A TARGETED DISRUPTION OF THE MURINE COMPLEMENT FACTOR-B GENE RESULTING IN LOSS OF EXPRESSION OF 3 GENES IN CLOSE PROXIMITY, FACTOR-B, C2, AND D17H6S45

Factor B is a serine protease, essential for the function of the alter native pathway of complement activation, To study further the importan ce of the alternative pathway of complement activation in vivo and to help elucidate any additional functions of factor B or its activation fragments we developed, by homologous recombination in embryonic stem cells, mice with a disrupted factor B gene. Factor B-deficient mice pr oduced no detectable factor B mRNA or protein and had no detectable fa ctor B enzymatic activity or alternative pathway function in their ser um, Further studies revealed that the two adjacent genes, complement c omponent C2 and D17H6S45, had been down regulated as a result of the d isruption, The down-regulation of C2 gene expression was sufficient to cause a complete loss of classical pathway function as determined by the failure of sera from the deficient mice to opsonize antibody-sensi tized sheep erythrocytes and by impairment of immune complex processin g in vivo, The resulting mouse is deficient in both factor B and C2, a nd hence the alternative and classical pathways of complement activati on, and adds to the repertoire of models for studying the in vivo role of complement in the immune system.