IL-15 Promotes the Survival of Anti-Inflammatory (M2), Immunoinhibitory (IL-10+) Dermal Macrophages in Human Eyelid Skin Under IFNγ-Dominated Inflammatory Conditions.

Interleukin (IL)-15 is primarily known as a pro-inflammatory and anti-apoptotic cytokine, which stimulates the proliferation and survival of key immunocytes, including macrophages (MACs). Yet, it remains unclear how IL-15 specifically impacts MACs in intact human skin, particularly immunoinhibitory, IL-10-producing/secreting M2 MACs (CD206(+)IL-10(+)). In the current pilot study, we explored this in organ-cultured healthy human eyelid skin in the presence of IFNγ (100 IU/mL) to mimic a pro-inflammatory signaling milieu found in several chronic immunodermatoses. Quantitative immunohistomorphometry showed that IFNγ significantly reduced the number of CD68(+)MACs, M2 CD206(+)MACs, and immunoinhibitory CD206(+)IL-10(+)MACs. Moreover, co-administering recombinant human (rh) IL-15 after inducing inflammation by IFNγ largely reversed the IFNγ-induced decline in MAC populations. To investigate if this was mediated via the private IL-15 receptor alpha (IL-15Rα), we successfully silenced IL-15Rα in human skin ex vivo. Indeed, co-administration of IL-15Rα siRNA abrogated the rhIL-15 protection of M2 CD206(+)MACs against IFNγ, but not of the CD206(+)IL-10(+)MAC subpopulation. These pilot data suggest that IL-15 maintains immunoinhibitory M2 CD206(+)IL-10(+)MACs in human skin under IFNγ-dominated inflammatory conditions. Therefore, it deserves to be explored whether IL-15 or IL-15Rα agonists can exert therapeutic benefit in chronic inflammatory dermatoses by preserving the intracutaneous pool of anti-inflammatory dermal M2 MACs.