Mechanisms of Allergen Sensing and the Development of Allergic Sensitization

National Institute Of Allergy And Infectious Diseases

Investigators

Abstract

During FY2025, we have advanced our understanding of how the immune system detects allergens and initiates type 2 inflammation, challenging long-standing paradigms of innate immune recognition. While classical innate immunity relies on the detection of conserved microbial patterns, our findings support an alternative model in which the immune system senses allergens through their capacity to disrupt tissue homeostasis, particularly via protease activity. Specifically, we demonstrated that cysteine protease activity within house dust mite (HDM) extracts, linked to the major allergen Der p 1, is a dominant trigger for innate immune activation and subsequent Th2-driven allergic inflammation. This supports the broader notion that the immune system has evolved to recognize protease activity as a signal of potential tissue damage or stress, which, while protective in some contexts, can be maladaptive in the setting of environmental allergens. A central achievement of this year’s work was the identification of a previously unrecognized population of lung macrophages that plays a critical role in this protease-sensing mechanism. We characterized a unique subset of perivascular Ly6G⁺ macrophages (Ly6G⁺ MΦ) residing in the alveolar interstitium, positioned between alveoli and capillaries, where they are optimally located to intercept inhaled allergens. These cells originate from GMP-derived, CCR2⁺ monocytes via a non-classical pathway and are transcriptionally defined by high expression of the nuclear receptor Nr4a1 (Nur77). We found that Ly6G⁺ MΦ detect allergen-derived cysteine protease activity through the PAR2 receptor, triggering their activation and proliferation. While previous studies implicated PAR2 in allergic responses, our data clarify the cellular mechanism by identifying Ly6G⁺ MΦ as the key initiators of PAR2-mediated allergen sensing in the lung. Functionally, Ly6G⁺ MΦ are distinct from classical alveolar and interstitial macrophages and exhibit transcriptomic signatures indicative of cell adhesion, migration, tissue remodeling, and IL-13/IL-4-driven activation. Their role extends beyond initial sensing; they also orchestrate the downstream migration of migratory dendritic cells (mDCs) to mediastinal lymph nodes (mLNs), a critical step for priming allergen-specific Th2 responses. Intriguingly, while inflammatory stimuli such as LPS or viral infection strongly upregulate CCR7 expression on mDCs, allergen exposure leads to CCR7 downregulation, a paradox given the efficient mDC migration observed in allergic settings. We resolved this paradox by identifying cysteinyl leukotrienes (cysLTs) produced by Ly6G⁺ MΦ as a necessary signal that sustains CCR7-dependent mDC migration despite low CCR7 expression. This mechanism appears specific to allergen-driven responses and is not required in the context of strong inflammatory cues. Importantly, we demonstrated that pharmacological inhibition of cysLT production selectively impairs mDC migration and Th2 priming following allergen exposure without affecting immune responses to pathogens. These findings have direct clinical relevance, as CysLT1R antagonists such as montelukast and zafirlukast are commonly used to treat asthma, although their mechanisms of action have remained incompletely defined. Our work suggests that these drugs may act in part by interfering with early macrophage-derived cysLT signals that promote allergen-specific immune activation. However, since CysLT2R and CysLT3R may compensate when CysLT1R is blocked, our data support the potential utility of pan-cysLT synthesis inhibitors as more comprehensive therapeutic agents to suppress allergic inflammation. Together, our findings from FY2025 provide new mechanistic insight into how allergens are sensed and how type 2 inflammation is initiated and sustained. By uncovering a key role for Ly6G⁺ macrophages and the cysLT pathway in orchestrating allergic responses, this work opens new avenues for targeting early innate events in asthma and other allergic diseases. A manuscript compiling all these findings is currently under revision at Nature Communications, where a revised version is being prepared for the editors and reviewers in response to their comments. In addition, during FY2025 we further examined the mechanisms governing dendritic cell migration in type 2 inflammation through a comprehensive review article that synthesizes current literature to better understand how this process differs from other types of immune responses. Conventional dendritic cells (cDCs) are critical antigen-presenting cells that initiate and regulate T cell responses, playing essential roles in immunity to pathogens, tolerance to innocuous antigens, tumor surveillance, and self-tolerance. For cDCs to fulfill their function in immune surveillance, their migration from peripheral tissues to draining lymph nodes (dLNs) is essential, as it enables them to convey the immunological state of peripheral tissues to antigen-specific T cells in the lymph nodes and thus shape the ensuing immune response. This migration is primarily directed by the chemokine receptor CCR7, which is typically upregulated in response to homeostatic or inflammatory signals. However, in the context of type 2 immune responses, such as those elicited by allergens or helminth infections, a paradox emerges: cDCs migrate efficiently to dLNs despite exhibiting reduced CCR7 expression. In our review, we discuss this unique feature of Th2 responses and propose that additional signaling mechanisms compensate for diminished CCR7 expression in this context. In particular, we highlight the role of membrane-derived bioactive lipid mediators, including eicosanoids, sphingolipids, and oxysterols, which synergize with CCR7 signaling to promote cDC migration and support Th2 cell differentiation. This work provides an integrated view of how type 2 immunity employs alternative or supplementary cues to regulate cDC trafficking, distinguishing it from other immunological settings. The review offers insight into the specialized control of cDC migration in allergic and parasitic contexts and its implications for the development of targeted immunomodulatory strategies. A manuscript summarizing these findings has been published, providing an accessible resource for the field. Finally, during FY2025 we also conducted an in-depth review of the functional heterogeneity of type 2 conventional dendritic cells (cDC2s), focusing on their ontogeny, plasticity, and context-dependent roles in shaping adaptive immune responses. Unlike other dendritic cell subsets, cDC2s are remarkably versatile and capable of directing a broad range of T cell responses, including those underlying type 2 immunity. Understanding the basis for this versatility is critical, as it informs how cDC2s contribute to immune regulation during health, inflammation, infection, and allergic disease. In our published review, we examined emerging evidence that cDC2s are not a uniform population, but rather comprise multiple sublineages arising from distinct developmental origins or shaped through environmental adaptation. We discussed how tissue-derived signals may override lineage-imprinted programs, reprogramming cDC2 function in situ and enabling plasticity in response to dynamic cues. This evolving view of cDC2 biology highlights the importance of considering both ontogeny and microenvironmental conditioning in defining their function. Clarifying the principles that govern cDC2 heterogeneity has significant implications for the development of precise immunotherapies that aim to harness or modulate dendritic cell function. This review was published earlier this year and provides a timely synthesis of a rapidly advancing field with broad relevance for immunology and translational medicine.