Review: Lymphocytes, cytokines, chemokines and the T-helper 1-T-helper 2 balance in canine atopic dermatitis.

Background: The development of atopic dermatitis (AD) and other cutaneous hypersensitivities involves the activation and differentiation of allergen-specific lymphocytes. Although hypersensitivity is often considered to be a 'T-helper 2-polarized' lymphocyte response, recent evidence suggests that clinical disease is associated with the development of multiple lymphocyte phenotypes.

Objectives: The purpose of this paper is to review recent advances in the understanding of the roles of lymphocytes, cytokines and noncytokine factors in the pathogenesis of canine AD.

Methods: Citation databases, abstracts and proceedings from international meetings published between 2001 and 2013 were reviewed in this update. Where necessary, older articles were included for background information.

Results: The development of canine AD is associated with changes in both cutaneous and circulating lymphocyte populations. These lymphocyte responses are characterized by the production of a complex variety of cytokines, including not only T-helper 2 but also T-helper 1, T-helper 17 and regulatory T-cell responses. In addition, microarray gene expression analysis has enabled the identification of a number of noncytokine factors that appear to be associated with atopic inflammation. These include the calcium-binding protein S100A8, serum amyloid A and a number of protease inhibitors, as well as genes involved in epidermal barrier formation, innate immunity receptors, cell cycle proteins and apoptosis.

Conclusions: The development of AD in dogs is characterized by the development of a delicate balance between a variety of T-cell phenotypes and inflammatory mediators, including cytokines, chemokines and noncytokine factors.