# Single Cell Analysis Reveals Dynamic Changes of Distinct Cell Populations in Human Nickel Allergy

## TL;DR

This study uses single-cell RNA sequencing to identify cell types involved in early and late immune responses in human nickel allergy.

## Contribution

The study identifies distinct cell populations and molecular pathways involved in the early and late phases of nickel hypersensitivity in human skin.

## Key findings

- Early hypersensitivity involves TLR4+ endothelial cells, suprabasal keratinocytes, and CCR7+ dendritic cells.
- Late hypersensitivity is marked by infiltration of KLF2+ central memory T cells and TLR4+CD163+CCR7− DC3 cells.
- Skin-resident T cells do not contribute to the early response but change significantly after 72 hours.

## Abstract

Metal allergies are prime examples of delayed‐type hypersensitivity divided into two phases: in the sensitization phase, initial contact with an allergen leads to activation of skin‐resident cells and formation of metal‐reactive T cells. during elicitation, these T cells mount an immune response resulting in clinically apparent eczema within 72 h after exposure. Two main mechanisms have been implicated in the initiation of metal hypersensitivity: direct or indirect activation of innate immune receptors such as Toll‐like receptor 4, and conditional innate immune activation via the NLRP3 inflammasome. Yet, the responsible cell type(s) mediating these responses are unknown. Moreover, it is unclear whether the elicitation phase is mainly dominated by infiltration of circulating metal‐responsive T cells or if tissue‐resident T cells contribute.

Here, we analyzed the relevance of different cell types in human nickel hypersensitivity by single‐cell RNA sequencing and immunofluorescence analysis of skin samples of nickel‐sensitized donors epicutaneously exposed to diluent and nickel for 8 or 72 h.

Nickel specifically activated distinct populations of endothelial cells, suprabasal keratinocytes, fibroblasts, and
CCR7

+
dendritic cells, co‐expressing the TLR4‐interacting proteogylcan
DCN
and CCR7 ligand
CCL21
within 8 h. Skin‐resident T cells were not involved in the early hypersensitivity response, as their gene expression remained unaltered 8 h after nickel exposure. However, substantial changes in the cutaneous T cell compartments occurred after 72 h, with massive infiltration of KLF2
+ central memory T cells being a recurrent feature of both nickel‐sensitized patients and individuals allergic to the glucocorticoid contact allergen budesonide.

We investigated cell types mediating the early and late inflammatory responses in human Ni2+‐allergy by scRNA‐Seq. Early responsiveness was mainly mediated by TLR4+TAGLN− endothelial cells indirectly activating TLR4−KRT16+ keratinocytes, and CCR7+DCN+ DCs. Late responsiveness was crucially driven by infiltration of KLF2+ central memory T cells and TLR4+CD163+CCR7− DC3 cells. CCR7, C‐C chemokine receptor type 7; DC, dendritic cells; EC, endothelial cells; IF, immunofluorescence; KC, keratinocytes; KLF2, Krüppel‐like factor 2; Ni2+, nickel; scRNA‐Seq, single cell RNA sequencing; SELL, L‐selectin; TAGLN, transgelin; TCM, central memory T cell; Th, T helper cell; TLR4, Toll‐like receptor 4.