Antiviral Vaccine Research
With annual HIV infection rates of 1.3 million people a year, development of an HIV vaccine remains a world-wide urgency. The HIV vaccine research group is therefore exploring novel vaccination strategies and aims at a better understanding of immune responses that are induced by HIV or are required for protection. Currently, the following projects are pursued:
After a prime immunization with a licensed vaccine, the structural protein components are taken up and processed by antigen-presenting cells (APC) which further activate naïve CD4+ T helper cells specific for vaccine-derived peptides complexed on MHC-II molecules of the APCs (1). The activated T cells proliferate and ultimately differentiate into CD4+ T helper memory (Thm) cells and T follicular helper (Tfh) cells (2). Upon boosting with VLPs, that display HIV-1 Env on the surface and incorporate immunodominant peptides from the licensed vaccine, the nanoparticles are taken up by Env-specific B cells in a B cell receptor (BCR)-dependent manner (3). After processing of the VLPs, both Env-derived and heterologous peptides are presented on MHC-II molecules on the B cell surface (4). Previously induced CD4+ T cells specific for the heterologous peptides may now provide help for the Env-specific B cells (5). This intrastructural help leads to differentiation of the B cells into plasma cells that secrete Env-specific antibodies (6).
Improving antibody responses to particulate vaccines by intra-structural help
Antigen-specific T follicular helper cells provide critical signals for cognate B-cells to differentiate into memory B-cells and plasma cells. In the context of particulate vaccines composed of different proteins, T helper cell epitopes and B-cell epitopes do not need to be covalently linked. HIV-Env specific B-cells for example can internalize virus-like particles (VLPs) containing HIV Env and Gag. This leads to presentation of Env- and Gag-derived peptides on their MHC-II molecules explaining our observation that Gag-specific T helper cells can provide help for Env-specific antibody responses. In mouse experiments, Gag-specific T helper cells not only enhanced anti-Env IgG levels up to 100-fold but also modulated the IgG subtype response. A major aim of our research is therefore to develop novel vaccine platforms that exploit T helper cells induced by common licensed vaccines to improve Fc-effector functions and the mucosal antibody response to heterologous viral antigens.
As a proof-of-concept, we introduced T helper cell epitopes of tetanus toxoid (TT) into HIV VLPs and immunized naïve mice or mice that had been immunized by Tetanol, a commercially available Tetanus vaccine, with HIV VLPs. Much higher HIV Env IgG1 antibody responses were observed in Tetanol-immunized mice that received VLPs containing the TT T helper cell epitopes than in naïve mice receiving the same TT-VLPs or Tetanol-primed mice receiving VLPs lacking the TT epitopes. Since the IgG1 subtype response induced in Tetanol-primed mice has less efficient Fc-effector functions we also identified T helper cell epitopes of an experimental H1 recombinant tuberculosis subunit vaccine in combination with the liposomal adjuvant CAF01.
To provide ISH, immunodominant MHC-II restricted peptides from the H1 vaccine were genetically incorporated into the HIV 1 Gag protein and used for HIV VLP production. ISH effects on Env-specific antibody levels and B cell differentiation were analyzed in mice primed against H1 and boosted with VLPs. In contrast to non-primed mice, a significant increase of Env-specific IgG levels for up to 26 weeks after the last immunization was observed. This increase was largely caused by elevated IgG2b and IgG2c levels in mice that received H1 priming.
Additionally, ISH enhanced the frequency of Env-specific long-lived plasma cells in the bone marrow. We also inserted murine T helper cells epitopes of the Spike protein of SARS-CoV-2 into the HIV-VLPs and observed strong intrastructural help effects. Using mucosal immunization regimens against the spike protein we are currently exploring the hypothesis that T helper cell responses induced by mucosal immunization can improve mucosal antibody responses after subsequent systemic booster immunizations with VLPs by intrastructural help.
Upon expression in E. coli, recombinant L1 proteins accumulate in irregular aggregate structures that do not spontaneously reassemble after cell lysis (left panel). Instead, a disassembly-reassembly process in vitro is typically required. To this end, L1 aggregates are first dissociated into soluble pentamers under the influence of reducing agents (center panel). Gradually removing the reducing agents via dialysis against a low-acidic buffer environment with high salt concentrations finally promotes the reassembly into functional L1 VLPs that may be conjugated with HIV-1 Env (right panel).
Particulate HPV vaccine plattform
To extend the concept of intrastructural help to non-enveloped viruses, we established an E-coli-based production system for L1 VLPs of HPV-16. Different coupling strategies were explored to coat the L1 VLPs with HIV-Env, and the immunogenicity of these experimental vaccines are currently explored.
Sterilizing immunity after passive immunization
Antibodies targeting the HIV-1 Env protein have been shown to prevent systemic infection in non-human primate models of AIDS. To explore whether these antibodies can block infection of the “first cell” at the portal of viral entry, challenge viruses based on simian immunodeficiency virus (SIV) were developed that use HIV-1 Env for entry into target cells during the first replication cycle, but then switch to SIV Env for all subsequent rounds of the infection. Passive immunization of monkeys with a broadly neutralizing antibody binding to Env of HIV-1, but not SIV, prior to rectal exposure to the switching challenge virus led to a >100-fold reduction of HIV-1 Env-mediated infection events. Using this challenge model under non-neutralizing conditions did not reveal a major reduction of HIV-1 Env mediated first infection events. Consistently, the same broadly neutralizing antibody with mutations inactivating the interaction with Fc-receptors reduced the HIV-1 Env-mediated infection events to a similar extent.
Passive immunization strategies against SARS-CoV-2
In response to the SARS-CoV-2 pandemic, we established in close collaboration with Prof. Ralf Wagner (Regensburg) and Dr. Philipp Steininger (Clinical Diagnostics) different cohorts for studies on the sero-epidemiology (TiKoCo), the cellular and humoral immune responses to infection and vaccination, and the clincial course of break-through infections (CoVaKo). In collaboration with Prof. Hans-Martin Jäck from the Division of Molecular Immunology und Prof. Thomas Winkler from the Chair of Genetics, we also rapidly generated human neutralizing monoclonal antibodies by immunizing TRIANNI mice genetically modified to express the human immunoglobulin variable region repertoire. The antibodies reduced viral loads in mice and non-human primates, but rapid emergence of viral escape mutants limited the applicability of the SARS-CoV-2 antibodies identified. In collaboration with Prof. Dirk Grimm and Sirion Biotech, we also developed a myotropic AAV-based vector, that provided high serum concentrations of one of the SARS-CoV-2 neutralizing antibodies for more than a year after a single injection in a preclinical model.
