Humoral immune responses in PECAM-1^−/−mice upon challenge with T-cell–independent antigen, DNP-Ficoll, and T-cell–dependent DNP-KLH.

(A) Serum Ig levels were determined in 8-week-old nonimmunized wild-type (PECAM-1^+/+) (n = 10) and PECAM-1^−/− (n = 10) animals using a capture antibody ELISA (asterisk denotes statistical significance; paired Studentt test; P < .001). (B) Eight-week-old PECAM-1^−/− mice produced increased levels of IgG_2a, IgG_2b, IgG₃, and IgA upon antigen challenge with a T-cell–independent antigen, DNP-Ficoll. Mice were immunized with 20 μg alum-precipitated DNP-Ficoll intraperitoneally on day 0 and bled on days 7, 14, and 21. DNP-specific antibodies were determined using a DNP-BSA capture ELISA and isotype-specific antisera (asterisk denotes statistical significance; paired Student t test; P < .001; n = 8). (C) T-cell–dependent antigen DNP-KLH induces comparable antibody responses in 8-week-old wild-type (PECAM-1^+/+) and PECAM-1^−/− mice. Mice were immunized with 100 μg alum-precipitated DNP-KLH intraperitoneally on days 0 and 21 and bled on days 7, 14, 21, and 35. DNP-specific antibodies were determined using a DNP-BSA capture ELISA and isotype-specific antisera (paired Student t test; P > .05; n = 8). Sera isolated from PECAM-1^+/+ and PECAM-1^−/− mice at days 7, 14, and 21 were diluted from 1:1000, while at day 35 sera were diluted from 1:10 000 before assay.