PTX3 can enhance IgG responses to microbial carbohydrates by activating a TI pathway. (A) ELISA of serum SP-reactive IgG2b, IgG2c, and IgG3 from Tcrb^−/− and Tcrb^−/−Ptx3^−/− mice after i.v. injection (d7) of live SP. RU, relative units. (B) Flow cytometric analysis of frequency (left) and absolute number (right) of splenic class-switched PBs expressing IgG2b or IgG2c and splenic NBh cells from Tcrb^−/− and Tcrb^−/−Ptx3^−/− mice after i.v. injection (d7) of live SP. (C) ELISA of serum SP-reactive IgG2b, IgG2c, and IgG3 from Tcrb^−/− mice after i.v. injection (d7) of live SP with or without exogenous PTX3. (D) ELISA of serum SP-reactive IgG2b and IgG3 from Tcrb^−/−Ptx3^−/− mice after i.v. injection (d7) of SP in the presence of exogenous PTX3. (E) ELISA of serum Pneumovax-specific IgM from WT, Ptx3^−/−, and PTX3-treated Ptx3^−/− mice at day 7 after i.v. immunization with Pneumovax. (F) ELISA of serum TNP-reactive IgG2b and IgG2c from WT mice after i.v. injection (d7) of TNP-LPS with or without exogenous PTX3. Data summarize at least two experiments with at least three mice in each experimental group. Error bars, SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001 (two-tailed Student’s t test).