Skin Treg proliferation requires cell contact with fibroblasts but does not require antigen presentation or costimulation. (A) Skin-resident T cells were cultured in IL-2 and IL-15 for 1 week either in contact with dermal fibroblasts (left panel) or separated from the monolayer by a 0.4 μm transwell membrane (right panel). (B) Dermal fibroblasts do not express HLA-DR, DP, or DQ, nor do they express costimulatory molecules CD80 (B7-1) or CD86 (B7-2). Isotype controls (heavy black line) and test antibodies (⊡) histograms are shown. (C) Blockade of HLA-DR, DP, and DQ with neutralizing antibodies did not reduce production of FOXP3⁺ Tregs from explant cultures. Explant cultures were maintained in IL-2 and IL-15 for 3 weeks; neutralizing antibody was included throughout the culture period and was added with each feeding. Values shown represent the means and SDs of duplicate measurements. Similar results were produced using 2 additional skin donors.