CD49a Expression Defines Tissue-Resident CD8+ T Cells Poised for Cytotoxic Function in Human Skin

Abstract

Tissue-resident memory T (Trm) cells form a heterogeneous population that provides localized protection against pathogens. Here, we identify CD49a as a marker that differentiates CD8⁺ Trm cells on a compartmental and functional basis. In human skin epithelia, CD8⁺CD49a⁺ Trm cells produced interferon-γ, whereas CD8⁺CD49a^- Trm cells produced interleukin-17 (IL-17). In addition, CD8⁺CD49a⁺ Trm cells from healthy skin rapidly induced the expression of the effector molecules perforin and granzyme B when stimulated with IL-15, thereby promoting a strong cytotoxic response. In skin from patients with vitiligo, where melanocytes are eradicated locally, CD8⁺CD49a⁺ Trm cells that constitutively expressed perforin and granzyme B accumulated both in the epidermis and dermis. Conversely, CD8⁺CD49a^- Trm cells from psoriasis lesions predominantly generated IL-17 responses that promote local inflammation in this skin disease. Overall, CD49a expression delineates CD8⁺ Trm cell specialization in human epithelial barriers and correlates with the effector cell balance found in distinct inflammatory skin diseases.

Keywords: CD49a; IL-15; Skin; cytotoxicity; granzyme B; immunopathology; perforin; psoriasis; tissue resident T cells; vitiligo.

Graphical abstract

Figure 1

Healthy Human Skin Contains a Population of Clonally Enriched CD49a⁺ Trm Cells in Epidermis (A–C) Phenotypic analysis of live CD3⁺TCRγδ⁻ T cells from blood, dermis, and epidermis from healthy donors (n = 20). (A) Representative FACS plots of CD103, CD69, and CD49a expression in epidermal CD4 or CD8 T cells. (B and C) Frequencies of CD103⁺CD69⁺ (B) and CD103⁺CD69⁺CD49a⁺ (C) in CD4⁺ or CD8⁺ T cells. Each point represents data from an individual donor. Median depicted. Two-tailed Wilcoxon tests, ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001. (D) Confocal image of cryopreserved healthy skin depicting CD8 (red), Keratin5/6 (blue), and collagen IV (white). Scale bar, 100 μm. (E) Confocal images of epidermal CD8⁺CD49⁻ (upper panel) and CD8⁺CD49⁺ (lower panel) cells relative to the collagen IV-rich basement membrane. CD8 (red), CD49a (green) DAPI (blue), collagen IV (white) are depicted. Scale bars, 10 μm. Images are representative of ten healthy donors. (F–J) Analysis of TCRβ CDR3 region in sorted CD8⁺ T cells from blood, dermis, and epidermis of healthy donors (n = 5). (F) Pie charts show the frequency of TCR Vβ families in T cell subsets from three donors. The top three Vβ families in epidermal CD8⁺CD103⁺CD49a⁺ are marked by color-coded asterisks. (G) The cumulative frequency of reads among top 20 clonotypes from epidermal CD8⁺CD103⁺CD49a^– (left) and CD8⁺CD103⁺CD49a⁺ (right) Trm cells. (H) Proportion of shared clonotypes (left) and cumulative frequency of shared reads (right) between CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm cells. (I) Log scatterplot from donor 1–3 showing proportion of reads for each unique clonotype in CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm cells. Major shared clones are highlighted with red circles. (J) Total frequency of productive reads of the top ten clonotypes of CD8⁺CD103⁺CD49a^– (left) and CD8⁺CD103⁺CD49a⁺ (right) Trm cells in the total reads of CD8⁺ T cell subsets in blood, dermis and epidermis. Mean ± SD is depicted. See also Figure S1.

Figure 2

Distinct Transcriptional Profiles of CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm Cells (A–E) RNA-seq analyses of CD8⁺ T cell subpopulations sorted from blood, dermis, or epidermis of healthy individuals. (A and B) Principal component analysis of the top 200 differentially regulated genes among (A) all six subpopulations or (B) four skin-derived subpopulations after mean centering to correct for batch effects. Dots indicate samples of different subpopulations from a total of 6 donors, as indicated. Principal component 1 and 2 represents the largest source of variation, combined accounting for (A) 70.6% and (B) 43.1% of the total variation, respectively. Color-coded eclipses indicate the 95% confident area of particular T cell subpopulations. (C–E) Transcriptome analysis of CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm cells sorted from healthy epidermis (n = 7). (C) MA-plot (Log₂ fold change against Log₂ count per million) showing differential gene expression between epidermal CD8⁺CD103⁺CD49a⁻ and CD8⁺CD103⁺CD49a⁺ Trm cells. Each gene was symbol-coded as indicated according to their adjusted p values generated using EdgeR with FDR correction. (D) Functional annotation analysis using DAVID tool listing gene sets upregulated (upper) or downregulated (lower) of CD8⁺CD103⁺CD49a⁺ Trm cells as compared to CD8⁺CD103⁺CD49a^– Trm cells. (E) Heatmap showing gene expression of selected genes associated with functional annotations. Each column represents an individual donor. Row-mean centered relative expression values are shown. (F) Quantitative real-time PCR measurements of transcript levels of specific genes, as indicated, in epidermal CD8⁺CD103⁺CD49a^– relative to CD8⁺CD103⁺CD49a⁺ Trm cell subsets. Fold-change was calculated against the mean expression of the CD8⁺CD103⁺CD49a^– Trm cells. Mean ± SD depicted. (G) Surface expression of CXCR3, CCR6 and IL-23 receptor (IL23R) in CD8⁺CD103⁺CD49a^– or CD8⁺CD103⁺CD49a⁺ Trm cells, as assessed by flow cytometry. Data are presented with respect to frequency of positive cells and their respective median florescence intensity (MFI) in subsets, as indicated. Two-tailed Wilcoxon tests. ^∗p < 0.05, ^∗∗p < 0.01. See also Figure S1.

Figure 3

IL-2 and IL-15 Induce Perforin and Granzyme Expression Specifically in Epidermal CD8⁺CD103⁺CD49a⁺ T Cells (A) Representative FACS plot of perforin and granzyme B expression in relation to CD49a expression in epidermal CD8⁺CD103⁺ Trm cells with or without IL-15 (20 ng/mL) stimulation for 48 hr. (B) Perforin (black) and granzyme B (gray) expressing cells among epidermal CD8⁺CD103⁺ Trm cells following 48 hr incubation with medium, IL-2 (20 ng/ml), IL-7 (20 ng/ml), IL-15 (20 ng/ml), IL-1β (20 ng/ml), IL-6 (20 ng/ml), IL-23 (20 ng/ml), IL-12, (50 ng/ml), IFN-α (2,000 U/ml). Mean ± SD is depicted. Kruskal-Wallis with Dunn’s multiple comparison tests. (C) Proportion of perforin and granzyme B expressing cells among epidermal CD8⁺CD103⁺CD49a^– or CD8⁺CD103⁺CD49a⁺ Trm cells upon IL-15 stimulation (n = 12). Two-tailed Wilcoxon test. (D) CD8⁺CD103⁺ Trm cells were sorted and stimulated with IL-15 (20 ng/ml) for 1, 2, 4, 8, or 48 hr. Mean ± SD of the increase (Δ) in perforin (left) and granzyme B (right) expressing cells among CD8⁺CD103⁺CD49a^– (inverted triangle) or CD8⁺CD103⁺CD49a⁺ (upright triangle) Trm cells as compared to unstimulated (n = 4). Two-tailed paired t test with Holm-Sidak correction. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001. See also Figure S2.

Figure 4

CD49a Expression Identifies Trm Cells with Cytotoxic Potential in Diverse Epithelial Tissues (A) SPICE charts of CD103, CD69, and CD49a expression in CD4⁺ (upper) and CD8⁺ (lower) T cells from peripheral blood, skin dermis and epidermis, full thickness gut (ileum), and ectocervical submucosa and epithelia. (B) Frequency of perforin and granzyme B-expressing cells epidermal Trm cells from gut, cervical submucosa and epithelium, as indicated, in the absence or presence of IL-15 stimulation (20 ng/ml) for 48 hr. Two-tailed Wilcoxon tests were used to test for statistical significance, ^∗p < 0.05. See also Figure S3.

Figure 5

IL-15 Stimulation Induces Strong Cytotoxic Function by CD8⁺CD103⁺CD49a⁺ Trm Cells (A) Specific lysis by sorted effector cells of P815 target cells in the presence of anti-CD3 antibody. Unstimulated (n = 3, left) or IL-15 stimulated (n = 6, right) blood derived CD57⁺CD8⁺ or CD57⁻CD8⁺ T cells, epidermal CD8⁺CD103⁺CD49a⁺, or CD8⁺CD103⁺CD49a^– Trm cells at effector to target ratios as displayed. Mean ± SD depicted. (B) Perforin (left) and granzyme B (right) expressing Trm cells following stimulation with IL-15 (20 ng/ml), anti-CD3 (1 μg/ml), or IL-15 + anti-CD3 in the presence of P815 cells (n = 4). Mean ± SD depicted. RM-ANOVA test with Holm-Sidak’s multiple comparisons test was employed for statistical significance. ^∗p < 0.05, ^∗∗p < 0.01. See also Figure S4.

Figure 6

Specialization with Respect to IL-17 and IFN-γ Production between Epidermal CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm Cells (A) Representative FACS plots of IFN-γ, IL-17A, and CD49a expression in epidermal CD8⁺CD103⁺ T cells. (B) SPICE charts depict expression of IFN-γ, IL-17A, TNF, and IL-2 in PMA plus ionomycin stimulated blood-derived total CD8⁺ T cells, dermal CD8⁺CD103^– and CD8⁺CD103⁺ T cells, as well as epidermal CD8⁺CD103⁺CD49a^– and CD8⁺CD103⁺CD49a⁺ Trm cells (n = 6). (C) Proportion of IFN-γ, IL-17A, IL-2, or TNF expressing cells among indicated T cell subpopulations (n = 6). ANOVA test with Holm-Sidak’s multiple comparisons. Mean ± SD depicted. (D) Proportion of IFN-γ- or IL-17A-expressing CD8⁺CD103⁺CD49a^– or CD8⁺CD103⁺CD49a⁺ Trm cells, as indicated, following sorting of epidermal explanted CD8⁺CD103⁺ Trm cells and combinations of stimulation with anti-CD3 antibody and IL-15 (20 ng/mL). (E) Sorted CD8⁺CD103⁺ Trm cells were seeded in non-coated or collagen IV-coated wells and cultured in the presence or absence of IL-15 (20 ng/mL) for 20–24 hr before anti-CD3 stimulation. Bar charts show the frequency of IFN-γ- and IL-17A-expressing cells among CD8⁺CD103⁺CD49a^– or CD8⁺CD103⁺CD49a⁺ T cells (n = 5), as indicated. Mean ± SD depicted. ANOVA test with Holm-Sidak’s multiple comparisons. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001. See also Figure S5.

Figure 7

Functional Dichotomy of CD8⁺CD103⁺CD49a⁺ and CD8⁺CD103⁺CD49a^– Trm Cells Is Preserved in Vitiligo and Psoriasis (A) Representative FACS plots show CD103 and CD49a expression on dermal and epidermal CD8⁺ T cells from healthy skin, vitiligo, and psoriasis lesions. (B) Frequency of CD49a⁺ cells among dermal or epidermal CD8⁺CD103⁺ Trm cells in skin from healthy donors (HC, n = 20), vitiligo (V, n = 13), or psoriasis lesions (Ps, n = 11). (C and D) Proportion of perforin or granzyme B expressing cells among (C) dermal or (D) epidermal CD8⁺CD103⁺ Trm cells from healthy skin (n = 13), vitiligo (n = 9), or psoriasis lesions (n = 8). (E) Representative contour plots of dermal CD8⁺CD103⁺CD49a⁻ (upper) or CD8⁺CD103⁺CD49a⁺ (lower) Trm cells in vitiligo lesions. (F) Proportion of perforin and granzyme B co-expressing cells among Trm cells from healthy skin or vitiligo lesions, as indicated. (G) Proportion of IFN-γ or IL-17A expressing cells among PMA/ionomycin stimulated epidermal CD8⁺CD103⁺ T cells from healthy skin (n = 12), vitiligo (n = 7), or psoriasis lesions (n = 8). (H) Representative contour plots showing IFN-γ/IL-17A expression in PMA/ionomycin stimulated epidermal CD8⁺CD103⁺CD49a⁻ (upper) and CD8⁺CD103⁺CD49a⁺ T cells (lower) from vitiligo (left) and psoriasis (right) lesions. (I and J) Proportion of IFN-γ and IL-17A expressing epidermal CD8⁺CD103⁺ T cells from (I) vitiligo or (J) psoriasis lesions. Two-tailed Wilcoxon tests in (G, J, and K) and Kruskal-Wallis tests with Dunn’s multiple comparison tests in (B–E and H). ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001. See also Figure S6.