Atopic dermatitis is a common pruritic
skin disease in which barrier dysfunction and cutaneous
inflammation contribute to pathogenesis. Mechanisms underlying the associated
inflammation are not fully understood, and although Langerhans cells expressing the nonclassical major histocompatibility complex (MHC) family member CD1a are known to be enriched within lesions, their role in clinical disease pathogenesis has not been studied. We observed that house dust mite (HDM)
allergen generates neolipid
antigens presented by CD1a to T cells in the blood and skin lesions of affected individuals. HDM-responsive CD1a-reactive T cells increased in frequency after birth in individuals with
atopic dermatitis and showed rapid effector function, consistent with
antigen-driven maturation. In HDM-challenged human skin, we observed
phospholipase A2 (PLA2) activity in vivo. CD1a-reactive T cell activation was dependent on HDM-derived PLA2, and such cells infiltrated the skin after
allergen challenge. Moreover, we observed that the skin barrier
protein filaggrin, insufficiency of which is associated with atopic
skin disease, inhibited PLA2 activity and decreased CD1a-reactive PLA2-generated neolipid-specific T cell activity from skin and blood. The most widely used classification schemes of
hypersensitivity suggest that nonpeptide stimulants of T cells act as
haptens that modify
peptides or
proteins; however, our results show that HDM
proteins may also generate neolipid
antigens that directly activate T cells. These data define PLA2 inhibition as a function of
filaggrin, supporting PLA2 inhibition as a therapeutic approach.