Interleukin 22 (IL-22) is a cytokine produced by the T(H)-17 lineage of helper T cells and NK-22 subset of natural killer cells that acts on epithelial cells and keratinocytes and has been linked to skin homeostasis and inflammation. Here we characterize a population of human skin-homing memory CD4(+) T cells that expressed the chemokine receptors CCR10, CCR6 and CCR4 and produced IL-22 but neither IL-17 nor interferon-gamma (IFN-gamma). Clones isolated from this population produced IL-22 only and had low or undetectable expression of the T(H)-17 and T helper type 1 (T(H)1) transcription factors RORgammat and T-bet. The differentiation of T cells producing only IL-22 was efficiently induced in naive T cells by plasmacytoid dendritic cells in an IL-6- and tumor necrosis factor-dependent way. Our findings delineate a previously unknown subset of human CD4(+) effector T cells dedicated to skin pathophysiology.

The enormous diversity of the naive T cell repertoire is instrumental in generating an immune response to virtually any foreign antigen that can be processed into peptides that bind to MHC molecules. The low frequency of antigen-specific naive T cells, their high activation threshold, and the constrains of antigen-processing and presentation have hampered analysis of naive repertoires to complex protein antigens. In this study, libraries of polyclonally expanded naive T cells were used to determine frequency and antigen doseresponse of human naive CD4 + T cells specific for a variety of antigens and to isolate antigen-specific T cell clones. In the naive repertoire, T cells specific for primary antigens, such as KLH and Bacillus anthracis protective antigen, and for recall antigens, such as tetanus toxoid, cytomegalovirus, and Mycobacterium tuberculosis purified protein derivative, were detected at frequencies ranging from 5 to 170 cells per 10 6 naive T cells. Antigen concentrations required for half-maximal response (EC50) varied over several orders of magnitude for different naive T cells. In contrast, in the memory repertoire, T cells specific for primary antigens were not detected, whereas T cells specific for recall antigens were detected at high frequencies and displayed EC50 values in the low range of antigen concentrations. The method described may find applications for evaluation of vaccine candidates, for testing antigenicity of therapeutic proteins, drugs, and chemicals, and for generation of antigen-specific T cell clones for adoptive cellular immunotherapy.

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