It’s a lot of work to be nonallergic

Edward Jenner developed the first successful vaccine based on the observation that persons exposed to poxvirus pathogens who did not succumb to the disease became protected from further encounters.¹ In more recent trials, much has been learned about correlates of health and protection from controlled challenges in patients with infectious diseases as diverse as influenza and malaria, in which the study of exposed subjects who mount an appropriate immune response might hold the key to development of treatment and vaccination alike.

The article by Ahuja et al² in this issue of the Journal underlines that this might be the case for allergic diseases as well, namely that important lessons are to be learned not only by studying patients with allergic disease but also by investigating the molecular mechanisms associated with immune responses in nonallergic subjects who are exposed and do not have IgE sensitization, clinical symptoms, or both.

The study shows that there is an immune response to house dust mite (HDM) allergen challenge in both allergic and nonallergic subjects. This means the nonallergic status is not based on lack of recognition and response to the allergen but is associated with a distinct and protective response. Similar conclusions have recently been reached in a study of T-cell recognition of seasonal pollen exposure.³ This seems to indicate that subjects who are exposed and nonallergic have an immune response to an allergen and that the response might actually be protective.

The investigators studied the effect of nasal allergen challenge (NAC) in subjects allergic (M+) and nonallergic (M−) to HDMs, performing immunologic characterization and using the power of mRNA profiling to determine changes in transcriptional patterns associated with the challenge. After NAC, M− subjects manifested an adaptive “healthy” response with increased expression of genes related to the epidermal/epithelial barrier and reduced expression of genes involved in inflammation compared with the responses of patients with HDM allergy (M+).

A rich set of data was collected, including time-of-flight mass cytometry, to perform immune phenotyping of PBMCs and both cross-sectional and longitudinal analysis of transcriptional profiles in M+ and M− participants. Furthermore, the authors performed a modular framework analysis⁴ to interpret the changes in transcriptional patterns observed. In peripheral blood markers of CD4⁺ and CD8⁺ T-cell activation were downregulated in M− but increased in M+ subjects. In M− subjects genes that promoted epidermal/epithelial barrier function, such as filaggrin, were upregulated, whereas chemokines and innate immunity genes (interferon) were downregulated.

An important aspect of the study was that the challenge of M− subjects was safe. During HDM challenge, only M+ subjects had allergic rhinoconjunctivitis symptoms, and at least during the follow-up period reported, no adverse reactions were noted. This suggests that such an approach can be used more generally to perform similar studies in this and other allergen systems. Follow-up studies could use similar strategies to recruit study participants from populations with previous exposures, indicating that allergen exposure in these subjects would be unlikely to result in development of allergic disease.

In the study by Ahuja et al,² the functionality of HDM-specific T cells was not monitored (although CD69/HLA-DR⁺ cells as a marker of activation were measured). Discordance between M− and M+ subjects was observed in T_H1 (IFN-γ⁺/CXCR3⁺) versus T_H2 (IL-4⁺/CCR4⁺) memory cells and regulatory T cells (CD25^hiCD127⁻CCR4⁺). The authors point out that it is possible that in M+ subjects T_H2 responses to HDMs, lack of a Treg response, or both can contribute to the differences observed in epithelial transcriptional profiles. However, it is also possible that in M− subjects the response toward preservation of the epidermal/epithelial barrier might be sufficient to prevent the inflammatory response.

The fact that nonallergic subjects harbor allergen-specific T cells has been known for quite some time. A study by Hinz et al³ noted that seasonal exposure to timothy grass (TG) allergens had opposite effects on TG-specific T cells in allergic and nonallergic donors. TG-specific T-cell responses were enhanced by the seasonal allergen exposure in donors with TG allergy. Conversely, TG-specific T cells were detectable out of season but were specifically downmodulated in the pollen season in nonallergic donors. Thus it seems allergen exposure of nonallergic subjects results in both enactment of a barrier-specific program and modulation of antigen-specific T-cell responses.

NAC has been used for many years for a variety of investigational purposes. One particular application is of great potential significance in the development of allergen-specific immunotherapies (AITs), in which NAC has been used to objectively measure clinical responses to the same extract administered as an immunotherapeutic.⁵ In the study by Ahuja et al,² immune response activation markers were measured after 3 hours. Several studies of NAC in the context of immunotherapy suggest that allergen-specific T-cell responses are initially increased a few hours after challenge but decrease afterward, which is possibly reflective of migration in the nasal tissues, lymphoid sites, or both.^5–7 It will be of interest to examine the kinetics of immune responses both in allergic and nonallergic subjects to determine whether additional molecular events are associated with nonallergic T-cell responses, such as lack of response downmodulation/tissue migration of the allergen-specific T cells in nonallergic compared with allergic subjects. The study describes a “healthy” response to allergen exposure. Successful AIT is also associated with modulation of the immunologic response to allergen encounters, both in the course of controlled administration and in terms of subsequent natural exposure. It will be interesting to investigate whether the NAC response of subjects who benefitted from AIT in terms of clinical symptoms resembles that observed in M− donors.

The study by Ahuja et al² brings to the forefront the importance of the mucosal barrier and nasal epithelium in allergen sensitization. In addition, many studies report that defects in the skin barrier are early critical events in the pathogenesis of atopic dermatitis and development of other diseases, such as food allergy, asthma, allergic rhinoconjunctivitis, and eosinophilic esophagitis.^8–10 This in turn implies that strategies to safeguard and repair defects in epidermal/epithelial barrier function could be of significant therapeutic value. Finally, the authors point out that their study has potential implications for biomarker discovery. Research aiming at studying phenotypes or transcriptional profiles associated with different disease severity or therapeutic responses should consider allergen exposure as a relevant variable.

In conclusion, the concept that is arising is that nonallergic subjects mount “healthy” responses to allergen exposure. These responses involve both a “resilient” epidermal/epithelial barrier function and regulation of reactivity at the T-cell level, orchestrating specific transcriptional profiles. A more in-depth understanding of these responses might sharpen our understanding of different phenotypes associated with disease and have important implications for biomarker development, evaluation of AIT efficacy, and development of new therapeutic interventions.