Expression of anticardiolipin cofactor, human β₂-glycoprotein I, by a recombinant baculovirus/insect cell system

A full-length cDNA coding a human β₂-glycoprotein I (β₂-GPI) was introduced into the baculovirus genome to construct a recombinant baculovirus. Spodoptera frugiperda (Sf9) cells were infected with the recombinant baculovirus. A protein (mol. wt 43000) reactive with anti-β₂-GPI antisera was produced in the insect cells and secreted into the culture medium. The recombinant β₂-GPI was purified from the culture supernatant by sequential cardiolipin (CL)-affinity column chromatography and gel filtration. The N-terminal amino acid sequence of the protein was identical to that of the native β₂-GPI purified from human sera, and a putative signal peptide was cleaved from the secreted form of the recombinant protein. The purified recombinant protein had a cofactor activity which enhances CL binding of anticardiolipin antibodies (aCL) in systemic lupus erythematosus (SLE) patients, as well as the native β₂-GPI. Thus, the β₂-GPI expressed in insect cells is an immunologically active cofactor.