Immature anti-insulin B cells are generated in vitro using an IL-7-driven bone marrow culture system. Flow cytometry on freshly isolated bone marrow (A) and after culture with IL-7 (B–F). Expression of B220, IgM, AA4.1, CD21, and CD23 are as shown for B220+ 7-aminoactinomycin D-negative gated cells on the day indicated. The IL-7-driven culture system used to generate naive 125Tg immature B cells is shown in the schematic in Fig. 2A. All data are representative of more than eight experiments. The average ± SD of B220+-gated cells is: A, pro- and pre- (B220+IgMa–): 14.7 ± 10.6%, immature (B220midIgMa+): 61.0 ± 10.2%, mature recirculating (B220highIgMa+): 24.4 ± 10.2%; B, B220+IgMa+: 95.3 ± 6.1; C, B220+IgMa+: 97.9 ± 1.2%; D, B220+AA4.1+: 91.3 ± 2.9%; E, B220+ CD21+: 0.03 ± 0.03%; and F, B220+CD23+: 0.20 ± 0.17%.