B lymphocyte selection and survival in systemic lupus

Increased mutations of CD72 transcript in B-lymphocytes from adolescent patients with systemic lupus erythematosus

Recent studies have shown that B cells play a central role in the pathogenesis of systemic lupus erythematosus (SLE). Abnormal expression of molecules engaging in B-cell receptor (BCR) signaling and resultant hyperactivity of B cells has been reported in both mouse models of lupus and patients with SLE. CD72 on B cells is unique in that it regulates BCR signaling both positively and negatively. We analyzed the expression of CD72 protein and mRNA in peripheral blood B cells from adolescent patients with SLE. The expression level of CD72 on B cells of the patients was decreased compared with that on B cells of controls. Sequence analysis of CD72 mRNA showed significantly increased nucleotide mutations, including both nucleotide substitutions and deletions. Almost all (95.6%) of the CD72 transcripts from the patients had different nucleotide sequences from those of the wild type. About half (41.3%) of the mutations were point mutations located close to the sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM), which negatively regulates BCR signaling. These results indicate that increased nucleotide mutation of CD72 mRNA accounts for the decreased expression level of CD72 in B cells, and it might be related to hyperactivity of B cells in patients with SLE.

Defective B cell tolerance checkpoints in systemic lupus erythematosus

A cardinal feature of systemic lupus erythematosus (SLE) is the development of autoantibodies. The first autoantibodies described in patients with SLE were those specific for nuclei and DNA, but subsequent work has shown that individuals with this disease produce a panoply of different autoantibodies. Thus, one of the constant features of SLE is a profound breakdown in tolerance in the antibody system. The appearance of self-reactive antibodies in SLE precedes clinical disease, but where in the B cell pathway tolerance is first broken has not been defined. In healthy humans, autoantibodies are removed from the B cell repertoire in two discrete early checkpoints in B cell development. We found these checkpoints to be defective in three adolescent patients with SLE. 25–50% of the mature naive B cells in SLE patients produce self-reactive antibodies even before they participate in immune responses as compared with 5–20% in controls. We conclude that SLE is associated with abnormal early B cell tolerance.