Impaired secondary and tertiary immune responses to a T-cell–dependent antigen in B7-H2^–/– mice. Wild-type (•) and mutant (○) mice were immunized with 100 μg alum-precipitated T-cell–dependent antigen NP-CG for the primary response (A), reboosted 30 days later with 5 μg of the antigen for the secondary response (B), and reboosted 60 days after the initial immunization for the tertiary response (C). Sera were collected from the mice 7 days after the primary immunization, 7 and 14 days after the secondary, and 5 days after the tertiary challenges. NP-specific antibodies of the IgM, IgG1, and IgG2a classes were quantified. The value for each mouse was plotted, and the dilution of the immune sera was indicated. Preimmune sera were negative for the antigen-specific antibodies and were not shown. Statistical significance was established using a paired 2-tailed Student t test, * indicates P < .1 and ** indicates P < .01. The asterisks for the tertiary response are applicable only to the 3 lowest serum dilutions in each graph. Horizontal bars indicate the mean.