ALLOANTIGEN PRESENTATION BY B-CELLS - ANALYSIS OF THE REQUIREMENT FORB-CELL ACTIVATION

This paper describes a model for investigation of the functional impli cations of B-cell activation for antigen presentation. Mixed lymphocyt e cultures were used to assess the ability of freshly isolated B cells , mitogen-activated B cells and Epstein-Barr virus (EBV)-transformed B -cell lines to stimulate the activation and proliferation of allogenei c T cells under a variety of experimental conditions. It was found tha t resting B cells presented antigen poorly, while activated cells were highly immunogenic. Paraformaldehyde fixation completely eliminated a ntigen presentation by resting B cells, despite constitutive expressio n of class II MHC antigens. However, fixation had little effect on ant igen presentation by activated B cells that expressed B7-1 and B7-2 in addition to class II major histocompatibility complex (MHC) molecules . Arrest of B-cell activation by serial fixation after treatment with F(ab')(2) fragments of goat anti-human IgM produced cells with variabl e antigen-presenting capacity. Optimal antigen presentation was observ ed for cells fixed 72 hr after the initiation of B-cell activation. Al though both B7-1 and B7-2 antigen expression increased after B-cell ac tivation, it was found that the rate of T-cell proliferation correlate d most closely with B7-2 expression. Stimulation of T cells by fixed a ctivated B lymphocytes could be blocked by antibodies directed at clas s II MHC molecules, indicating involvement of the T-cell antigen recep tor. In addition, T-cell proliferation was inhibited by antibodies spe cific for B7-1 and B7-2 and by the fusion protein CTLA4-Ig, demonstrat ing a requirement for CD28 signal transduction. The sole requirement o f B7 family expression for antigen presentation by B lymphocytes was s hown by demonstration of T-cell stimulation by fixed resting B cells i n the presence of CD28 antibody as a source of artificial costimulatio n.