Defective immune response and severe skin damage following UVB irradiation in interleukin-6-deficient mice

Abstract

Interleukin-6 (IL-6), a multifunctional cytokine, is induced in the acute-phase reaction following ultraviolet (UV) irradiation of humans and mice. Using IL-6-deficient (IL-6-/-) mice, we investigated the role of IL-6 in immunosuppression and inflammatory responses caused by UVB (280-320 nm) radiation. The IL-6-/- mice had a defective contact hypersensitivity (CHS) in response to the sensitizers 2,4-dinitrofluorobenzene and oxazolone. The injection of recombinant IL-6 (rIL-6) into these mice resulted in a marked recovery of the CHS. Serum IL-6 was significantly elevated by UV irradiation of wild-type B6 J/129Sv (IL-6+/+) mice but was not detectable in IL-6-/- mice. Interestingly, there was no induction of serum interleukin-10 (IL-10) by UV irradiation of IL-6-/- mice, whereas UV exposure caused a significant increase in serum IL-10 levels in IL-6+/+ mice. Injection of rIL-6 into IL-6-/- mice increased IL-10 to levels similar to those of IL-6+/+ mice. Being different from IL-6+/+ mice, no epidermal proliferation was found at 48 hr in the IL-6-/- mice, but delayed cell proliferation was observed at 72 hr after UV exposure. Immunohistochemical analysis demonstrated that the epidermis was capable of synthesizing IL-6 at 72 hr after UV irradiation of IL-6+/+ mice. In addition, the IL-6-positive cells appeared to be Langerhans' cells, which were detected with dendritic cell-reactive S-100 antibody. The present study strongly suggests that IL-6 may play a crucial role in the alteration of cutaneous immune responses following UV exposure, and provides evidence that IL-6 is a potent inducer of IL-10. Furthermore, IL-6 production induced by UV radiation appears to be an important early signal for repair of UV-caused skin damage.

Figure 1

Reduced contact hypersensitivity (CHS) to 2,4-dinitrofluorobenzene (DNFB) in interleukin (IL)-6^−/− mice with and without exposure to UV radiation. A severely impaired CHS reaction was found in the IL-6^−/− mice compared with IL-6^+/+ mice. UV exposure suppressed the CHS response in IL-6^+/+ mice. The data represent mean±SD for five mice. **Indicates a significant difference (P <0·01) from non-UV-irradiated IL-6^+/+ mice.

Figure 2

Restoration of contact hypersensitivity (CHS) to oxazolone in recombinant interleukin-6 (rIL-6)-injected mice. IL-6^−/− mice injected with rIL-6 showed significant restoration of the CHS response to oxazolone. The data represent mean±SD for six mice.** and **a indicate a significant difference (P < 0·01) from IL-6^+/+ and IL-6^−/− mice, respectively.

Figure 3

Changes in serum interleukin (IL)-6 and IL-1β levels in IL-6^−/− and IL-6^+/+ mice at various time-points following UV exposure. (a) Serum IL-6 was elevated by UVB irradiation and peaked at 48 hr in IL-6^+/+ mice, but was not detectable in IL-6^−/− mice. (b) Serum IL-1β increased slightly following UV exposure in IL-6^−/− mice in comparison with IL-6^+/+ mice. The data represent mean±SD for three mice. *Indicates a significant difference (P < 0·05) from IL-6^+/+ or IL-6^−/− mice.

Figure 4

Induction of serum interleukin (IL)-10 in IL-6^−/− mice injected with recombinant interleukin-6 (rIL-6) at various time-points following UV exposure. There was no induction of serum IL-10 in IL-6^−/− mice after UV exposure, although IL-10 was dramatically induced by UV irradiation of IL-6^+/+ mice. Injection of rIL-6 significantly induced serum IL-10 levels at 24, 48 and 72 hr following UV exposure in IL-6^−/− mice. The data represent mean±SD for four mice. *Indicates a significant difference (P <0·05) from IL-6^−/− mice.

Figure 5

Defective epidermal proliferation in interleukin (IL)-6^−/− mice following UV exposure. Unlike IL-6^+/+ mice, IL-6^−/− mice had no epidermal hyperplasia until 48 hr post-UVB, followed by a delayed increase at 72 hr post-UVB. The data represent mean±SD for three mice. **Indicates a significant difference (P <0·01) from IL-6^+/+ mice.

Figure 6

Greater skin damage by UV exposure in interleukin (IL)-6^−/− mice compared with IL-6^+/+ mice. (a–c) Skin of IL-6^+/+ mice at 24, 48 and 72 hr after UV exposure, respectively. (d–f) Skin of IL-6^−/− mice at 24, 48 and 72 hr after UVB exposure, respectively. Haematoxylin and eosin (H&E) staining showed the absence of epidermal proliferation at 48 hr following UV exposure and the appearance of delayed epidermal proliferation at 72 hr in the IL-6^−/− mice. IL-6^−/− mice had greater severity of skin damage on gross appearance than IL-6^+/+ mice. Bar, 50 μm.

Figure 7

Positive immunostaining for interleukin (IL)-6 and dendritic cells at 72 hr following UV exposure in IL-6^+/+ mice. (a) Immunostaining showed the induction of IL-6 at 72 hr after UV irradiation. Arrowheads indicate the IL-6 positive cells. (b) Immunostaining with S-100 antibody confirmed that IL-6 was induced by UV irradiation in epidermal dendritic cells, probably Langerhans' cells (arrowheads). Bar, 50 μm.