Inflammed skin harbours Th9 cells.

Various effector CD4 + T-lymphocyte subsets have been characterized, such as T-helper (Th)1, Th2, regulatory CD4 + T cells (Treg), Th17, and Th22, which show distinct patterns of cytokine release. Specifically, the cytokine interleukin (IL)-9 has been regarded largely as an exclusive Th2 cytokine. However, a new subset of the Th population, named " Th9 " , which is separate from Th2, producing IL-9 in large quantities has been described (1, 2). More recently, new information concerning Th9 cells confirmed their uniqueness with regard to the unusual production kinetics of IL-9 and the short retention of these cells in affected target tissues (3). For the generation of Th subpopulations, transcription factors, formerly believed to be specific of each subset, are required, such as Tbet for Th1 cells, GATA-3 for Th2 cells, Foxp3 for Treg, and ROR-γt for Th17 cells. A recent publication (4) further assessed the Th9 identity by showing that the transcription factor PU.1 is required for the development of Th9 lineage; indeed, PU.1 binds directly to the IL-9 promoter to trigger specific chromatin modifications (5). Since PU.1 was formerly known to be expressed in myeloid lineage and B lymphocytes, and to be required for normal macrophage and granulocyte differentiation, the utility of PU.1 as an immunohistochemical marker for skin was restricted to primary skin neoplasms of non-lymphoid cell origin (6). To the best of our knowledge, PU.1 expression has never been investigated to identify Th9 cells within cutaneous lesions. The aim of the present study was therefore to investigate whether CD4 + PU.1 + cells (4, 5) are recognizable in skin sections, specifically within some inflammatory cutaneous infiltrates. Ten formalin-fixed, paraffin-embedded skin samples, previously collected at the time of biopsy, were utilized: they included 4 cases of chronic lesions of atopic dermatitis (AD), 3 cases of chronic plaque psoriasis, and 3 cases of chronic lesions of allergic contact dermatitis (ACD) to nickel. For PU.1 staining, the anti-PU.1 antibody " G148-74, ab 48587 " (Abcam, Cambridge, UK) was used, and for IL-9 staining the anti-IL-9 antibody " ab111915 " (Abcam) was used. A previously described technique was carried out for single-labelling immunocytochemistry (7). Similarly, the anti-CD3 antibody " anti-CD3 mAb " (Neomarkers, Fremont, CA, USA) and the same technique were used to reveal CD3 + cells. In order to reveal CD4 + PU.1 + cells (4, 5), a double-staining immunocytochemical technique was performed (7). Negative controls for PU.1 antibody …