A Rationally Designed Bovine IgA Fc Scaffold Enhances Accumulation of a VH-Fc Fusion Without Compromising Binding to Enterohemorrhagic

We previously isolated a single domain antibody (VH) that binds Enterohemorrhagic (EHEC) with the end-goal being the enteromucosal passive immunization of cattle herds. To improve the yield of a chimeric fusion of the VH with an IgA Fc, we employed two rational design strategies, supercharging and introducing disulfide bonds, on the bovine IgA Fc component of the chimera. After mutagenizing the Fc, we screened for accumulation levels after transient transformation in leaves. We identified and characterized five supercharging and one disulfide mutant, termed '(5 + 1)Fc', that improve accumulation in comparison to the native Fc. Combining all these mutations is associated with a 32-fold increase of accumulation for the Fc alone, from 23.9 mg/kg fresh weight (FW) to 599.5 mg/kg FW, as well as a twenty-fold increase when fused to a VH that binds EHEC, from 12.5 mg/kg FW tissue to 236.2 mg/kg FW. Co-expression of native or mutated VH-Fc with bovine joining chain (JC) and bovine secretory component (SC) followed by co-immunoprecipitation suggests that the stabilizing mutations do not interfere with the capacity of VH-Fc to assemble with JC and FC into a secretory IgA. Both the native and the mutated VH-Fc similarly neutralized the ability of four of the seven most prevalent EHEC strains (O157:H7, O26:H11, O111:Hnm, O145:Hnm, O45:H2, O121:H19 and O103:H2), to adhere to HEp-2 cells as visualized by immunofluorescence microscopy and quantified by fluorometry. These results collectively suggest that supercharging and disulfide bond tethering on a Fc chain can effectively improve accumulation of a VH-Fc fusion without impacting VH functionality.