The prevalence of cashew allergy has been rising in industrial countries.¹ More critically, accidental contact with cashew-containing food is frequently associated with highly severe anaphylactic reactions in allergic individuals.² These elements highlight the need for a safe treatment able to minimize the impact of this life-threatening allergy.

In a recent study published in Allergy , our group evaluated the relevance of investigational epicutaneous immunotherapy (EPIT) to protect against anaphylaxis in a mouse model of cashew allergy.³ We demonstrated that epicutaneous patches containing cashew allergens were able to modulate cashew-specific antibody responses and decrease cashew-specific Th2 responses in cashew-sensitized animals. More importantly, EPIT was able to reduce mast cell reactivity and to afford protection against anaphylactic symptoms following oral challenge, suggesting that EPIT may be a safe and efficacious approach to treat cashew allergy. During this work, we demonstrated that epicutaneous patches were able to deliver cashew allergen to skin dendritic cells (DC) using cashew protein extract conjugated to Fluoroprobe-647 (cashew-F647). We showed a significant increase of cashew-specific epidermal Langerhans cells, cDC1 and cDC2 in the local draining lymph nodes of mice that received patches containing cashew-F647 for 48 hours.

In the present work, we aimed to better characterize allergen capture at the skin level while evaluating the capacity of cashew allergen to modulate DC activation in sensitized or naïve animals. To that end, epicutaneous patches containing cashew-F647 were applied to cashew-sensitized or naïve mice for 6 hours (antibody titers measured before patch application are depicted on Figure S1 ). Then, skin was homogenized, and cells were analyzed by flow cytometry (Figure 1 ). Langerhans cells and cDC2 were the two main subsets of skin DC able to take up cashew allergen, in agreement with previously published data.⁴ Remarkably, a significant increase of cashew-positive Langerhans cells, cDC2 and cDC1 was observed in sensitized mice compared to naïve animals. Note that the difference was more pronounced for Langerhans cells, with a 47% increase of allergen-positive cells among this subset. To further address the activation status of these cashew-positive DC, the expression of CD86 and PD-L2 was measured in each skin DC subset (Figure 2 ). A significant decrease in CD86 expression was measured for cashew-positive Langerhans cells and cDC1 isolated in sensitized mice compared to naïve mice. Conversely, a significant increase of PD-L2 expression was observed in cashew-positive Langerhans cells and cDC2 isolated in sensitized animals.

Previous data demonstrated that PD-L2 expression by skin DCs, especially Langerhans cells and cDC2, is a key element leading to the generation of Tregs and to the acquisition of tolerance to topically administered allergen.^4,5 Here, we demonstrated that the expression of PD-L2 and CD86 on skin DC following topical allergen application depends upon preexisting immunological status. Furthermore, these results support the fact that a strong Th2 context promotes the generation of tolerogenic skin DC characterized by a decrease in CD86 expression and an increase of PD-L2 expression, as previously shown by our group using OVA as a model allergen.⁶

Further investigations are warranted to decipher the mechanisms by which specific preexisting immunity impacts the capacity of skin DC to capture topically administered allergens and to modify their activation status. Our previous results suggest that preexisting IgG could efficiently promote allergen uptake (unpublished data), but the mechanisms leading to the modulation of CD86 and PD-L2 are still unknown.

Overall, these data provide additional insight into the mechanisms of action of EPIT for cashew allergy treatment. They suggest that the acquisition of tolerance to epicutaneous allergen would be more efficient in highly sensitized individuals and that the underlying mechanisms are embedded in the skin.