Broadly neutralizing antibodies (bnAbs) against HIV-1 protect from infection and reduce viral load upon therapeutic applications. However no vaccine was able so far to induce bnAbs demanding their expensive biotechnological production. For clinical applications, nanobodies (VHH) derived from heavy chain only antibodies from Camelidae, may be better suited due to their small size, high solubility/stability and extensive homology to human VH3 genes. Here we selected broadly neutralizing nanobodies by phage display after immunization of dromedaries with different soluble trimeric envelope proteins derived from HIV-1 subtype C. We identified 25 distinct VHH families binding trimeric Env, of which 6 neutralized heterologous primary isolates of various HIV-1 subtypes in a standardized in vitro neutralization assay. The complementary neutralization pattern of two selected VHHs in combination covers 19 out of 21 HIV-1 strains from a standardized panel of epidemiologically relevant HIV-1 subtypes. The CD4 binding site was preferentially targeted by the broadly neutralizing VHHs as determined by competition ELISAs and 3D models of VHH-Env complexes derived from negative stain electron microscopy. The nanobodies identified here are excellent candidates for further preclinical/clinical development for prophylactic and therapeutic applications due to their potency and their complementary neutralization patterns covering the majority of epidemiologically relevant HIV-1 subtypes.

In the absence of an active prophylactic vaccine against HIV-1, passively administered, broadly neutralizing antibodies (bnAbs) identified in some chronically infected persons were shown to prevent HIV-1 infection in animal models. However, passive administration of bnAbs may not be suited to prevent sexual HIV-1 transmission in high-risk cohorts, as a continuous high level of active bnAbs may be difficult to achieve at the primary site of sexual transmission, the human vagina with its acidic pH. Therefore, we used Lactobacillus, a natural commensal in the healthy vaginal microbiome, to express bn nanobodies (VHH) against HIV-1 that we reported previously. After demonstrating that recombinant VHHA6 expressed in E. coli was able to protect humanized mice from mucosal infection by HIV-1Bal, we expressed VHHA6 in a soluble or in a cell-wall-anchored form in Lactobacillus rhamnosus DSM14870. This strain is already clinically applied for treatment of bacterial vaginosis. Both forms of VHHA6 neutralized a set of primary epidemiologically relevant HIV-1 strains in vitro. Furthermore, VHHA6 was still active at an acidic pH. Thus, lactobacilli expressing bn VHH potentially represent an attractive vector for the passive immunization of women in cohorts at high risk of HIV-1 transmission.

Nanobodies or VHH are the smallest naturally occurring antibody fragments derived from heavy chain only antibodies from Camelidae. Due to their physicochemical properties (high stability, high affinity and target specificity, extended CDR3 loops and their small size allowing to enter into protein cavities), nanobodies are very suited for preventive and therapeutic applications. We recently selected nanobodies with broad neutralizing capacity against primary HIV-1 strains of different subtypes from phage immune libraries generated from dromedaries immunized with HIV-1 subtype C gp140 SOSIP Env glycoproteins (Koch et al., 2017, in press). Two nanobodies with complementary neutralization pattern neutralized 19 out of 21 pseudoviruses in the standard TZM-bl assay. Epitope mapping data by competition ELISAs as well as negative-stain EM reconstructions with trimeric SOSIPs identified the CD4 binding site as the major target. A new selection performed on a next-generation optC SOSIP664 plus sCD4 with libraries generated at a late timepoint (7 months after the initial 7 weeks immunization cycle) identified 2 new nanobodies, which are currently being analyzed for neutralization. We further proved functionality of the nanobodies at acidic pH (as found in the vagina) and identified nanobody combinations resulting in increased breadth of neutralization in vitro. In view of preventive applications at vaginal sites of HIV-1 transmission, we are expressing the best nanobodies in a membrane-bound form and as a secreted version from lactobacilli (L. rhamnosus), which colonize the human vagina. Finally, nanobodies will be analyzed in a humanized mouse model of HIV-1 infection for their HIV-neutralizing capacity in vivo.

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