Humanization of an anti-human IL-6 mouse monoclonal antibody glycosylated in its heavy chain variable region

Interleukin-6 (IL-6) inhibitors are good potential therapeutic agents in human patients, and anti-IL-6 antibodies are among the best candidates. Here, we have successfully humanized mouse monoclonal antibody SK2, which specifically binds to IL-6 and strongly inhibits IL-6 functions. Since this antibody possesses N-linked carbohydrates on Asn-30 of V_H region, which seems to be very close to an antigen-binding site, influence of these carbohydrates on antigen-binding was investigated. A biosensor study showed that the mouse SK2 Fab and its deglycosylated fragments had almost equal Kd (K_on/K_off), 26.8 nM (1.05×10⁶12.81×10⁻²) and 24.7 nM (1.28×10⁶13.15×10⁻²), respectively. Furthermore, a mutant chimeric SK2 antibody, in which the N-glycosylation site was removed from the V_H region, showed a Kd of 11 nM, almost similar to that of the original chimeric SK2 antibody, determined by Scatchard analysis with ¹²⁵I-IL-6. These data indicate the carbohydrates of mouse SK2 V_H region do not significantly influence antigen-binding activity. In the next step, two versions of each humanized SK2 V_L and V_H regions were carefully designed based on the amino acid sequences of human REI and DAW, respectively. Only one alteration, Tyr to Phe, was made at position 71 in the two light chains, according to the canonical residue for L1. A N-glycosylation site was introduced on the two heavy chains, by changing Ser to Asn at position 30. All four combinations of humanized light and heavy chains could bind to IL-6 as well as the chimeric SK2 antibody. The light chain first version, however, could not efficiently inhibit IL-6 binding to its receptor, indicating the importance of the L1 loop conformation for the inhibitory activity of SK2 antibody. In contrast, both versions of the heavy chains were comparable, in yielding good humanized SK2 antibodies, suggesting that the glycosylation of the SK2 V_H region has no influence in recreating a functional antigen-binding site in this humanization.