T cell epitopes of the La/SSB autoantigen in humanized transgenic mice expressing the hLa class II haplotype DRB1*0301/DQB1*0201

Objective T cells are implicated in the production of anti‐La/SSB and anti‐Ro/SSA autoantibodies commonly associated with the DR3/DQ2 haplotype in systemic lupus erythematosus and Sjögren's syndrome. This study was undertaken to investigate the DR3/DQ2‐restricted T cell response to wild‐type human La (hLa) and a truncated form of mutant La. Methods Humanized transgenic mice expressing HLA–DRB1*0301/DQB1*0201 (DR3/DQ2) were immunized with recombinant antigen and examined for development of autoantibodies and T cell proliferation against overlapping peptides spanning the La autoantigen. HLA restriction and peptide binding of identified T cell epitopes to DR3 or DQ2 were determined using blocking monoclonal antibodies and a direct binding assay. Results DR3/DQ2‐transgenic mice generated an unusually rapid class‐switched humoral response to hLa with characteristic spreading to Ro 52 and Ro 60 proteins following hLa protein immunization. Seven T cell determinants in hLa were restricted to the HLA–DR3/DQ2 haplotype. Six epitopes tested were restricted to HLA–DR and bound DR3 with semiconserved DR3 binding motifs. No DQ restriction of these epitopes was demonstrable despite efficient DQ binding activity in some cases. No neo–T cell epitopes were identified in mutant La; however, T cells primed with mutant La exhibited a striking increase in proliferation to the epitope hLa 151–168 compared with T cells primed with hLa. Conclusion Multiple DR3‐restricted epitopes of hLa have been identified. These findings suggest that truncation of La produced by somatic mutation or possibly granzyme B–mediated cleavage alters the immunodominance hierarchy of T cell responsiveness to hLa and may be a factor in the initiation or maintenance of anti‐La autoimmunity.