Stable HIV Decoy Receptor Expression after in Vivo HSC Transduction in Mice and NHPs: Safety and Efficacy in Protection from SHIV

We aim to develop an in vivo hematopoietic stem cell (HSC) gene therapy approach for persistent control/protection of HIV-1 infection based on the stable expression of a secreted decoy protein for HIV receptors CD4 and CCR5 (eCD4-Ig) from blood cells. HSCs in mice and a rhesus macaque were mobilized from the bone marrow and transduced by an intra-venous injection of HSC-tropic, integrating HDAd5/35++ vectors expressing rhesus eCD4-Ig. In vivo HSC transduc-tion/selection resulted in stable serum eCD4-Ig levels of X100 mg/mL (mice) and >20 mg/mL (rhesus) with half maximal inhibitory concentrations (IC50s) of 1 mg/mL measured by an HIV neutralization assay. After simian-human-immunodefi- ciency virus D (SHIV.D) challenge of rhesus macaques injected with HDAd-eCD4-Ig or a control HDAd5/35++ vector, peak plasma viral load levels were X50-fold lower in the eCD4-Ig-ex-pressing animal. Furthermore, the viral load was lower in tis-sues with the highest eCD4-Ig expression, specifically the spleen and lymph nodes. SHIV.D challenge triggered a selective expansion of transduced CD4+CCR5+ cells, thereby increasing serum eCD4-Ig levels. The latter, however, broke immune tolerance and triggered anti-eCD4-Ig antibody responses, which could have contributed to the inability to eliminate SHIV.D. Our data will guide us in the improvement of the in vivo approach. Clearly, our conclusions need to be validated in larger animal cohorts.