The human cytomegalovirus (HCMV) is well-recognized as a clinically important pathogen. Infections with HCMV during pregnancy and subsequent transmission to the fetus are among the most frequent and severe causes of congenital disease and childhood sequelae. Congenital HCMV infection (cCMV) has been labeled as the second most frequent cause of mental retardation in the western world, led only by Down‘s syndrome. An estimated 0.5–2.2 % of infants born in countries of the western hemisphere are congenitally infected with HCMV. 10-20% of these suffer from acute disease; another 10-20% will develop sequelae like sensorineural hearing loss later in life. Annual costs for the treatment and care of congenitally infected infants are estimated to range up to 1 billion USD in the USA alone. Antiviral drugs may ameliorate symptoms after birth but are insufficient to improve the course of the disease and to eliminate the virus.
A second serious problem associated with significant morbidity, mortality and prolonged hospitalisation is HCMV reactivation in hematopoietic stem cell or solid organ transplant recipients. Antiviral therapy or antiviral prophylaxis is associated with serious adverse events, development of drug resistance, and late progression to HCMV disease. In addition, both pre-emptive and prophylactic interventions require prolonged monitoring, thus consuming resources on both the clinical and the economical level. Consequently, the development of a safe and effective vaccine against HCMV has been given top priority in biomedical research. Several efforts have been made to establish a vaccine for both the prevention of cCMV and to attenuate the consequences of HCMV reactivation in immunosuppressed individuals. Testing of some of these candidate vaccines in clinical studies has met with limited success. We thus designed a strategy for a HCMV vaccine (Figure 1).
A promising vaccine candidate, developed in our laboratory is based on subviral particles of HCMV, denominated as Dense Bodies (DBs; Gogesch et al., 2019). DBs are released from HCMV infected culture cells and can be purified from the culture supernatants (Pepperl-Klindworth et al., 2003). DBs are efficiently taken up by a variety of human and murine cell types of hematopoietic and non-hematopoietic origin, thereby delivering their proteinaceous content into the cytosol. They contain dominant antigens that induce both humoral and cellular immune responses against HCMV (Becke et al., 2010; Pepperl et al., 2000). In immunization experiments in mice, HCMV-specific neutralizing antibodies were induced to titers comparable to those observed during natural infection in humans (Becke et al., 2010; Pepperl et al., 2000). These antibodies could still be detected one year after immunization. In an HLA-A2 transgenic mouse model, DB-immunization led to the induction of high levels of HCMV-specific cytotoxic T-lymphocytes. Furthermore, pronounced T-helper lymphocyte responses were induced (Pepperl-Klindworth et al., 2002; Pepperl et al., 2000). The fact that these immune reactions were elicited by DBs in the absence of an adjuvant highlights the outstanding antigenic potential of these particles (Becke et al., 2010). The exceptional immunogenicity of DBs is related to their capacity to induce activation and maturation of monocyte-derived immature dendritic cell (Sauer et al., 2013). In further experiments, we could demonstrate that DBs are accessible to genetic engineering and may accommodate additional heterologous peptides or proteins (Mersseman et al., 2008; Pepperl-Klindworth et al., 2003).
Based on a laboratory strain, we generated a prototype seed virus strain for DB-production that expresses the pentameric glycoprotein complex gH/gL/pUL128-131(PC; Lehmann et al., 2019). The PC has been identified as a crucial component of the HCMV virion that mediates viral entry into a broad spectrum of host cells, including epithelial cells, endothelial cells, and dendritic cells. The PC has also been found to be a major target of the humoral response, as a large proportion of the neutralizing antibody capacity in convalescent human sera has been found to be directed against this complex. DBs of the PC-expressing HCMV strain indeed induced a neutralizing antibody response against HCMV that was superior to that induced by PC-negative strain DBs (Lehmann et al., 2019). Current work focusses on adding additional safety features in order to provide a safe seed strain for production of DBs under Good Manufacturing Practice (GMP)-conditions (Gogesch et al., 2019).
A scalable and robust purification procedure for GMP-compliant vaccine production and a trial flow for phase I clinical studies have already been established. Following the GMP-compliant production and Good Laboratory Practice (GLP)-compliant pre-clinical studies, phase I trials will be initiated.
Becke, S., Aue, S., Thomas, D., Schader, S., Podlech, J., Bopp, T., Sedmak, T., Wolfrum, U., Plachter, B., Reyda, S., 2010. Optimized recombinant Dense Bodies of human cytomegalovirus efficiently prime virus specific lymphocytes and neutralizing antibodies without the addition of adjuvant. Vaccine 28(38), 6191-6198.
Gogesch, P., Penner, I., Krauter, S., Büscher, N., Grode, L., Aydin, I., Plachter, B., 2019. Production Strategies for Pentamer-Positive Subviral Dense Bodies as a Safe Human Cytomegalovirus Vaccine. Vaccines (Basel) 7(3).
Lehmann, C., Falk, J.J., Büscher, N., Penner, I., Zimmermann, C., Gogesch, P., Sinzger, C., Plachter, B., 2019. Dense Bodies of a gH/gL/UL128-131 pentamer repaired Towne strain of human cytomegalovirus induce an enhanced neutralizing antibody response. J Virol 93(17).
Mersseman, V., Besold, K., Reddehase, M.J., Wolfrum, U., Strand, D., Plachter, B., Reyda, S., 2008. Exogenous introduction of an immunodominant peptide from the non-structural IE1 protein of human cytomegalovirus into the MHC class I presentation pathway by recombinant Dense Bodies. J. Gen. Virol 89(Pt 2), 369-379.
Pepperl-Klindworth, S., Frankenberg, N., Plachter, B., 2002. Development of novel vaccine strategies against human cytomegalovirus infection based on subviral particles. J. Clin. Virol 25 Suppl 2, 75.
Pepperl-Klindworth, S., Frankenberg, N., Riegler, S., Plachter, B., 2003. Protein delivery by subviral particles of human cytomegalovirus. Gene Ther 10(3), 278-284.
Pepperl, S., Münster, J., Mach, M., Harris, J.R., Plachter, B., 2000. Dense Bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression. J. Virol 74(13), 6132-6146.
Sauer, C., Klobuch, S., Herr, W., Thomas, S., Plachter, B., 2013. Subviral Dense Bodies of human cytomegalovirus stimulate maturation and activation of monocyte-derived immature dendritic cells. J. Virol 87(20), 11287-11291.