THE THYMUS-INDEPENDENT ANTIGEN ALPHA(1-3) DEXTRAN ELICITS PROLIFERATION OF PRECURSORS FOR SPECIFIC IGM ANTIBODY-PRODUCING CELLS (MEMORY CELLS), WHICH ARE REVEALED BY LPS STIMULATION IN SOFT AGAR CULTURES AND DETECTED BY IMMUNOBLOT

Single antibody-forming cells (AFC) specific for alpha(1-3) dextran (D ex) from i.p.-immunized BALB/c mice were enumerated in soft agar cultu res by blotting on antigen-precoated membranes and subsequent staining via enzyme-coupled anti-IgM antibodies. Short cultures (2 h) revealed AFC as harvested ex vivo, while in long-term cultures (4 days), in th e presence of lipopolysaccharide (LPS) as B cell mitogen, cells or col onies developed by differentiation in vitro. Whereas the spleen contai ned most AFC ex vivo in a sharp-peak response at 4 and 5 days after i. p. injection of Dex in aqueous solution, peritoneal exudate cells (PEC ) contained only very few AFC. However, the same PEC population develo ped Dex-specific cells or colonies after 4 days of culture. The isotyp e of antibodies was IgM. The frequency of these Dex-specific LPS-induc ible precursor cells rose exponentially in the course of the immune re sponse to a broad plateau and was still, 11 weeks after Dex injection, approximately 40-fold higher than in non-immunized mice. Since these cells increased in frequency after antigen injection, and since they c ould not be detected as AFC during 2 h ex vivo, they were regarded as memory cells. They seemed to be arrested in vivo, but could be induced to differentiation and/or proliferation in vitro. Although these cell s had the functional characteristics of memory cells as defined above, they produced anti-Dex antibodies of IgM isotype. Their population mi ght be critical for the protection of the peritoneal cavity against mi crobial invasion from the intestines, and it may be significant in thi s context that we could evoke a peritoneal memory cell response only w hen antigen was injected intraperitoneally, but not intravenously. In athymic BALB/c-nu/nu mice only few of these Dex-specific memory cells were found. It is possible that T cells exert a regulatory influence o n this pathway of differentiation.