Lupus IgG VH4.34 antibodies bind to a 220-kDa glycoform of CD45/B220 on the surface of human B lymphocytes.

Anti-lymphocyte autoantibodies are a well-recognized component of the autoimmune repertoire in human systemic lupus erythematosus (SLE) and have been postulated to have pathogenic consequences. Early studies indicated that IgM anti-lymphocyte autoantibodies mainly recognized T cells and identified CD45, a protein tyrosine phosphatase of central significance in the modulation of lymphocyte function, as the main antigenic target on T cells. However, more recent work indicates that lupus autoantibodies can also recognize B cells and that CD45 may also represent their antigenic target. In particular, IgM Abs encoded by V(H)4.34 appear to have special tropism for B cells, and strong, but indirect evidence suggests that they may recognize a B cell-specific CD45 isoform. Because V(H)4.34 Abs are greatly expanded in SLE, in the present study we investigated the antigenic reactivity of lupus sera V(H)4.34 IgG Abs and addressed their contribution to the anti-lymphocyte autoantibody repertoire in this disease. Our biochemical studies conclusively demonstrate that lupus IgG V(H)4.34 Abs target a developmentally regulated B220-specific glycoform of CD45, and more specifically, an N-linked N-acetyllactosamine determinant preferentially expressed on naive B cells that is sterically masked by sialic acid on B220-positive memory B cells. Strikingly, our data also indicate that this reactivity in SLE sera is restricted to V(H)4.34 Abs and can be eliminated by depleting these Abs. Overall, our data indicate that V(H)4.34 Abs represent a major component of the lupus IgG autoantibody repertoire and suggest that the carbohydrate moiety they recognize may act as a selecting Ag in SLE.