Cloning and expression of a murine Fab fragment recognizing a defined linear epitope of Chironomus thummi thummi major allergen Chi t 1-9.

We have cloned and expressed in bacteria the genes coding for the Fab fragment of the monoclonal antibody (MoAb) M3. M3 is a murine IgG1 antibody reactive with the major allergen Chi t 1-9 of Chironomus thummi thummi. The major allergen Chi t 1-9 is known to be an aggressive inhalant allergen and causes type-I allergy. The immunoglobulin (Ig) fragment genes were cloned as a synthetic dicistronic operon. In this operon each gene is preceded by a bacterial signal sequence to direct the recombinant protein to the periplasmic space of the bacteria. The cloned genes were expressed in Escherichia coli using the strong T7-RNA-polymerase-based system. Sequence analysis revealed that the light and heavy chains of MoAb M3 belong to the V kappa II and V kappa IIC group of the Ig family, respectively. Genes of the V kappa II and V kappa IIC group are known to be used in response to haptens. The apparent affinity constants of the parent antibody M3 and the recombinant Fab fragment are nearly equivalent.