Thursday, November 12, 2015: 10:36 AM
251D (Salt Palace Convention Center)
The HIV envelope spike, comprised of gp41 and gp120 proteins assembled as a homotrimeric complex, is the only exposed target on the viral membrane for neutralizing antibodies. Over the past 15 years, a number of HIV envelope trimer mimics have been engineered as potential immunogens for the induction of broadly neutralizing antibodies (bNAbs) against HIV infection. Recently, a stable and homogeneous HIV env trimer, BG505 SOSIP.664 (SOSIP), has been developed and shown to express epitopes for bNAbs and not for non-NAbs. It is generally thought that presentation of env trimers in a highly multivalent form from the surface of a particle may contribute to the development of strong and durable bNAb responses in immunization. To this end, we coupled SOSIP to the surface of liposomal nanoparticles in order to (1) multivalently present SOSIP in a fashion mimicking its display on the viral surface, (2) control SOSIP orientation and prevent antibodies from developing to the SOSIP base, (3) improve draining to lymph nodes and (4) incorporate various adjuvants and helper epitopes to enhance humoral responses. Unilamellar liposomes containing 5% Ni-NTA lipids were synthesized by rehydration and membrane extrusion, followed by post-synthesis coupling of 6xHis-tagged SOSIP (each gp140 monomer contains a C-terminal 6xHis tag). SOSIP trimer was efficiently (~50-65% loading) and reproducibly coupled to the surface of liposomes with 50, 100, and 200 nm diameter liposomes having 13-17, 52-75, and 236-280 trimers per particle, respectively, as determined by ELISA. CryoEM imaging revealed homogeneously distributed and oriented SOSIP on the surface of liposomes irrespective of particle size and lipid composition. Further, the incorporation of monophosphoryl lipid A (MPLA) adjuvant directly into the liposomes did not affect SOSIP loading or other liposome characteristics, including size, shape and lamellarity. One key aspect in particulate vaccine design is to ensure the desired antigenic profile of the antigen is not compromised by the conjugation or encapsulation methods used. SOSIP trimer antigenicity, determined by ELISA on intact liposomes, was maintained upon liposome coupling, displaying epitopes targeted by known bNAbs (i.e. VRCO1, PGT145, PGT151) and not epitopes targeted by non-NAbs (i.e. B6, 447D). Injections of labeled liposomes in balb/c mice showed that all 3 liposome sizes trafficked to draining lymph nodes by 24 hrs, though 50 nm vesicles accumulated most efficiently. Mice were immunized with SOSIP-coupled liposomes containing 10µg SOSIP and 10µg MPLA; both post-prime and post-boost serum SOSIP-specific IgG titers revealed that 100 nm liposomes resulted in the strongest immune response compared to 50 and 200 nm liposomes or soluble SOSIP. Ongoing studies will investigate the role of lipid composition and bilayer fluidity, as well as antigen density to determine optimal particulate vaccine conditions in mice. These results show promise for the use of SOSIP-coupled liposomes in larger animal models and as a potential component of an eventual prophylactic HIV vaccine.