A high throughput lentivirus sieving assay identifies neutralization resistant Envelope sequences and predicts in vivo sieving

Whole-body PET imaging of simian immunodeficiency virus using gp120-targeting probes fails to reveal regions of specific uptake in rhesus macaques

PURPOSE

Following the initial reports demonstrating the feasibility of immunoPET imaging of simian immunodeficiency virus (SIV) using gp120-targeting monoclonal antibodies in non-human primates, replication efforts of the imaging system in human immunodeficiency virus (HIV)-infected individuals have yielded conflicting results. Herein, we used two anti-gp120 antibodies, 7D3 and ITS103.01LS-F(ab')2, to interrogate the reproducibility of gp120-targeting probes for immunoPET imaging of SIV in rhesus macaques.

METHODS

The binding affinity estimates of 89Zr radiolabeled 7D3 and ITS103.01LS-F(ab')2 to SIV gp120, and the in-vitro and ex-vivo binding specificities of [89Zr]Zr-7D3 and [89Zr]Zr-ITS103.01LS-F(ab')2 to SIV Env expressing cells, primary cells, and tissue sections from uninfected and SIV-infected macaques were obtained through competition assays. The biodistributions of [89Zr]Zr-7D3 and [89Zr]Zr-ITS103.01LS-F(ab')2 were performed with static PET scans up to 6 days post-injection in 20 rhesus macaques and the standardized uptake values in various tissues were compared between SIV-infected and uninfected controls.

RESULTS

Despite the demonstrated nanomolar affinity of [89Zr]Zr-7D3 and [89Zr]Zr-ITS103.01LS-F(ab')2 to SIV gp120, and strong binding specificity to SIV gp120 cell lines, we observed no discernible differences in their binding in primary cells, tissue sections of secondary lymphoid organs, in-vivo probe uptake between SIV-infected and uninfected macaques, or ex-vivo validation necropsies. While the probes remained stable in-vivo, only [89Zr]Zr-ITS103.01LS-F(ab')2 in chronic plasma retained its binding specificity to SIV gp120, with [89Zr]Zr-7D3 experiencing a > 97% reduction in binding to gp120 due to competition from endogenous antibodies at the 7D3 binding site.

CONCLUSION

The overall absence of specific uptake suggests inadequate binding potential (ligand affinity x target molarity) for these probes to effectively image SIV or HIV in-vivo, warranting further investigation into the lack of reproducibility observed with earlier non-human primate SIV imaging and conflicting human studies.

Antibody-dependent cellular cytotoxicity, infected cell binding and neutralization by antibodies to the SIV envelope glycoprotein

Antibodies specific for diverse epitopes of the simian immunodeficiency virus envelope glycoprotein (SIV Env) have been isolated from rhesus macaques to provide physiologically relevant reagents for investigating antibody-mediated protection in this species as a nonhuman primate model for HIV/AIDS. With increasing interest in the contribution of Fc-mediated effector functions to protective immunity, we selected thirty antibodies representing different classes of SIV Env epitopes for a comparison of antibody-dependent cellular cytotoxicity (ADCC), binding to Env on the surface of infected cells and neutralization of viral infectivity. These activities were measured against cells infected with neutralization-sensitive (SIVmac316 and SIVsmE660-FL14) and neutralization-resistant (SIVmac239 and SIVsmE543-3) viruses representing genetically distinct isolates. Antibodies to the CD4-binding site and CD4-inducible epitopes were identified with especially potent ADCC against all four viruses. ADCC correlated well with antibody binding to virus-infected cells. ADCC also correlated with neutralization. However, several instances of ADCC without detectable neutralization or neutralization without detectable ADCC were observed. The incomplete correspondence between ADCC and neutralization shows that some antibody-Env interactions can uncouple these antiviral activities. Nevertheless, the overall correlation between neutralization and ADCC implies that most antibodies that are capable of binding to Env on the surface of virions to block infectivity are also capable of binding to Env on the surface of virus-infected cells to direct their elimination by ADCC.

Mechanistic basis of post-treatment control of SIV after anti-α4β7 antibody therapy

Treating macaques with an anti-α4β7 antibody under the umbrella of combination antiretroviral therapy (cART) during early SIV infection can lead to viral remission, with viral loads maintained at < 50 SIV RNA copies/ml after removal of all treatment in a subset of animals. Depletion of CD8⁺ lymphocytes in controllers resulted in transient recrudescence of viremia, suggesting that the combination of cART and anti-α4β7 antibody treatment led to a state where ongoing immune responses kept the virus undetectable in the absence of treatment. A previous mathematical model of HIV infection and cART incorporates immune effector cell responses and exhibits the property of two different viral load set-points. While the lower set-point could correspond to the attainment of long-term viral remission, attaining the higher set-point may be the result of viral rebound. Here we expand that model to include possible mechanisms of action of an anti-α4β7 antibody operating in these treated animals. We show that the model can fit the longitudinal viral load data from both IgG control and anti-α4β7 antibody treated macaques, suggesting explanations for the viral control associated with cART and an anti-α4β7 antibody treatment. This effective perturbation to the virus-host interaction can also explain observations in other nonhuman primate experiments in which cART and immunotherapy have led to post-treatment control or resetting of the viral load set-point. Interestingly, because the viral kinetics in the various treated animals differed—some animals exhibited large fluctuations in viral load after cART cessation—the model suggests that anti-α4β7 treatment could act by different primary mechanisms in different animals and still lead to post-treatment viral control. This outcome is nonetheless in accordance with a model with two stable viral load set-points, in which therapy can perturb the system from one set-point to a lower one through different biological mechanisms.

Some macaques treated with an anti-α4β7 monoclonal antibody along with antiretroviral therapy during the early stages of simian immunodeficiency virus infection had their viral load become undetectable (below 50 SIV RNA copies/ml) after all treatment was stopped, whereas animals not given the antibody all had their viral loads rebound to high levels. Using a mathematical model, we examined four potential ways in which the antibody could have altered the balance between viral growth and immune control to maintain an undetectable viral load. We show that a shift to controlled infection can occur through multiple biologically reasonable mechanisms of action of the anti-α4β7 antibody.