C1q levels were determined in the group of 45 patients with systemic lupus erythematosus and compared with parameters evaluating apoptosis in lymphocytes and their T and B subpopulations. C1q levels were analysed by radial immunodiffusion according to Mancini using commercial kits, apoptotic indices of lymphocytes were determined by flowcytometry using double staining method. Mean values of C1q in the whole group were 173±44,46 mg/l, median was 180 mg/l, in two cases the levels of C1q were undetectable (below 23 mg/l), and in six cases the levels exceeded 230 mg/l. In the group of active patients the mean levels of complement component C1q reached 138 mg/l, SD±43,2 mg/l, and median was 145 mg/l. In the group of patients with moderate activity the mean levels of C1q reached 202 mg/l, SD±27,5 mg/l, and median was 208 mg/l. In the group of patients with lupus nephritis the mean C1q values were 144 mg/l, SD±31 mg/l, and median was 150.1 mg/l. The differences between active and inactive patients as well as between patients with lupus nephritis and inactive patients were statistically significant (p=0.0005 and p=0.001). Using the regression analysis the significant correlations were found between C1q and C3 and C4 complement component levels (r=0.49 and 0.42), C1q and apoptotic index CD19 (r=0.4), and inverse relations were revealed between C1q and ECLAM or SLEDAI indices (r=-0.4 or -0.46), levels of antinucleosomal antibodies (r=-0.42, p=0.0001), and apoptotic index of lymphocytes or the total phagocytic activity (r=-0.37 or -0.35, p=0.05 for both). No correlations were demonstrated between C1q levels and the index of damage SLICC (r=-0.01), levels of anti dsDNA antibodies (r=-0.2), and number of apoptotic T lymphocytes (r=-0.29). This suggests that C1q could be a suitable parameter for follow-up of the activity of systemic lupus erythematosus, which can add to traditionally used C3 and C4 complement components in clinical practice.

        Key words: systemic lupus erythematosus, complement, C1q, apoptosis, lymphocytes