Single-chain antibodies that target lentiviral vectors to MHC class II on antigen-presenting cells

Lentiviral vectors are promising vaccines because they can transduce and express antigens in dendritic cells in vivo, leading to potent immunization. To improve the safety and efficacy of lentivector vaccination, we sought to target vector transduction to antigen-presenting cells by modifying the viral envelope. To do this we screened a nonimmunized human single-chain antibody phage display library for phage that bound mouse bone marrow-derived dendritic cells (BMDCs) and isolated three single-chain antibodies (scFvs) that bound to more than 20% of cells in the BMDC culture. The three scFvs also bound to dendritic cells, macrophages, monocytes, and B cells from mouse spleen, but not to neutrophils, eosinophils, or T cells. Immunoblotting demonstrated that two unique scFvs, C2 and C7, recognized MHC class II. We constructed chimeric envelope proteins, by fusing these two scFvs to the amino terminus of the amphotropic murine leukemia virus envelope (MLV-A). These chimeric envelopes were expressed on the surface of lentiviral vector particles and enhanced infection (5- to 10-fold) of BMDC cultures, compared with lentiviral vectors with unmodified MLV-A envelope. Similarly, the chimeric envelopes enhanced (10- to 20-fold) the infection of primary lymph node class II-positive cells. One of the envelopes, C2, gave increased interferon-gamma production from splenocytes of vaccinated mice compared with MLV-A, achieving a level similar to that obtained with vesicular stomatitis virus glycoprotein G, when used to deliver an ovalbumin model antigen gene. These results demonstrate that surface-targeting lentiviral vector transduction of antigen-presenting cells gives efficient and potentially safer immunization.