DISCUSSION
This study integrates human clinical data, transcriptomic analyses, and
mouse models to investigate the effect of UVB on antigen-specific Treg
induction. Our results demonstrate that UVB promotes the differentiation
of antigen-activated CD4+ T cells into Tregs and
enhances Treg clonal expansion, leading to antigen-specific immune
suppression. By analyzing PBMCs from psoriasis patients by CyTOF, we
identified a distinct CD4+ T cell cluster expressing
Treg-associated molecules and TCR activation phenotypes (Foxp3, CD25,
HLA-DR, TIGIT, and Tim3), which significantly increased after UVB
treatment and predicted therapeutic response. This cell subset was
further confirmed to be antigen-specific Tregs induced by UVB in mouse
models of OVA-induced skin inflammation and allogeneic skin
transplantation. Adoptive transfer experiments demonstrated the
important role of UVB-induced antigen-specific Tregs in disease
modulation. Finally, scRNA-seq and scTCR-seq analyses from patient PBMCs
revealed that UVB treatment led to Treg differentiation from
CD4+ T cells and Treg clonal expansion.
UVB irradiation can induce immune tolerance and effectively treat
inflammatory skin diseases.19-21 The mechanisms
underlying UVB-induced immunosuppression are multifactorial and complex.
Immediate effects, including DNA damage, membrane lipid oxidation, and
isomerization of chromophores (such as urocanic acid), induce cell
growth arrest and apoptosis.22,23 Delayed effects
include decreased inflammatory cytokine production, inhibition of
antigen-presenting cell (APC) activity, and induction of
Tregs.7,24 In our results, we found UVB
treatment increased the frequency of a specific cluster of Tregs
(cluster 11) expressing significantly higher HLADR, TIGIT, and TIM3, the
constitutional TCR activation markers, in psoriasis patients (Fig ure1D,E). The induction of antigen-specific Tregs by UVB was
further proved by using OVA tetramers in a mouse model of OVA-induced
skin inflammation (Fig ure 2G), as well as by an animal model of
allogeneic skin transplantation (Fig ure 3). These results show
that UVB can significantly induce antigen-specific Tregs in the
periphery to exert antigen-specific immune suppression in an
inflammatory context.
The therapeutic potential of Tregs has generated significant enthusiasm,
in part due to the targeted suppression capabilities of antigen-specific
Tregs, which avoid the global immunosuppression mediated by polyclonal
Tregs. It is well known that TGF-β can increase Foxp3 expression
in naive T cells with antigen stimulation in
vitro.25,26 This TGF-β-dependent Foxp3 expression is
not attainable from effector or memory T cells and is not sustainable by
the presence of proinflammatory cytokines.25,26Akamatsu et al. reported that the inhibition of CDK8/19 converted
antigen-specific effector/memory T cells into Foxp3-expressing
Tregs.27 The converted Tregs could inhibit autoimmune
and allergic immune responses in animal models.27 Our
adoptive transfer experiments found mice receiving UVB-induced
OVA-specific Tregs did not develop skin inflammation under OVA
stimulation conditions, indicating the sustainability of Foxp3
expression and suppressive function by these Tregs in vivo (Fig ure4). By single-cell trajectory and pseudotime analyses on patient
PBMCs, we found a trajectory originating from naive T cells,
transitioning into Tmem or Teff cells, and ultimately differentiating
into Tregs (Figure 6A). This was confirmed by analyzing
CD4+ T cells in the skin (Fig ure 6B), as well
as by the pseudotime analysis (Fig ure 6C,D).
UVB has numerous effects on local and systemic immunity. Our
findings in mice indicated that UVB suppressed immune responses to
topically applied model antigens by inducing antigen-specific Tregs.
Several other studies have reported the systemic immunosuppression of
UVB — Sasaki et al. found that UVB exposure prevented
atherosclerosis in atherosclerotic mice8; Hayashiet al. suggested that UVB exposure suppressed angiotensin
II-induced abdominal aortic aneurysm in mice by expanding
Tregs2; and Breuer et al. indicated that UVB
attenuated the systemic immune response in the central nervous
system.1 To better understand whether Tregs induced in
the local site are similar to circulating Tregs, we undertook bulk RNA
sequencing from skin-sorted Tregs in imiquimod-induced psoriasiform
mice. With pathway analysis, we found DEGs associated with biological
functions related to cell stability, cellular localization, and cell
proliferation were upregulated in skin Tregs of UVB-treated mice
(Fig ure 5A). When compiling the results with the RNA-seq
profiles from patient PBMCs, we found similar expression patterns in
genes involved in both positive and negative regulation of Treg
differentiation, maintenance, and function, as well as in TCR activation
signaling (Fig ure 5D,E).
Transcriptomic analysis of mouse skin and human blood Tregs
under UVB treatment revealed three significantly down-regulated genes,
including VAV1 , IDH1 , and RAB35 (Figure
5E). The VAV1 gene encodes the guanine