Type I interferon-associated cytotoxic inflammation in lichen planus

Lichen planus (LP) is an inflammatory autoimmune skin disease of unknown origin. Evidence has accumulated that autoreactive cytotoxic CD8(+) T lymphocytes cause destruction of keratinocytes. Recent studies suggested that type I interferons (IFNs) play a central role in cytotoxic skin inflammation by increasing the expression of IP10/CXCR10 and recruiting effector cells via CXCR3. Here, we investigated whether type I IFNs are also involved in the pathogenesis of LP. Skin biopsies of altogether 17 donors (seven LP and 10 healthy controls) were analyzed by immunohistochemistry using monoclonal antibodies against CD3, CD4, CD8, CD20, CD68, CXCR3, granzyme B, IP10/CXCL10, CD123, and the MxA protein, which is specifically induced by type I IFNs. Our analysis revealed a significant expression of the MxA protein in all LP skin biopsies, indicating involvement of type I IFNs. Expression of MxA was closely associated with the recruitment of CXCR3(+) and granzyme B+ lymphocytes, indicating a Th1-biased cytotoxic immune response. Strong expression of the CXCR3 ligand, the interferon-inducible protein IP10/CXCL10, links type I IFN expression and recruitment of CXCR3(+) lymphocytes. Plasmacytoid dendritic cells (pDCs) appear to be a major source of type I IFNs in LP. Our observations support the hypothesis that lesional type I IFNs produced by pDCs plays an important role in chronic cytotoxic inflammation of LP by recruiting cytotoxic effector lymphocytes via IP10/CXCR3 interactions.