Allelic Exclusion in Transgenic Mice That Express the Membrane form of Immunoglobulin μ

Abstract

Antibody-producing cells display a special form of regulation whereby each cell produces immunoglobulin from only one of its two sets of antibody genes. This phenomenon, called allelic exclusion, is thought to be mediated by the product of one heavy chain allele restricting the expression of the other. Heavy chains are synthesized in two molecular forms, secreted and membrane bound. In order to determine whether it is specifically the membrane-bound form of the immunoglobulin M (IgM) heavy chain (mu) that mediates this regulation, transgenic mice were created that carry a human mu chain gene altered so that it can only direct the synthesis of the membrane-bound protein. The membrane-bound form of the human mu chain was made by most of the B cells in these animals as measured by assays of messenger RNA and surface immunoglobulins. Further, the many B cells that express the human gene do not express endogenous mouse IgM, and the few B cells that express endogenous mouse mu do not express the transgene. Thus, the membrane-bound form of the mu chain is sufficient to mediate allelic exclusion. In addition, the molecular structures recognized for this purpose are conserved between human and mouse systems.

Double-strand signal sequence breaks in V(D)J recombination are blunt, 5'-phosphorylated, RAG-dependent, and cell cycle regulated.

“…Hardy, and D. Baltimore, in prep.). These mice were mated with human-~-transgenic mice (Nussenzweig et al 1987), and the genotypes of the progeny were determined by PCR assays.…”

Murine lambda gene rearrangements: the stochastic model prevails over the ordered model.

“…The H chain genes rearrange first, followed by the light ones (H -L). Furthermore, the expressed membranebound form of /A chain (um) which signals cessation of H chain locus rearrangement has been shown to activate rearrangement of the x locus (Alt et al, 1982;Alt et al, 1984;Reth et al, 1985;Iglesias et al, 1987;Nussenzweig et al, 1987;Reth et al, 1987;Mantz et al, 1988), thus cumulating positive and negative functions. With respect to the x/X isotypic exclusion, there is a considerable body of evidence for an ordered regulated model in which x is ( Oxford University Press expressed before X (x -X).…”

Immunoglobulin gamma 2b transgenes inhibit heavy chain gene rearrangement, but cannot promote B cell development.

“…Both P. and 6 heavy chains can promote feedback inhibition at the stage of heavy chain gene rearrangement and, in combination with light chains, at the stage of V(D)J recombinase shutoff (3,4) . It is clear from work with transgenic mice that the membrane form of p, (Am) is required for both types of feedback inhibition, and that the secreted form (As) does not mediate feedback (5,6). The p, and S proteins have related transmembrane domains and identical short cytoplasmic tails (Lys-Val-Lys) (7) .…”