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Effect of oral eicosapentaenoic acid on epidermal Langerhans cell numbers and PGD2 production in UVR‐exposed human skin: a randomised controlled study

Langerhans cells (LCs) are sentinels of skin's immune system, their loss from epidermis contributing to UVR suppression of cell‐mediated immunity (CMI). Omega‐3 polyunsaturated fatty acids show potential to reduce UVR suppression of CMI in mice and humans, potentially through modulation of LC migration. Our objectives were to examine whether eicosapentaenoic acid (EPA) ingestion influences UV‐mediated effects on epidermal LC numbers and levels of immunomodulatory mediators including prostaglandin (PG)D2, which is expressed by LC. In a double‐blind randomised controlled study, healthy individuals took 5‐g EPA‐rich (n=40) or control (n=33) lipid for 12 weeks; UVR‐exposed and unexposed skin samples were taken pre‐ and postsupplementation. Epidermal LC numbers were assessed by immunofluorescence for CD1a, and skin blister fluid PG and cytokines were quantified by LC‐MS/MS and Luminex assay, respectively. Presupplementation, UVR reduced mean (SEM) LC number/mm2 from 913 (28) to 322 (40) (P<.001), and mean PGD2 level by 37% from 8.1 (11.6) to 5.1 (5.6) pg/μL; P<.001), while IL‐8 level increased (P<.001). Despite confirmation of EPA bioavailability in red blood cells and skin in the active group, no between‐group effect of EPA was found on UVR modulation of LC numbers, PGD2 or cytokine levels postsupplementation. Thus, no evidence was found for EPA reduction of photoimmunosuppression through an impact on epidermal LC numbers. Intriguingly, UVR exposure substantially reduced cutaneous PGD2 levels in humans, starkly contrasting with reported effects of UVR on other skin PG. Lowered PGD2 levels could reflect LC loss from the epidermis and/or altered dendritic cell activity and may be relevant for phototherapy of skin disease.