Expression and role of IL-15 in post-burn hypertrophic scars

Hypertrophic scarring is a skin disorder that occurs after wounding and thermal injury. There is accumulating evidence that immunologic processes such as infiltration of activated T lymphocytes and altered cytokine production may play a role in the formation of hypertrophic scars. Interleukin-15, a cytokine identified as a T cell growth factor, also acts as a chemoattractant for T cells and has pro-inflammatory properties. We investigated the expression and the role of this cytokine in hypertrophic scarring. IL-15 expression was compared in skin biopsies of hypertrophic scars (HS) both in active (AHS) and in remission (RHS) phases, in normotrophic scars (NTS) and in normal skin using reverse transcriptase-polymerase chain reaction and immunohistochemistry. IL-15 expression in HS was significantly higher than in NTS or normal skin. Furthermore, AHS expressed higher levels of IL-15 than RHS. Immunohistologic analysis of AHS samples showed strong IL-15 immunoreactivity in keratinocytes and Langerhans cells in the epidermis and in macrophages, fibroblasts, and dermal dendritic cells in the dermis. High levels of IL-15 expression in AHS correlated with abundant infiltration of activated CD3+ cells. Ex vivo experiments indicate that IL-15 can sustain the proliferative response of T cells derived from AHS but not from RHS and NTS. In addition, IL-15 prevents both cytokine deprivation and activation-induced apoptosis of T cells derived from AHS. Taken together, these results suggest that IL-15 can be involved in the recruitment, proliferation, and apoptosis inhibition of T cells in AHS. The findings that the evolution from an AHS to a RHS is associated with a decrease in IL15 expression, and with a loss of IL-15 responsiveness in ex vivo-cultured T cells, indicate that this cytokine plays an important role in the biology of pathologic scar formation.