SUMMARYIn bullous pemphigoid (BP), the binding of BP180-specific antibodies to their hemidesmosomal target antigen is not sufficient for blister formation, but must be accompanied by the release of proteases. Using plasminogen activator (PA) knock-out mice, the PA system has previously been shown to be a prerequisite for blister formation in experimental murine BP. Here, we found elevated levels of plasmin and tPA, but not of uPA, in blister fluid from BP patients ( n = 7) compared to blisters from patients with toxic epidermal necrolysis ( n = 4) and suction blisters in healthy controls ( n = 7). Subsequently, we addressed the question whether keratinocytes release PA in response to the binding of anti-BP180 antibodies. Treatment of cultured normal human keratinocytes with BP IgG, but not with control IgG, led to both increased protein and mRNA levels of tPA, but not of uPA, as determined by ELISA and RT-PCR, respectively. The specificity of this finding was confirmed using BP180-deficient keratinocytes from a patient with generalized atrophic benign epidermolysis bullosa, where no tPA release was observed after stimulation with BP IgG. Our results show the elevated expression and release of tPA from normal human keratinocytes upon stimulation with antibodies to human BP180. Keratinocytes, by secreting tPA, may thus play an active role in blister formation of BP.

The expression of BP180 has previously been demonstrated to be influenced by both calcium (Ca(2+)) concentration and binding of anti-BP180-antibodies in cultured keratinocytes of the skin squamous cell carcinoma line DJM-1. Here, BP180 expression was studied in cultured normal human epidermal keratinocytes by confocal laser scanning microscopy. We exploited an experimental system, in which BP180 was previously shown to mediate, upon incubation with anti-BP180 antibodies, a specific signal-transducing event that leads to the release of inflammatory mediators, such as IL-8 from cultured normal human epidermal keratinocytes (NHEK). We found that without addition of BP180-specific IgG, BP180 is predominantly expressed on the cell surface irrespective of the Ca(2+) concentration. In contrast, cell surface BP180 was greatly reduced in NHEK kept in high Ca(2+) medium after incubation with BP180-specific IgG for 12 h compared to low Ca(2+) medium. This effect was seen with antibodies to both N- and C-terminal fragments of the BP180 ectodomain, respectively. In addition, a slightly higher BP180 expression was found in NHEK cultured in low compared to high Ca(2+) medium by Western blotting. Interestingly, in contrast to NHEK kept under low Ca(2+ )conditions, in NHEK grown in high Ca(2+) medium, no elevated levels of IL-8 were released after treatment of cells with anti-BP180 IgG compared to normal IgG. Our data indicate that the Ca(2+)-modulated expression of BP180 is functionally relevant. This finding sheds further light on the complex pathomechanism in blister formation of BP180-related autoimmune blistering skin diseases.