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 f ound in the sera of patients with Sjogren's syndrome and SLE. The anti -Ro/SSA and anti-La/SSB antibodies often appear together but are not c ross-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 coexist ed with IgG anti-Iio 60 kDa antibodies in nearly half of the sera. A s imilar fraction also contained IgM anti-La/SSB antibodies. The frequen cy of sera with IgM antibodies of both specificities was that expected from random overlap. A predominating IgG1 anti-Ro 60 kDa response was found in all patients, but anti-Re 60 kDa antibodies of the other IgG subclasses were present also in a high number of sera. This is in con trast to the reported IgG subclass distribution of anti-La/SSB antibod ies. Mapping of IgM and IgG1-4 antibody recognition of different parts of the Ro 60 kDa protein was also performed. IgM and IgG1-4 antibodie s of all sera reacted with the central part of the Ro 60 kDa protein, encompassing amino acid residues 181-320.