The 55-6 murine B cell hybridoma line not constitutively expressing CD40 was treated withincreasing amounts of intact
anti-mouse surface immunoglobulin G
antibody (
anti-mIgG) eithernot preincubated or preincubated for 48 h with lipopolysaccharide (LPS). In vitro, cross-linkingof surface immunoglobulin G (sIgG) with the whole molecule of
anti-IgG
antibodies inducedthe expression of CD69, CD40, and CD19 surface
antigens on 55-6 cells. The effect of sIgGligation was dose-dependent, and preincubation with LPS enhanced their responsiveness to
anti-mIgG stimulation. The expression of these surface molecules reached the maximum valueduring the first part of the cell cycle, corresponding to the position of the G1 peak of the DNAdistribution. Stimulation of cells with
anti-mIgG did not induce changes either in the numberof viable cells or in the fraction of cells undergoing proliferation (mitosis). However,preincubation of 55-6 cells with LPS for 48 h before stimulation with
anti-mIgG increased boththe maximum specific growth rate (
max) and the percentage of cells in the G2/M phase, incomparison with non-preincubated cells. Moreover, on cells preincubated with LPS prior to
anti-mIgG treatment, specific IgG2a production rate was enhanced signific
antly compared tothat obtained in control cultures. The correlation between the
antibody production rate and theamount of IgG that is detectable on the cell surface was analyzed by flow cytometry. A goodcorrelation between secreted and surface IgG was observed, and the results of cell cycle analysesdemostrated that the 55-6 hybridoma cell line has a subst
antially higher sIgG content in G1phase.