Allergic rhinitis (AR) and bronchial asthma (BA) are inflammatory diseases of the upper and lower respiratory systems, respectively, and represent a significant burden on healthcare systems worldwide (Cardell et al. 2016; Nurmagambetov et al. 2018; Avdeeva et al. 2020; Roland et al. 2021; Håkansson et al. 2023). Estimates of the prevalence of AR vary from 5 to 50%, depending on the methodology and population studied (Oliveira et al. 2020; Alqahtani 2020). Epidemiological studies with a more restrictive definition of AR, incorporating IgE plasma levels to confirm the presence of atopic immune responses, report prevalences of 10–12% (Tschopp et al. 1998; Salo et al. 2011). The reported prevalence of BA ranges from 5.4 to 17.9% (Song et al. 2022).

Available data indicate that the pathogeneses of AR and BA are linked. In 1997, J. Grossman published the “one airway, one disease” theory, which states that asthma and allergic rhinitis are “described better as a continuum of inflammation involving one common airway” than as distinct and separate entities (Grossman 1997). Data from the SAPALDIA cohort study published the following year show that 60% of patients with BA also suffer from AR (Tschopp et al. 1998). In 1999, data from the ECRHS showed that patients with BA are likely to be co-diagnosed with AR (Leynaert et al. 1999), and updated data from the same study confirmed AR as a risk factor for BA (Shaaban et al. 2008). More recent studies show a similar connection (Jacob et al. 2016; Acevedo-Prado et al. 2022).

Additional evidence for the one airway theory of AR and BA lies in the morphology and cytology of the respiratory epithelium, a key tissue in the inflammatory processes of the upper and lower respiratory tracts. The major component of the respiratory epithelium throughout the airways is the ciliated columnar epithelial cells, which make up 50–80% of the tissue by cell count. Additional major cell types include goblet cells, which produce mucus (Jaramillo et al. 2018); basal cells, which are the major population of stem cells in this tissue (Cumplido-Laso et al. 2023); and chemosensory cells, which serve as initiators of physiological responses to external stimuli by interfacing with the immune and nervous systems (Hollenhorst and Krasteva-Christ 2023).

Evidence for the interplay between AR and BA has also been found in the clinical setting. A 3-year, randomized, open-label trial of immunotherapy in 205 children with AR showed that those receiving immunotherapy were less likely than the control group to develop BA (Möller et al. 2002). Follow-up studies showed that the protective effect was maintained for up to 10 years after therapy (Niggemann et al. 2006; Jacobsen et al. 2007).

Omalizumab is an anti-IgE antibody that received marketing authorization for use in asthma in 2005. Results from the pivotal clinical trial showed a statistically significant reduction in clinically relevant symptoms after 28 weeks of treatment in patients with severe persistent asthma (Humbert et al. 2005). Subsequent meta-analyses show that the use of omalizumab is also associated with a meaningful improvement in AR symptoms, overall quality of life, and a reduction in medication use for AR (Tsabouri et al. 2014; Yu et al. 2020).

Dupilumab is an anti-interleukin-4 (IL-4) receptor alpha (IL-4Rα) antibody, which reduces Th2-type inflammation by decreasing the availability of the receptor for proinflammatory cytokines IL-4 and IL-13. In a randomized, double-blind, placebo-controlled study, dupilumab was shown to reduce corticosteroid use by 70% compared to a 42% reduction in the placebo group. Patients treated with dupilumab experienced 59% fewer asthma exacerbations compared to those receiving placebo (Rabe et al. 2018). Two post hoc analyses of the clinical data for dupilumab showed that patients with AR receiving dupilumab experienced a statistically significant reduction in symptoms (Wenzel et al. 2016; Busse et al. 2020).

Innate and adaptive immune responses are broadly classified into three types, which are induced and characterized by the presence of specific cytokines (Fig. 1). Each type of immune response is tailored to defend against different types of pathogens, but when dysregulated, it may give rise to inflammatory, autoimmune, or atopic diseases.

In the case of asthma, it is known that dysregulated immune responses of different types can result in very similar symptoms. Endotypes of asthma include type 2-high asthma, which is characterized by high blood and sputum eosinophil counts (Ogulur et al. 2021). Type 2-low asthma is characterized by the absence of type 2 biomarkers and is often associated with the presence of sputum neutrophils (Sze et al. 2020). Efforts to understand the pathways behind type 2-low asthma are important, as this endotype represents 30–50% of patients with refractory asthma, and there are few specific therapy options available (Hinkset al. 2021).

Mutations affecting the IL17A gene have been found to be associated with both asthma and AR (Wang et al. 2012; Du et al. 2016; Lee and Song 2023). In addition, IL17A protein levels have been found to be consistently elevated in patients with asthma compared to healthy volunteers (Wong et al. 2001; Chien et al. 2013; Tao et al. 2015; Chen et al. 2016; Lv et al. 2016; Hatta et al. 2017), while IL17A is also elevated in the plasma of patients with acute or refractory asthma compared to patients with mild asthma (Chien et al. 2013; Tao et al. 2015). The same correlations hold for IL17A in AR patients compared to healthy volunteers and when comparing relatively more severe AR with milder AR (Ciprandi et al. 2008; Tang et al. 2014; Degirmenci et al. 2018; Erkan et al. 2020).

IL17A is expressed in several immune cell types upon activation, including innate lymphoid cells (ILC), natural killer (NK) cells, CD8⁺ T cells, γδ T cells, and, importantly, Th17 cells. It performs its intercellular signaling function as a homodimer or as a heterodimer with IL17F (Liu et al. 2013a; Goepfert et al. 2017). Receptors for the IL17A homo- or heterodimer are found in diverse tissues and organs; however, the best-characterized response to IL17A signaling occurs in epithelial cells, endothelial cells, and fibroblasts (Gaffen 2009; Nirula et al. 2016), where IL17A signaling precipitates expression of chemokines CXCL1 and CXCL8, cytokines IL1β, IL6, and TNF, and metalloproteases with direct antimicrobial functions (Rex et al. 2023). The chemokines and cytokines induced by IL17A attract and activate macrophages and neutrophils, which affect the removal of harmful pathogens in a protective immune response and contribute to the pathology of autoimmune or atopic diseases when dysregulated (Mills 2023).

Genome-wide association studies (GWAS) have identified 128 mutations associated with asthma, including many that affect IL33, its receptor IL1RL1, or its downstream effectors (El-Husseini et al. 2020). IL33 is constitutively expressed in many tissues throughout the body, including the respiratory system (Moussion et al. 2008; Fagerberg et al. 2014). IL33 is released into the intercellular space in response to tissue or cell damage, where it induces an immune response, the specifics of which depend on the types of immune cells present and the cytokine milieu (Funakoshi-Tago et al. 2008; Salmond et al. 2012). IL33 is subject to several post-translational regulatory mechanisms, including nuclear localization during homeostasis (Carriere et al. 2007); activating proteolysis in the intercellular space (Lefrançais et al. 2014; Scott et al. 2018); deactivating proteolysis during apoptosis (Lingel et al. 2009; Liu et al. 2013b); deactivating oxidation in the intercellular space (Lingel et al. 2009; Liu et al. 2013b); and sequestration by circulating dummy receptors (Hayakawa et al. 2007). These mechanisms suggest IL33’s key role as a highly localized but generalized activating signal to the immune system in cases of tissue damage.

A series of experiments published in 2014 (Mizutani et al. 2014) suggests a mechanism for the interaction between IL33 and IL17A in BA. Both anti-IL33 and anti-IL17A antibodies decrease neutrophilic inflammation in mice; however, the administration of anti-IL33 antibodies does not affect IL17A levels. Further, mice begin to express elevated levels of IL17A after repeated intrabronchial exposure to IL33. After sensitization by intrabronchial IL33 administration, the combined administration of IL33 and IL17A results in higher levels of neutrophilic inflammation than the administration of IL33 or IL17A alone. These results suggest an interaction in which repeated stresses expose the respiratory microenvironment to IL33, ultimately leading to activation of an IL17A-driven immune response that exacerbates inflammation triggered by future IL33-mediated responses to external stimuli. Subsequent publications appear to support this proposed interaction (Vocca et al. 2015; Cheng et al. 2022).

This hypothesis contends that, since AR and BA are manifestations of the same inflammatory process, investigation of the expression levels of IL17A and IL33 in patients with AR and BA may provide valuable insights into their interactions and help assess their value as biomarkers for different endotypes of the disease.

Five subjects were enrolled in each of the following three groups: five patients with AR, five patients with both AR and BA, and five healthy controls. The mean age of patients with AR was 29.6 ± 6.7 years, the mean age of patients with AR+BA was 36.8 ± 11.6 years, and the mean age of healthy controls was 30.6 ± 9.76 years. Each group included three males and two females. Inclusion criteria included age ≥ 18 years, moderate or severe seasonal or perennial AR, and mild, newly diagnosed BA with long-standing AR. Subjects were selected from the existing database of an ongoing national study conducted at the Allergology Clinic of the “Aleksandrovska” University Hospital in Sofia, where they provided written informed consent. The study was conducted in accordance with the Declaration of Helsinki and approved by the local Ethics Committee. Subjects were selected using a stratified random sampling approach to ensure that the sample was representative of the overall study population.

Serum and nasal lavage fluid (NLF) samples were collected as part of the national study. Serum samples were obtained from venous blood, allowed to settle for 20 minutes, and then centrifuged at 1,200 rpm for 15 minutes. The samples were subsequently frozen at –70 °C. NLF was obtained by instilling 10 mL of sterile saline solution into each nostril. After 10 seconds in a reclined position, subjects were asked to blow their nose into sample containers. Samples were centrifuged at 1,200 rpm for 15 minutes and frozen at –20 °C. Both serum and NLF samples were collected outside the pollen season and in the absence of active symptoms for each subject.

Protein levels of IL17A and IL33 measured by ELISA are presented in Tables 1, 2 below.

Comparison of protein levels at different anatomical sites within patient groups revealed no significant difference in protein levels between serum and NLF in healthy volunteers (see Tables 1, 2). In contrast, patients with AR+BA showed significantly higher IL17A levels in serum than in NLF (p = 0.003) and higher IL33 levels in NLF than in serum (p = 0.005). Patients with AR only did not show statistically significant differences in IL17A (p = 0.051) or IL33 levels (p = 0.742) between serum and NLF.

Comparison of protein levels between patient groups showed significantly increased IL17A in NLF compared to healthy volunteers in both AR patients (p < 0.001) and AR+BA patients (p < 0.001), and significantly decreased IL33 in NLF compared to healthy volunteers for both AR patients (p = 0.042) and AR+BA patients (p = 0.033). Serum protein levels compared with those of healthy volunteers showed a significant elevation of IL-17A in AR+BA patients only (p = 0.002). A comparison of protein levels between AR-only and AR+BA patients revealed no significant differences.

Figs 2–5 show graphical representations of these and additional selected comparisons.

RT-PCR of total RNA from serum and nasal lavage fluid (NLF) of patients with allergic rhinitis (AR) or allergic rhinitis with bronchial asthma (AR+BA) showed a 2–2.5-fold increase in the expression of IL17A in both patient groups and both sample types, compared to a 1–1.5-fold increase in healthy volunteers (p < 0.01 for AR serum; p < 0.01 for ARNLF; p < 0.001 for AR+BA serum; p < 0.001 for NLF) (see Fig. 6).

RT-PCR of total RNA from serum and NLF of patients with AR or AR+BA showed a 2.2–2.8-fold increase in the expression of IL33 in both patient groups and both sample types, compared to an approximately 1.5-fold increase in healthy volunteers (p < 0.01 for AR serum; p < 0.01 for ARNLF; p < 0.001 for AR+BA serum; p < 0.01 for NLF) (see Fig. 7).