Since the development of modern vaccines, many diseases have been globally eradicated. Yet vaccines for several widespread diseases, including HIV, malaria, and tuberculosis, remain elusive. While traditional vaccine approaches have failed against these infectious diseases, researchers have turned towards the development of engineered subunit vaccines. Specifically, my postdoctoral work in the Irvine lab at MIT has focused on the design and optimization of general B-cell targeted and disease-inspired nanoparticulate vaccines.
Lipid-based nanoparticles have been widely characterized for the delivery of small molecules, peptides and proteins. Liposomes containing surface-bound antigens are similarly being developed as vaccines and have been shown to induce strong immune responses when compared to soluble antigens by facilitating cross-presentation and enhancement of humoral responses. Yet so far no studies have focused on how specific liposome design considerations modulate immune responses at the single cell level to enhance CD4+ T cell help and antibody production through interaction and activation of specific B cells. In the first half of my poster, I will describe how we evaluated the role of liposome composition on the direct binding and internalization of liposomes with surface-displayed peptide antigens to antigen-specific B cells. Focusing on the roles of surface PEGylation and antigen tethering, we optimized a particle formulation which bound to antigen-specific B cells with the highest intensity. B cell activation in vitro, assessed at the single-cell level using flow cytometry, was shown to be a direct function of antigen-specific binding. The incorporation of TLR4 agonist, monophosphoryl lipid A (MPLA), into the liposome bilayer also significantly enhanced early activation (day 1) and helped sustain activation and B cell viability at later times (up to 4 days) in an antigen-specific manner. Interestingly, antigen-specific antibody titers in mice immunized with antigen-bearing liposomes containing MPLA were found to correlate directly to in vitro B cell binding profiles for a variety of liposome formulations. The results obtained in these studies will further the development and understanding of effective particle-based peptide vaccines.
Next I will describe a lipid-based nanoparticle vaccine designed specifically for HIV prophylaxis. 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. SOSIP trimer was efficiently (~50-65% loading) and reproducibly coupled to the surface of 5% Ni-NTA containing liposomes of various sizes via C-terminal 6xHis tags. CryoEM imaging revealed homogeneously distributed and oriented SOSIP on the surface of liposomes irrespective of particle size and lipid composition. 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). Upon immunization in mice, 100 nm liposomes resulted in the highest serum SOSIP-specific IgG titers 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.
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