Are Anti-BP180 IgG1 or IgG4 Autoantibodies Pathogenic?

Bullous pemphigoid (BP) is an acquired autoimmune skin disease characterized by autoantibodies against two hemidesmosomal antigens, BP230 (BPAG1) and BP180 (BPAG2) and subepidermal blisters (Stanley et al, 1981; Diaz et al, 1990; Stanley, 1999). In the skin lesions of these patients basal keratinocytes detach from the underlying dermis at the level of the lamina lucida. BP antihemidesmosomal autoantibodies can bind to the dermalepidermal junction and activate the complement system.The majority of BP sera react with epitopes within the NC16A domain of BP180 (Giudice et al, 1993). The serum levels of autoantibodies to BP180 NC16A are correlated with the severity of BP (Haase et al, 1998). Gammon et al ¢rst showed that BP autoantibodies, with the help of complement and leukocytes, induce derma-epidermal junction (DEJ) separation in a skin organ culture system (Gammon et al, 1982). Further studies using a similar approach revealed that antihuman BP180NC16A antibodies and neutrophils are responsible for this tissue injury (Sitaru et al, 2002). Liu et al, using an IgG passive transfer approach, ¢rst provided in vivo evidence that rabbit antibodies directed against the murine BP180NC16A homolog are pathogenic in mice (Liu et al, 1993). Subsequently, this ¢nding was extended to the BP hamster model, in which neonatal hamsters injected with rabbit antihamster BP180 develop BP skin lesions (Yamamoto et al, 2002). These in vitro and in vivo data demonstrate that antibodies speci¢c for the BP180NC16A domain are pathogenic. It is clear that the binding of anti-BP180 antibody to its target is the critical ¢rst step in subepidermal blister formation in BP. What still remains unclear is which IgG subclass(es) are pathogenic and directly related to the severity of the disease. It is of great interest and clinical signi¢cance to know the answer since the diierent IgG subclasses have distinct functional properties. One of them is the ability to activate complement. IgG3 is the most eiective complement activator, followed by IgG1 and IgG2, while IgG4 is not capable of ¢xing complement. Studies on this issue, however, have painted a controversial picture. For example, Bernard et al and La⁄tte et al demonstrated that antiBP180 autoantibody IgG1 is the predominant IgG subclass, whereas Dopp et al concluded that anti-BP180 autoantibody IgG4 is the predominant IgG subclass in BP (Bernard et al, 1990; Dopp et al, 2000; La⁄tte et al, 2001). The factors causing this controversy are most likely the diierences in their assay systems (i.e., immunoblotting, immuno£uorescence, or ELISA), substrates in their assays (epidermal extracts that contain both BP230 and BP180, recombinant BP180 antigen expressed in prokaryotic or eukaryotic cells), and more importantly, the selected BP patient populations (patients in active, remission, untreated or treated states). In the new work on page 1065 of this issue, Hofmann et al developed an ELISA assay using baculovirus-derived recombinant BP180 containing NC16A domain (BP180-N) and analyzed 116 patients with clinically active BP and 6 BP patients in remission. They found that in BP180-N-reactive sera from the active BP, 85% were IgG1-positive (including both IgG1and IgG4-positive), with 45% being IgG1 exclusively, while 46% were IgG4positive (including both IgG1and IgG4-positive), with only 6% being IgG4-positive only. More importantly, anti-BP180-N IgG1 was predominant in the acute phase of BP (52% IgG1 vs 4% IgG4), while IgG4 was predominantly detected in BP patients in remission (50% IgG4 vs 17% IgG1). Consistent with these observations, the levels of anti-BP180-N IgG1 alone in BP patients’ sera re£ect the disease severity: 49% in acute stage, 35% in chronic stage, and 17% in remission. In contrast, of patients with anti-BP180-N-reactivity alone 5% were in acute phase, 13% in chronic phase, and 50% in remission. Taken together, these ¢ndings identify anti-BP180-N IgG1 autoantibodies as the predominant IgG subclass that is associated with acute onset of BP. Since IgG1 antibodies are capable of ¢xing complement, the signi¢cance of these ¢ndings is that they identify the link between anti-BP180 autoantibodies and complement in their pathogenic role in BP. In 1967, Jordon et al (Jordon et al, 1967) reported in their hallmark paper that in addition to IgG, complement component C3 is deposited at the DEJ in the perilesional skin of BP patients. Since then, components of both the classical and alternative complement pathways have been detected at the dermal-epidermal junction of patients’ skin and blister £uids. Based upon these immunohistological observations, it has been hypothesized that the complement-¢xing BP autoantibodies and complement activation play a pathological role in the initiation of BP. Now, Hofmann et al provide convincing results, strongly suggesting that BP autoantibody IgG1 binds to the pathogenic epitope within the NC16A domain and activates the complement system, thus initiating BP. Liu et al provide direct evidence that complement activation is essential for BP skin lesions in the murine passive transfer model of BP (Liu et al, 1995). Mice de¢cient in complement are resistant to the pathogenic activity of antimurine BP180 antibodies. Further studies revealed that complement activation leads to mast cell degranulation and subsequent neutrophil recruitment. In¢ltrating neutrophils release neutrophil elastase and other proteolytic enzymes and cause DEJ separation (Liu et al 2000).The importance of neutrophil elastase in the direct cleavage of BP180 and subsequent subepidermal blistering in human BP is also documented by Verraes et al (Verraes et al, 2001). They demonstrated that neutrophil elastase is present in the lesional skin and blister £uids of BP and that BP180 degradation by blister £uids depends on neutrophil elastase activity. Thus, the immune in£ammation induced by complement-¢xing autoantibodies could be the key pathogenic mechanism in BP. Interestingly, the authors of this report also uncover a dual IgG1 and IgG4 response to BP180-N more frequently in BP patients with extensive skin involvement than patients with localized BP. This observation suggests that anti-BP180-N IgG4 autoantibodies may sustain the disease phenotype. The direct pathogenic role of autoantibodies independent of complement and in£ammatory cells has been demonstrated in other autoimmune blistering diseases, including pemphigus and