The synergistic effects of anti-IgM and monoclonal anti-Ia antibodies in induction of murine B lymphocyte activation

MHC class II molecules on murine B lymphocytes have been shown to serve as recognition molecules in B cell-T cell interaction. The demonstration that a variety of B-cell stimuli such as anti-Ig, lipopolysaccharide, and interleukin-4 induce hyper-Ia expression has led to the proposal that Ia molecules may serve a role in B-cell activation. However, the question remains whether Ia molecules play a direct or indirect role in B-cell activation. In the present study it has been shown that Ia molecules may play a direct role in providing growth signals to B cells. Affinity purified monoclonal anti-Ia antibodies against both IA (MKD6) and IE (14.4.4) region encoded Ia molecules were able to enhance anti-μ induced B-cell proliferation in a synergistic manner. Anti-Ia antibodies alone had minimal effects on B-cell proliferation. Second, not all monoclonal anti-Ia antibodies, such as the anti-IA antibody 10.3.6.2, can induce this synergy. Third, the synergistic effects of anti-Ia on the anti-μ activation can be demonstrated under serum-free culture conditions. Finally, the effects of anti-Ia antibodies on B-cell activation are not due to induction of interleukin-1 secretion in the cultures nor are due to interaction with the Fc receptors. Since such positive stimulatory effects of anti-Ia antibodies were not reported previously, rigorous steps were taken to demonstrate the reproducibility and specificity of the phenomenon. In over 20 experiments utilizing serum-free culture conditions, we have been able to consistently demonstrate that anti-Ia antibodies augmented anti-μ induced B-cell proliferation by 2.6 fold, on the average. In addition, the anti-Ia antibody induced augmentation of B-cell proliferation is also allele specific and does not require participation by T cells and adherent cells. All antibody preparations used in this study were also shown to be free of endotoxin as demonstrated by the Limulus Amebocyte Assay. The synergistic effects are specific to anti-Ia and anti-μ antibodies, since antibodies to Lyb2 failed to augment the response to anti-μ. The synergy between anti-Ia and anti-μ can be demonstrated with monoclonal (BET2) anti-μ or affinity purified goat anti-μ or (Fab)₂ fragments of the anti-μ antibodies. These results suggest that B-cell surface Ia molecules may function as signal transducer molecules as well as recognition molecules which are important for B-cell activation.