Chronic Rhinosinusitis (CRS) is characterized by inflammation of the sinonasal mucosa lasting longer than 12 weeks, leading to negative impact in the patient's quality of life.1 CRS poses a significant burden to both the patient and society due to its high prevalence, vast, and costly symptoms, high indirect costs, and not entirely effective treatments.1

The mechanisms that lead to Eosinophilic CRSwNP (ECRS) are not fully established in the literature. There is a clear need for further studies to understand ECRS physiopathology and to identify any perspective of new treatments.2 CRSwNP may be presented with either predominant type 1, type 2, or type 3 immune responses, and several times the inflammation may be mixed.3–5

Type 2 CRSwNP tend to be much more resistant to current therapies, exhibiting higher rates of recurrence than the other endotypes.6 It is characterized by overexpression of the cytokines IL-4, IL-5, and IL-13 and activation and recruitment of eosinophils and mast cells.1 Moreover, the amount of eosinophilic infiltration and the intensity of the inflammatory response are reported to be closely related to the prognosis and severity of the disease.7

The complexity of this pathology makes the clinical and experimental study models very problematic. Therefore, it is reasonable to assess ECRS in such a model that embraces most of the related events in a cost-effective way. The murine model has been helpful to study ECRS, because it allows the evaluation of the pattern of inflammation, epithelial remodeling, and collagen deposition.8 The availability of transgenic animals is useful for genetic and pathogenic studies. Finally, the murine model is the in vivo model dealing with the smallest animal, being an advantage in both ethical and financial perspectives.

However, there are limitations when transposing animal models findings to humans. The size of mice makes it challenging for studies involving surgical models or drugs involving implants and/or stents. Also, mouse nasal polyps are smaller in size and number, whereas they often occupy a considerable percentage of the nasal cavity9 in humans.8,9

Over the years, several murine models for CRSwNP have been developed. In 2011, the first murine model was described in BALB/c animals, with Intranasal (IN) Staphylococcus aureus Enterotoxin B (SEB),10 and Intraperitoneal (IP) and IN Ovalbumin (OVA).10 Since then, several models have been proposed, in both C57BL and BALB/c animals.9–35 However, there is no consensus in the literature on which murine model induces a more robust eosinophilic response and a higher production of polyps.

In this review, we pool data and discuss murine models that induced ECRS presented in the literature. We also compare these studies regarding the tissue eosinophilia and the number of polypoid lesions present in the mucosa of the sinuses, which are fundamental to the evaluation of these studies.

From the selected articles, 11 induced ECRS with SEB10,14,37–45; 1 with AP24; 1 compared two models, the first induced with HDM and the latter with SEB29; and 2 induced ECRS with SEB in transgenic animals.11,13 Additionally, in 5 articles, the models were used to assess different therapies to inhibit polyp formation (Resveratrol alone37 or associated with Mucoadhesive Nanostructured Microparticles,14 Cyclosporine,40 Chloroquine,43 and Tofacitinib.44

BALB/c mice were used in 13 experimental models (total number: 159 animals), whereas B57BL/6 were chosen in 8 (total number: 61 animals). All the animals were 4‒5-week-old at the beginning of the experimental assays.

Animal models are especially important to study the physiopathology of a specific disease (in this case, ECRS), and to evaluate the effect of possible new therapies. In this aspect, murine models of ECRS have advantages and disadvantages, like any other animal models. The major advantages of murine models are: 1) They are cheap; 2) They are easy to handle; and 3) Many reagents and antibodies are easily available. Transgenic or knockout mice, important for studies of the pathophysiological mechanisms of the disease, are also available.11,13

In our review, the murine model most used involved BALB/c animals and the combination of IP and IN OVA with IN SEB.10,37–41,43–46 The OVA concentration remained constant in all experimental groups: IP: 25 µg and IN: 3%. The ideal SEB concentration to induce eosinophilic polyps was 10 ng. The reduction of SEB concentration to 5 ng considerably decreased polyp induction,10 whereas the increase to 500 ng did not change the number of eosinophilic polyps per field.10

The duration of the models that used BALB/c animals and the combination of IP and IN OVA with IN SEB ranged from 103 to 186 days.10,37–41,43–46 The duration of 186 days produced more robust eosinophilic polyps.41 It is important to note that the average lifespan of BALB/c mice ranges from 180 days to 365 days, and that mice would end the long-lasting protocol with around 214 days of age. With this, we conclude that the duration of the protocol of 103 days brings satisfactory results with lower chance of animal loss due to natural death.10,37–41,43–46

Using the same protocol with IP and IN OVA and IN SEB, but with C57BL/6 mice, an increase of either the dose of SEB13 and/or OVA11,29 was necessary to achieve a significant number of eosinophilic polyps in sinonasal mucosa. One possible reason for this is that C57BL/6 mice have attenuated allergic airway hyperresponsiveness compared to BALB/c mice, which has been demonstrated for asthma models.8 To achieve the same polyp and eosinophil index, the SEB dose had to be adjusted to 20 ng13 or the OVA concentration to 6%.11,29 The increase of SEB seemed to be more efficient inducing ECRS in C57BL/6 mice than the increase in OVA.

Another model with C57BL/6 mice used ID and IN HDM either alone or combined with IN SEB.29 HDM alone did not induce polyps but stimulated eosinophilia, whereas the combination of HDM with SEB induced polyps.29 In summary, it seemed that the use of HDM alone can serve as a model for the study of allergic rhinitis, but not for nasal polyps. Moreover, the combination of HDM and SEB was not as efficient as OVA + SEB protocol to induce ECRS.10,37–41,43–46

The combination of IN AP with IP and IN OVA was enough to produce polyps and eosinophilia,24 but the assays that lasted 53 and 67 days failed to produce robust nasal polyps. As the duration of the experiment increased, more polyps were observed. Possibly, if authors used the time of 103 days, they might find the same number of polyps as the combination of OVA and SEB.

In summary, IP and IN OVA associated with IN SEB is the most used protocol in both BALB/c and C57BL/6 mice to induce ECRS, as it also seems to be the most efficient to produce both polyps and eosinophils at nasal tissue. The IP and IN OVA, associated with IN AP model was also a good model to induce ECRS in C57BL/6 mice. In contrast, the combination of ID and IN HDM and IN SEB was the least efficient model to produce eosinophilic nasal polyps in mice.

IP and IN OVA associated with SEB seems to produce the most robust eosinophilic sinonasal inflammation, especially in BALB/c mice.

The present study was supported by FAPESP (process number: 2019/05843-2), and in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ‒ Brasil (CAPES) ‒ Finance Code 001.

The authors declare no conflicts of interest.