Soluble Fas Ligand is essential for blister formation in Pemphigus

Pemphigus is a blistering disease characterized by pemphigus autoantibodies (PVIgG)
directed mostly against desmogleins (Dsgs), resulting in the loss of keratinocyte adhesion
(acantholysis). Yet, the mechanisms underlying blister formation remain to be
clarified. We have shown previously that anti-Fas ligand (FasL) antibody (Ab) prevents
PVIgG-induced caspase-8 activation and Dsg cleavage in human keratinocytes, and
that sera from pemphigus patients contain abnormally increased levels of FasL. Here,
we demonstrate that recombinant FasL induces the activation of caspases prior to Dsg
degradation, and anti-FasL Ab prevents acantholysis in cultured keratinocytes. Moreover,
the silencing of FasL reduces PVIgG-induced caspase-8 activation and Dsg3 cleavage.
Following injection of PVIgG into mice, FasL is upregulated at 1–3 h and is followed by
caspase-8-mediated keratinocyte apoptosis, before blister formation. The administration
of anti-FasL Ab after PVIgG injection blocks blister formation in mice. Furthermore, we
injected PVIgG into two different gene-targeted mutant mice that selectively lack either
secreted soluble FasL (sFasL), FasLΔs/Δs mice, or the membrane-bound form of FasL
(mFasL), FasLΔm/Δm mice. After PVIgG treatment, blisters are only visible in FasLΔm/Δm animals,
lacking mFasL, but still producing sFasL, similar to wild-type (C57BL/6) animals.
By contrast, a significant decrease in the relative acantholytic area is observed in the
FasLΔs/Δs animals. These results demonstrate that soluble FasL plays a crucial role in the
mechanisms of blister formation, and blockade of FasL could be an effective therapeutic
approach for pemphigus.