The detection of IgG and IgA autoantibodies to desmocollins 1-3 by enzyme-linked immunosorbent assays using baculovirus-expressed proteins, in atypical pemphigus but not in typical pemphigus

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Abstract
We have shown previously that human desmocollin (Dsc) 1 is recognized by IgA autoantibodies of subcorneal pustular dermatosis (SPD) type IgA pemphigus. However, the presence of IgG anti-Dsc autoantibodies is still controversial, and antibodies to Dsc2 and Dsc3 have not been clearly identified. To investigate this by producing recombinant proteins consisting of the entire extracellular domains of human Dsc1, 2 and 3 in baculovirus, and to use them to establish an enzyme-linked immunosorbent assay (ELISA). By this ELISA, we examined in total 165 cases of various types of autoimmune bullous diseases, as well as 23 normal controls. None of 45 sera of classical pemphigus showed either IgG or IgA antibodies to any Dsc. In contrast, one atypical pemphigus serum showed both IgG and IgA antibodies to Dsc1, which were adsorbed by incubation with Dsc1 baculoprotein. Furthermore, this ELISA detected both IgA and IgG anti-Dsc3 antibodies in one atypical case, and IgA antibodies to both Dsc2 and Dsc3 in another. This reactivity was confirmed by positive IgA immunofluorescence with Dsc2 and Dsc3 expressed on COS-7 cells. These results show that both IgG and IgA autoantibodies against all of Dsc1-3 are present in the sera of particular cases of nonclassical pemphigus, except for IgG antibodies to Dsc2, but that they are not detected in classical pemphigus. Unexpectedly, although IgA antibodies of all of eight SPD type IgA pemphigus sera reacted with Dsc1 expressed on COS-7 cells, only one serum was positive in Dsc1 ELISA for IgA. This result indicates either that Dscs expressed by baculovirus may not adopt the correct conformation or that Dscs may need association with other molecules to express all the epitopes for autoantibodies.