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Virus-Like Particles as a Flexible Platform for Mucosal Vaccine Delivery


Through funding in part from the NSF’s Integrated Graduate Education and Research Traineeship program on Integrating Nanotechnology with Cell Biology and Neuroscience, Zoe Hunter has primarily been working with a Qbeta bacteriophage virus-like particle (VLP) platform, and attaching peptides from the cellular HIV coreceptor, CCR5, to its surface to determine the efficacy of this vaccine in protecting against HIV infection.

VLPs are nanostructures that are comprised of viral structural proteins arrayed in a highly repetitive multivalent format. When overexpressed, viral coat proteins spontaneously self-assemble into particles that are indistinguishable from infectious virus. This structure is highly immunogenic, because the humoral immune system responds vigorously to antigens displayed in the dense, repetitive arrays that are characteristic of viral surfaces. Therefore, VLPs attractive for inducing antibody responses against molecules that are normally difficult to invoke immune responses against. Such molecules could include carbohydrates, small peptides, or other haptens, and self-proteins. The laboratory of Dr. Bryce Chackerian, Ms. Hunter’s advisor, has developed a portfolio of display technologies that can be used to deliberately modify VLPs so that they essentially function as nanoparticle scaffolds for antigen presentation. Any number of viruses could, in principle, provide a scaffold for high-density display of target antigens, and indeed many different VLP types have been adapted for this purpose.

As an alternative to conventional HIV vaccines, Ms. Hunter is working on developing a vaccine that targets CCR5, a self-molecule that is critically involved in HIV acquisition and is not subject to antigenic variation. Unlike viral antigens, in which variants are rapidly selected in response to host immune pressures, CCR5 is a cellular protein and is, therefore, genetically stable. It is expected that a vaccine that induces antibodies against CCR5 – either by limiting its expression on the cell surface or by blocking virus-receptor interactions – could block viral replication and prevent viral transmission. Because HIV infection usually occurs at mucosal surfaces, it is highly likely that CCR5 antibodies at the site of transmission may be more effective at blocking viral entry.

In collaboration with Dr. Hugh Smyth at the UNM College of Pharmacy, Ms. Hunter has examined the ability of VLP-based immunogens to induce mucosal immune responses upon pulmonary vaccination using aerosolized VLP-vaccines. Aerosol delivery to the lung has a number of advantages. First, the lower respiratory tract contains abundant antigen-presenting cells, which play important roles in priming adaptive immune responses. Second, although the mucosal immune system is, by and large, compartmentalized, pulmonary vaccination results not only in local mucosal responses in the lung, but also can give rise to strong mucosal responses in the genital/vaginal mucosa.

Ms. Hunter has investigated mucosal antibody responses induced by pulmonary vaccination of Qbeta-CCR5 VLPs using aerosolized vaccines. Fig. 1 illustrates the Qbeta-antigen complex linked using SMPH. Upon attachment of peptide and subsequent aerosolization, the resultant particles have to remain both intact and small enough to reach the lower respiratory tract of rodents (ideally, under 6 µm in diameter). As shown in Fig. 2, particles retained classical VLP morphology following aerosolization and were well within the size limitations necessary for pulmonary delivery. Pulmonary delivery in rats resulted in the induction of systemic (serum-associated) IgG against CCR5 at similar levels as when rats were immunized intramuscularly.

The pulmonary immunization protocols resulted in ~10-fold higher IgA responses in the sera (Fig. 3). Rats immunized solely via the IM route had low IgG levels in the bronchial and uterine lavages and in feces, and undetectable IgA. In contrast, rats immunized with solely the aerosolized vaccine elicited mucosal IgA responses in the lung, feces, and uterus, but only had detectable IgG in the uterus (Fig. 4). Therefore, VLP-based vaccines are compatible with pulmonary delivery and induce high-titer systemic antibodies as well as local and systemic mucosal antibody responses. The mucosal immune system is largely compartmentalized, making the route of vaccine delivery a key consideration when seeking to induce regional mucosal immunity. Although the method of immunizing via a pulmonary route was effective at inducing a local response in the lung as well as a secondary response in the genital tract, it was also of interest to investigate the strength of the mucosal immune response following direct immunization of the genital tract.

To this end, Ms. Hunter is currently developing an intravaginal spray composed of VLPs. Preliminary studies have shown that the VLPs, when delivered in an aerosolized spray, are more effective at inducing local and systemic immunity than a previously reported method involving deposition of droplet-sized particles suspended in a gel.

Address Goals

The work on mucosally-administered VLP-based vaccines has made it possible to extend the studies to a more physiologically relevant macaque model. It is highly likely that intravaginal inoculation with Qbeta-CCR5 vaccines will result in strong genital mucosal antibody responses. Using this model, viral challenges will be conducted following immunization in order to determine the efficacy of the vaccines at limiting or preventing SIV/HIV infection in vivo. More broadly, this work significantly expands upon the flexibility and applicability of vaccine design, contributing to efforts to eradicate the disease on a global scale and in a safe and cost-effective manner.