Pascale Alard

Hapten-specific tolerance induced by acute, low-dose ultraviolet B radiation of skin requires mast cell degranulation.

The deleterious effects of ultraviolet B radiation (UVR) on cutaneous immunity are mediated in part by cytokines released from cutaneous cells following radiation exposure. On the one hand, TNF‐α has been advocated as the primary mediator of failed contact hypersensitivity induction, and, on the other hand, IL‐10 has been held responsible for tolerance. While keratinocytes exposed toUVR have been found to produce both TNF‐α and IL‐10, there is reason to question whether these major cellular constituents of the epidermis are the relevant source of immunomodulatory cytokinesafter UVR. Dermal mast cells also produce TNF‐α and IL‐10, and we have recently reported that mast cell‐derived TNF‐α is required for UVR‐induced impairment of CH induction. In this study, we have examined whether mast cells are also a relevant source of IL‐10 in UVR‐dependent tolerance. We found that (a) UVR fails to induce tolerance in mast cell‐deficient mice, and (b) that tolerance occurs if mast cells are triggered to degranulate after ligation of the IgE receptor. Both types of tolerance were neutralized with anti‐IL‐10 antibodies, are hapten specific, and are associated with regulatory lymphoid cells. We conclude that mast cells are required in UVR‐induced tolerance and may be one of the major sources of IL‐10 that mediates the tolerance induced by acute, low‐dose UVR.

Neural Influences on Induction of Contact Hypersensitivity

ABSTRACT: Contact hypersensitivity (CH)‐induction begins when cutaneous antigen‐presenting cells (APC) capture hapten that has been applied epicutaneously, and the process prepares hapten for presentation to T‐cells. APCs are functionally plastic, are influenced by the microenvironment in which they reside, and their functional properties have a profound effect on the phenotype of the hapten‐specific T‐cells that they activate. Ultraviolet B radiation (UVR) distorts the cutaneous microenvironment, thereby altering local APC function, and changing the immune outcome from sensitization to unresponsiveness. Although UVR induces keratinocytes to produce TNFα and IL‐10 (cytokines that have been implicated in failed CH‐induction and tolerance, respectively, after UVR), dermal mast cells turn out to be the source of these immunomodulatory cytokines. Mast cell degranulation is triggered by CGRP released from UVR‐exposed cutaneous nerve termini. Even in normal skin, cutaneous nerves influence the immune response to haptens. Substance P released from cutaneous nerves acts as an adjuvant, raising the immunogenicity of epicutaneously applied haptens. Thus, the nerves and the neuropeptides that these processes release contribute to the cutaneous microenvironment. By altering APC function, cutaneous nerves can dictate the quality and the quantity of immune responses to antigens of the skin.

Evidence that ultraviolet B radiation transiently inhibits emigration of Langerhans cells from exposed epidermis, thwarting contact hypersensitivity induction.

Langerhans cells (LC) play a critical role in the induction of contact hypersensitivity (CH), and ultraviolet B radiation (UVR) impairs CH induction in UVB‐susceptible (UVB‐S) mice via a TNF‐α‐dependent mechanism. A possible explanation of this effect is that UVR impairs CH in UVB‐S mice by immobilizing LC transiently in the epidermis and upper dermis, thereby preventing their timelymigration to draining lymph nodes. To test this hypothesis we examined in vitro and in vivo the effects of in vivo UVR on migration of Ia+ cells from skin of UVB‐S and UVB‐resistant (UVB‐R) mice. Dorsal surfaces of ears of mice were irradiated with 400 J/cm2 UVB and either explanted in vitro or transplanted orthotopically to the thoracic wall of syngeneic recipients. After 24, 48, and 72 h the epidermis was recovered from these explants/grafts and the number of Ia+ cells determined by immunohistochemistry. Culture medium obtained after explants were removed was also evaluated for content of Ia+ cells. The density of Ia+‐bearing cells in the epidermis of cultured untreated skin explants and of grafted skin fell progressively for both UVB‐S and UVB‐R skin during the observation period. The rate of decline in Ia+ cells density during this interval was greatly impaired if the skin was exposed to UVR prior to excision; this effect was seen equally in UVB‐S and UVB‐R skin. Recovery of Ia+ cells in the medium after removal of cultured untreated skin explants was maximum after 24 h andcomparable in UVB‐S and UVB‐R skin. However, the number of Ia+ cells recovered in the medium from UVB‐exposed skin was significantly reduced only if the skin donor was UVB‐S. We conclude that the ability of UVR to impede LC migration from epidermis is significantly greater for UVB‐S mice, accounting in part for the failure of these mice after UVR to acquire CH.

Aqueous Humor Induces Transforming Growth Factor-β (TGF-β)-Producing Regulatory T-Cells

Purpose: The intraocular microenvironment is an immuneprivileged site where immunogenic inflammation is suppressed. Suppression of immunogenic inflammation has been associated with immunosuppressive factors found in aqueous humor produced by ocular tissues. To further understand the mechanisms suppressing immunogenic inflammation in the eye, we have examined the production of lymphokines by primed T-cells activated in the presence of aqueous humor. Methods: Enriched in vivo primed T-cells were T-cell receptor-stimulated in the presence of fresh aqueous humor. The culture supernatant was assayed for IFN-γ and IL-4 by sandwich ELISA. TGF-β production by T-cells stimulated in the presence of aqueous humor was assayed by a TGF-β bioassay of the culture supernatant and by quantitative RT-PCR for TGF-β1 mRNA expression. Aqueous humor-treated T-cells were assayed for their capacity to suppress IFN-γ production by stimulated, primed T-cells. Results: Aqueous humor-enhanced proliferation but irreversibly suppressed production of both IFN-γ and IL-4 by in vitro-activated, in vivo-primed T-cells. Aqueous humor induced in vivo primed T-cells to produce TGF-β in vitro, and these TGF-β-producing T-cells suppressed IFN-γ production by other T-cells activated in co-cultures. Conclusions: Aqueous humor alters the functional program of TCR-ligand-activated, primed T-cells, converting the cells to TGF-β-producing regulatory cells. Curr. Eye Res. 16: 900–908, 1997.