A single intradermal injection of IFN-γ induces an inflammatory state in both non-lesional psoriatic and healthy skin

Abstract

Psoriasis is a chronic, debilitating, immune-mediated inflammatory skin disease. As IFN-γ is involved in many cellular processes, including activation of dendritic cells (DCs), antigen processing and presentation, cell adhesion and trafficking, and cytokine and chemokine production, IFN-γ-producing Th1 cells were proposed to be integral to the pathogenesis of psoriasis. Recently, IFN-γ was shown to enhance IL-23 and IL-1 production by DCs and subsequently induce Th17 cells, which are important contributors to the inflammatory cascade in psoriatic lesions. To determine whether IFN-γ indeed induces the pathways expressed in psoriatic lesions, a single intradermal injection of IFN-γ was administered to an area of clinically normal, non-lesional (NL) skin of psoriasis patients and biopsies were collected 24 hours later. Although there were no visible changes in the skin, IFN-γ induced many molecular and histological features characteristic of psoriatic lesions. IFN-γ increased a number of differentially expressed genes in the skin, including many chemokines concomitant with an influx of T cells and inflammatory DCs. Furthermore, inflammatory DC products tumor necrosis factor (TNF), inducible nitric oxide synthase, IL-23, and TNF-related apoptosis-inducing ligand were present in IFN-γ-treated skin. Thus, IFN-γ, which is significantly elevated in NL skin compared with healthy skin, appears to be a key pathogenic cytokine that can induce many features of the inflammatory cascade of psoriasis.

Fig. 1. IFN-γ injection induces genomic expression of known IFN-γ-regulated chemokines

mRNA expression levels normalized to hARP for the T cell (CXCL9 and CXCL10) and DC (CCL2 and CX₃CL₁) chemokines in baseline, placebo and IFN-γ-injected healthy (black bars, n=10) and psoriatic skin (white bars, n=10). Error bars represent the mean ± SEM. IFN-γ-injected skin was compared to placebo. (**) P<0.01.

Fig. 2. IFN-γ injection induces dermal inflammatory myeloid DCs

Representative immunohistochemistry of (a) CD11c⁺ and (b) CD1c⁺ cells in psoriasis skin. (c) Quantification of CD11c⁺, CD1c⁺, and inflammatory DCs (CD11c⁺ minus CD1c⁺ cells) in healthy (black circles) and psoriasis skin (white circles). Each circle represents a different patient. IFN-γ-injected skin is compared to placebo. (***) P<0.001. (d) Two-color immunofluorescence of CD1c⁺ DCs (red) and CD11c⁺ DCs (green). The white line delineates the dermal epidermal junction. Scale bar = 100μm.

Fig. 3. IFN-γ injection induces expression of known products of inflammatory myeloid DCs

(a–c) Two-color immunofluorescence of CD11c⁺ DCs with inflammatory products: (a) TRAIL, (b) TNF, and (c) iNOS in baseline, placebo and IFN-γ-injected psoriasis skin. The white line delineates the dermal epidermal junction. Scale bar = 100μm. (d) mRNA expression levels normalized to hARP for additional DC cytokines in healthy (black bars, n=10) and psoriatic skin (white bars, n=10). Error bars represent the mean ± SEM. IFN-γ-injected skin is compared to placebo. (*) P<0.05, (**) P<0.01.

Fig. 4. IFN-γ injection induces lymphoid-like structures and expression of known lymphoid genes and proteins

(a) mRNA expression levels normalized to hARP for the lymphoid genes in healthy (black bars, n=10) and psoriatic skin (white bars, n=10). Error bars represent the mean ± SEM. IFN-γ-injected skin is compared to placebo. (**) P<0.01, (***) P<0.001. (b) Two-color immunofluorescence of CD11c⁺ DCs (red) and CD3⁺ T cells (green), in psoriasis skin. White arrows denote DC/T cell clusters. The white line delineates the dermal epidermal junction. (c–d) Representative immunohistochemistry of (c) peripheral node addressin (PNAd) and (d) ICAM-1 in psoriasis skin. Scale bar = 100μm.

Fig. 5. The gradient of IFN-γ mRNA expression parallels T cell and DC infiltration as psoriasis becomes more severe

(a) IFN-γ mRNA expression in healthy (n=10), NL skin from mild (n=10, BSA<10%) versus moderate-to-severe (n=9, BSA>10%) psoriasis patients, IFN-γ-injected healthy (n=10) or mild NL psoriasis (n=10) skin and psoriasis LS (n=9) skin. Error bars represent the mean ± SEM. (**) P<0.01. (***) P<0.001. (b) Quantification of CD3⁺ T cells (blue bars) and CD11c⁺CD1c⁻ inflammatory DCs (red bars) counts per mm of skin in patients in (a).