Rational vaccine design requires understanding details of protective immunity against each virus. Yasuko Mori and associates from Japan have now identified CD4+ and H-2Kd restricted CD8+ T-cell epitopes essential for HHV-6B viral entry, opening new possibilities for vaccines and immunotherapy.
The Mori group from Kobe University previously discovered that CD134/OX40 is a specific receptor for HHV-6B, and that it binds to the HHV-6B gH/gL/gQ1/gQ2 complex. Glycoprotein gQ1 and gQ2 are unique to HHV-6 and HHV-7. They had also determined that the neutralizing antibodies screened in previous tests against HHV-6B almost all targeted a subunit of gQ1, BgQ1, suggesting that it is critical for virus entry and adaptive immunity.
The Mori group now has identified epitopes, distinct parts of BgQ1 recognized by T-cells, that are essential for HHV-6B viral entry. T-cell recognition of antigen peptides, like those of BgQ1, that are presented by class I HLA is an important part of adaptive immunity that allows the human immune system to attack viral intruders. The authors note that the necessity of T-cells to control HHV-6B replication is suggested by the higher incidence of persistent HHV-6B viremia in patients without robust T-cell responses. Consequently, identifying unique viral epitopes could lead to the design and development of new vaccines and immunotherapies.
The investigators used mice immunized with “codon optimized” plasmid DNA expressing BgQ1 by in vivo electroporation to identify the cytotoxic epitopes. The optimization of codon has been reported to result in enhanced cytotoxic T cell reactivity, by increasing the translational efficiency of plasmid DNA. They also used algorithm programs to predict epitopes.
Using these BALB/c mice that expressed BgQ1 in their splenocytes, 5 BgQ1 peptides were found to strongly stimulate T cells: CD8+ T cells were most responsive to the peptides P17 and P18 while CD4+ cells were most responsive to P16 and P43, and to a lesser extent, P11. The results indicated that these peptides contained CD4 and CD8-recognized epitopes that serve as targets for the immune response, and that could serve as targets for future immunotherapies. Their discovery could also lead to the development of a DNA vaccine which could provide long-lasting T cell specific immune responses.
Further experimentation determined that the H-2Kd MHC class 1a molecule was involved in presenting P17 and P18 to CD8+ cells. Finally, two computer-based programs were utilized to predict CD8+ T cell epitope candidates within P17 and P18. Four candidates were synthesized, and the 9-mer peptide AFCPMTSKL was found to be the optimal CD8+ T cell epitope on BgQ1.
It is increasingly clear that HHV-6A and HHV-6B are important in the pathogenesis of various human diseases, increasing interest in new therapies. Developing effective vaccines for herpesviruses remains a challenge, but DNA vaccine research has intensified after the realization that in vivo electroporation, a method to introduce macromolecules into cells by inducing brief electric pulses, can increase antigen delivery up to 1000-fold over “naked DNA” delivery alone (Sardesai 2011).
ASP0113, a DNA vaccine to prevent CMV reactivation in transplant patients failed a phase III trial in 2018. It had been developed by Vical and was licensed to the Japanese pharmaceutical company Astellas.
Read the full paper: Nagamata 2019.