In this issue of JEM, Schuijs et al. (https://doi.org/10.1084/jem.20240103) highlight a novel role for basophils during allergic immune responses to house dust mites (HDM). They reveal that interleukin-33 (IL-33)-activated basophils facilitate the recruitment and extravasation of Th2 cells into the lungs during a specific time frame via their interactions with pulmonary endothelial cells.

Asthma is one of the most common lung diseases, and type 2 immune inflammation is a hallmark feature (Hammad and Lambrecht, 2021; Harker and Lloyd, 2023). In this study, Schuijs and colleagues use a variety of genetic tools to dissect the role of basophils in mediating allergic lung inflammation (Schuijs et al., 2024). Their work reveals the particular role basophils play in coordinating immune responses following inhalation of allergen. Although basophils were not critical for the inception of allergen responses, they played a vital role in controlling T cell recruitment into the lung. Multiple studies have highlighted the origins and mechanisms of allergic immune responses, leading to the development of a new generation of agents that target type 2 immune responses (Saglani et al., 2023). However, a significant proportion of patients failed to respond to these therapies, highlighting the urgent need to better understand the dynamics of the cellular players and their molecular controllers in the lungs during asthma.

Régis Joulia and Clare M. Lloyd.

In the first part of the study, the authors used basophil reporter (Mcpt8-CreYFP) mice employing the well-known repeated house dust mite (HDM) challenge model (Hammad et al., 2010) to analyze the dynamics of basophil recruitment. By injecting fluorescently labeled anti-CD45 intravenously, they found basophils in pulmonary circulation with IgE present on their surface. This set of experiments was crucial in demonstrating that basophils are recruited to the lungs following HDM exposure but remain in circulation at the endothelial surface rather than migrating into parenchymal tissue regions. These data reinforce the notion that cell location influences their function and therefore pathological outcomes.

Basophils control immune cell recruitment in the lungs during allergic airway disease by activating endothelial cells. Using various genetic approaches, Schuijs et al. showed that IL-33 release leads to basophil activation and subsequent optimal environment to recruit other leukocytes to the site of inflammation.

To determine the overall contribution of basophils for HDM-induced inflammation, a mouse with global basophil depletion was employed (Ohnmacht et al., 2010). A marked reduction in most of the canonical physiological features of asthma was observed, with a decline in eosinophils, lymphocytes, cytokines, and chemokines, coupled with less mucus production and improved lung function. Allergic inflammation can be split into distinct stages with an initial sensitization phase followed by an allergen challenge period, and the authors went on to use a conditional depletion model to assess the temporal contribution of basophils to these phases. While basophils were redundant for the initial phase of Th2 priming and disease inception, they were required during the allergen challenge phase. Early deletion of basophils did not impact antigen presentation to T cells, aligning with previous work from this group placing dendritic cells (DCs) as primary antigen-presenting cells following inhaled HDM (Hammad et al., 2010). Other studies have shown that basophils express MHC class II and costimulatory molecules and are capable of antigen presentation (Sokol et al., 2009). These differences are likely explained by variations in experimental systems, particularly the choice of allergen, but important questions remain, particularly with respect to the human scenario where basophils are more abundant than in laboratory mice, and people tend to be sensitized to multiple complex allergens.

The value of the conditional depletion model was evident in the subsequent experiments, whereby basophil depletion during the challenge period led to a significant reduction in type 2 inflammatory parameters, including IgE. This was associated with a decrease in antigen uptake by different DC populations, as analyzed using fluorescent HDM. Although not addressed within the current study, the mechanism by which basophils might influence antigen capture or DC migration remains an interesting question.

The authors then explored the role of the high-affinity IgE receptor in this model. Surprisingly, neither global depletion nor basophil-specific depletion of FcεRIα affected immune cell recruitment. This is perhaps unexpected given the well-established importance of this pathway in driving allergic inflammation in the skin (Cheng et al., 2015). Nevertheless, it is possible that other mechanisms, such as FcγR, compensate for the absence of this receptor. An alternative mechanism for basophil activation was found to be via the alarmin IL-33 and its receptor, ST2, on the basophil surface. Blocking IL-33 signaling using soluble ST2 recapitulated the phenotype observed in basophil-deficient mice. Additionally, mice reconstituted with basophils deficient in ST2 or interleukin 4 (IL-4) showed a reduction in type 2 cytokines following HDM exposure. Uncontrolled basophil activation elicited by basophil-specific deletion of the ubiquitin-modifying molecule Tnfaip3 exacerbated inflammation in the lungs following HDM exposure. This was dependent on IL-33 signaling as soluble ST2 reduced this hyperinflammatory profile. These findings indicate that basophil intrinsic IL-33/ST2 signaling stimulates IL-4 production, thus facilitating Th2 cell recruitment and extravasation, and confirms the overall importance of this alarmin in the regulation of pulmonary immunity (Cayrol and Girard, 2022).

Given that basophils remain closely associated with the vasculature, the authors questioned whether the phenotype of endothelial cells was altered by activated basophils following HDM exposure. Indeed, endothelial cells from basophil-deficient mice expressed less VCAM-1 and ICAM-1, suggesting that basophils contribute to endothelial cell activation and create local inflammatory hotspots for lymphocyte migration. Although this idea is interesting and other cells such as mast cells can create “hot spots” for leukocyte recruitment (Joulia et al., 2022), additional work is required to analyze this. For example, are basophils recruited to the vasculature adjacent to the airway, or are they present in the capillaries? The limited number of basophils in mice restricts detailed spatial analyses, but more detailed phenotyping of endothelial cells in this model could be valuable. It may be that there are subpopulations of endothelial cells, in distinct locations, that influence inflammation in different ways. It is also intriguing to speculate how basophils remaining in the vasculature communicate with DCs inside the tissue to influence antigen capture. Lungs are highly vascularized, so even if basophils remain in the pulmonary vasculature, they will likely have a high influence on local pulmonary inflammation.

The role of basophils in most immune responses has been enigmatic, largely due to the lack of genetic tools specifically targeting these cells. New models, such as these cell-specific Mcpt8-Cre mice, enable better characterization of basophils and their contribution to immune-related pathologies. Overall, this study reveals the important role of basophils in regulating lung immune responses in a temporally and spatially controlled manner. Going forward, it will be important to identify the source of IL-33 responsible for basophil activation. In mice, IL-33 is primarily expressed by endothelial cells from the venular compartment and adventitial fibroblasts (Dahlgren and Molofsky, 2019; Saglani et al., 2013). It is not clear whether basophils are recruited to these adventitial regions following lung activation and are retained locally in these blood vessels. We and others have shown that there are differences in immunity across the life course, which influences asthma pathogenesis (Joulia et al., 2024; Saglani et al., 2018). It would be fascinating to include a fourth dimension in the study and explore the role of basophils in early life, where IL-33 is crucial in mediating pathology.

In summary, this study highlights the intricate relationship between lung basophils and their microenvironment. It confirms the essential role of alarmins including IL-33 in regulating these cellular interactions and sheds new light on the underappreciated basophils as central regulators of pulmonary immune responses.

This work was supported by funds from the Wellcome Trust (220254/Z/20/Z to C.M. Lloyd; 227236/Z/23/Z to R. Joulia) and ACTERIA EFIS Allergology 2023 to R. Joulia. The figure was created with Biorender.

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