In psoriasis, epidermal hyperplasia is driven by underlying immune activation, whether as a direct response to IL-20 family cytokines that induces hyperplasia and inhibits keratinocyte terminal differentiation or as an indirect response to immune-mediated injury to keratinocytes. The epidermal reaction in psoriasis is largely restored to normal with selective immune suppression. Hence, one might hypothesize that similar epidermal responses should occur in the presence of "generalized" cellular immune activation, in diseases with similar inflammatory infiltrate and epidermal hyperplasia, such as AD. In fact, psoriasis and AD share features of dense T-cells and dendritic cell infiltrates, as well as over-expression of IL-22 in skin lesions. These diseases also share similar epidermal hyperplasia in their chronic phases.
Work from the investigators group showed that IL-22 is a key cytokine in the pathogenesis of both AD and psoriasis. The investigators have demonstrated that in psoriasis, ustekinumab suppresses the production of IL-12, IL-23, and IL-22. Additionally, by RT-PCR the investigators demonstrated that the mRNA expression of p40 cytokine and the IL23R is up-regulated in AD as compared to both normal skin and psoriasis. The investigators therefore hypothesize that ustekinumab will suppress IL-22 and possibly also p40 production in AD lesions and reverse both the epidermal growth/differentiation defects and the underlying immune activation, and hence will suppress disease activity. Interestingly, p40 was also found to be significantly up-regulated in non-lesional AD skin as compared with normal skin.
Although AD is thought to be predominately a disease of Th2-type cells, in the chronic stage, there is large Th1 component. To date, the precise mechanism by which sequential activation of Th2 and Th1 cells in AD is achieved remains unknown. IL-12 induces the differentiation and maturation of human Th cells into Th1-type cells. Recent circumstantial evidence suggests that in AD patients IL-12 may facilitate a change from the Th2-type to a Th1 cytokine profile. IL-12 was recently shown to be highly elevated in pediatric AD and its levels were strongly associated with disease severity.
Expression of IL-12 p40 mRNA is significantly enhanced in lesional skin from AD, suggesting that the enhanced local production of IL-12 in dendritic cells and macrophages may be responsible for the increased production of IFN-γ in chronic lesions potentially suggesting that IL-12 may have a pivotal role in promoting inflammation in atopic dermatitis. Topical steroids which constitute a mainstay of therapy in AD are known to strongly down-regulate IL-12 expression, possibly also indicating that targeted anti IL-12 therapy might important role in treating AD.
Recently, the Th1/Th2 paradigm in autoimmunity and allergy has been revisited to include a role for a new population of IL-17-producing Th cells known as Th17. Th17 cells are characterized by the production of inflammatory cytokines such as IL-17A, IL-17F, IL-22, and IL-26. One of the key factors involved in naive Th-cell commitment to a Th17 phenotype is IL-23.
Patients with acute AD were found to have increased Th17 T-cells in peripheral blood by flow cytometry and intracellular cytokine staining 26 as well as by immunohistochemistry (IHC) in lesions. Since IL-23 is the major inducer of Th17 T-cells, as well as "T22" T-cells, neutralization of IL-23 could potentially result in both decreased Th17 signal in acute AD as well as decreased "T22/IL22" signal. Therefore the investigators postulate that ustekinumab in AD will act both inhibiting the IL-12-dependent Th1 shift in chronic AD stage as well as the pathogenic IL-22/"T22" axis in this disease.
