10 day ovalbumin exposure. Mice were sensitized, challenged and bled as described in Figure 1 for OA-specific IgG (A) and IgE (B) production (lineplots). In a second protocol mice were exposed to ovalbumin aerosol for 10 consecutive days and assayed on Day 12 (bargraph). Ovalbumin specific immunoglobulin production is expressed in ELISA U/ml. *p < 0.05: comparisons were made between CD23/ and CD23+/+ mice (n = 815).

Ten days OA-aerosol exposure also resulted in significant increases in OA-specific IgE levels which were comparable in the two groups of mice (Figure 2B). Mice that received 10 day-PBS exposure had no OA-specific immunoglobulins (data not shown).

Effects of Passive Sensitization with OVA-Specific and TNP-Specific IgE on Immunoglobulin Levels Following Airway Exposure to OA

We determined whether passive sensitization with OA-specific IgE would potentiate antibody responses following OA exposures of the airways. Mice received 100 µl of hybridoma supernatant intravenously, containing either OA-specific or TNP-specific IgE on three consecutive days prior to aerosol challenge with 1% OA on four consecutive days. Serum samples were collected for immunoglobulin assays 48 h after the last challenge. Figure 3 illustrates the levels of OA-specific IgG or IgE in the serum of either CD23^+/+ and CD23^/ mice on a log scale. Four airway challenges with OA alone, failed to induce OA-specific IgG or IgE production. In passively sensitized mice, OA-specific IgG was found only in the CD23^+/+ mice, while the CD23^/ mice had no detectable OA-specific IgG in their serum (Figure 3A). Mice passively sensitized with IgE without OA-airway challenge failed to develop IgG or IgE antibodies. There were significant differences in the IgG and IgE levels between mice receiving OA-IgE alone and mice receiving OA-IgE as well as four airway challenges in the CD23^+/+ group but not in the CD23^/ animals. OA-specific IgE was found in the serum of both CD23^+/+ and CD23^/ groups. Following OA-IgE injection and four days OA nebulization, the OA-specific IgE levels in CD23^/ mice were 25 ± 3 ELISA U/ml as opposed to 365 ± 24 ELISA U/ml in the CD23^+/+ mice. This difference is statistically significant (p < 0.01). Values in the control groups were less than 10 ELISA U/ml (Figure 3B).

View larger version (16K): [in this window] [in a new window]   Figure 3.   OA specific IgG (A) and IgE (B) levels in passively sensitized mice. Mice received intravenous injections of hybridoma supernatants containing OA-specific (solid bars) and TNP-specific (hatched bars) IgE prior to four aerosol challenges (neb) with 1% OA. Control groups received OA-specific IgE alone (grey bars) or 4 d nebulization alone (open bars). Serum was assayed on Day 10 (n = 816). *p < 0.05, **p < 0.01 CD23+/+ versus CD23/, ##p < 0.05 OA-IgE alone versus OA-IgE + 4 days OA nebulization.

Proliferative Response of Mononuclear Cells from Sensitized CD23^/ and CD23^+/+ Mice

We compared the capability of lymphocytes from CD23^/ and CD23^+/+ mice to proliferate in response to antigen. Mononuclear cell preparations from the spleens of both groups responded with dose-dependent proliferative responses following intraperitoneal sensitization. As indicated in Table 1, there were no significant differences between the groups. Mononuclear cells from nonsensitized mice did not show any antigen-induced proliferation (data not shown).

BAL and Lung Digest Cellular Content in Wild Type and CD23-Deficient Mice

Animals exposed to OA over 10 d demonstrated significant increases in eosinophil numbers in the lung digests. Although there was a trend for higher eosinophil counts in CD23^/ mice it did not reach statistical significance (p = 0.065) (Figure 4). Following this protocol in this mouse strain we were not able to find significant inflammatory changes in the BAL samples when compared with control mice receiving 10 d PBS. The BAL cellular content in these mice was virtually identical to the control values in both of the CD23^/ and CD23^+/+ groups.

View larger version (24K): [in this window] [in a new window]   Figure 4.   Lung digest cellular composition of mice receiving 10 days-OA exposure. Mice received OA aerosol exposure for 20 min on 10 consecutive days. Lung digestion was performed on Day 12 as described in Figure 5. **p < 0.01 OA-challenged mice versus PBS-challenged mice. Crossed bars = CD23+/+ OA challenged (n = 8); open bars = CD23+/+ PBS challenged (n = 5); solid bars = CD23/ OA challenged (n = 8); hatched bars = CD23/ PBS challenged (n = 6).

View larger version (20K): [in this window] [in a new window]   Figure 5.   BAL cellular composition of mice sensitized and challenged with OA. Mice were sensitized and challenged (i.p. + neb) as described in Figure 1. Control mice received three OA challenges (neb) without intraperitoneal sensitization. BAL was obtained on Day 28. Cytospin preparations of BAL were stained with Giemsa and counted under light microscope (×100 magnification). Three hundred cells were counted in each slide. Cells were identified as macrophage, neutrophil, lymphocyte or eosinophil. Total cell counts in each sample were corrected for the recovered volume. Data represent the absolute number of each cell type. **p < 0.01 sensitized/challenged mice versus mice challenged alone. Crossed bars = CD23+/+ sensitized and challenged (n = 9); open bars = CD23+/+ challenged alone (n = 5); solid bars = CD23/ sensitized and challenged (n = 9); hatched bars = CD23/ challenged alone (n = 6).

The numbers of total leukocytes and macrophages recovered from BAL of intraperitoneally sensitized mice were significantly higher than in nonsensitized animals, in both the CD23-deficient and wild-type animals (Figure 5). While there were no eosinophils and neutrophils in nonsensitized mice, the sensitized and challenged animals from both of the CD23^+/+ and CD23^/ groups demonstrated a marked increase in inflammatory cells, particularly eosinophils. The proportion of eosinophils reached approximately 63% in the wild-type and 72% in the CD23^/ mice.

The number of eosinophils in the lung digests obtained both from sensitized and challenged CD23^+/+ and CD23^/ mice were significantly elevated when compared with controls. Furthermore, CD23^/ mice had significantly higher eosinophil levels than the wild-type mice (Figure 6).

View larger version (22K): [in this window] [in a new window]   Figure 6.   Lung digest cellular composition of mice sensitized and challenged with OA. Lung cell extracts were prepared on Day 28 by enzymatic digestion of whole lungs. Cytospin preparations of BAL were stained and counted as described in Figure 4. Data represent the absolute number of each cell type. **p < 0.01 sensitized/challenged mice versus mice challenged alone. #p < 0.05 CD23/ versus CD23+/+ mice. Crossed bars = CD23+/+ sensitized and challenged (n = 9); open bars = CD23+/+ challenged alone (n = 5); solid bars = CD23/ sensitized and challenged (n = 9); hatched bars = CD23/ challenged alone (n = 6).

Tissue Eosinophil Influx Following Sensitization and Challenge of CD23^+/+ and CD23^/ Mice

In order to directly examine the level of eosinophilia in the peribronchial tissue we performed immunolabeling of eosinophil major basic protein (MBP) on formalin fixed, paraffin embedded tissue sections. The tissue samples were taken 48 h after the last OA-aerosol challenge. In both CD23^/ and in CD23^+/+ mice we failed to detect any positively stained cells following three OA nebulizations alone (Figure 7A). However, sensitized mice which received aerosol challenges developed a marked eosinophilia in both groups. The eosinophils (MBP⁺ cells) were accumulated in the peribronchial and perivascular submucosal tissue, while the lung parenchyma remained relatively eosinophil free. A few cells could also be seen penetrating the airway epithelium, traveling through to the lumen (Figure 7B). We observed the following morphological changes after sensitization and challenge of mice: (1) thickening of the submucosal tissue probably due to edematous changes and a large number of inflammatory cells; (2) the inflammatory cell infiltrate consisted mainly of eosinophils; (3) lymphoid tissue proliferation at certain sites. No basement membrane thickening, epithelium disruption or airway hyperplasia was observed probably due to the acute nature of our model (the mice were sacrificed 48 h after the last OA challenge). In all of these parameters, we were unable to identify any differences between CD23^+/+ and CD23^/ mice.

View larger version (113K): [in this window] [in a new window]   Figure 7.   Tissue eosinophil influx following sensitization and challenge of CD23+/+ and CD23/ mice. CD23/ mice were sensitized and challenged as described in Figure 1. Tissue samples were taken 48 h after the last OA challenge. Control mice received three OA nebulizations alone. Cryostat sections were cut and stained with a polyclonal anti-major basic protein (MBP) antibody. Immunolabeled (MBP+) eosinophils were studied under fluorescence microscopy (×200). The eosinophils (MBP+ cells) accumulated in the peribronchial and perivascular submucosal tissue. A few cells could also be seen penetrating the airway epithelium, traveling through to the lumen (B). No MBP+ cells were seen in mice challenged alone (A). CD23+/+ mice demonstrated a similar degree of eosinophil influx following sensitization and challenge with OA.

Tracheal Smooth Muscle Reactivity in CD23^+/+ and CD23^/ Mice

To monitor airway responsiveness in the different groups following all three modes of sensitization/challenge (10 d OA exposure, i.p. OA/alum injections and OA challenges, passive sensitization with IgE and OA challenges), we used electric field stimulation of tracheal preparations. ES₅₀ values from individual dose-response curves were calculated and the ratio (%) relative to naive controls are depicted in Figure 8. A decrease in ES₅₀ represents an increase in responsiveness (13).

View larger version (16K): [in this window] [in a new window]   Figure 8.   Passive and active sensitization to ovalbumin causes airway hyperresponsiveness measured by electrical field stimulation. Airway responsiveness was studied by electrical field stimulation of tracheal smooth muscle preparations. Results are expressed as the percent ES50 values of naive control mice used in the same experiments. Mean ES50 values of control naive mice were 3.58 ± 0.26 Hz in CD23/ mice (n = 10) and 3.75 ± 0.27 Hz in CD23+/+ mice (n = 10). (A) Groups of mice were passively sensitized with OA-specific IgE (solid bars, n = 16) or TNP-specific IgE (crossed bars, n = 6) prior to four exposures to OA. Control mice required four OA nebulizations alone (open bars, n = 6). (B) Mice were actively sensitized by intraperitoneal injection followed by aerosol challenges as described in Figure 1 (solid bars, n = 14), or received 10 days of OA exposure (hatched bars, n = 14). Control mice received three OA nebulizations alone (open bars, n = 9). Comparisons were made between sensitized and nonsensitized mice. *p < 0.05.

As shown in Figure 8A, passively sensitized mice which received OA-specific IgE prior to four OA-aerosol challenges demonstrated significant decreases in ES₅₀ when compared with mice which were injected with TNP-specific IgE or which received nebulized OA alone. Further, there were no differences between the CD23^+/+ and CD23^/ groups.

Active sensitization of CD23^+/+ and CDE23^/ mice following 10 days OA exposure as well as sensitization with OA/ alum and OA aerosol challenges resulted in significant and comparable decreases in ES₅₀ as illustrated in Figure 8B. Mice receiving 3 d OA exposure alone, demonstrated no difference from responsiveness of naive controls. Mice which were i.p. sensitized but challenged with PBS instead of OA showed no significant differences from the control animals (data not shown).

Airway Responsiveness to Methacholine in Wild Type and CD23-Deficient Mice

Figure 9 presents the dose-response curves of pulmonary resistance (R_L) plotted against concentrations of inhaled MCh in mice sensitized with OA/alum (or injected with PBS) followed by airway challenges. Nonsensitized but OA-challenged animals from the CD23^/ and CD23^+/+ groups were not significantly different and demonstrated changes in R_L in response to increasing concentrations of MCh, which were similar to normal C57/B6 mice (data not shown). Sensitization and challenge with OA resulted in significant increases in airway responsiveness. CD23^/ mice were more responsive than the CD23^+/+ mice as demonstrated by a two-way ANOVA test (p < 0.01). In addition, there was a significant difference between these two groups at 50 mg/ml MCh (p < 0.01) (Figure 9A).

View larger version (23K): [in this window] [in a new window]   Figure 9.   Airway responsiveness to MCh in CD23+/+ and CD23/ mice. Groups of mice were sensitized (i.p.) as described in Figure 1. Control mice received three aerosol challenge to OA alone. Increasing concentrations of nebulized methacholine (MCh) were administered through the tracheal cannula. (A) Data points represent the mean increase in RL relative to the baseline resistance, measured in response to PBS in individual mice in each group. *p < 0.05, **p < 0.01 sensitized versus nonsensitized mice, #p < 0.05 CD23/ versus CD23+/+ mice. Solid circles = CD23/ sensitized and challenged (n = 12); open circles = CD23/ challenged alone (n = 8); solid squares = CD23+/+ sensitized and challenged (n = 12); open squares = CD23+/+ challenged alone (n = 8). (B) PC100 and 200 (provocative concentrations of MCh which cause 100 and 200% increases, respectively, in lung resistance above baseline). Crossed bars = CD23+/+ sensitized and challenged (n = 12); solid bars = CD23/ sensitized and challenged (n = 12); *p < 0.05, **p < 0.01 CD23/ versus CD23+/+ mice. (C ) Changes in dynamic compliance (Cdyn) following nebulized MCh. Data points represent the percentage decrease relative to baseline values which were measured following PBS nebulization. *p < 0.05, **p < 0.01 sensitized versus nonsensitized mice.

PC100 and 200 (provocative concentrations of MCh which cause 100 and 200% increases in lung resistance above baseline) were calculated by log-linear transformation of the dose-response curves. Each of these PC values was significantly lower in CD23^/ mice than in their wild-type littermates (Figure 9B).

We have additionally compared the changes of dynamic compliance (C_dyn) in these mice. This parameter has several determinants including airway resistance to flow, tissue resistance to deformation and elastic recoil of the lung tissue; thus, it is useful in detecting changes in the small airways and peripheral tissue. While sensitization and challenge resulted in significant decreases in the C_dyn values (ANOVA p < 0.01) in both the CD23^+/+ and CD23^/ mice (Figure 9C), there were no significant differences detectable by C_dyn between CD23^+/+ and CD23^/ mice.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

CD23 has been widely implicated in the synthesis of IgE as well as in IgE-mediated immune and inflammatory functions (2). As a result, CD23 may have a pivotal role in the development and maintenance of allergic inflammation. We tested this hypothesis in a mouse model of allergic AHR and found that following sensitization and challenge with OA, neither production of IgE nor the development of airway inflammation and AHR were inhibited in CD23^/ mice.

We compared the allergic responses in CD23^+/+ and CD23^/ mice using three different sensitization protocols. First, mice received nebulization with OA for 10 consecutive days. This approach does not involve adjuvant treatment and antigen exposure is solely via the airways (12), mimicking natural conditions more closely. Further, the development of tracheal smooth muscle hyperresponsivness to electrical field stimulation in this model appears dependent on IgE production and eosinophilic infiltration of the airways (12, 20). Mice in this study produced moderately elevated OA-specific IgE levels, some eosinophilic inflammation in the peribronchial regions and tracheal smooth muscle hyperresponsiveness to electrical field stimulation, in both the CD23^/ and CD23^+/ groups. These data suggest that CD23 has no profound effect on the allergic changes in the 10 d OA-exposure model.

In a second protocol, we further amplified the allergic responses in order to be able to study and compare the kinetics of immunoglobulin production in CD23 deficient and wild-type mice, as well as to examine changes in airway inflammation and airways responsiveness to nebulized MCh in vivo. It has previously been shown that high levels of IgE and eosinophilic airway inflammation can be achieved by airway challenges of systemically (i.p.) sensitized animals (17, 21). Following this approach, the mice produced marked increases in OA-specific IgE and IgG, a significant airway inflammatory cell infiltration with predominance of eosinophils, tracheal hyperreactivity to electric field stimulation and increased responsiveness to MCh in vivo, again, in both groups of mice. However, the changes were significantly higher in the CD23-deficient mice suggesting an inhibitory effect of CD23 when high levels of IgE are involved in the allergic response.

Finally, in order to study the importance of CD23/IgE- mediated events on the development of allergic response independently, we used a third protocol in which CD23^+/+ and CD23^/ mice were passively sensitized with OA-specific IgE prior to OA challenges. A similar method has previously been described in our laboratory and was shown to induce tracheal smooth muscle hyperresponsiveness to electrical field stimulation (14). The role of CD23 in enhancing antigen presentation by binding antigen-IgE complexes seems to be the most consistent feature among human and murine immune responses. The absence of upregulation of OA-specific IgG1 and IgE production in CD23^/ mice following administration of anti-OA IgE supports these observations. However, this mechanism does not seem to play a role in the development of tracheal smooth muscle hyperresponsiveness.

There are a number of studies suggesting that CD23 is involved in increased IgE production presumably by interacting with its ligand, CD21 on the B cell surface (21). Further, in vivo anti-CD23 antibody treatment of rats was shown to inhibit specific IgE immune responses (22) supporting a role for CD23 as an enhancer of IgE production. However, the function of CD23 may be more complex as shown by seemingly controversial data. Actively sensitized CD23^/ mice in our studies produced significantly higher amounts of OA-specific IgE while their OA-specific IgG levels remained the same when compared with their CD23^+/+ littermates. These data suggest that presence of CD23 in this mouse model may exert a specific inhibitory effect to a certain extent on IgE production, following systemic (i.p.) sensitization. It has indeed been demonstrated that while a soluble form of CD23 increased the spontaneous as well as IL-4-induced IgE synthesis by B cells (23), ligation of the membrane form of CD23 inhibited IgE production (24). In addition, in humans, most of the regulatory function of soluble CD23 is ascribed to its IgE binding capacity but in mice there is no evidence to support that soluble CD23 retains IgE binding (25). Other laboratories using CD23-deficient mice have demonstrated that these animals have normal T and B cell development and function with normal or enhanced IgE production following sensitization to antigen (5) or parasite infection (26, 27). Further, using heterozygous (CD23^+/) populations of gene-targeted mice, an inverse relationship was revealed between the expression of CD23 and levels of IgE produced after parasite infections (26). Taken together, these data suggest that in murine systems, the inhibitory, membrane bound form of CD23 may be the dominating IgE-regulatory factor.

Antigen-IgE complexes bound to CD23 are described to facilitate antigen presentation to specific T cells, thereby amplifying T cell responses in vitro (5, 6). We have previously shown that such in vitro upregulation of T cell function following exogenous IgE treatment could be abolished after addition of anti-CD23 antibody, and was absent in cells from CD23 deficient mice (28). In vivo studies have also demonstrated an impaired capability of CD23^/ mice to utilize the enhancing effects of antigen-specific IgE treatment in immunoglobulin production (5). We found that passive sensitization with OA-specific IgE (but not anti-TNP IgE) antibody prior to repeated inhalational exposures to OA, was associated with significantly enhanced immune responses including OA-specific immunoglobulin (IgG and IgE) production in CD23^+/+ but not in CD23^/ mice. These data confirmed that in the absence of CD23, the enhancing effects of IgE on certain immune functions are indeed impaired. Interestingly, following this sensitization protocol, CD23^/ mice were still perfectly capable of developing tracheal smooth muscle hyperresponsiveness to electrical field stimulation, indicating that facilitated antigen presentation through IgE-CD23, may be of little importance in this murine model.

Sensitized and challenged mice developed a marked increase in the number of eosinophils both in the BAL fluid and in the airway submucosal tissue. In addition, quantitative evaluation of whole lung digests revealed a significant increase in the eosinophilia of CD23^/ mice when compared to wild-type littermates. The higher number of eosinophils and the fact that CD23^/ mice were able to produce significantly larger amounts of OA-specific IgE may account for their increased R_L response to MCh. Immunocytochemistry of the lung tissue showed that eosinophils were accumulated in the peribronchial-perivascular area in sensitized and challenged mice while the parenchyma was relatively eosinophil-free. While the precise mechanism whereby eosinophils affect function is not known, it is suggested that eosinophil granule products increase airway responsiveness to nonspecific stimuli in vivo. CD23 may play a role in IgE-mediated eosinophil degranulation (29). Our experiments however, did not support this hypothesis since the absence of CD23 did not inhibit MCh- responsiveness in the CD23^/ mice. Recently, two different mRNA species were described in humans, CD23a and CD23b (30). Both forms have been identified on B cells, but only CD23b was found on other cell types such as T cells, dendritic cells, platelets and eosinophils (3, 11). The presence of form b has been confirmed in the mouse, and it is not known whether eosinophils in this species express CD23.

Although availability of immunologically relevant transgenic or `knockout' mice have proven important for studying models of allergic sensitization (31), site-specific alterations of the mouse genome can often result in surprising phenotypes (32). While constructing the mutations carrying loss-of-function or null-alleles of the gene of interest is technically the simplest, there are a number of limitations to this approach. These include a redundancy in the genome that allows multiple genes to perform overlapping functions, as well as the ability of the genes to compensate for the absent function of the targeted gene. Since surface expression of CD23 was completely absent in the homozygous gene-deficient mice in the present study, the findings of increased functional consequences (such as heightened R_L) may be explained by compensatory mechanisms of other membrane structures that are able to bind and mediate functions of IgE in a similar fashion to that of CD23. Several different classes of such IgE-binding molecules have been found (the best known are FcRI and Mac-2). These molecules are structurally unrelated, they are encoded by different genes and may be detected on the cell surface, in the cytoplasm and the nucleus of various cell types, particularly after activation (33). Further studies are needed to clarify the involvement of these IgE-binding molecules in IgE-mediated functions and the possible regulatory relationships between them.

In the absence of functionally active CD23 in the CD23^/ mice we observed enhanced allergic responsiveness following i.p. sensitization and repeated airway challenge. There are at least two possible explanations: (1) The absence of CD23 may activate another, functionally similar gene which compensates or overcompensates for the loss of CD23. Evidence for this is still missing, although such compensation has been described following targeting of other genes (32). (2) If ligation of CD23 plays an inhibitory role on IgE production, its absence would lead to enhanced responses. Our findings on the effects of anti-CD23 antibody (B3B4) which suppresses IgE production and inhibits the development of airway inflammation and hyperresponsiveness suggest that binding of CD23 with this antibody results in negative regulatory signals (manuscript in preparation). This is supported by studies on CD23 transgenic mice which demonstrate a severely impaired capability to produce IgE (34).

In conclusion, the present experiments demonstrate that CD23 deficient mice are not impaired in their ability to produce IgE or to develop allergic airway inflammation and AHR following active allergic sensitization and challenge. The allergen-induced responses are in fact enhanced in these mice, implying that a negative regulatory effect of CD23 on allergic inflammation may be deficient in the CD23^/ animals.