Design and expression of polymeric immunoglobulin fusion proteins: A strategy for targeting low-affinity fc gamma receptors

We have developed a family of cloning vectors that direct expression of fus ion proteins that mimic aggregated immunoglobulin (IgG) (AIG) and immune co mplex function with respect to their interactions with Fc gammaR and that a llow for the inclusion and targeting of a second protein domain to cells ex pressing Fc gammaR. This was accomplished by expressing multiple linear cop ies of the hinge and CH2 domains (HCH2) of human IgG(1) fused to the framew ork region of human IgG(1). Convenient restriction sites allow for the faci le introduction of additional amino-terminal domains. The resulting molecul e is tripartite. The carboxyl-IgG(1) framework domain provides stability an d permits dimerization, and the intervening polymer provides increased effe ctor function and targeting to Fc gammaR while the amino-terminal domain ca n deliver an additional signal to cells expressing Fc gammaR. To demonstrat e the utility of the vectors, the extracellular domain of human CD8 alpha w as expressed as a HCH2 polymer fusion protein. The fusion proteins were sec reted in useful amounts from polyclonal cell lines established in Sf9 cells following transfection and selection with G418. The biological activity of the recombinant CD8 alpha -HCH2 polymers was determined and compared to th ose of AIG and an anti-CD16 monoclonal antibody using an in vitro assay. Th e activity of the fusion proteins positively correlates to the number of HC H2 units. The largest polymer tested was Severalfold more potent than AIG a t similar concentrations. The HCH2 polymers described here represent a new strategy in the design of recombinant proteins for the therapeutic targetin g of Fc gammaR in autoimmune disorders. (C) 2001 Academic Press.