B220⁻/IgM⁻ population III cells are IgG-secreting plasma cells.

(A) B220⁻/IgM⁻ cells were isolated as described in “Materials and methods,” cytocentrifuged onto slides, and stained with hematoxylin and eosin. Original magnification × 40. (B) B220⁻/IgM⁻ cells were purified, cytocentrifuged onto slides, fixed, stained with FITC-conjugated anti-Igλ, anit-Igκ, anti-Igμ, or anti-Igγ and examined by both phase and confocal microscopy. Original magnification × 40. (C) The number of plasma cells secreting the various IgG isotypes was determined by ELISPOT. The data are represented as the mean (± SEM) number of secreting cells per 50 cells for 4 independent plasma cell isolations. In each individual experiment, each isotype was performed in triplicate. (D) Purified plasma cells were fixed and stained with PI as described in “Materials and methods,” and analyzed by flow cytometry. A representative experiment of 3 is shown. Cell lines examined in parallel showed significant fractions in the S and G₂/M phases of the cell cycle (not shown), whereas the plasma cells were nearly all G₀/G₁.