Truncated IgM is present in serum of old L^−/− mice. (A) Level of μHC in serum of 2- to 14-month-old L^−/− mice determined by ELISA at 1/100 dilution. (B) A comparative analysis of IgM serum titers in L^−/− (▴), C-region deletion (CΔ²⁶ ) as a negative control (♦) and normal (NM) mice (■). Means and standard deviation were calculated from the 10 L^−/− mice in panel A and 7 NM mice. (C) Western analysis of serum antibodies from L^−/−, of different age in months (m) as indicated. Sera from L^−/−, μNR, and NM mice were purified by incubation with anti–mouse Ig coupled to Sepharose, separated on Ready-Gels under reducing conditions, and visualized with antibodies against mouse IgM. (D) Flow cytometric analysis of spleen cells surface-stained for IgM and B220. A representative presentation of a 1-year-old L^−/− mouse compared with NM and μNRL^−/− animals. The upper histograms show that, in addition to B220⁺ IgM⁺ cells, an IgM^bright B220⁻ population can be found in old L^−/− mice. This population is more abundant in μNRL^−/− mice. Plotting forward scatter (FSC) against IgM shows an IgM^bright population of small particles with percentages indicated.