Engineering C1q single-chain globular head variants for enhanced IgM binding

The initiation of the classical complement pathway begins with the binding of the globular head of complement component 1q (C1q) to antigen-bound immunoglobulin M (IgM). To investigate the binding mechanism and sites of C1q, a single-chain protein mimetic of the globular head of C1q and variants thereof were designed.

Two approaches were used to generate single-chain globular head C1q variants containing single point mutations potentially altering IgM/C1q binding. The rational protein engineering approach aimed to increase surface charge, considering the negatively charged IgM Cµ3 region and positively charged C1q globular heads. Further, a library of 646 variants with single point mutations in the C1q B-chain loops was designed and expressed using yeast surface display. Three rounds of panning in IgM-coated plates yielded twenty-eight sequenced yeast colonies. The His-tagged wild type variant and six of nine selected variants were stably expressed in Chinese hamster ovary cells and purified using immobilized-metal affinity chromatography. All variants were tested for IgM interaction in competition with serum-derived C1q and in a complement activation assay to evaluate the C1q competition potential of the single-chain globular head proteins.

Expression levels differed among the globular head C1q variants, and SDS-PAGE analysis revealed variations in migration mobility, suggesting conformational differences. Four variants showed enhanced IgM binding compared to the wild type variant indicated by improved C1q displacement in the competitive interaction assay. These results were further supported by an advanced complement activation assay, where these variants significantly inhibited complement activation. These findings underpin the critical role of specific amino acids for IgM/C1q interaction and highlight the potential of engineered C1q as a potent inhibitor or activator of the classical complement cascade.