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

Abstract

Background: 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.

Methods: 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.

Results: Peripheral blood mononuclear cells 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 T cells (mean 3.5%, n = 19), followed by Co²⁺ - and Ni²⁺ -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 gene segment family (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 were conserved among cross-reactive clonotypes.

Conclusions: 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.

Keywords: CD154 upregulation assay; T cell receptor (TCR) repertoire; TCR α-chain segment TRAV9-2; complementarity determining region 3 (CDR3) histidine; metal allergens Ni2+, Co2+, Pd2+.