Western blot analysis of the intracellular and extracellular IgM content of recombinant CHO and HEK cells (7 μg of total cell lysate per lane). Intracellular IgM content of CHO IgM-617 (lanes 2 and 7, upper blot) and CHO IgM-012 (lanes 4 and 9, upper blot) as well as HEK IgM-617 (lanes 2 and 7, bottom blot) cells mostly consists of lower IgM oligomers. No IgM signal could be detected in HEK IgM-012 cell lysates (lanes 4 and 9, bottom blot). In extracellular fractions of CHO IgM-617 (lanes 1 and 6, upper blot), CHO IgM-012 (lanes 3 and 8, upper blot), HEK IgM-617 (lanes 1 and 6, bottom blot), and HEK IgM-012 (lanes 3 and 8, bottom blot), IgM pentamers can be observed. HEK IgM-012 supernatant was concentrated 20-times prior to use. IgM from human serum was used as the positive control (lanes 5 and 10)