Engineering of recombinant human Fcγ receptor I by directed evolution

Human FcRI is a high-affinity receptor for human IgG. On the basis of its binding activity, recombinant human FcRI (rhFcRI) has several possible applications, including as a therapeutic reagent to treat immune complex-mediated disease and as a ligand in affinity chromatography for purification of human IgG. As the stability and production rate of rhFcRI are low, it would need to be engineered for use in such applications. In this study, we demonstrated engineering of rhFcRI by directed evolution through random mutagenesis and integration of mutations. Engineered rhFcRI was expressed by Escherichia coli. Screening identified 19 amino acid mutations contributing to the thermal stability and production rate of rhFcRI. By integration of these mutations, engineered rhFcRI containing all 19 amino acid mutations (enFcRd) was constructed and showed markedly enhanced thermal stability (transition midpoint temperature [Tm] 65.6C) and production rate (3.27 mg L-medium(1) OD6001) compared with wild-type rhFcRI (Tm 48.5C; production rate, 0.07 mg L-medium(1) OD6001) without a change in the specificities of binding to human IgG subclasses. Moreover, the binding affinity of enFcRd for human IgG1/? (equilibrium dissociation constant [K-D] 0.80 10(10) M) was higher than that of wild-type rhFcRI (K-D 1.23 10(10) M). Our study showed that substantial engineering of rhFcRI is possible.