Systemic autoimmune disease in humans and mice is characterized by loss of immunologic tolerance to a restricted set of self-nuclear antigens. Autoantigens, such as double-stranded (ds) DNA and the RNA-containing Smith antigen (Sm), may be selectively targeted in systemic lupus erythematosus because of their ability to activate a putative common receptor. Toll-like receptor 9 (TLR9), a receptor for CpG DNA, has been implicated in the activation of autoreactive B cells in vitro, but its role in promoting autoantibody production and disease in vivo has not been determined. We show that in TLR9-deficient lupus-prone mice, the generation of anti-dsDNA and antichromatin autoantibodies is specifically inhibited. Other autoantibodies, such as anti-Sm, are maintained and even increased in TLR9-deficient mice. In contrast, ablation of TLR3, a receptor for dsRNA, did not inhibit the formation of autoantibodies to either RNA-or DNA-containing antigens. Surprisingly, we found that despite the lack of anti-dsDNA autoantibodies in TLR9-deficient mice, there was no effect on the development of clinical autoimmune disease or nephritis. These results demonstrate a specific requirement for TLR9 in autoantibody formation in vivo and indicate a critical role for innate immune activation in autoimmunity.

Systemic lupus erythematosus (SLE) is the prototypical human autoimmune disease. Although the underlying causes of SLE remain unknown, the characteristic loss of immunologic tolerance to a restricted set of self-nuclear antigens is a common and defining feature of disease (1–3). Autoantibodies to macromolecular complexes of protein and nucleic acid, such as chromatin and small nuclear ribonucleoproteins (snRNPs), are predominant in SLE patients; antibodies directed against the individual components of double-stranded (ds) DNA, histones, and the several Smith antigen (Sm) polypeptides can also be found in many lupus patients (4–6). Because the ubiquitous autoantigens targeted in SLE all contain some form of nucleic acid, it is possible that the canonical antinuclear antibodies of lupus arise because these autoantigens can stimulate invariant receptors that recognize conserved nucleic acid determinants (3).