NIgM in sera of 3′RR-deficient mice and wt mice. (A) Indirect immunofluorescence assay for detection of NIgM. Sera from wt and 3′RR-deficient mice were investigated on Hep-2 cell-coated slides. Because reduced circulating IgM levels were found in 3′RR-deficient mice, sera were adjusted at 20-μg/mL IgM. MLR/lpr and RAGγC sera were used as positive and negative controls, respectively (1/10 dilution). Representative experiments from 1 to 5 sera per genotypes. Original magnification ×20. (B) Data, expressed as means ± SEM of the indicated number (n) of values, were analyzed using Prism software (GraphPad Software, La Jolla, CA). Significance was calculated with a nonparametric Mann-Whitney U test. PBS signal was arbitrary quoted to 1. (C) ELISA for detection of NIgM against kidney cell lysates. Sera from 3′RR-deficient and wt mice were adjusted at 20-μg/mL IgM. Data are reported as means ± SEM of 4 sera. *P = .02, Mann-Whitney U test for significance. A RAGγC serum was used as a negative control (1/10 dilution) to define baseline value.