ARTHRITIS & RHEUMATISM Vol. 63, No. 8, August 2011, pp 2436–2444 DOI 10.1002/art.30401 © 2011, American College of Rheumatology Anti-C1q Antibodies Are Associated With Systemic Lupus Erythematosus Global Activity but Not Specifically With Nephritis A Controlled Study of 126 Consecutive Patients Yasuhiro Katsumata, Kohei Miyake, Yasushi Kawaguchi, Yuko Okamoto, Manabu Kawamoto, Takahisa Gono, Sayumi Baba, Masako Hara, and Hisashi Yamanaka Objective. Several studies have shown that anti- C1q antibodies correlate with the occurrence and activ- ity of nephritis in systemic lupus erythematosus (SLE). However, the significance of anti-C1q antibodies in SLE has not been fully characterized. The aim of this study was to investigate associations between anti-C1q anti- bodies and clinical and serologic parameters of SLE. Methods. An enzyme-linked immunosorbent as- say kit was used to measure anti-C1q antibodies in the sera of 126 consecutive patients with active SLE who were admitted to our university hospital from 2007 through 2009. Sera obtained from patients with high titers of anti-C1q antibodies at the initial evaluation (n  20) were reevaluated following treatment. Control sera were obtained from patients with other auto- immune diseases and from normal healthy control subjects (n  20 in each group). Associations between anti-C1q antibodies and clinical and serologic par- ameters of SLE were statistically analyzed. Results. Anti-C1q antibodies were detected in the sera of 79 of 126 patients with SLE. The prevalence and titers of anti-C1q antibodies were significantly (P < 0.0001) higher in SLE patients than in patients with rheumatoid arthritis, patients with systemic scle- rosis, and normal healthy control subjects. The preva- lence and titers of anti-C1q antibodies were not signif- icantly associated with active lupus nephritis (P  0.462 and P  0.366, respectively). Anti-C1q antibody titers were significantly correlated with SLE Disease Activity Index 2000 scores and the levels of anti–double- stranded DNA antibodies, C3, C4, CH50, and C1q (P < 0.0001 for all comparisons). Moreover, anti-C1q antibody titers significantly decreased as clinical dis- ease was ameliorated following treatment (P  0.00097). Conclusion. These findings indicate that anti-C1q antibodies are associated with SLE global activity but not specifically with active lupus nephritis. Systemic lupus erythematosus (SLE) is an auto- immune disease characterized by the presence of auto- antibodies. The diagnosis of SLE can be difficult, be- cause SLE is a great imitator of other diseases (1). Autoantibodies are clearly central to the pathogenesis of SLE, with different autoantibodies associated with dif- ferent clinical features (2). Among these autoantibodies (100), several have been associated with disease activ- ity (1). Although anti–double-stranded DNA (anti- dsDNA) antibodies are the most extensively studied autoantibodies in SLE, others play a role in clinical manifestations such as autoimmune hemolytic anemia, thrombocytopenia, skin disease, and neonatal lupus (3). C1q is a complex molecule consisting of a collag- enous portion with globular heads, morphologically re- sembling a bundle of tulips. C1q is the first component of the classical pathway of complement activation, and its main function is to clear immune complexes (ICs) Supported by a grant from the Japanese Ministry of Health, Labor and Welfare (Grant for Intractable Diseases). Yasuhiro Katsumata, MD, PhD, Kohei Miyake, MD, PhD, Yasushi Kawaguchi, MD, PhD, Yuko Okamoto, MD, Manabu Kawamoto, PhD, Takahisa Gono, MD, PhD, Sayumi Baba, MD, Masako Hara, MD, PhD, Hisashi Yamanaka, MD, PhD: Tokyo Women’s Medical University, Tokyo, Japan. Address correspondence to Yasuhiro Katsumata, MD, PhD, Institute of Rheumatology, Tokyo Women’s Medical University, 10-22 Kawada-cho, Shinjuku-ku, Tokyo 162-0054, Japan. E-mail: katsumata@ior.twmu.ac.jp. Submitted for publication August 30, 2010; accepted in revised form April 7, 2011. 2436