Tissue‐resident memory T (T_RM) cells are abundant in the memory T cell pool and remain resident in peripheral tissues, such as the skin, where they act as alarm sensors or cytotoxic killers. T_RM cells persist long after the pathogen is eliminated and can respond rapidly upon reinfection with the same antigen. When aberrantly activated, skin‐located T_RM cells have a profound role in various skin disorders, including vitiligo and melanoma. Autoreactive T_RM cells are present in human lesional vitiligo skin and mouse models of vitiligo, which suggests that targeting these cells could be effective as a durable treatment strategy for vitiligo. Furthermore, emerging evidence indicates that induction of melanoma‐reactive T_RM cells is needed to achieve effective protection against tumor growth. This review highlights seminal reports about skin‐resident T cells, focusing mainly on their role in the context of vitiligo and melanoma, as well as their potential as therapeutic targets in both diseases.

Autoreactive CD8⁺ T cells play a pivotal role in melanocyte destruction in autoimmune vitiligo. Immunotherapy for melanoma often leads to autoimmune side-effects, among which vitiligo-like depigmentation, indicating that targeting immune checkpoints can break peripheral tolerance against self-antigens in the skin. Therapeutically enhancing immune checkpoint signaling by immune cells or skin cells, making self-reactive T cells anergic, seems a promising therapeutic option for vitiligo. Here, we review the current knowledge on the PD-1/PD-L1 pathway in vitiligo as new therapeutic target for vitiligo therapy.

The Opal multiplex technique is an established methodology for the detection of multiple biomarkers in one section. The protocol encompasses iterative single stainings and heating-mediated removal of the primary and secondary antibodies after each staining round, leaving untouched the Opal fluorophores which are deposited onto the antigen of interest. According to our experience, repetitive heating of skin sections often results in tissue damage, indicating an urgent need for milder alternatives to strip immunoglobulins. In this study, we demonstrate that considerable heating-related damage was found not only in skin but also in tissues of different origin, mostly characterized by low cell density. Importantly, the morphology remained fully intact when sections were repetitively exposed to β-mercaptoethanol-containing stripping buffer instead of multiple heating cycles. However, target epitopes appeared sensitive at a differential degree to multiple treatments with stripping buffer, as shown by loss in staining intensity, but in all cases, the staining intensity could be restored by increment of the primary antibody concentrations. Application of β-mercaptoethanol-containing stripping buffer instead of heating for antibody removal markedly improved the quality of the Opal multiplex technique, as a substantial higher number of differently colored cells could be visualized within a well-conserved morphological context:

Accumulating studies have indicated immune-based destruction of melanocytes in both segmental vitiligo (SV) and non-SV (NSV). Whereas SV often occurs unilaterally during childhood and stabilizes after an initial period of activity, the disease course of NSV is usually slowly progressive, with new lesions occurring bilaterally during life. This suggests an involvement of distinct pathophysiology pathways, specifically increased systemic immune activation in patients with NSV but not in patients with SV. This research aimed to identify the differences in immune cells in the blood of patients with SV and NSV through immunophenotyping of circulating cells. Regulatory T cells were unaffected in patients with SV compared with that in healthy controls but decreased in patients with NSV. In patients with NSV, the reduction in regulatory T cells was associated with the presence of other systemic autoimmune comorbidities, which were less present in SV. Similarly, the absence of a melanocyte-specific antibody response in patients with SV suggests less involvement of B-cell immunity in SV. These data show that in contrast to patients with NSV, no increased systemic immunity is found in patients with SV, indicating that SV pathogenesis is associated with a localized cytotoxic reaction targeting epidermal melanocytes.

Checkpoint inhibitor (CI) therapy has revolutionized treatment for non-small cell lung cancer (NSCLC). However, a proportion of patients do not respond to CI therapy for unknown reasons. Although the current paradigm in anti-tumor immunity evolves around T cells, the presence of tertiary lymphoid structures and memory B cells has been positively correlated with response to CI therapy in NSCLC. In addition, double negative (DN) (CD27^- IgD^-) B cells have been shown to be abundant in NSCLC compared to healthy lung tissue and inversely correlate with the intratumoral presence of memory B cells. Nonetheless, no study has correlated DN B cells to survival in NSCLC. In this study, we evaluated the presence and phenotype of B cells in peripheral blood with flow cytometry of patients with NSCLC and mesothelioma before receiving CI therapy and correlated these with clinical outcome. Non-responding patients showed decreased frequencies of B cells, yet increased frequencies of antigen-experienced CD21- DN (Atypical) B cells compared to responding patients and HC, which was confirmed in patients with mesothelioma treated with CI therapy. These data show that the frequency of CD21- DN B cells correlates with lack of response to CI therapy in thoracic malignancies. The mechanism by which CD21- DN B cells hamper CI therapy remains unknown. Our findings support the hypothesis that CD21- DN B cells resemble phenotypically identical exhausted B cells that are seen in chronic infection or function as antigen presenting cells that induce regulatory T cells.

Mounting evidence shows that the PD‐1/PD‐L1 axis is involved in tumor immune evasion. This is demonstrated by anti‐PD‐1 antibodies that can reverse tumor‐associated PD‐L1 to functionally suppress anti‐tumor T‐cell responses. Since type I and II interferons are key regulators of PD‐L1 expression in melanoma cells and IFN‐γ‐producing CD8⁺ T cells and IFN‐α‐producing dendritic cells are abundant in vitiligo skin, we aimed to study the role of PD‐1/PD‐L1 signalling in melanocyte destruction in vitiligo. Moreover, impaired PD‐1/PD‐L1 function is observed in a variety of autoimmune diseases. It is, therefore, hypothesized that manipulating PD‐1/PD‐L1 signalling might have therapeutic potential in vitiligo. The PD‐1⁺ T cells were abundantly present in situ in perilesional vitiligo skin, but expression of PD‐L1 was limited and confined exclusively to dermal T cells. More specifically, neither melanocytes nor other epidermal skin cells expressed PD‐L1. Exposure to IFN‐γ, but also type I interferons, increased PD‐L1 expression in primary melanocytes and fibroblasts, derived from healthy donors. Primary human keratinocytes only showed increased PD‐L1 expression upon stimulation with IFN‐γ. More interestingly, melanocytes derived from non‐lesional vitiligo skin showed no PD‐L1 upregulation upon IFN‐γ exposure, while other skin cells displayed significant PD‐L1 expression after exposure. In a vitiligo skin explant model, incubation of non‐lesional vitiligo skin with activated (IFN‐γ‐producing) T cells from vitiligo lesions was previously described to induce melanocyte apoptosis. Although PD‐L1 expression was induced in epidermal cells in these explants, this induction was completely absent in melanocytes. The lack of PD‐L1 upregulation by melanocytes in the presence of IFN‐γ‐producing T cells shows that melanocytes lack protection against T‐cell attack during vitiligo pathogenesis. Manipulating PD‐1/PD‐L1 signalling may, therefore, be a therapeutic option for vitiligo patients.