B-1a cells are not involved in the PS3 Ab response in the presence or absence of CpG1668. For production of allotype chimeras, irradiated C57BL/6 mice were reconstituted with BM from C57BL/6 mice Igh^b and with B-1a cells from C57BL/6 mice Igh^a. Five weeks after reconstitution, chimeric mice (3 per group) were immunized intraperitoneally with PBS, PS3 alone, or PS3 followed by injection 2 days later with 80 μg CpG1668. Seven days after immunization, blood samples were collected, and peritoneal washings were harvested and analyzed by fluorescence-activated cell sorting staining. (A) Percentage of B-1a and B-1b cells expressing IgM^a or IgM^b in a mice immunized with PBS alone. R1 corresponds to the total B-1 population in the peritoneal washings. Percentages of each quadrant are indicated. (B) The total IgM titers of both allotype (IgM^a and IgM^b) were determined by ELISA. (C) The PS3-specific IgM titers of both allotype (IgM^a and IgM^b) were determined by ELISA. Results are expressed as means ± SEM of the values collected in 3 individual mice.