Effect of sCD23 on growth of SMS-SB cells. For each of the experiments illustrated in Fig 1, SMS-SB cells were harvested from log-phase cultures in PFMH_II , washed, and seeded at low cell density (2.5 × 10⁴ cells/mL) in the same medium; recombinant cytokines were included at 50 U/mL, and the final culture volume was 0.1 mL. The source of recombinant sCD23 used in these experiments was a Sf9 supernatant which was used at a final dilution of 1:100 in the cultures. The 24-hour cultures were pulsed with 0.3 μCi/well [³H]-TdR for 6 hours before harvest. All cultures were established in triplicate, and each experiment illustrated is representative of at least three independent repeats. (A) Presents the data for the effects of a range of individual cytokines, and stimulation indices were calculated according to the equation: Stimulation Index = Mean CPM incorporated in presence of cytokine / Mean cpm incorporated in absence of cytokine. (B) Presents a dose response curve using affinity-purified recombinant sCD23 produced in E coli.9 (C) Shows the effect of inclusion of 40 μg of the IgG fractions of RB55 anti-CD23 or nonimmune rabbit IgG on proliferation of SMS-SB cells cultured for 48 hours in the presence, or absence, of 100 ng/mL sCD23.