Engineering Secretory (S)IgA to Facilitate Affinity Purification for Large Scale Downstream Processing

Secretory (S)IgA class antibodies play a vital role in first line immunological defence at mucosal surfaces. Passive immunisation with monoclonal SIgA antibodies could offer a completely novel application for a biologics market currently dominated by systemic IgG class monoclonal antibodies. Recent advances have made large scale manufacture of SIgA mAbs feasible, but downstream affinity purification remains a hurdle because IgA antibodies usually have poor affinity for Protein A, the most widely used affinity ligand for antibodies. This research addresses this problem by engineering chimaeric secretory component (SC), containing IgG Fc domains (CH₂─CH₃) which confer Protein A binding. SC is a component of SIgA and consists of five immunoglobulin-like domains. We demonstrate that expressing recombinant monoclonal SIgA with SC domains D2-D3 or D3-D4 replaced by IgG Fc domains (CH₂─CH₃) enables Protein A affinity purification of SIgA, without affecting antibody assembly or antigen binding functionality. Inserting the Protein A binding site into SC, results in purification of SIgA and free SC, which could then be separated according to size, but importantly not SIgA assembly intermediates such as dimeric IgA, that has a similar molecular size to SIgA. The use of SC-Fc fusions could streamline SIgA mAb purification and unlock their clinical potential.