New concepts in herpes simplex virus vaccine development: notes from the battlefield

Abstract

The recent discovery that T cells recognize different sets of herpes simplex virus type 1 and type 2 epitopes from seropositive symptomatic and asymptomatic individuals might lead to a fundamental immunologic advance in vaccine development against herpes infection and diseases. The newly introduced needle-free mucosal (i.e., topical ocular and intravaginal) lipopeptide vaccines provide a novel strategy that might target ocular and genital herpes and possibly provide 'heterologous protection' from HIV-1. Indeed, mucosal self-adjuvanting lipopeptide vaccines are easy to manufacture, simple to characterize, extremely pure, cost-effective, highly immunogenic and safe. In this review, we bring together recent published and unpublished data that illuminates the status of epitope-based herpes vaccine development and present an overview of our recent approach to an 'asymptomatic epitope'-based lipopeptide vaccine.

Figure 1. Selecting protective asymptomatic CD4 and CD8 epitopes from any HSV-1 target antigen

Step 1 indicates selection of promiscuous CD4 and CD8 T-cell epitopes from HSV tegument protein UL25 by computational algorithms. Step 2 shows further screening of T-cell epitopes based on their high-affinity in vitro binding with soluble HLA class I and class II molecules. Step 3 indicates further selection of CD4 and CD8 T-cell epitopes based on their in vitro T-cell antigenicity using T cells from HSV-seropositive individuals. Step 4 shows the most antigenic CD4 and CD8 epitopes are divided into asymptomatic (green) and symptomatic (red) epitopes as recognized by T cells from HSV-seropositive asymptomatic and symptomatic patients. From this point onward, all symptomatic epitopes will be rejected from further studies (see symbol ‘X’). Step 5 shows the in vivo screening of all asymptomatic CD4 and CD8 epitopes in terms of their T-cell immunogenicity is tested in ‘humanized’ HLA double-transgenic mice. Step 6 shows the final selection of best protective asymptomatic CD4 and CD8 epitopes as tested in HLA double-transgenic mice. Step 7 shows the chemical ligation of the best selected protective asymptomatic CD4 and CD8 epitopes along with a lipid moiety (palmitic acid molecule) at the N-terminus. The inlay shows the ultimate construct of a multivalent lipopeptide (LipoVac) bearing several pairs of CD4–CD8 epitopes from HSV tegument protein UL25 that are synthesized in tandem with GPGPG sequences (spacers) and then covalently linked at the N-terminal with a lysine (K) that is precoupled with one PAM. HSV: Herpes simplex virus; PAM: Palmitic acid moiety.

Figure 2. Operating cellular mechanisms for mucosal immunization with lipopeptide vaccines

Lipopeptide antigens applied to intravaginal mucosal epithelium are taken up by mucosal resident antigen-presenting cells (APCs), especially DCs/pDCs, which are activated and migrate out of the mucosal inductive sites into the mucosal effector sites of draining lymph nodes where antigen presentation to T cells occurs, resulting in a strong local mucosal immune response. The mucosal immunization with a multivalent lipopeptide vaccine targeting TLR-2 induces an immune mechanism that bridges innate immunity to the ‘asymptomatic’ epitope-specific adaptive T-cell response. DCs are activated through recognition of the lipopeptide by TLR-2 receptors molecules. This triggered activation of the MyD88-dependent pathway and leads to the production of inflammatory cytokines (IL-12, IL-6 and TNF-α), and to phenotypic maturation of APCs (i.e., upregulation of costimulatory and MHC molecules on DCs surface). The multiepitope lipopeptide vaccine is captured, processed and presented by MHC molecules on the matured DCs to T lymphocytes (signal 1: MHC/TCR interaction). This is not sufficient to activate naive T lymphocytes, which need an additional signal from costimulatory molecules (signal 2: B7/CD28), of which expression on DCs is induced by TLR-2 stimulation. Activated T lymphocytes become further differentiated to effector T lymphocytes by stimulation with cytokines such as IL-12. DC: Dendritic cell; HSV: Herpes simplex virus; HTL: Helper T lymphocyte; MyD88: Myeloid differentiation primary response gene 88; pDC: Plasmacytoid dendritic cell; TCR: T-cell receptor; TLR: Toll-like receptor.