Summary The rapid mutation rate of HIV-1 results in many thousands of viral strains, thus thwarting current vaccine efforts. Although HIV-1 infection can be controlled by anti-retroviral therapy (ART), the virus rebounds within weeks of ART cessation because complete elimination of HIV-1 is prevented by latent viral reservoirs. Broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope spike (Env) that have been isolated from a subset of HIV-1–infected donors are protective against HIV-1 infection and can lower the viral load after infection, and it has been suggested that bNAbs could play a role in eliminating the viral reservoir. However, HIV-1 can evade even the most potent bNAbs by mutation. Here we seek to engineer bNAbs to be resistant to viral mutation so they could be used to eliminate viral reservoirs. Our strategy relies upon harnessing avidity effects to prevent viral resistance to bNAbs at both an individual and population level. We hypothesize that HIV-1 hinders IgGs from using both antigen-binding Fabs to bind bivalently. This is accomplished by the small number and low density of HIV-1 Env spikes, which prevent most IgGs from inter-spike crosslinking (bivalent binding between spikes), and the architecture of the Env trimer, which impedes intra-spike crosslinking (bivalent binding within a spike). We suggested that predominantly monovalent binding expands the range of HIV-1 mutations permitting Ab evasion, whereas reagents capable of bivalent binding through intra-spike crosslinking would be more potent across multiple strains of HIV-1. This hypothesis was supported by our demonstration of up to 100-fold increases in geometric mean potency achieved with our first generation intra- spike crosslinking reagents (homo- and hetero-diFabs joined by rigid DNA linkers). These results support the hypothesis that HIV's low spike density contributes to vulnerability of HIV-1 bNAbs to spike mutations and suggests that the ideal anti-HIV therapeutic for eliminating HIV reservoirs would utilize avidity to achieve intra- spike crosslinking because this sort of therapeutic would reduce the concentration required for sterilizing immunity and be resistant to Env mutations. Here we propose to design, produce, and evaluate second generation intra-spike crosslinking reagents with two improvements: (i) they will contain an IgG Fc to mediate effector functions and increase the serum half-life, and (ii) the DNA will be replaced by structured protein linkers. We will also evaluate the effects of Fc substitutions designed to enhance Fc-mediated effector functions through tighter binding to activating FcγR receptors and improve serum half-life through enhanced binding to FcRn, including a novel computational design strategy to improve binding to FcRn under conditions that promote increased IgG half-life. These more potent bNAbs could be used therapeutically at lower concentrations and thus reduce cost and/or production time, increase the number of pat...