Ni2+, Co2+, Pd2+ specific T cells and TCR. Unique and common TCR repertoire features of nickel, cobalt and palladium-specific human CD154+CD4+ T cells

Apart from Ni2+, Co2+ and Pd2+ ions commonly trigger T cell-mediated allergic contact dermatitis. However, in vitro frequencies of metal-specific T cells and the mechanisms of antigen recognition remain unclear. Here, we utilized a CD154 upregulation assay to quantify Ni2+, Co2+ and Pd2+ specific CD4+ T cells in peripheral blood mononuclear cells (PBMC). Involved αβ T cell receptor (TCR) repertoires were analyzed by high-throughput sequencing. PBMC incubation with NiSO4, CoCl2 and PdCl2 increased frequencies of CD154+CD4+ memory T cells that peaked at ~400 µM. Activation was TCR-mediated as shown by the metal-specific restimulation of T cell clones. Most abundant were Pd2+ specific cells (mean 3.5%, n = 19), followed by Co2+ and Ni+ specific cells (0.6%, n = 18 and 0.3%, n = 20) in both allergic and non-allergic individuals. A strong overrepresentation of the gene segment TRAV9-2 was unique for Ni2+ specific TCR (28% of TCR) while Co2+ and Pd2+ specific TCR favorably expressed TRAV2 (8%) and the TRBV4 family segments (21%), respectively. As a second, independent mechanism of metal ion recognition, all analyzed metal-specific TCR showed a common overrepresentation of a histidine in the complementarity determining region 3 (CDR3; 15% of α-chains, 34% of β-chains). The positions of the CDR3 histidine among metal-specific TCR mirrored those in random repertoires and was conserved among cross-reactive clonotypes. Induced CD154 expression allows a fast and comprehensive detection of Ni2+, Co2+ and Pd2+ specific CD4+ T cells. Distinct TCR repertoire features underlie the frequent activation and cross-reactivity of human metal-specific T cells.