3.2 UVB induces OVA-specific Tregs in mouse model of OVA-induced skin inflammation
We hypothesized that UVB-induced activated Tregs are antigen-specific. To investigate the impact of UVB on the induction of antigen-specific Tregs, we induced skin inflammation in mice using ovalbumin (OVA) as a model antigen; the experimental protocol is shown in Figure 2A. OVA stimulation caused significant skin inflammation, characterized by redness, increased transepidermal water loss (TEWL), epidermal thickening, and inflammatory responses, and all were alleviated by UVB treatment (Figure 2B-D). To assess immune responses, cells isolated from the skin were restimulated with PMA and ionomycin, and intracellular cytokine staining with flow cytometric analysis was performed. Increased frequencies of IL-4-, IL-5-, and IL-22-producing CD4+T cells (gated as shown in Figure S3A) were noted in OVA-stimulated mice, and UVB treatment significantly reduced these inflammatory T cells (Figure 2E). These findings demonstrate that UVB inhibits inflammation induced by a model antigen.
We further analyzed skin-draining lymph node (dLN) cells (Figure S3B). UVB increased the frequency of CD25+Foxp3+ Tregs and upregulated the expression of TCR-activation markers, including CD25, ICOS, CTLA-4, TIGIT, Tim3, CD39, and Helios (Figure 2F). Similar findings were also detected in the skin (Fig S3C). These findings suggested enhanced antigen specificity. To test this, we used OVA323-339tetramers to identify OVA-specific Tregs (gated as shown in Figure S3D). We found that OVA-specific Tregs in UVB-treated mice were significantly increased (Figure 2G). To further investigate the role of Tregs in UVB-mediated immune suppression, we depleted Tregs in Foxp3DTR-GFP mice using diphtheria toxin (Figure S4A). Treg depletion under UVB treatment exacerbated skin inflammation and increased IL-4-producing CD4+ T cells (Figure S4B-D). In summary, UVB-induced immunosuppression is mediated by an increase in antigen-specific Tregs.