Binding of pemphigus vulgaris IgG to antigens in desmosome core domains excludes immune complexes rather than directly splitting desmosomes

Background Pemphigus vulgaris (PV) is characterized by autoantibodies against desmoglein (Dsg) 3 or both Dsg1 and Dsg3, i.e. desmosomal adhesion molecules. Objectives We examined whether or not PV IgG binding to Dsg3 directly impairs the adhesion of desmosomes. Methods For immunofluorescence microscopy, keratinocytes were first incubated with PV IgG for 30 min in low Ca²⁺ medium, in which no desmosomes were formed, and then for 1 h in high Ca²⁺ medium to generate desmosomes. For immunoelectron microscopy, after a 30‐min incubation with PV IgG in low Ca²⁺ medium, cells were incubated with antihuman IgG with 5‐nm gold particles for 5 min; after washing, cells were further incubated in high Ca²⁺ medium for 1 h. For tracing of PV IgG/Dsg3 immune complexes formed in the desmosomal core domain, cells were first incubated with PV IgG for 5 min to allow PV IgG to bind the desmosomal core domain and were further incubated with PV IgG‐free medium for different times. Results Immunofluorescence microscopy revealed that PV IgG bound in a random‐punctate pattern on the cell surface in low Ca²⁺ medium was translocated to the cell–cell contacts forming a dotted‐linear distribution, suggesting desmosome generation even in the presence of PV IgG. Immunoelectron microscopy revealed that half‐desmosome‐like structures decorated with gold particles in low Ca²⁺ keratinocytes coupled to form desmosomes and gold particles were sandwiched in the desmosomal core domain after Ca²⁺ switch, even though their surfaces were covered with PV IgG/antihuman IgG 5‐nm gold particles. In the tracing experiments, although PV IgG demonstrated a dotted‐linear distribution along the cell–cell contacts colocalized with desmoplakin (DPK) after a 30‐min tracing, it disappeared from cell–cell contacts after a 5‐h tracing, leaving DPK and desmocollin 3. Conclusions These results suggest that the PV IgG/Dsg3 immune complexes are excluded from the desmosomal core domain rather than directly splitting the desmosome.