The 55-6 murine B cell hybridoma line not constitutively expressin
g CD40 was treated withincreasin
g amounts of intact anti-mouse surface immuno
globulin G antibody (anti-mI
gG) eithernot preincubated or preincubated for 48 h with lipopolysaccharide (LPS). In vitro, cross-linkin
gof surface immuno
globulin G (sI
gG) with the whole molecule of anti-I
gG antibodies inducedthe expression of CD69, CD40, and CD19 surface anti
gens on 55-6 cells. The effect of sI
gGli
gation was dose-dependent, and preincubation with LPS enhanced their responsiveness to anti-mI
gG stimulation. The expression of these surface molecules reached the maximum valuedurin
g the first part of the cell cycle, correspondin
g to the position of the G1 peak of the DNAdistribution. Stimulation of cells with anti-mI
gG did not induce chan
ges either in the numberof viable cells or in the fraction of cells under
goin
g proliferation (mitosis). However,preincubation of 55-6 cells with LPS for 48 h before stimulation with anti-mI
gG increased boththe maximum specific
growth rate (
ges/entities/m
gr.
gif">
max) and the percenta
ge of cells in the G2/M phase, incomparison with non-preincubated cells. Moreover, on cells preincubated with LPS prior toanti-mI
gG treatment, specific I
gG2a production rate was enhanced si
gnificantly compared tothat obtained in control cultures. The correlation between the antibody production rate and theamount of I
gG that is detectable on the cell surface was analyzed by flow cytometry. A
goodcorrelation between secreted and surface I
gG was observed, and the results of cell cycle analysesdemostrated that the 55-6 hybridoma cell line has a substantially hi
gher sI
gG content in G1phase.