ABCB5 Identifies Immunoregulatory Dermal Cells

Abstract

Cell-based strategies represent a new frontier in the treatment of immune-mediated disorders. However, the paucity of markers for isolation of molecularly defined immunomodulatory cell populations poses a barrier to this field. Here, we show that ATP-binding cassette member B5 (ABCB5) identifies dermal immunoregulatory cells (DIRCs) capable of exerting therapeutic immunoregulatory functions through engagement of programmed cell death 1 (PD-1). Purified Abcb5(+) DIRCs suppressed T cell proliferation, evaded immune rejection, homed to recipient immune tissues, and induced Tregs in vivo. In fully major-histocompatibility-complex-mismatched cardiac allotransplantation models, allogeneic DIRCs significantly prolonged allograft survival. Blockade of DIRC-expressed PD-1 reversed the inhibitory effects of DIRCs on T cell activation, inhibited DIRC-dependent Treg induction, and attenuated DIRC-induced prolongation of cardiac allograft survival, indicating that DIRC immunoregulatory function is mediated, at least in part, through PD-1. Our results identify ABCB5(+) DIRCs as a distinct immunoregulatory cell population and suggest promising roles of this expandable cell subset in cellular immunotherapy.

Figure 1

Characterization of ABCB5⁺ dermal cell subpopulations in human skin. (A) Representative immunofluorescence staining of ABCB5 protein expression (green) in human skin (left and center-left, arrows identify ABCB5⁺ cells). The specificity of the anti-ABCB5 staining was assessed using an isotype-matched control antibody (center-right) and by a peptide competition assay (right, pre-incubation of the ABCB5 antibody with a peptide containing the amino acid sequence of the antibody binding epitope (Frank et al., 2003; Ksander et al., 2014)). Nuclei are counterstained with DAPI (blue). d, dermis; e, epidermis. (B) Quantification of ABCB5⁺ cell frequency in human skin determined by immunofluorescence staining, as above, and counting of n=8–10 independent skin sections per donor (n=3). (C) Immunofluorescence double staining (right panels) of ABCB5 (green) with CD31 or CD34 (red). Nuclei are counterstained with DAPI (blue). (D) Representative flow cytometry of ABCB5 expression in human skin and representative dual-color flow cytometric analyses examining co-expression of ABCB5 with the MSC antigens CD29, CD44, CD49e, CD73, CD105 or CD166, the endothelial lineage marker CD31, the stem cell marker CD34, or the hematopoietic lineage marker CD45. (E) Representative immunofluorescence double staining of ABCB5 (red) and PD-1 (green) expression (top panels; arrows: ABCB5⁺PD-1⁺ cells) and of CD45 (red) and PD-1 (green) expression (bottom panels; arrows: PD-1⁺CD45⁻ cells; asterix: PD-1⁺CD45⁺ cell) in human skin specimens, with nuclei counterstained in blue. (F) Flow cytometry of PD-1 expression by CD45⁺ vs. CD45⁻ cells present in single cell suspension derived from human skin biopsies (left panels: representative plots; right panel: mean percentages, n=4 patients). (G) PD-1 mRNA expression in human patient-derived DIRC short-term cultures (< 5 passages), as determined by RT-PCR amplification of the full PD-1 CDS. Human PBMCs were used as a positive control. (H) PD-1 mRNA expression determined by real-time PCR in ABCB5⁺ vs. ABCB5⁻ human skin isolates negative for the pan lymphocyte marker CD45. (I) Representative flow cytometry plots (left) and mean percentages (right) of PD-1 protein expression by ABCB5⁺ vs. ABCB5⁻ human skin cells.

Figure 2

Characterization of Abcb5⁺ dermal cell subpopulations in murine skin. (A) Representative immunofluorescence staining of Abcb5 protein expression (red) in murine skin (left and center). The specificity of the anti-Abcb5 staining was assessed using an isotype-matched control antibody (right). Arrows in center panel identify Abcb5⁺ cells. d, dermis; e, epidermis; hf, hair follicle. (B) Representative flow cytometry plots (left) and mean percentages (right) of PD-1 protein expression by Abcb5⁺ vs. Abcb5⁻ murine (BALB/c) skin cells and (C) representative immunofluorescence double staining of Abcb5 (green) and PD-1 (red) expression (top panels) and of CD45 (green) and PD-1 (red) expression (bottom panels) in murine skin specimens, with nuclei counterstained in blue. Insets: high magnification. (D) Immunohistochemical (IHC) analysis of ABCB5 expression across the anastomosis of a human skin to Rag2^−/− mouse xenograft using an anti-ABCB5 mAb that recognizes both human ABCB5 and murine Abcb5. Isotype-matched negative controls established dermal cell binding of ABCB5 mAb as specific. (E) Representative IHC images depicting co-expression of ABCB5 with human MHC class I in human skin xenografts, as above. h, human ABCB5⁺hMHC class I⁺ cell; m, murine Abcb5⁺hMHC class I⁻ cell. (F) Representative flow cytometry of surface Abcb5, CD29, CD44, CD49e, CD73, CD105, CD166, CD31, CD34 and CD45, and of (G) PD-1 expression by early passage murine DIRC cultures. (H) PD-1 mRNA expression by murine DIRCs as determined by RT-PCR amplification of the full murine PD-1 CDS and (I) representative flow cytometry plots (left) and mean percentages (right) of PD-1 protein expression by late passage murine DIRC cultures.

Figure 3

Immunoregulatory effects of Abcb5⁺ DIRCs. (A) Diagram depicting the procedure for i.v. DIRC administration (day 0) and (i) abdominal heterotopic cardiac allotransplantation or (ii) spleen cell isolation for determination of ex vivo T-cell proliferation by MLR (day 7). Cardiac allograft survival was (iii) monitored daily by palpation, with rejection defined as the complete cessation of detectable beating. For evaluation of lymphocytic infiltration and preservation of myocardium by immunohistochemistry, (iv) cardiac allograft were harvested on day 17. (B) [³H]thymidine incorporation (mean±SE) of alloimmune stimulated C57BL/6 splenocytes plotted against increasing stimulator to responder ratios. Stimulators: naïve irradiated BALB/c (left) or C3H/HeJ (right) splenocytes. Responders: Untreated, C57BL/6 DIRC- or BALB/c DIRC-treated C57BL/6 recipient splenocytes. (C) [³H]thymidine uptake (mean±SE) of mitogen-stimulated splenocytes in experimental groups as in (B). (D) Kaplan-Meier analysis of graft survival in BALB/c DIRC-treated C57BL/6 recipients of BALB/c (left) or C3H/HeJ donor hearts (center), and C57BL/6 DIRC-treated C57BL/6 recipients of BALB/c donor hearts (right), or of (E) Abcb5⁺ DIRC- vs. matched Abcb5⁻ BALB/c-derived dermal cell-treated C57BL/6 recipients of BALB/c donor hearts. (F) Representative flow cytometry plots (left) and mean percentages (right) of splenic CD4⁺CD25⁺Foxp3⁺ Treg cells in untreated vs. BALB/c DIRC-treated C57BL/6 recipient mice. (G) Left panel: Quantification of Foxp3⁺ Treg cells in cardiac allografts of untreated, recipient type-, donor type-, or third party strain DIRC-treated C57BL/6 recipients. Bars represent means ±SE of the total number of Foxp3-expressing cells per field of view. Triplicate images for n=3 mice per group were analyzed. Right panels: A representative picture of Foxp3 immunohistochemical staining on intracardiac infiltrates for each experimental group is shown.

Figure 4

Immunomoregulatory effects of PD-1 expressed by DIRCs. (A) Stable PD-1 knockdown or scramble control BALB/c DIRCs were generated by using shRNA gene silencing. Confirmation of PD-1 knockdown was determined by RT-PCR (left), real-time PCR (center), or by flow cytometry (right). (B) [³H] thymidine incorporation (mean±SE) of alloimmune stimulated C57BL/6 splenocytes at stimulator to responder ratios of 1:5 and 1:1, respectively. Stimulators: naïve irradiated BALB/c (left) or C3H/HeJ (right) splenocytes. Responders: Untreated, scrambled control- or PD-1 knockdown BALB/c DIRC-treated C57BL/6 recipient splenocytes. (C) [³H]thymidine uptake (mean±SE) of mitogen-stimulated splenocytes in experimental groups as in (B). (D) Representative flow cytometry plots (left) and mean percentage ± SE (right) of splenic CD4⁺CD25⁺Foxp3⁺ Treg frequencies in untreated, scrambled control- vs. PD-1 knockdown BALB/c DIRC-treated C57BL/6 recipients, or in (E) untreated vs. wildtype BALB/c DIRC-treated PD-L1(−/−) knockout (KO) C57BL/6 recipients. (F) Kaplan-Meier analysis of graft survival in untreated, scrambled shRNA control- vs. PD-1 knockdown BALB/c DIRC-treated C57BL/6 recipients of BALB/c donor hearts.