Georgetown University Hospital, Washington, District of Columbia, USA

In this issue of the Journal,^{Mallbris et al (2005)} describe an effect of sunlight on the human innate immune system that could be of great clinical significance. The Karolinska team reports that the UVB band of ultraviolet light stimulates expression of hCAP18, the precursor of the antimicrobial peptide LL-37, in human skin. The group conducted a clinical study in which they exposed the skin of eight individuals to a minimal erythematous dose of both UVA (300–400 nm) and UVB (280–315 nm) wavelengths, at separate body sites, and biopsied these sites 24 h after exposure (as well as prior to UV exposure). These biopsies were then analyzed for the abundance of hCAP18 mRNA and Vitamin D receptor protein. The investigators report that each of the individuals exposed to UVB light exhibited an increase in the amount of both hCAP 18 mRNA and Vitamin D receptor in their skin. A positive correlation between induction of the skin antimicrobial peptide and the Vitamin D receptor was noted, suggesting that hCAP18 expression was influenced positively by the increase in Vitamin D receptor levels. The effect was not observed following UVA exposure. The magnitude of the response appeared not to correlate with the complexion of the individual (e.g., people who tan easily and rarely burn in sun vs those who tan poorly and sunburn easily). There was a surprising variation among individual responses in both the magnitude of the response to UV exposure as well as with respect to the levels of both hCAP18 mRNA and vitamin D receptor protein measured in the unexposed skin.

This report follows a prior clinical study by the same team (^{Weber et al, 2005}) in which the topical administration of a Vitamin D analog (calcipotriol) onto the skin of volunteers was shown to increase the expression of fully processed LL 37 peptide within 24 h of administration. The authors demonstrated in vitro that the responsiveness of the hCAP18 gene in human keratinocytes to Vitamin D was dependent on the presence of a Vitamin D receptor-DNA binding sequence located about 500 nucleotides upstream from the gene, confirming an earlier study conducted in vitro by another group (^{Wang et al, 2004}). In vitro, both 1,25 dihydroxy Vitamin D₃ and 25 hydroxy Vitamin D₃, but not the Vitamin D precursor 7 dehydrocholesterol (7DHC) exhibited stimulatory activity.

Together, these reports strongly suggest that exposure of human skin to UVB light can stimulate expression of the antimicrobial peptide LL 37, likely through a chain of events that involves Vitamin D.

The discovery that sunlight, through the mediation of Vitamin D, activates an arm of innate immunity within the skin of man could have far-ranging implications with respect to our understanding of certain human skin conditions and their treatment.

Caucasians are surprisingly prone to sunburn. Ultraviolet radiation within the UVB bandwidth causes photochemical alterations in both the DNA and membrane lipids of epidermal cells (^{Halliday, 2005}). Furthermore, low doses of UVB appear to coordinately suppress keratinocyte proliferation and stimulate cellular maturation. Surprisingly, low doses of UVB also suppress certain arms of the adaptive immune system (^{Halliday, 2005}). For example, UVB treatment depresses acute and delayed hypersensitivity reactions. Certain chronic inflammatory skin conditions, such as psoriasis, are treated with measured doses of UVB, in part for this reason. UVB is believed to cause immunosuppression by direct action on immune cells within the skin, as well as indirectly. Vitamin D, converted by UVB in skin from 7DHC has potent immunosuppressant properties. Indeed, the active forms of Vitamin D have been shown to inhibit T cell proliferation, inhibit expression of MHC class II proteins on antigen presenting cells, and depress the activity of Langerhans cells (^{Lehmann et al, 2004}). In addition, sunlight stimulates the keratinocyte and the melanocyte to express MSH, a hormone that not only plays a role in the pigmentation of skin but also serves to suppress epidermal inflammation (^{Brogden et al, 2005}).

Thus, sunlight, which so readily damages skin normally, also "quiets" the adaptive immune system within the skin, in part through the activation of the Vitamin D signaling pathway. We presume that without the induction of UVB-associated immunosuppression we would suffer severe inflammation following "normal" exposure to sunlight. The reports by Mallbris et al, Weber et al, and Wang et al tell us that while the adaptive system is being "toned down", UVB, via the Vitamin D pathway, also causes a "non-inflammatory" antimicrobial component of the innate immune system to be "turned on".

LL-37 is one of several abundant antimicrobial peptides expressed by the keratinocytes and the epidermal glands of man (^{Gallo, 2005}). Antimicrobial peptides are widely expressed throughout nature and provide a means by which multicellular organisms can defend themselves rapidly against microbial assault (^{Zasloff, 2002}). In mammals, most antimicrobial peptides appear to be synthesized in epidermal layers above the stem cell layer and accumulate within the superficial layers of the epidermis and on the skin surface. Several antimicrobial peptides such as LL-37, -defensins 2 and 3, and psoriasin are inducible, usually following skin injury or exposure to microbial products (^{Gallo, 2005}). It is also likely that antimicrobial peptides play an important role in the control of microbial flora on the intact and damaged epithelial surface, both by suppressing the growth of certain microorganisms and by "stabilizing" the numbers and diversity of commensal flora (^{Zasloff, 2002}). In addition to their anti-infective activity, antimicrobial peptides exhibit biological activities that provide benefit in the setting of skin injury, such as attraction for certain immune cells, epithelial repair, and angiogenesis (^{Koczulla et al, 2003}). In certain diseases, like psoriasis, intense expression of several antimicrobial peptides is noted; presumably, for this reason, skin infections in this condition are rarely observed. In atopic dermatitis (^{Gallo, 2005}), in contrast, as well as in chronic leg ulcers (^{Heilborn et al, 2003}) and thermal burns (^{Gallo, 2005}), depressed expression of antimicrobial peptides has been reported, likely to be the mechanism underlying the association of the infections that frequently complicate these skin conditions.

The observation by Mallbris et al that UVB stimulates expression of LL-37 in epidermis is not without some surprise. It leads us to speculate that acute exposure of the skin to sunlight might tip the "immune balance" in favor of innate immune defense. Functional deficits in the defense of the skin against microbes caused by suppression of local adaptive immunity could be compensated for by stimulation of antimicrobial peptide expression (see Figure 1). We note that UVB can stimulate innate immunity in yet other ways. MSH, although a potent immunosuppressant itself—as well as a "hormone" or "neuropeptide"—also has, independently, potent antifungal activity; thus, stimulation of "tanning" is accompanied by the generation of local antifungal defenses. Pain, associated with sunburn, involves stimulation of nocioceptive C-type fibers in the epidermis, which release substance P when activated. Substance P is a potent pro-inflammatory peptide with neutrophil attractant properties. Substance P also exhibits direct antimicrobial activity, suggesting that when pain is associated with sunburn damage, sensory nerves are discharged to further coordinate specific defensive scenarios within the innate branch of immunity (^{Brogden et al, 2005}).

Figure 1.

Full figure (15K)

This report provokes many questions and thoughts. In the study reported by Malbris et al epidermal hCAP18 expression ranged over 100 fold across the study population, in both unexposed skin and at sites exposed to UVB light. Do these variations relate to the proclivity of an individual to skin cancer? Many natural and synthetic antimicrobial peptides exhibit cytotoxic activity against transformed cells (^{Gallo, 2005}), likely a result of the enrichment of anion phospholipids on their surface; might those individuals who respond to UV exposure with "robust" expression of LL-37 stand a lesser chance of having a transformed cell survive in the epidermis than those that rely more heavily on activation of an adaptive immune response to clear the cancerous cell? In addition to LL-37, which other antimicrobial peptides are induced by Vitamin D and UVB? Would the judicious use of Vitamin D reduce inflammation and/or scarring in the setting of epidermal wound repair by augmenting antimicrobial peptide expression? Burns appear to predispose to a state of Vitamin D deficiency, due to a depression of Vitamin D synthesis in both damaged and uninjured skin caused by an unknown mechanism (^{Klein et al, 2004}). Are burn victims generally impaired in antimicrobial peptide expression? Should we consider treatment of burn victims with topical Vitamin D? The diabetic foot ulcer and the venous stasis ulcer are chronic wounds often complicated by complex polymicrobial infections. Would the judicious use of either UVB or Vitamin D prior to skin breakdown in patients pre-disposed to these wounds (to stimulate local expression of LL-37) alter the natural history of their skin disease? Since atopic dermatitis has been associated with depressed expression of LL-37, should judicious use of Vitamin D and/or UVB be considered?