Mast cells: The forgotten players in type 2 inflammation — From nasal cytology to chronic rhinosinusitis with nasal polyps and beyond

Abstract

Mast cells represent key regulators within type 2 inflammation and contribute significantly to the pathophysiology of chronic inflammatory airway diseases. Nasal cytology has enabled their consistent identification, highlighting how their interaction with eosinophils influences inflammatory persistence, tissue remodeling, and disease severity in chronic rhinosinusitis with nasal polyps (CRSwNP). In contrast, conventional hematoxylin–eosin histopathology does not reliably visualize mast cells, potentially limiting comprehensive cellular characterization. Through cytological assessment, distinct inflammatory endotypes have been defined, including nonallergic rhinitis with mast cells and nonallergic rhinitis with eosinophils and mast cells, which may represent stages along a continuum progressing toward CRSwNP. The Clinical–Cytological Grading integrates cellular patterns with clinical parameters, identifying eosinophil–mast cell coinfiltration as a marker associated with increased disease severity and recurrence risk. Evidence from both sinonasal and extra-nasal mucosae, including eosinophilic esophagitis, further supports the relevance of mast cells within the broader network of type 2 inflammation. As therapeutic strategies increasingly target specific inflammatory pathways, accurate cellular endotyping becomes essential for refining risk stratification and optimizing biologic therapy selection. Integrating nasal cytology with histological and molecular approaches offers a pragmatic framework for advancing precision medicine in type 2 inflammatory diseases.

1. Introduction

The understanding of chronic airway inflammation has traditionally centered on eosinophils. However, the development of nasal cytology has demonstrated that this perspective is incomplete. Mast cells, among the most complex and multifunctional immune cells, often remain under-recognized in routine tissue assessment, as conventional hematoxylin–eosin (H&E) staining may underestimate their presence without specific histochemical or immunohistochemical techniques.

Beyond their classical role in immediate hypersensitivity reactions, mast cells are increasingly recognized as key regulators of chronic inflammation and tissue homeostasis. Through the release of cytokines, chemokines, lipid mediators, and proteases, they modulate epithelial integrity, vascular permeability, and immune cell recruitment, thereby influencing both the initiation and persistence of type 2 inflammatory responses [1].

This invisibility is not only technical but also conceptual. While mast cell infiltration in organs such as the skin, heart, brain, or intestine is widely recognized as clinically relevant, their presence in the nasal mucosa has historically received comparatively less attention in both research and therapeutic frameworks. This relative under-recognition has contributed to a long-standing marginalization of the nose within inflammatory disease paradigms [2]. Such reductionism has contributed to an oversimplified paradigm in which type 2 inflammation is frequently equated with eosinophilia alone. In lower airway diseases such as asthma, sputum analyses have demonstrated the existence of distinct inflammatory phenotypes that extend beyond eosinophilia, supporting a broader interpretation of airway inflammation [3].

These observations support the relevance of mast cell assessment across the unified airway, in line with the concept of integrated upper and lower airway inflammation as described in the united airway model [4, 5]. While eosinophils undeniably play a central role, accumulating biological evidence indicates that mast cells actively cooperate within the type 2 inflammatory network, functioning as amplifiers through immunoglobulin E (IgE)-dependent and cytokine-mediated mechanisms [6, 7]. Reconsidering the role of mast cells is therefore not merely a matter of descriptive pathology, but a step toward a more integrated interpretation of chronic airway diseases. By combining cytological observation with clinical evolution and emerging therapeutic insights, it becomes possible to move beyond a purely eosinophilic paradigm and toward a more comprehensive model of type 2 inflammation.

Over the past 2 decades, nasal cytology has restored scientific attention to the nasal mucosa, revealing it as a true immunological laboratory [8]. Through a simple, noninvasive, and reproducible method, it has been possible to identify new nosological entities, particularly nonallergic rhinitis with mast cells (NARMA) and nonallergic rhinitis with eosinophils and mast cells (NARESMA) [9] (Fig. 1A, B). These forms are not merely diagnostic categories but represent links along the inflammatory continuum from rhinitis to chronic rhinosinusitis with nasal polyps (CRSwNP), highlighting the contribution of mast cells to inflammation persistence, tissue remodeling, and disease chronicity [10, 11]

Figure 1.

(A) Nonallergic rhinitis with mast cells and (B) nonallergic rhinitis with eosinophils and mast cells. Nasal cytology showing mast cells (M) and eosinophils (E), stained with May–Grünwald Giemsa. Original magnification: ×1000.

Nonallergic rhinitis with eosinophilia syndrome (NARES) was originally described as a non-IgE-mediated rhinitis characterized by nasal eosinophilia in the absence of systemic allergic sensitization [12]. In this context, the cytological pattern of NARES (Fig. 2), characterized by predominant eosinophilic infiltration, represents an earlier yet potentially progressive stage within the inflammatory spectrum summarized in Table 1. The transition from isolated eosinophilic predominance to mixed eosinophil–mast cell infiltration underscores the dynamic evolution of type 2 inflammation and supports the concept of mast cell involvement as a marker of disease amplification and progression.

Figure 2.

Nonallergic rhinitis with eosinophils. Nasal cytology showing numerous eosinophils (E), partially degranulated. Stained with May–Grünwald Giemsa. Original magnification: ×1000.

Table 1.

Clinical and cytological characteristics of type 2–oriented nonallergic rhinitis phenotypes and their potential evolution toward chronic rhinosinusitis with nasal polyps

Phenotype	Dominant cytological pattern	Clinical severity	Frequent comorbidities	Treatment response profile	Potential evolution
NARES	Eosinophilic predominance	Moderate-to-severe	Asthma, atopy	Generally responsive to intranasal corticosteroids	Possible progression to CRSwNP in selected patients
NARMA	Mast cell predominance	Variable, often persistent	Nonallergic hyperreactivity	Variable response to antihistamines and corticosteroids	Evolution toward CRSwNP remains under investigation
NARESMA	Mixed eosinophil–mast cell infiltration	Severe	Asthma, aspirin sensitivity, allergy	Higher likelihood of systemic therapy and biologic treatment indication	Increased risk of progression to CRSwNP and postoperative recurrence

CRSwNP, chronic rhinosinusitis with nasal polyps; NARES, nonallergic rhinitis with eosinophilia syndrome; NARESMA, nonallergic rhinitis with eosinophils and mast cells; NARMA, nonallergic rhinitis with mast cells.

2. The cytological revolution: seeing inflammation beyond eosinophils

Nasal cytology allows a direct, dynamic, and nontraumatic assessment of the inflammatory microenvironment [13]. Unlike histopathology, which offers a static snapshot, cytology enables longitudinal monitoring and quantitative evaluation of cellular composition.

The technique requires no anesthesia and is performed by gently scraping the intermediate portion of the inferior turbinate [8]. The collected sample is stained with May–Grünwald–Giemsa, which clearly highlights inflammatory cell populations, including eosinophils, neutrophils, lymphocytes, and mast cells, when examined at ×1000 magnification [8]. This simple and reproducible method provides detailed cellular characterization within the nasal mucosa and has enabled the development of Clinical–Cytological Grading (CCG) system, capable of predicting disease severity and relapse in CRSwNP [14]^.

On this basis, the CCG was developed. It integrates cytological findings (eosinophils, neutrophils, mast cells) with clinical data, comorbidities (asthma, allergy, aspirin sensitivity), and recurrence risk. Among all inflammatory profiles, the eosinophil–mast cell pattern has proven particularly informative in predicting disease severity and postoperative recurrence [15], defining an endotype in which the cooperation between eosinophils and mast cells amplifies and sustains type 2 inflammation. Importantly, the CCG does not merely quantify cellular infiltration but contextualizes it within clinical behavior, integrating clinical and cytological perspectives. The identification of eosinophil–mast cell coinfiltration has been associated with higher symptom burden, increased recurrence rates, and greater need for systemic corticosteroids, suggesting that this combined pattern reflects a biologically more aggressive endotype rather than a simple additive inflammatory phenomenon [16].

These findings support the hypothesis that mast cells are not only present within the inflammatory infiltrate but may influence disease trajectory. Their intraepithelial localization (Fig. 3B) suggests a strategic position at the interface between the external environment and the immune system, where they may contribute to barrier dysfunction and sustained type 2 signaling. Beyond their diagnostic value, these cellular forms (NARES, NARMA, and NARESMA) have been hypothesized to represent early stages within an inflammatory continuum that may, in some patients, evolve toward CRSwNP [17].

Figure 3.

Histological findings of recurrent chronic rhinosinusitis with nasal polyps. (A) Hematoxylin–eosin staining showing a dense eosinophilic infiltrate in the lamina propria. (B) The same specimen stained for CD117 reveals numerous mast cells (brown staining) predominantly located within the epithelial layer. Original magnification: ×400.

This hypothesis is supported by their shared cytological pattern, characterized by eosinophil and mast-cell infiltration, and by previous reports suggesting a possible transition from NARES to the so-called Vidal’s syndrome, historically considered a prepolypous condition [18].

While this progression has not yet been definitively demonstrated, the cytological overlap and the persistence of a type 2 inflammatory profile make these entities part of a unified spectrum of mucosal inflammation. The main clinical and cytological distinctions among these phenotypes are summarized in Table 1. From a translational perspective, recognizing these entities as part of a continuum underscores the importance of early phenotypic and endotypic characterization. Identifying eosinophil–mast cell patterns at the rhinitis stage may enable closer clinical monitoring and, in selected cases, earlier therapeutic intervention aimed at preventing progression toward more severe chronic inflammatory phenotypes such as CRSwNP.

3. The histopathological blind spot

As early as 2010, the study documented the cytological heterogeneity of nasal polyposis, delineating not only eosinophilic patterns but also mast cell–predominant and mixed eosinophil–mast cell forms, the latter accounting for more than 30% of analyzed cases [19]. These findings indicate that mast cells in CRSwNP are neither a recent nor anecdotal observation, but rather a well-established component of the disease that has long been underestimated.

In routine clinical practice, histological reports of nasal polyps are often strikingly uniform: they describe an eosinophilic infiltrate with a mild lymphoplasmacytic component, as if the inflammatory cytotype of polyposis were exclusively eosinophilic. Mast cells are rarely mentioned or quantified, despite their biological relevance. The reason is primarily technical: routine H&E staining fails to adequately highlight mast cells, which require specific histochemical or immunohistochemical markers such as toluidine blue or anti-tryptase antibodies or CD117 for reliable detection [20, 21]. Over time, this technical limitation has evolved into a conceptual bias, reducing type 2 inflammation to an exclusively eosinophilic phenomenon and obscuring the cooperative role of mast cells.

Yet numerous studies have demonstrated that eosinophils and mast cells form a functional axis, maintaining inflammation through reciprocal mediators, including interleukin (IL)-5, IL-9, IL-13, tryptase, and major basic protein, that promote edema, remodeling, and recurrence [7, 15, 16]. In addition to this reciprocal activation, mast cells interact closely with epithelial-derived alarmins, including thymic stromal lymphopoietin [22] and IL-33, and with group 2 innate lymphoid cells [23, 24]. These interactions position mast cells within upstream regulatory circuits of type 2 inflammation, suggesting that they contribute not only to amplification but also to the maintenance and spatial organization of the inflammatory microenvironment. Beyond inflammatory amplification, mast cells may also contribute directly to structural remodeling in CRSwNP [7]. Through the release of proteases such as tryptase and chymase, as well as profibrotic and angiogenic mediators, mast cells may influence extracellular matrix turnover and vascular remodeling within polyp tissue [21, 25]. This structural contribution further supports the concept that mast cells are involved not only in inflammatory persistence but also in the chronic architectural transformation characteristic of nasal polyposis. To overcome this blind spot, histopathologists should adopt specific stains (Giemsa, toluidine blue, Alcian blue) and immunohistochemical markers (tryptase, CD117) to accurately identify and quantify mast cells (Fig. 3A, B). Only then will it be possible to validate cytological evidence histologically and achieve a truly integrated morphological understanding of CRSwNP. Clinical evidence from biologic therapies further supports the relevance of the mast cell axis in CRSwNP. Biologics targeting type 2 pathways have significantly improved clinical outcomes; however, their mechanisms of action also provide important pathophysiological insights. For example, dupilumab, by blocking IL-4 and IL-13 signaling, induces substantial clinical improvement even in cases where eosinophilic infiltration is not completely abolished, suggesting that additional cellular contributors may influence disease persistence [26]. Similarly, the dexpramipexole study in CRSwNP, which markedly reduced tissue and blood eosinophil counts without translating into significant clinical improvement [27], provided further evidence that eosinophil depletion alone is insufficient to modify disease trajectory. These findings suggest that additional inflammatory cells, including mast cells, may sustain the pathological process independently of eosinophil burden. Furthermore, omalizumab, through IgE neutralization and downstream modulation of IgE-dependent effector cells, has demonstrated significant clinical efficacy in CRSwNP [28]. While these observations do not represent direct mechanistic proof, they reinforce the concept that type 2 inflammation cannot be fully explained by eosinophils alone.

This limitation is not confined to the sinonasal mucosa. A similar bias is evident in other mucosal diseases, such as eosinophilic esophagitis (EoE), where conventional H&E staining highlights eosinophils but does not adequately identify mast cells. When CD117 or other specific markers are applied, eosinophil–mast-cell coinfiltration becomes evident [29] (Fig. 4A, B). As in CRSwNP, this pattern likely characterizes more severe, relapsing, and therapy-resistant phenotypes. In this context, mast cell infiltration and transforming growth factor β1 expression have been implicated in esophageal tissue remodeling and smooth muscle dysfunction [29]. Moreover, the clinical efficacy of IL-4/IL-13 pathway blockade with dupilumab [30] suggests that type 2 inflammation in EoE, as in CRSwNP, reflects a coordinated cellular network rather than a purely eosinophil-driven process. Collectively, these data further support the need for integrated cellular assessment rather than reliance on eosinophil quantification alone.

Figure 4.

Histological findings of eosinophilic esophagitis. (A) Hematoxylin–eosin staining showing numerous eosinophils (E) infiltrating the esophageal epithelium. (B) The same specimen stained for tryptase shows numerous mast cells (brown staining) distributed within the epithelial tissue. Original magnification: ×400.

Therefore, an integrated and translational approach combining cytology, histology, and molecular biology is warranted. The combined evaluation of eosinophils and mast cells should be considered a key diagnostic and prognostic parameter across type 2 inflammatory diseases. In this context, nasal cytology may serve as a practical and accessible model for understanding the immunopathology of mucosal inflammation throughout the body [31].

4. Toward an integrated morphological approach

The systematic inclusion of mast cell evaluation in both cytological and histological protocols represents a decisive step toward an integrated morphological medicine. Only by combining the cytological perspective, which captures dynamic cellular activity, with the histological perspective, which documents tissue architecture, can we fully understand the complexity of type 2 inflammation [15]. Such integration is particularly relevant within the unified airway concept, where upper and lower airway inflammation share common cellular and molecular pathways [32]. Evaluating mast cell involvement across different mucosal districts may help clarify disease extension and systemic inflammatory burden, reinforcing the translational value of nasal cytology as a window into broader airway pathology.

In this framework, the CCG assumes a pivotal role. Its extension to tissue samples could lead to a parallel histological grading system, correlating microscopic findings with clinical behavior. Such integration would promote shared diagnostic language among cytologists, pathologists, and clinicians, restoring to the cell its biological meaning within the functional context of the organ and the individual [33]. In parallel, the progressive shift toward noninvasive monitoring strategies in airway diseases opens new perspectives. Biomarkers such as fractional exhaled nitric oxide (FeNO) are widely used to reflect type 2 inflammatory activity [34], while emerging approaches based on nasal breath analysis and volatile organic compound profiling are under active investigation in the framework of treatable traits and precision medicine [35]. Although these techniques require further validation and standardization [36], their integration with cytological evaluation may enhance disease monitoring and therapeutic stratification.

The development of biologic therapies targeting upstream epithelial cytokines [37] further underscores the importance of accurate cellular phenotyping. As therapeutic strategies move beyond downstream effector pathways, precise morphological characterization of the inflammatory microenvironment becomes essential for interpreting clinical responses and refining personalized treatment approaches [38].

From this perspective, mast cells should not be regarded as a minor component of the inflammatory infiltrate, but rather as central regulators within the type 2 inflammatory network.

5. Future directions: toward noninvasive monitoring and integrated endotyping

The field of airway inflammation is progressively moving toward minimally invasive strategies able to monitor disease activity over time and across different mucosal districts. Within the unified airway framework, which conceptualizes upper and lower airway diseases as interrelated manifestations of a single inflammatory continuum [32], the assessment of mast cell–related inflammation should not be confined to tissue specimens but complemented by surrogate markers reflecting ongoing type 2 activity in both upper and lower airways. In lower airway diseases, induced sputum–based cellular analyses have contributed to the characterization of inflammatory phenotypes [39], and although sputum evaluation has been less consistently applied to mast cells, these approaches reinforce the concept that cellular profiling beyond eosinophil quantification may refine risk stratification and therapeutic decisions. Translating such minimally invasive phenotyping strategies to upper airway disease remains challenging but represents a promising avenue for advancing personalized management across type 2 inflammatory disorders.

At the same time, noninvasive biomarkers such as FeNO are increasingly incorporated into clinical pathways to estimate type 2 inflammatory activity and treatment response [34]. Although FeNO does not specifically quantify mast cell activation, its integration with cytological endotyping may improve longitudinal monitoring and facilitate interpretation of fluctuations in disease control.

Looking forward, emerging technologies based on nasal breath analysis and volatile organic compound profiling are under active investigation [35]. If future studies demonstrate robust and reproducible associations with defined cellular endotypes, including mast cell–rich patterns, these tools could support broader implementation of endotyping strategies worldwide, particularly in settings where cytology is not yet routinely available.

From a clinical standpoint, refining mast cell–based endotyping may have implications for therapeutic stratification. As biologic therapies expand and treatment algorithms become increasingly phenotype-driven [40], identifying patients with prominent eosinophil–mast cell coinfiltration could help anticipate differential response patterns and long-term recurrence risk. In this context, cytological evaluation may serve not only diagnostic purposes but also decision-support functions within precision medicine frameworks.

Another relevant perspective concerns the integration of cytological endotyping within clinical decision algorithms. As treatment strategies in CRSwNP and other type 2 diseases become increasingly structured around disease severity, comorbidities, and biomarker profiles, incorporating mast cell–based patterns into stratification models may further refine risk prediction. Patients displaying persistent eosinophil–mast cell coinfiltration could represent a subgroup with higher relapse potential and distinct therapeutic needs, particularly in the context of biologic therapy selection and postoperative management.

Moreover, the longitudinal reproducibility of nasal cytology offers a unique opportunity to monitor cellular shifts over time, providing a dynamic complement to static histological evaluation and systemic biomarkers. Such an approach may help distinguish transient inflammatory fluctuations from stable endotypic traits, thereby improving both short-term management and long-term outcome prediction.

Finally, the expanding landscape of biologic therapies, including agents targeting upstream epithelial cytokines, further emphasizes the need for precise morphological and cellular characterization. In this context, nasal cytology may continue to serve as a pragmatic reference method, while emerging noninvasive approaches evolve as complementary tools supporting precision medicine across type 2 inflammatory diseases.

6. Conclusions

Mast cells should no longer be regarded as secondary participants in type 2 inflammation but as active regulators of the mucosal microenvironment. Their consistent identification through nasal cytology contributes to a more comprehensive and dynamic understanding of chronic inflammatory airway diseases.

The traditional emphasis on eosinophils has provided important insights into type 2 inflammation; however, growing evidence indicates that eosinophils and mast cells act in concert, shaping disease persistence, tissue remodeling, and recurrence. A balanced interpretation of airway inflammation, therefore, requires consideration of both cellular components.

The integration of nasal cytology with advanced histological and molecular approaches offers a more complete characterization of inflammatory endotypes. In this context, CRSwNP may serve as a valuable pathophysiological model for exploring mechanisms of mucosal immunity across different organ systems. Recognizing eosinophil–mast cell endotypes in CRSwNP, as well as in EoE and other type 2 disorders, may support more refined risk stratification and therapeutic selection.

As biologic therapies increasingly target specific inflammatory pathways, accurate cellular phenotyping becomes essential not only for understanding disease mechanisms but also for guiding treatment decisions and monitoring response over time. The integration of cytological assessment with emerging biomarkers represents a pragmatic and scalable strategy for advancing precision medicine in airway inflammation.

Ultimately, renewed attention to mast cell biology broadens the interpretative framework of type 2 inflammation and reinforces the role of the upper airway as a key site for translational immunological research.

Acknowledgments

Not applicable.

Conflicts of interest

The authors have no financial conflicts of interest.

Author contributions

All authors contributed to the conception of the manuscript, participated in the drafting and critical revision of the text, and approved the final version of the manuscript.