Residual NKT cells in SAP^−/−.BALB/c are competent stimulators of antibody production. (A) NKT cells in thymus, spleen and liver isolated from SAP^−/−.BALB/c or SAP^−/−.B6 mice, were identified with CD1d tetramer loaded with the αGalCer analog PBS57 and anti-TCRβ. (B) The number of NKT cells in different organs of SAP^−/−.BALB/c and SAP^−/−.B6 mice (n = 3). NKT cells in control WT mice are indicated in the bottom panel. (C) Comparison of activation/differentiation markers on splenic and liver NKT cells (CD1d TET⁺ TCRβ⁺) of SAP^+/+ and SAP^−/−.BALB/c mice. (D) Anti-NP IgG titers in the serum of SAP^−/−.BALB/c or SAP^−/−.B6 mice 9 days after intraperitoneal immunization with NP-KLH alone or with NP-KLH plus αGalCer. Hapten-specific serum IgG titers were determined by ELISA. Representative of at least 2 independent experiments with 5 animals per group. (E) The number of GC B cells (B220⁺ Fas⁺ GL7⁺) in the spleen was determined by flow cytometry 9 days after immunization as in (D). Representative of at least 2 independent experiments with 5 animals per group. (F) SAP^−/−.BALB/c mice were immunized with NP-KLH with or without αGalCer. Forty days after primary injections both groups were rechallenged with NP-KLH, and after 5 days anti-NP IgG levels were determined.