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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. [ABSTRACT FROM AUTHOR]

Background: Nickel is the most frequent cause of T cell–mediated allergic contact dermatitis worldwide. In vitro, CD4+ T cells from all donors respond to nickel but the involved αβ T cell receptor (TCR) repertoire has not been comprehensively analyzed. Methods: We introduce CD154 (CD40L) upregulation as a fast, unbiased, and quantitative method to detect nickel‐specific CD4+ T cells ex vivo in blood of clinically characterized allergic and non allergic donors. Naïve (CCR7+ CD45RA+) and memory (not naïve) CD154+ CD4+ T cells were analyzed by flow cytometry after 5 hours of stimulation with 200 µmol/L NiSO4., TCR α‐ and β‐chains of sorted nickel‐specific and control cells were studied by high‐throughput sequencing. Results: Stimulation of PBMCs with NiSO4 induced CD154 expression on ~0.1% (mean) of naïve and memory CD4+ T cells. In allergic donors with recent positive patch test, memory frequencies further increased ~13‐fold and were associated with markers of in vivo activation. CD154 expression was TCR‐mediated since single clones could be specifically restimulated. Among nickel‐specific CD4+ T cells of allergic and non allergic donors, TCRs expressing the α‐chain segment TRAV9‐2 or a histidine in their α‐ or β‐chain complementarity determining region 3 (CDR3) were highly overrepresented. Conclusions: Induced CD154 expression represents a reliable method to study nickel‐specific CD4+ T cells. TCRs with particular features respond in all donors, while strongly increased blood frequencies indicate nickel allergy for some donors. Our approach may be extended to other contact allergens for the further development of diagnostic and predictive in vitro tests. [ABSTRACT FROM AUTHOR]

Background: Chemical allergies are T cell-mediated diseases that often manifest in the skin as allergic contact dermatitis (ACD). To prevent ACD on a public health scale and avoid elicitation reactions at the individual patient level, predictive and diagnostic tests, respectively, are indispensable. Currently, there is no validated in vitro T cell assay available. The main bottlenecks concern the inefficient generation of T cell epitopes and the detection of rare antigen-specific T cells. Methods: Here, we systematically review original experimental research papers describing T cell activation to chemical skin sensitizers. We focus our search on studies published in the PubMed and Scopus databases on non-metallic allergens in the last 20 years. Results: We identified 37 papers, among them 32 (86%) describing antigen-specific human T cell activation to 31 different chemical allergens. The remaining studies measured the general effects of chemical allergens on T cell function (five studies, 14%). Most antigen-specific studies used peripheral blood mononuclear cells (PBMC) as antigen-presenting cells (APC, 75%) and interrogated the blood T cell pool (91%). Depending on the individual chemical properties, T cell epitopes were generated either by direct administration into the culture medium (72%), separate modification of autologous APC (29%) or by use of hapten-modified model proteins (13%). Read-outs were mainly based on proliferation (91%), often combined with cytokine secretion (53%). The analysis of T cell clones offers additional opportunities to elucidate the mechanisms of epitope formation and cross-reactivity (13%). The best researched allergen was p-phenylenediamine (PPD, 12 studies, 38%). For this and some other allergens, stronger immune responses were observed in some allergic patients (15/31 chemicals, 48%), illustrating the in vivo relevance of the identified T cells while detection limits remain challenging in many cases. Interpretation: Our results illustrate current hardships and possible solutions to monitoring T cell responses to individual chemical skin sensitizers. The provided data can guide the further development of T cell assays to unfold their full predictive and diagnostic potential, including cross-reactivity assessments. [ABSTRACT FROM AUTHOR]