Permanent suppression of NP-specific IgG⁺ memory cells and transient suppression of IgM memory B cells caused by “helpless” antigen NP-MSA. Mice were primed with NP-KLH in alum; and 6 weeks later, mice were injected with NP-MSA (n = 6) or unhaptenated MSA (n = 5, week 2; n = 4, week 8). Two or 8 weeks later, mice were boosted with 20 μg of NP-KLH and ELISPOT was performed after 4 more days. Initially (week 2), suppression of both IgG⁺ (A) and IgM⁺ (C) B cells was observed. By week 8, whereas IgG⁺ memory cells remain suppressed (B), IgM memory recovered (D). Horizontal bars represent median values. **Statistical significance (P < .01). *Statistical significance (P < .03). n/s indicates not significant. These data are representative of 2 independent experiments. (E-F) Loss of NP-specific IgG⁺ memory cells after exposure to “helpless” antigen. Mice were primed and boosted with NP-KLH and then injected either with 200 μg of NP-MSA or 200 μg MSA (control) 3 weeks later. After a further 6 days, splenocytes were taken and analyzed by FACS. B cells were gated on CD19 and the IgD⁺ and IgD⁻ populations and then analyzed further for antigen binding and for expression of IgG1. (E) Representative FACS profile showing loss of NP-specific IgG1⁺ but not IgG⁻ B cells in mice injected with NP-MSA versus those injected with MSA (i-ii). The numbers of NP-specific B cells (gated) per 10⁶ lymphocytes (i-ii) are shown. IgG⁻ NP binding cells were positive for surface IgM in both controls and mice injected with NP-MSA (iii-iv). The quadrant statistics of IgG1⁻ gates are shown (iii-iv). (F) Enumeration of NP-specific IgG⁺ and IgG⁻ memory cells in the 2 groups. Horizontal bars represent median values. **Statistical significance (P < .01). These are combined data of 2 independent experiments.