AAV-delivered eCD4-Ig protects rhesus macaques from high-dose SIVmac239 challenges

This study evaluates the protective efficacy of eCD4-Ig, an antibody-like entry inhibitor designed to mimic the HIV-1 and SIV receptor (CD4) and coreceptor (CCR5), against SIVmac239—a highly pathogenic and difficult-to-neutralize viral strain in rhesus macaques. Researchers hypothesized that an AAV-delivered therapeutic could provide stable, long-term prophylaxis, avoiding the need for repeated infusions required by conventional broadly neutralizing antibodies (bNAbs).

The researchers created a rhesus-adapted version of the inhibitor, rh-eCD4-Ig I39N. They delivered this using AAV1 vectors into the quadriceps muscles of rhesus macaques, resulting in sustained systemic expression of the inhibitor. To test its protective capacity, the treated animals were subjected to serial intravenous challenges with SIVmac239, beginning at doses that effectively infected all eight control animals. The study monitored infection status, viral loads, and the emergence of potential viral escape mutations.

All four treated macaques were protected from the initial challenge doses that infected all control animals. While the treated animals eventually succumbed to infection after receiving significantly escalated challenge doses, they exhibited markedly lower peak and post-peak viral loads compared to controls. This suggests the inhibitor continued to suppress the virus even after breakthrough infection occurred. Genetic analysis revealed that the virus evolved resistance through two distinct pathways: one involving mutations in the CD4-binding site that reduced affinity for the inhibitor, and another involving compensatory mutations that allowed the virus to better utilize native rhesus CD4, though these mutations came with significant fitness costs to viral replication.

This research demonstrates that eCD4-Ig is a potent and broadly neutralizing candidate for HIV-1 prophylaxis. By utilizing AAV-mediated gene transfer, this approach offers a one-time intervention that can provide durable protection. Unlike traditional vaccine strategies that attempt to elicit a variable immune response, this approach delivers a consistent, highly potent inhibitor, establishing a strong foundation for future human clinical applications.