Checkpoint Modifiers

Mechanism of Action​ of Glycoprotein D (gD)

Herpes simplex virus gD is a genetically encoded checkpoint modifier adjuvant. It works at the site of T cell induction, promoting enhanced CD8+ T cell responses and broadened responses by recognizing subdominant epitopes that generally fail to reach the threshold for T cell activation. We believe that inducing potent, prolonged, broad, and highly functional CD8+ T cell responses can translate into novel, adaptable, and accessible therapies to fight disease, alone or in combination with other treatments.

Herpes Simplex Virus gD The Genetically Encoded Checkpoint Modifier Adjuvant

The gD BTLA-HVEM blockade can enhance and broaden T cell activation.1–4

HVEM Complex in Regulating T cell Activation

1. BTLA-HVEM: inhibition

2. LIGHT-HVEM: stimulation

3. BTLA-HVEM-LIGHT ligation sends a dominant inhibitory signal to limit activation signals from TCR binding


BTLA & gD Share HVEM Binding Site


gD BTLA-HVEM Blockade Enhances and Broadens T cell Activation

4. Following intramuscular injection, VRON-infected antigen presenting cells travel to regional draining lymph nodes

5. Within antigen presenting cells, the adenovirus vector produces fusion protein of gD + antigen of choice

6. Degradation of incorrectly produced fusion protein releases peptides from the antigen, which, upon binding to MHC class I, are recognized by CD8+ T cells

7. The gD fusion protein translocates to the cell surface, where it blocks BTLA-HVEM interaction, thereby increasing T cell receptor signaling and allowing for co-stimulation through LIGHT

Anti-PD-1 Monoclonal Antibody Plus gD-containing Combination Demonstrated Delayed Tumor Growth

In the preclinical melanoma (Melapoly) model, anti-PD-1 monoclonal antibody plus gD-containing combination demonstrated delayed tumor growth1,5

Benefits of gD’s Mechanism of Action​

Preclinical studies in HIV, HBV, HCV, malaria, COVID-19, ebola, and in certain cancers (i.e. melanoma) have demonstrated the safety and immunogenic benefits of gD.6–10 This includes data demonstrating that gD amplifies and broadens CD8+ T cell responses to different disease-specific antigens.6–10 gD has shown promising preclinical efficacy, not only in our anti-HBV immunotherapy (VRON-0200), but also in cancers associated with HPV and melanoma.1,2,11,12 It is believed to have lower risk of “off target” side effects.

Checkpoint modification by gD lowers the CD8+ T cell activation threshold​.

Checkpoint Modification (gD): Amplifies CD8+ T cell Responses

Our VIACT™ platform includes the checkpoint modifier gD plus a disease specific antigen insert. gD plus the antigen insert improved immunogenicity in animal models by amplifying CD8+ T cell responses compared with the antigen insert alone and controls​.1,2,11,12

Results reported as medians. HBV and HPV analysis via one-way ANOVA; Melanoma via two-way ANOVA with Sidak correction. *p-value between 0.001–0.01; **p-value between 0.001–0.01; ***p-value >0.0001. NBgD has a deletion to gD eliminating the herpes virus entry mediator binding site.

Checkpoint Modification (gD): Broadens CD8+ T cell Responses

Animal models of cancer and infectious disease showed that gD plus an antigen insert induced a broad antigen-specific CD8+ T cell response versus treatment with gD alone​.1,11,13,14

*Blood was collected at different time points after vaccination and measured for immune responses to individual melanoma-associated antigen epitopes by intracellular cytokine staining for production of IFN-γ, TNF-α, and IL-2.

Checkpoint Modification (gD): Improved Efficacy

Mice vaccinated with VRON-0200 HBV or VRON-0100 HPV (gD plus antigen of choice, in this case, core and pol, and HPV-16 E7/6/5 antigens, respectively), delivered via ChiVax™ showed viral load decline or antitumor activity versus those immunized with gDPolN, supporting the value of lowering the T cell activation threshold.1,11,12,15

*Histology is from the thyroid glands.

AdC, chimpanzee adenovirus serotype 68; AdC6, heterologous chimpanzee adenoviral viral vector of serotype 6; Ad, adenovirus; Ag, antigen; ANOVA, analysis of variance; APC, antigen presenting cell; BL, baseline; BTLA, B-and T-lymphocyte attenuator; CD, cluster of differentiation; E7, HPV E7 oncoprotein; gD, glycoprotein D; HBV2, HBV core & pol; HBV, hepatitis B virus; HCV, hepatitis C virus; HIV, human immunodeficiency virus; HPV, human papillomavirus; hTrp, human tyrosinase-related protein; HVEM, herpes virus entry mediator; IFN, interferon; IL, interleukin; I.P., intraperitoneal injection; IV, intravenous; LIGHT, lymphotoxin-like, exhibits inducible expression, and competes with herpes simplex virus glycoprotein D for HVEM, a receptor expressed by T lymphocytes; Melapoly, melanoma antigens (Trp-1, Trp-2, gp100, mutated BRAFV600E); MHC, major histocompatibility complex; mTrp, mouse tyrosinase-related protein; PD-1, programmed cell death protein-1; PolN, N terminus of polymerase; SEM, standard error of mean; TCR, T cell receptor; tet, tetramer; TNF, tumor necrosis factor; Trp, tyrosinase-related protein; VRON-0200, gD fused to HBV core & pol.

  1. Luber A, et al. ESMO TAT 2021:Abstract 143.
  2. Xiang ZQ, et al. ASCO-SITC Clinical Immuno-Oncology Symposium 2020:Abstract 71.
  3. Stiles KM, et al. J Virol. 2010;84:11646–60.
  4. Virion Therapeutics. Data on file.
  5. Zhang Y, et al. Cancer Cell. 2017;32:377–91.
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  9. Tiono AB, et al. PLoS One. 2018;13:e0208328.
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  12. Hasanpourghadi M, et al. EASL 2021:Abstract OS-2478.
  13. Hasanpourghadi M, et al. Virol J. 2021;18:242.
  14. Novikov M, et al. BioRxiv. Published online January 1, 2022. DOI: 10.1101/2022.02.23.481620.
  15. Lasaro M, et al. Mol Ther. 2011;19:1727–36.