D3.334 - Characterization of novel nickel specific cd4+ regulatory T cells from a nickel sensitized joint implant failure patient

Nickel (Ni²⁺) is a component of the most common orthopaedic alloys used in implants, and sensitization to Ni²⁺ is a frequent factor in joint replacement failure. Metal ions attached to self-peptides cause post-translational protein modifications and create powerful neoantigens for T cells by altering antigen charge and structure. The exact processes by which Ni²⁺-specific Treg cells recognize and are activated by Ni²⁺ are still unknown. Our objective was to characterize the binding and metal specificities of Ni²⁺-specific CD4 Treg cells derived from a knee replacement patient with failure due to nickel sensitization.

Using scRNA-seq on Ni²⁺-stimulated PBMCs from a joint implant failure patient, we identified two Ni²⁺-reactive Treg cells, cloned the Treg TCRs to 5KC T cell avatars, and characterized Ni²⁺-reactivity and specificity using the NFAT-ZsGreen reporter system and IL-2 secretion. Site-specific mutagenesis identified essential Ni²⁺ binding sites.

The two Ni²⁺-reactive Treg cells differed greatly in their HLA requirements and metal reactivities that triggered activation. The CZD9.1 T cell receptor (TCR) responds to multiple HLA-DR, HLA-DP and HLA-DQ alleles, but can only be activated by Ni²⁺and palladium, both group 10 metals. The CZD9.2 TCR is restricted to HLA-DR7, but can be stimulated by multiple adjacent period 4 transition metals, including Ni²⁺, cobalt, copper, and zinc, as well as palladium. The two histidines in the CZD9.1 TCRβ chain CDR3 region are critical for Ni²⁺-specific stimulation, which is ablated by site specific mutagenesis, although a specific peptide on antigen-presenting cells is still required to respond to Ni²⁺. 

Ni²⁺-specific Treg cells derived from a Ni²⁺ sensitized joint failure patient employ different mechanisms of Ni²⁺ recognition. The ability of the CZD9.1 TCR to bind multiple different HLA’s may be due to the histidines that stabilize and coordinate Ni²⁺ ions directly, whereas in the CZD9.2 TCR, acidic amino acids of CDRb3 may be the Ni²⁺ ligand. Differences in Ni²⁺ employed coordination chemistry likely determine the different TCR metal and HLA specificities, and such plasticity may explain the ubiquity of Ni²⁺ allergy.