Evaluating an anti-idiotype derived germline-targeting immunogen designed to elicit VRC01-class precursors in human antibody transgenic mice

ABSTRACT An effective HIV-1 vaccine will likely need to elicit broadly neutralizing antibodies (bNAbs) that bind relatively conserved regions of the otherwise highly variable envelope glycoprotein. Among these, VRC01-class bNAbs are a reproducible antibody class that bind the CD4-binding site through genetic features encoded by the VH1-2 heavy chain and a light chain containing a rare five-amino-acid-long complementarity-determining region 3 (CDRL3). We previously developed a germline-targeting immunogen, iv4/iv9, derived from anti-idiotypic monoclonal antibodies (ai-mAbs) that target these genetic signatures of VRC01-class bNAbs. Here, we applied structure-guided modification of iv4/iv9 that improved selective binding to VRC01 precursors in vitro and carried out immunizations in ATX-GK mice. These mice are transgenic for human antibody genes, producing a diverse, genetically human antibody repertoire. We found that ATX-GK mice harbor VRC01 precursors at frequencies lower than those found in humans. Class-switched VRC01 precursors were detected in a minority of mice immunized with a modified iv4/iv9 immunogen. Collectively, these results indicate that the ATX-GK mice have utility to evaluate VRC01-class germline-targeting immunogens but are a stringent model due to a low frequency of VRC01-class B cells. They further suggest that the ai-mAb immunogens evaluated herein will require further optimization to reproducibly prime VRC01-class B cells in ATX-GK mice. IMPORTANCE An effective HIV-1 vaccine will likely need to elicit broadly neutralizing antibodies (bNAbs) that bind relatively conserved regions of the otherwise highly variable envelope glycoprotein. Among these, VRC01-class bNAbs are a reproducible antibody class that bind the CD4-binding site through genetic features encoded by the VH1-2 heavy chain and a light chain containing a rare five-amino-acid-long complementarity-determining region 3 (CDRL3). We previously developed a germline-targeting immunogen, iv4/iv9, derived from anti-idiotypic monoclonal antibodies that target these genetic signatures of VRC01-class bNAbs. Here, we evaluate the B-cell response to immunization with iv4/iv9 and with a structure-guided modified iv4/iv9 in ATX-GK mice. These modifications are intended to improve the selectivity of VRC01 precursors in a diverse polyclonal B cell repertoire. These mice are transgenic for human antibody genes, producing a diverse, genetically human antibody repertoire. We found that ATX-GK mice harbor VRC01 precursors at frequencies lower than those found in humans. Class-switched VRC01 precursors were detected in a minority of animals immunized with the modified iv4/iv9. Collectively, these results indicate that although the iv4/iv9 immunogen did not reproducibly elicit VRC01-class B cells, ATX-GK mice have utility to evaluate VRC01-class germline-targeting immunogens but are a stringent model due to a low frequency of VRC01-class B cells.