Ly7 cells were cultured for 14 days in the presence or absence of IL-21 on either control 3T3 fibroblasts or 3T3-CD40L stable transfectants. A. On day 7 and 14, aliquots of cells were stained and phenotypic changes were examined based on CD20 and CD38 expression. B. Culture supernatants taken on day 0, 4, 7 and 14 were analyzed by ELISA for production of total IgM and IgG antibodies. Note the different y-axis scale for IgM and IgG. Student t-tests were performed to compare the Ig titer from a given treatment to that of the control culture (3T3) on the same day. *, P < 0.05; **, P < 0.01. Results are representative of 4 independent experiments.