CRE-LOXP-MEDIATED GENE REPLACEMENT - A MOUSE STRAIN PRODUCING HUMANIZED ANTIBODIES

Background: The bacteriophage-derived Cre-loxP recombination system op erates efficiently in mammalian cells. This system is particularly use ful in gene-targeting experiments in the mouse, and has already been u sed to generate 'clean' deletions of target genes in the germ line, as well as to inactivate target genes in a conditional manner (based on regulated expression of the Cre recombinase). In principle, Cre-loxP-m ediated recombination should also allow gene replacement, and thus the introduction of virtually any kind of mutation into the genome. Resul ts: We used the Cre-loxP system, in mouse embryonic stem cells, to rep lace the mouse gene C gamma 1, which encodes the constant region of th e heavy chain of IgG1 antibodies, with its human counterpart. The muta tion was transmitted through the mouse germ line, and the resulting mu tant mice were crossed with mice expressing K light chains with a huma n, instead of a mouse, constant region. Mice homozygous for both mutat ions produce humanized, K-chain-bearing IgG1 antibodies at the same le vel and efficiency as wild-type mice produce murine IgG1 antibodies. T hese animals should enable the ex vivo production of humanized, chimer ic monoclonal antibodies specific for any antigen to which the mouse c an respond. Conclusions: Cre-loxP-mediated gene replacement is a simpl e and efficient general method of targeted mutagenesis in the mouse.