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GM-CSF produced by the airway epithelium is required for sensitization to cockroach allergen

Airway epithelial cells are among the first to encounter inhaled allergens and can initiate allergic responses by producing pro-Th2 innate cytokines. In this study, we investigated the role of epithelial-derived cytokines in sensitization to a clinically relevant allergen, cockroach allergen (CRA). Among the epithelial-derived cytokines, GM-CSF played a central role in the initiation of Th2 allergic responses to CRA. We show that initial exposure to CRA directly activated airway epithelial cells through a TLR4-MyD88-dependent pathway and MyD88 signaling in epithelial cells induced upregulation of GM-CSF during sensitization. Epithelial-derived GM-CSF was required for allergic sensitization and selectively restored Th2 responses in the absence of MyD88. Thus, we demonstrate that epithelial-derived GM-CSF is a critical early signal during allergic sensitization to CRA.


Introduction
Allergic asthma is a chronic lung disease driven by an immune response against allergens, which results in reversible airflow obstruction, mucus hypersecretion, and the recruitment of eosinophils and Th2 cells to the airway 1 . Airway epithelial cells are among the first to detect the presence of inhaled allergens through pattern recognition receptors (PRRs) 2 , and in response to allergens, produce cytokines that can initiate type 2 responses 3,4 . Although transgenic overexpression of epithelial-derived cytokines such as thymic stromal lymphopoietin (TSLP), IL-25, IL-33, and GM-CSF are each sufficient to induce a Th2 allergic response against an antigen [5][6][7][8] , mounting evidence suggests that the requirement for these cytokines in allergic responses to various allergens is distinct.
TSLP is induced in a large number of allergic responses [9][10][11][12][13][14] and is required in ragweeddriven allergic inflammation. Yet TSLP has been shown to be dispensable for allergic sensitization to house dust mite (HDM), in which IL-33 appears to play a more central role 10,15 . GM-CSF, which can be produced by a number of different cell types including epithelial cells, has also emerged as an important regulator of allergic inflammation 15 . Patients with asthma have increased levels of GM-CSF expression; in mice, transgenic overexpression of GM-CSF serves as a potent adjuvant that permits sensitization to ovalbumin 8 and sub-threshold levels of HDM 16 . Antibody blockade of GM-CSF during sensitization reduces allergic inflammation to HDM and Blomia tropicalis mite allergen 15,17,18 , suggesting an important role for GM-CSF early in the allergic response.
In this study, we focus on cockroach allergen (CRA), an important allergen associated with allergic asthma and asthma morbidity, especially in low-income urban populations [19][20][21] . CRA can activate protease-activated receptor 2 (PAR2) [22][23][24] and toll-like receptors (TLRs) 25 to induce the release of inflammatory cytokines. However, TSLP is not required for allergic responses to CRA 26 , while IL-25 is only required for chronic allergic responses to CRA 27 . Our studies demonstrate that MyD88-dependent induction of GM-CSF in epithelial cells was required for sensitization to inhaled CRA. Epithelial-derived GM-CSF specifically promoted Th2-mediated allergic responses, but not Th17-associated neutrophilia, downstream of the MyD88 signaling pathway. Interestingly, while induction of GM-CSF has been shown to occur downstream of IL-1 in response to HDM, CRA-induced GM-CSF was IL-1 independent. In contrast to the requirements for IL-33 in HDM-induced asthma, IL-33 only played a minimal role in response to CRA. Overall, our study demonstrates that epithelial-derived GM-CSF is a key mediator of sensitization to CRA, but that induction of GM-CSF by CRA is mediated by pathways distinct from HDM.

Intranasal sensitization and challenge with CRA results in airway inflammation with features of allergic asthma
We first established a model of CRA-mediated allergic airway disease in which mice were sensitized and challenged with the allergen in the absence of an adjuvant (Fig. 1a). Following CRA challenge, mice developed airway inflammation with eosinophils representing the main infiltrating cell type (Fig. 1b). There was also a significant increase in the recruitment of CD4 T cells, DCs, and neutrophils to the airway (Fig. 1b). Consistent with an allergic response, histological analysis of lung sections showed extensive goblet cell metaplasia and mucus production (Fig. 1c). In addition, we detected elevated levels of CRAspecific IgE antibodies in the serum (Fig. 1d) as well as increased lung mRNA expression of T H 2 and T H 17 cytokines, with no effect on IFNFγ (Fig. 1e). Depletion of CD4 T cells prior to sensitization and challenge protected mice from the development of allergic inflammation in response to CRA ( Supplementary Fig. 1). These results show that sensitization and challenge with CRA leads to the development of airway inflammation that closely resembles allergic asthma.

MyD88 is required for the development of allergic airway inflammation to CRA
A previous study showed that MyD88 is required in bone marrow-derived DCs for production of IL-6 and IL-23 after exposure to CRA, but it is unknown if MyD88 is required for allergic responses to CRA 25 . Myd88−/− mice sensitized and challenged with CRA developed markedly less airway inflammation compared to wild type (WT) mice; the number of eosinophils, neutrophils, and CD4 T cells were strongly reduced in the BAL (Fig.  2a). Histological analysis confirmed this reduction in immune cell infiltrates and revealed less mucus production in the lung (Fig. 2b). Furthermore, both T H 2 and T H 17 cytokine levels were significantly lower in the BAL fluid of Myd88−/− mice (Fig. 2c). These results reveal an important role for the MyD88 signaling pathway in the development of allergic airway inflammation to CRA.
Next, we investigated the role of individual TLRs in sensing CRA and found no significant differences in airway eosinophilia between WT and Tlr2−/− mice ( Supplementary Fig. 2a).
In contrast, TLR4-deficient mice exhibited significantly reduced airway eosinophilia, mucus production, and T H 2 cytokine production (Supplementary Fig. 2b-d). Because LPS is a TLR4 agonist that can promote T H 2 responses at low doses 28 , we measured the concentration of contaminating LPS in CRA. We found that each dose of CRA contained 11 ng of LPS (data not shown) and asked if this low level of LPS was sufficient to drive sensitization to an antigen. In contrast to CRA-treated mice, mice sensitized and challenged with 11 ng of LPS plus ovalbumin failed to develop airway eosinophilia ( Supplementary  Fig. 3). This suggests that, while LPS might contribute, it alone does not drive sensitization to CRA.
In addition to TLRs, MyD88 is also required for signaling pathways downstream of the IL-1 receptor family. Members of the IL-1 cytokine family, such as IL-1α and IL-33, have been shown to be important for sensitization to HDM 15 . When IL1R signaling was blocked with anakinra, an IL-1 receptor antagonist, levels of airway eosinophilia were similar to that of the control group ( Supplementary Fig. 4a). These results demonstrate that IL-1α and IL1β are not required for allergic response to CRA. Consistent with this data, we also found that IL-1α was not upregulated in response to low or high dose of CRA (data not shown). Next, we compared WT and casp1−/− mice to determine the role of IL-18 and IL-1β. Caspase 1 is required for processing pro-IL-1β and pro-IL-18 into their active forms. Again, we found no significant differences in eosinophil numbers between WT and casp1−/− mice after CRA challenge ( Supplementary Fig. 4b). However, when IL-33-deficient mice were sensitized and challenged with CRA, there was a significant decrease in airway eosinophilia compared to WT mice. Surprisingly, the numbers of CD4 T cells, DCs, and neutrophils remained elevated in IL-33-deficient mice ( Supplementary Fig. 4c). These results show that among the IL-1 cytokines, IL-33 played a limited role in CRA-induced airway eosinophilia.

MyD88 is required in lung epithelial cells for allergic inflammation to CRA
MyD88 is expressed broadly in both immune and structural cells. To determine which of these cellular compartments were responsible for MyD88-dependent responses to CRA, we established reciprocal bone marrow (BM) chimeras in which WT or Myd88−/− BM cells were transferred into irradiated WT or Myd88−/− hosts. WT → WT BM chimeric mice developed a robust allergic response to CRA. In contrast, Myd88−/− Myd88−/− → BM chimeric mice developed reduced eosinophilia, which confirmed the critical role of MyD88 in CRA-induced allergic inflammation ( Supplementary Fig. 5). A similar reduction in eosinophilia was observed when MyD88 was absent in structural cells (WT → Myd88−/−).
In contrast, absence of MyD88 in the radiosensitive, hematopoietic compartment (Myd88−/− → WT) did not affect the degree of allergic inflammation to CRA ( Supplementary Fig. 5).
These results indicate that MyD88 expression is required in lung radioresistant cells for a T H 2 allergic response against CRA.
MyD88 is expressed by various structural cells in the lung such as epithelial cells and fibroblasts 29 . Because epithelial cells are uniquely positioned to sense inhaled allergens and can initiate T H 2 allergic responses, we hypothesized that MyD88 expression is required in epithelial cells to promote sensitization to CRA. To test this hypothesis, we generated mice that lacked MyD88 specifically in lung epithelial cells by crossing Myd88 fl/fl mice with mice expressing Cre recombinase under the control of the 3.7-kb human surfactant protein C (SPC) promoter 30,31 . The SPC promoter is expressed in the primary lung buds starting at E10.5 and directs recombination in all airway and alveolar epithelial cells 30,[32][33][34] . When SPC-Cre/Myd88 fl/fl mice were sensitized and challenged with CRA, they developed significantly reduced airway eosinophilia and goblet cell metaplasia (Fig. 3a, b). In addition, restimulation of cells isolated from the lungs of these mice revealed a significant decrease in the percentage and number of IL-5 + and IL-13 + CD4 T cells (Fig. 3c). On the other hand, deletion of MyD88 in CD11c+ cells and CD4 T cells had no effect on the degree of airway inflammation following sensitization and challenge with CRA ( Supplementary Fig. 6). Altogether, these results demonstrate that MyD88 signaling in lung epithelial cells, but not hematopoietic cells, is important for initiation of allergic responses against CRA.

GM-CSF is produced by lung epithelial cells in a MyD88 dependent manner
Following allergen exposure, epithelial cells can initiate and regulate T H 2 responses through production of prototypical cytokines such as TSLP, IL-25, and IL-33 2 . However, we found no significant differences in CRA-induced airway eosinophilia between WT, Tslp−/−, and Il17rb−/− mice ( Supplementary Fig. 7), while IL-33-deficient mice only displayed a reduction in eosinophilia ( Supplementary Fig. 4c).
When we examined additional epithelial-derived cytokines induced by CRA, we found elevated levels of GM-CSF in the BAL as early as two hours post-sensitization (Fig. 4a). To assess if upregulation of GM-CSF was dependent on MyD88 signaling in lung epithelial cells, we analyzed the expression of GM-CSF in SPC-Cre/Myd88 fl/fl mice after sensitization with CRA. SPC-Cre/Myd88 fl/fl mice failed to upregulate GM-CSF in response to CRA, suggesting that MyD88 signaling in lung epithelial cells triggers the release of GM-CSF (Fig. 4b).
To confirm that epithelial cells produced GM-CSF in response to CRA, we established organotypic air-liquid interface (ALI) cultures of primary epithelial cells from WT and Myd88−/− tracheas. When WT ALI cultures were stimulated with CRA, there was a significant increase in GM-CSF levels in the culture medium on the basolateral side (Fig.  4c). In contrast, ALI cultures established from Myd88−/− mice failed to upregulate GM-CSF after stimulation with CRA (Fig. 4c). Altogether, these results demonstrate that airway epithelial cells produce GM-CSF in a MyD88-dependent manner following exposure to CRA.

GM-CSF is required during allergic sensitization, but not challenge, in CRA-driven allergic airway disease
To investigate the role of epithelial-derived GM-CSF in regulating CRA-driven allergic airway disease, mice were treated with a neutralizing αGM-CSF antibody at either sensitization or challenge. Blocking GM-CSF at sensitization strongly reduced airway inflammation after CRA challenge, with eosinophil numbers in the BAL significantly lower compared to isotype control treatment (Fig. 5a). There was also a reduction in the number of CD4 T cells, DCs, and neutrophils (Fig. 5a). Furthermore, mRNA expression of type 2 cytokines and serum CRA-specific IgE levels were also significantly reduced (Fig. 5c, d). In contrast, blocking GM-CSF at challenge failed to reduce airway eosinophilia in mice sensitized and challenged with CRA (Fig. 5e). These results reveal a crucial role for GM-CSF during sensitization, but not challenge, in CRA-induced allergic airway disease.

GM-CSF is required for optimal dendritic cell function and Th2 priming
Given that the allergic response to CRA is CD4 T cell driven, we postulated that GM-CSF played a role in DC function during priming. To explore this possibility, we asked how neutralization of GM-CSF affected the maturation of DCs after exposure to CRA. Mice were sensitized with fluorescently-labeled CRA in the presence or absence of the αGM-CSF antibody, and the number of CRA + DCs in the draining lymph node was determined by flow cytometry. Neutralization of GM-CSF significantly reduced the delivery of fluorescently labelled CRA to the draining lymph node. There was a reduction in both the percentage and total number of A647-CRA + DCs in the lung-draining lymph node 24 hours after CRA administration (Fig. 6a). Additionally, surface expression of the costimulatory molecule CD80 was markedly decreased on A647 + DCs (Fig. 6b). These findings suggest that GM-CSF regulates the ability of DCs to acquire antigen, upregulate costimulatory molecules, and migrate to the draining lymph node during sensitization. To examine if GM-CSF blockade affected the ability of DCs to prime T H 2 cells, we sensitized mice with CRA and collected lung-draining lymph node cells on day 3. When these cells were restimulated with CRA, there was a significant reduction in the production of T H 2 cytokines from αGM-CSF treated mice (Fig. 6c). Overall, these results demonstrate that GM-CSF is required for optimal function of DCs and Th2 priming during CRA sensitization.

GM-CSF selectively rescues T H 2 allergic airway response to CRA in Myd88−/− mice
To determine the function of GM-CSF downstream of MyD88 signaling, GM-CSF was administered intranasally to Myd88−/− mice during CRA sensitization. Treatment with GM-CSF largely restored airway inflammation in Myd88−/− mice to levels observed in WT mice. This was due to a significant increase in the number of eosinophils and CD4 T cells, whereas recruitment of neutrophils to the BAL was not recovered by GM-CSF treatment (Fig. 7a, b). Furthermore, GM-CSF selectively restored T H 2, but not T H 17 responses, in Myd88−/− mice. In vitro restimulation of lung cells with PMA/ionomycin revealed a significant increase in the number of IL-5 + and IL-13 + CD4 T cells, but not IL-17A + or Sheih et al. Page 5 IFNγ + CD4 T cells (Fig. 7d). Consistent with the recovery of T H 2 responses, CRA-specific IgE levels were also restored by GM-CSF treatment (Fig. 7c). These results demonstrate that epithelial-derived GM-CSF specifically promotes T H 2-mediated eosinophilic inflammation following exposure to CRA.

Discussion
The airway epithelium coordinates allergic responses by producing pro-Th2 cytokines such as TSLP, IL-25, IL-33, and GM-CSF. Although studies using HDM allergen have provided valuable insights into the role of epithelial-derived cytokines in mucosal sensitization, understanding pathways of sensitization initiated by other clinically relevant allergens remain important given that allergens are structurally and functionally distinct. Here, we focused on cockroach allergen, an important factor in the development of asthma for innercity children. We uncovered a critical role for epithelial-derived GM-CSF in allergic sensitization to CRA, in which GM-CSF specifically promoted Th2 priming by regulating the functional maturation of DCs. Interestingly, while IL-1α initiates the release of GM-CSF in response to HDM, IL-1α was not required for CRA-induced GM-CSF. Additionally, IL-33 only played a minor role in allergic response to CRA whereas IL-33 is critical for allergic sensitization to HDM. Thus, our study demonstrates that epithelial-derived GM-CSF is a critical early signal for the initiation of Th2 allergic responses to CRA.
Activation of PAR2 by CRA-derived proteases is important for initiation of allergic responses to CRA. However, in the absence of PAR2, allergic response to CRA is partially reduced but nonetheless intact, which suggests a role for other PRRs in the recognition of CRA 35 . In this study, we have identified TLR4-MyD88 as a major signaling pathway that promotes sensitization to CRA. We show that MyD88 signaling was required in lung epithelial cells for upregulation of GM-CSF during sensitization. Previous studies have shown that CRA-derived proteases can also induce GM-CSF production by epithelial cells through a PAR2 dependent mechanism 36 . Our findings suggest that in addition to PAR2, TLR4-MyD88 signaling pathway is another important signaling pathway that mediates production of GM-CSF in response to CRA.
TLR4 stimulation with a low dose of LPS can promote Th2 responses to inhaled antigens 28 .
Here we show that the TLR4-mediated response to CRA extends beyond an immune response to LPS and antigen since the low level of contaminating LPS in CRA was not sufficient to induce Th2 allergic responses. Instead, it is possible that other components of CRA or the release of endogenous ligands mediates this TLR4-dependent response to CRA. Bla g 1, a major cockroach allergen, contains a hydrophobic cavity that binds to lipids known to activate TLR4 37 . Therefore, Bla g 1 may lower the threshold of LPS needed to generate a T H 2 allergic response by enhancing TLR4 binding. Alternatively, TLR4 may be activated by an endogenous ligand released in response to CRA such as high mobility group box 1 (HMGB1) 38 . This ability to trigger TLR4 is a characteristic shared by many allergens, and our findings further support the idea that TLR4 signaling is essential for the development of allergic inflammation.
Previous studies have shown that CRA contains a TLR2 agonist 39 . However, we found that TLR2 was not required for allergic responses to CRA. This finding is consistent with a study by Page et al. in which TLR2−/− mice displayed increased airway inflammation against CRA 40 . Although we did not find a protective role for TLR2, which could be due to differences in the route of sensitization and administration protocol, our data demonstrates that TLR2 is not required for allergic sensitization to CRA.
Activation of airway epithelial cells is a critical first step in the initiation of responses to aeroallergens. In this study, we demonstrate that MyD88 expression in radioresistant cells was necessary and sufficient for allergic airway response to CRA. Furthermore, we determined that MyD88 signaling in lung epithelial cells, but not in CD11c+ alveolar macrophages or DCs, was required for sensitization to CRA. Our findings indicate that MyD88 signaling in epithelial cells plays a predominant role in facilitating allergic responses to CRA. In contrast, a recent study in HDM showed that TLR4 signaling in both lung epithelial cells and hematopoietic cells contributes to the allergic response 41 . It is likely that this difference in the contribution of epithelial cells to CRA-versus HDM-driven allergic responses leads to further differences in the induction and requirement of epithelialderived cytokines.
In this study, we found key differences in the requirement for epithelial-derived cytokines in allergic response to CRA, compared to other allergens. We show that TSLP and IL-25 were dispensable for the development of CRA-induced airway inflammation. Although TSLP has been considered a master regulator of Th2 responses, our data indicate a redundant role for TSLP and IL-25 in airway sensitization and is consistent with studies in HDM and peanut allergy 10 . Additionally, IL-33 only played a minor role in allergic responses to CRA; in stark contrast, IL-33 is essential for allergic sensitization to HDM 10,15 and many other clinically relevant allergens such as Alternaria alternata, peanut allergen, cypress pollen, and bee venom 10,[42][43][44] . Although the current paradigm considers TSLP, IL-25, and IL-33 to be critical drivers of Th2 allergic responses, we present a novel pathway in which all three epithelial-derived cytokines are not crucial for sensitization to CRA.
Inhaled allergens and other noxious stimuli such as viruses, cigarette smoke, and diesel exhaust fumes can upregulate the expression of GM-CSF in the airway [45][46][47][48] . Increased levels of GM-CSF amplifies the risk of allergic sensitization by lowering the threshold of allergen required for allergic responses 16 . Additionally, neutralizing GM-CSF at sensitization reduces allergic inflammation to HDM and Blomia tropicalis mite allergen 15,17,18 . However, in contrast to HDM in which both IL-33 and GM-CSF are required for allergic responses, we found that epithelial-derived GM-CSF was the primary driver of allergic sensitization to CRA. Furthermore, in the absence of MyD88, delivery of exogenous GM-CSF was sufficient to initiate an allergic response to CRA, which suggests that IL-33 is not required for GM-CSF mediated responses to CRA. Interestingly, we also show that GM-CSF selectively initiates the Th2, but not the Th17, arm of the immune response to CRA, which indicates a directed role for GM-CSF that extends beyond a proinflammatory action.
Although epithelial-derived GM-CSF plays an important role in sensitization to both HDM and CRA, there are key differences in the pathways that lead to production of GM-CSF by these distinct allergens. Willart et al. showed that sensitization to HDM is driven by epithelial-derived IL-1α, which acts upstream in the cytokine cascade to initiate the release of GM-CSF in an autocrine manner 15 . However, the requirement for IL-1α in HDM-driven allergic responses depends the allergen dose and administration protocol 49 . In contrast, IL-1α was not upregulated in response to CRA and was not required for allergic responses to CRA, suggesting that GM-CSF production is independent of IL-1α. Despite differences in the pathways of sensitization initiated by CRA versus HDM, these pathways converge on GM-CSF, which suggests a central role for GM-CSF in allergic sensitization.
In conclusion, we have determined the role of epithelial-derived cytokines in CRA-driven allergic responses. We uncovered a novel pathway in which epithelial-derived GM-CSF, but not TSLP, IL-25, or IL-33, is essential for allergic sensitization to CRA. Upregulation of GM-CSF in lung epithelial cells occurs through a MyD88-dependent pathway that does not require IL-1α. Downstream of MyD88, GM-CSF selectively initiates a Th2 allergic response, but not Th17-mediated responses. Overall, our findings establish a central role for epithelial-derived GM-CSF in allergic sensitization to CRA and suggest that GM-CSF might be an important therapeutic target.

Reagents
Lyophilized whole body German cockroach extract was purchased from Greer laboratories (XPB46D3A4, Lot # 259066) and resuspended in PBS to a protein concentration of 2 mg/ml. Low endotoxin ovalbumin was purchased from Worthington Biochemical Corporation (LS003061). LPS from E. coli 026:B6 was purchased from Sigma (L2654). Human IL-1ra (anakinra) was purchased from the Virginia Mason hospital pharmacy. Recombinant GM-CSF was purchased from Peprotech (315-03).

Cockroach allergen induced allergic airway model
On days 0-2, mice were anesthetized with isoflurane and sensitized intranasally with 40 μg of CRA (Greer, B46) in a total volume of 20 μl PBS. After 15 days, mice were challenged intranasally with 40 μg of CRA on two consecutive days and airway inflammation was assessed 72 hours later. Control mice were treated with PBS under the same protocol. For neutralization experiments, anti-GM-CSF antibody (Clone MP1-22E9, Biolegend) was injected 5 hours before the first sensitization or challenge dose. 120 μg of antibody was administered i.p. and 30 μg of antibody was administered intranasally. Anakinra, IL-1ra, was i.p. administered at 25 mg/kg with each dose of CRA for a total of five times. To deplete CD4 T cells, 150 μg of anti-CD4 antibody (GK1.5, Bio X Cell) was i.p. administered on days -1, 2, and 16. To determine if GM-CSF can recover T H 2 allergic inflammation in Myd88−/− mice, mice were intranasally treated with 50 ng of GM-CSF plus CRA during sensitization and were only challenged with CRA.

Evaluation of airway inflammation
Mice were euthanized by i.p. injection with 1 ml of 2.5% Avertin. Bronchoalveolar lavage (BAL) was performed four times, each time with 1 ml of PBS. Next, BAL was centrifuged at 250 x g for 5 min to collect the cell pellet. Cells were resuspended in FACS buffer and counted using a hemocytometer. Differential cell counts were performed by flow cytometry. The first ml of BAL fluid collected for ELISA.
After BAL, lungs were perfused by injecting 3 ml of PBS into the right ventricle of the heart until the lungs turned white. The right upper lobe of the lung was placed in 1 ml of RNALater and stored at −80°C for later analysis by qPCR. The remaining lung tissue was fixed in 10% neutral buffered formalin. Tissues were embedded in paraffin, sectioned, and stained with H&E and periodic acid-Schiff. Serum was collected and analyzed for CRAspecific antibodies by ELISA.
To determine differential cell counts in the lung, single cell suspensions were prepared by cutting the lung into fine pieces using a scissor. Next, lung tissue was digested in 50 μg/ml of Liberase TM (Roche) and 10 U/ml of DNase I (Sigma) in RPMI-1640 at 37°C for 30 min. Digested lung samples were filtered through a 100-μm cell strainer and washed with equal volumes of RPMI plus 20% FBS. Red blood cells were lysed with ACK lysis buffer and lung cells were centrifuged at 250 x g for 5 min. Cells were resuspended in FACS buffer and counted using a hemocytometer. Differential cell counts were performed by flow cytometry. Lung cells were stimulated for 4 h with PMA (50 ng/ml), ionomycin (500 ng/ml), and GolgiPlug ™ (BD Biosciences) at the recommended concentration.

A647-CRA and Migration of DCs
A647 was conjugated to CRA using the Alexa Fluor 647 microscale protein labeling kit per manufacturer's instructions (Life Technologies). DC uptake of A647-CRA and migration to the draining lymph node was assessed by administering 11 μg of A647-CRA plus 39 μg of unconjugated CRA intranasally. Twenty four hours later, lung draining lymph node was collected and single cell suspension was prepared. Lymph nodes were teased apart using needles and digested in 1 ml of digest solution containing 50 μg/ml of Liberase TM (Roche) and 10 U/ml of DNase I in RPMI-1640. After digesting for 25 minutes at 37°C, 100 μl of 0.1 M EDTA was added to each sample and digested for five additional minutes at 37°C. Next, digested lymph nodes were crushed with a 1 ml syringe and filtered through a 100-μM cell filter. Cells were resuspended in FACS buffer and analyzed by flow cytometry.

Evaluation of sensitization to CRA
On days 0-2, mice were sensitized with 40 μg of CRA intranasally. On day 3, lung draining lymph nodes were collected and single cell suspensions were prepared as described above. Cells were plated in a 96-well flat bottom plate at 5×10 6 cells/ml and restimulated with 0 or 10 μg/ml of CRA. The cell culture supernatant was collected on day 7 and cytokine levels were analyzed by ELISA.

Bone marrow chimeras
Mice were irradiated twice with 4.5 Gy and at least 2 × 10 6 bone marrow cells were transferred i.v. by retro-orbital injection. Mice were used for experiments after 8-10 weeks to allow for bone marrow reconstitution.

Flow Cytometry
To prevent nonspecific binding of antibodies, cells were incubated with anti-CD16/CD32

Air-liquid interface cultures
Primary airway epithelial cells were cultured as previously described 51 . Mice were euthanized by CO 2 and the trachea was collected and digested in 0.2% pronase at 4°C overnight. Mouse tracheal epithelial cells (MTEC) were isolated and cultured on collagencoated transwells. After one week, MTEC cultures were transitioned to air-liquid interface (ALI) by removing medium from the upper compartment and maintained at ALI for 2 weeks. Cells were exposed to 15 μg of CRA for 24 hours and medium from the basal compartment was collected for ELISA.

ELISA
To detect CRA-specific IgE, plates were coated overnight with 6.25 μg/ml CRA in PBS at 4°C. Plates were washed and blocked with 1% BSA in PBS for 2 hours at room temperature. Serum samples were added to the plates and incubated for 2 hours at room temperature.
Plates were washed and biotin-conjugated anti-mouse IgE (R35-118) was added at 1:250 dilution and incubated for 2 hours at room temperature. Plates were washed and incubated with streptavidin-HRP for 30 minutes. 1-Step Ultra TMB-ELISA substrate (Pierce) was added and incubated at room temperature in the dark. The reaction was stopped with 2 N H 2 SO 4 and OD was measured at 450 nm.
To detect cytokines, the following capture and detection antibody pairs were used: The concentration of cytokines in the BAL fluid were detected using these antibodies and the protocol described above.

Supplementary Material
Refer to Web version on PubMed Central for supplementary material.  MyD88 is required for the development of allergic airway inflammation to CRA. C57BL/6 and Myd88−/− mice were sensitized and challenged with CRA as described in the Methods.  epithelial cells were cultured at air-liquid interface. Cells were exposed to 15 μg of CRA or PBS for 24 hours and medium from the basal compartment was collected. GM-CSF levels were measured by ELISA. Data represent mean ± SEM of one experiment and are representative of two independent experiments. Comparisons were made using unpaired Student's t test. * p < 0.05, ** p < 0.01, *** p < 0.001. GM-CSF is required during allergic sensitization, but not challenge, in CRA-driven allergic airway disease. C57BL/6 mice were treated with rIgG2a or a neutralizing anti-GM-CSF antibody, five hours prior to sensitization. Mice were then sensitized and challenged with CRA and airway inflammation was analyzed 72 hours later. (a) BAL differential cell counts were determined by flow cytometry. (b) PAS staining of formalin-fixed and paraffinembedded lung sections. Original magnification, x20. (c) Serum levels of CRA-specific IgE.
(d) RNA was isolated from the lung and mRNA expression of IL-4, IL-5, and IL-13 was determined by qPCR. All expression values are relative to PBS control samples and normalized to the housekeeping gene GAPDH. Data represent combined data of two independent experiments, mean ± SEM (n = 3-10 mice per group). (e) C57BL/6 mice were treated with rIgG2a or a neutralizing anti-GM-CSF antibody, five hours prior to challenge. After allergen challenge, BAL differential cell counts were determined by flow cytometry. Data represent combined data of two independent experiments, mean ± SEM (n = 5-9 mice per group). Comparisons were made using unpaired Student's t test. * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 6.
GM-CSF is required for optimal dendritic cell function and Th2 priming. C57BL/6 mice were treated with rIgG2a or a neutralizing anti-GM-CSF antibody, five hours prior to intranasal administration with A647-CRA. The draining lymph node was collected 24 hours later and cells were analyzed by flow cytometry to assess the uptake of CRA and maturation of DCs. DCs were gated as CD11c+ and MHCII+.