Lentiviral vectors engineered for direct In Vivo delivery to resting T cells

Abstract Direct in vivo T cell engineering offers a new paradigm for CAR T delivery. We have evaluated several approaches to engineer lentiviral (LV) vectors capable of delivering CAR transgenes directly and selectively to T cells in vivo, within the lymphoid niche. Our goal was to develop an LV delivery platform that following intravenous administration can selectively bind and activate T cells, deliver and integrate the (CAR) transgene into genomic DNA, and drive CAR expression on the T cell surface. Further requirements were that the retargeted LV should be associated with minimal off-target transduction of tumor cells and normal host tissues and should be manufacturable at titers sufficient for human application. Our strategy for retargeting has been to modify the trimeric VSV-G protein, which is well established for efficient LV pseudotyping. Of the approaches evaluated, three retargeting platforms have proven to be effective: direct covalent display, indirect covalent display, and G-capping. Direct covalent display. This (LV-169) approach uses chimeric VSV-G protein that contains a CD3-targeting ligand displayed as a N-terminal extension of VSV-G protein that has been mutated to ablate (blind) its interaction with the LDL receptor. Chimeric and nonchimeric blinded G proteins are co- incorporated into LV particles within mixed envelopes which bind to CD3. Upon binding to cell surface CD3, the engineered particles activate T-cells and enter acidifying endosomes, whereupon they trigger membrane fusion and deliver their payloads. LV-169 displaying an anti-human CD3 scFv selectively transduced resting human T cells in PBMC cultures, in fresh human whole blood and following intravenous administration to humanized mice. In immunocompetent Cre reporter mice, surrogate vectors displaying anti-murine CD3 scFv selectively transduced lymph node resident T cells, with off-target transduction being limited to tissue-resident macrophages. To eliminate the incorporation of CAR protein into the LV-169 envelope, which can lead to highly undesirable transduction of tumor cells expressing the CAR target, we placed the CAR transgene in reverse to 5'LTR orientation, downstream of a T-cell specific promoter/enhancer. In vivo efficacy of LV-169-BCMA-CAR was demonstrated in humanized mice bearing disseminated OPM-2 myeloma tumors. Clinical translation of this construct is now underway. Indirect covalent display. In this G-protein engineering approach, a short peptide (SPY-tag) is displayed as an N-terminal extension of the LDLR-blinded VSV-G protein. The SPY-tagged LV vectors are then covalently modified to display a CD3-targeting ligand by simply mixing them at room temperature with a bifunctional fusion protein consisting of the SPY-catcher and the CD3 ligand. Under these conditions, the SPY-catcher and the SPY-tag spontaneously form a covalent isopeptide bond. The CD3 ligand-modified LV particles were shown to specifically activate and transduce T cells ex vivo within human PBMC cultures. G-capping. In the final approach, the VSV-G protein is not genetically modified. Instead, a “capping” protein crosslinker that binds directly to the unmodified G protein on preformed LV particles is used to mask their LDLR binding sites and redirect them to interact with CD3. Mixing of the CD3-targeted capping protein with traditional G-pseudotyped LV particles leads to efficient binding of the cap to the trimeric G proteins. The cap-modified LV are unable to interact with LDLR but interact efficiently with CD3. CD3-capped LV particles were highly specific and efficient in human T cell activation and transduction in PBMC cultures. The association between the G protein and the bound cap is highly stable during freeze-thaw cycles and at room temperature, a critical attribute of this platform to facilitate the manufacture of a clinically and commercially viable LV product. In summary, we have developed three translationally relevant platform technologies for the generation of LV particles capable of delivering CAR transgenes specifically and efficiently to resting niche-resident T cells after intravenous administration. Manufacturing studies are underway and, in the case of the LV-169 platform, clinical translation is already in progress.