IgM and IgG Subclass Distribution of Human Anti‐Ro/SSA 60 kDa Autoantibodies

The Ro/SSA and La/SSB antigens are common targets for autoantibodies found in the sera of patients with Sjögren's syndrome and SLE. The anti‐Ro/SSA and anti‐La/SSB antibodies often appear together but are not cross‐reactive. This paper describes the humoral autoimmune response to the Ro/SSA 60 kDa protein moiety with respect to the presence of IgM and IgG1‐4 antibodies. IgM antibodies to the Ro 60 kDa protein coexisted with IgG anti‐Ro 60 kDa antibodies in nearly half of the sera. A similar fraction also contained IgM anti‐La/SSB antibodies. The frequency of sera with IgM antibodies of both specificities was that expected from random overlap. A predominating igGl anti‐Ro 60 kDa response was found in all patients, but anti‐Ro 60 kDa antibodies of the other IgG subclasses were present also in a high number of sera. This is in contrast to the reported IgG subclass distribution of anti‐La/SSB antibodies. Mapping of IgM and IgG 1–4 antibody recognition of different parts of the Ro 60 kDa protein was also performed. IgM and IgGl‐4 antibodies of all sera reacted with the central part of the Ro 60 kDa protein, encompassing amino acid residues 181–320.