AN EXAMPLE OF IDIOTYPIC MIMICRY

We have evaluated the impact of transgenic immunoglobulin (TGIg) expre ssion on endogenous antibody repertoires. The transgenic system was ch osen as to allow for normal recombination of endogenous Ig genes, secr etion of TGIg from early development on, and distinguishing the TGIg f rom endogenous Ig by several serological markers on the C and V region s of the molecules. The transgenic construct encodes a complete anti-( 4-hydroxy-3-iodo-5-nitrophenyl) acetyl (NP) antibody molecule carrying a well-defined idiotype, bearing a lambda 1 light chain and a chimeri c heavy chain encoded by a human alpha 2 C region devoid of its membra ne exon, and the murine B1.8 VDJ-region. Endogenous antibody repertoir es were analyzed in mitogen-driven limiting dilution cultures, in sing le-cell assays for naturally activated Ig-secreting cells, and in hybr idomas derived by direct fusion of spleen cells from unmanipulated ani mals. The results show that a very high frequency of splenic resting B cells and plasma cells in transgenic animals produce IgM with B1.8-cr oss-reactive idiotypes. This was confirmed by hybridoma analysis which also established that the levels of transgene expression and of idiot ype-positive IgM production by the same cell are neat correlated. The affinities of idiotype-positive endogenous Ig varied, but were general ly several orders of magnitude lower than the transgene-encoded idioty pe. V regions from idiotype-cross-reactive IgM heavy chains showed mar ked diversity in sequences that were all different from the transgenic B1.8. These results are compatible with idiotypic mimicry resulting f rom intercellular selection based on degenerate, whole V region reacti vities.