The speckled epidermal nuclear immunofluorescence of mixed connective tissue disease seems to develop as an in vitro phenomenon.

The pathogenesis of speckled epidermal nuclear immunofluorescence in patients with mixed connective tissue disease (MCTD) was studied by reproducing this reaction in guinea-pigs, using serum samples containing high titre antibody to ribonucleoprotein (RNP). Immunofluorescence studies on specimens obtained from guinea-pig skin into which serum samples containing high titre RNP antibody had been injected intradermally, revealed positive epidermal nuclear staining for IgG. This speckled immunofluorescence was demonstrable immediately after injection and remained so for 24 or 48 h. The pattern of fluorescence was similar in all cases, and there was no penetration of RNP antibody through the cell membrane. The epidermal nuclear fluorescence was not detected with sera at a dilution of 1:100 or more. These results provide strong evidence that the epidermal nuclear immunofluorescence observed in patients with high titre antibody to RNP develops as an in vitro phenomenon.