Cytoplasmic levels of IgE(Δ) in human myeloma cells in the different phases of the cell cycle

The cytoplasmic levels of IgE (λ and e chains) of human myeloma cells in vitro in different stages of the cell cycle were determined by cytophotometric techniques. Asynchronously proliferating cells were pulsed with 3H or 14C-labeled thymidine. After fixation the cells were “stained” with fluorescein-conjugated anti-λ or anti-λ sera. Fluorescence intensities, representing the amounts of λ and e chains respectively, were determined by microspectrofluorometry. The same individual cells were then analyzed by dry mass and Feulgen DNA determinations using microinterferometry and microspectrophotometry, respectively. S cells were identified by autoradiography. This methodology thus enabled cell cycle analyses without using synchronization procedures. The amounts of λ and e chains correlated well indicating a balanced relationship between them during the cell cycle. In 4 out of 7 experiments the IgE concentration ( = amount of IgE in relation to the cellular mass) fluctuated during interphase: it mostly increased during G1 followed by a decrease in S, sometimes with a recovery during G2. In the other 3 experiments no periodicity was found during the cell cycle, i.e. the cytoplasmic IgE accumulated at the same rate as the overall cellular proteins. Thus, previous published assumptions that Ig synthesis is always restricted to certain periods of the cell cycle, could not be confirmed. Instead, our data indicate that the cell cycle expression of IgE(λ) may be variable in a clonal cell population during in vitro conditions. The explanation for this variability has ot been found. Using the same methodology, surface-bound Ig however, always shows a noncyclic expression. A nonproliferating fraction of cells (9 %) was found. These cells, predominantly G1 cells, did not differ from proliferating G1 cells with respect to IgE content. Neither was there any relationship between morphology (immature or mature plasma cells) and IgE levels.