Antimitochondrial autoantibodies in pemphigus vulgaris: a missing link in disease pathophysiology.

A loss of epidermal cohesion in pemphigus vulgaris (PV) results from autoantibody action on keratinocytes (KCs) activating the signaling kinases and executioner caspases that damage KCs, causing their shrinkage, detachment from neighboring cells, and rounding up (apoptolysis). In this study, we found that PV antibody binding leads to activation of epidermal growth factor receptor kinase, Src, p38 MAPK, and JNK in KCs with time pattern variations from patient to patient. Both extrinsic and intrinsic apoptotic pathways were also activated. Although Fas ligand neutralizing antibody could inhibit the former pathway, the mechanism of activation of the latter remained unknown. PV antibodies increased cytochrome c release, suggesting damage to mitochondria. The immunoblotting experiments revealed penetration of PVIgG into the subcellular mitochondrial fraction. The antimitochondrial antibodies from different PV patients recognized distinct combinations of antigens with apparent molecular sizes of 25, 30, 35, 57, 60, and 100 kDa. Antimitochondrial antibodies were pathogenic because their absorption abolished the ability of PVIgG to cause keratinocyte detachment both in vitro and in vivo. The downstream signaling of antimitochondrial antibodies involved JNK and late p38 MAPK activation, whereas the signaling of anti-desmoglein 3 (Dsg3) antibody involved JNK and biphasic p38 MAPK activation. Using KCs grown from Dsg3(-/-) mice, we determined that Dsg3 did not serve as a surrogate antigen allowing antimitochondrial antibodies to enter KCs. The PVIgG-induced activation of epidermal growth factor receptor and Src was affected neither in Dsg3(-/-) KCs nor due to absorption of antimitochondrial antibodies. These results demonstrated that apoptolysis in PV is a complex process initiated by at least three classes of autoantibodies directed against desmosomal, mitochondrial, and other keratinocyte self-antigens. These autoantibodies synergize with the proapoptotic serum and tissue factors to trigger both extrinsic and intrinsic pathways of cell death and break the epidermal cohesion, leading to blisters. Further elucidation of the primary signaling events downstream of PV autoantigens will be crucial for the development of a more successful therapy for PV patients.