A single mutation turns a non-binding germline-like predecessor of broadly neutralizing antibody into a binding antibody to HIV-1 envelope glycoproteins

Broadly neutralizing antibodies (bnAbs) against human immunodeficiency virus (HIV)-1 are rare in natural infection and elicitation of HIV-1 bnAbs has not been achieved by any vaccine candidates. We and others have reported that HIV-1 bnAbs are highly diversified from their germline-like predecessors, and the germline-like predecessors of bnAbs lack measurable binding to HIV-1 envelope (Env) glycoproteins, suggesting that Env structures containing the epitopes of bnAbs may not initiate somatic maturation pathway, which may partially explain the rarity of HIV-1 bnAbs. To determine the minimum mutations required for converting non-binding germline-like predecessors to Env-binding antibodies, we started with the bnAb b12 as a prototype and generated six “chimeric” scFv b12 variants by sequentially replacing the heavy chain V-segment (HV), D(J)-segment [HD(J)] in the heavy chain variable region (VH), and the whole light chain variable region (VL) in b12 germline-like predecessor with the mature counterparts. We tested the recombinant scFv variants for binding and neutralizing activities. Results showed that a single point mutation in germline D-segment was enough to convert nonbinding germline-like b12 to an Env-binding antibody. Replacement with either mature HV or mature VL also made the germline-like b12 bind to Env, but none of single segment replacements conferred neutralization ability to the germline antibody. Mature VL in combination with mature HD(J), or mature HV, or both conferred increasing neutralization activity to the germline antibody. However, hybrid scFv, mature VH / germline VL did not neutralize the virus, suggesting the importance of mature VL in neutralizing the virus. These results may have implications for vaccine development.