Combined Immunostimulation and Conditional Cytotoxic Gene Therapy Provide Long-term Survival in a Large Glioma Model

Abstract

In spite of preclinical efficacy and recent randomized, controlled studies with adenoviral vectors expressing herpes simplex virus-1 thymidine kinase (HSV1-TK) showing statistically significant increases in survival, most clinical trials using single therapies have failed to provide major therapeutic breakthroughs. Because glioma is a disease with dismal prognosis and rapid progression, it is an attractive target for gene therapy. Preclinical models using microscopic brain tumor models (e.g., V 0.3 mm 3 ) may not reflect the pathophysiology and progression of large human tumors. To overcome some of these limitations, we developed a syngeneic large brain tumor model. In this model, administration of single therapeutic modalities, either conditional cytotoxicity or immunostimulation, fail. However, when various immunostimulatory therapies were delivered in combination with conditional cytotoxicity (HSV1-TK), only the combined delivery of fms-like tyrosine kinase ligand (Flt3L) and HSV1-TK significantly prolonged the survival of large tumor-bearing animals (z80%; P V 0.005). When either macrophages or CD4 + cells were depleted before administration of viral therapy, TK + Flt3L therapy failed to prolong survival. Meanwhile, depletion of CD8 + cells or natural killer cells did not affect TK + Flt3L efficacy. Spinal cord of animals surviving 6 months after TK + Flt3L were evaluated for the presence of autoimmune lesions. Whereas macrophages were present within the corticospinal tract and low levels of T-cell infiltration were detected, these effects are not indicative of an overt autoimmune disorder. We propose that combined Flt3L and HSV1-TK adenoviralmediated gene therapy may provide an effective antiglioma treatment with increased efficacy in clinical trials of glioma. (Cancer Res 2005; 65(16): 7194-204)

Immunosuppressive Myeloid Cells’ Blockade in the Glioma Microenvironment Enhances the Efficacy of Immune-Stimulatory Gene Therapy

“…[17][18][19][20] The gene therapy approach entails using adenoviruses (Ads) encoding herpes simplex virus type-I thymidine kinase (Ad-TK) and Fms-like tyrosine kinase 3 ligand (Flt3L), which are injected into the tumor followed by ganciclovir (GCV) administration. Thymidine kinase converts GCV to its active metabolite, which leads to tumor cell lysis, with concomitant release of tumor antigens and damage-associated molecular pattern molecules, i.e., HMGB1.…”

“…Thymidine kinase converts GCV to its active metabolite, which leads to tumor cell lysis, with concomitant release of tumor antigens and damage-associated molecular pattern molecules, i.e., HMGB1. 18,19,21,22 Flt3L mediates dendritic cell (DC) recruitment and expansion into the tumor microenvironment (TME). The DCs pick up the tumor antigen, traffic it to the draining lymph nodes, and prime a robust anti-tumor cytotoxic and memory T cell response, leading to tumor regression and long-term survival.…”

“…The DCs pick up the tumor antigen, traffic it to the draining lymph nodes, and prime a robust anti-tumor cytotoxic and memory T cell response, leading to tumor regression and long-term survival. 18,19 Gliomas have been shown to employ a variety of mechanisms to suppress the immune system, such as downregulation of MHC class I molecules, production of transforming growth factor-b (TGF-b), vascular endothelial growth factor (VEGF), prostaglandin E2, and IL-10, expression of ligands of checkpoint receptors, such as PD-1, and accumulation of immunosuppressive cells, such as myeloidderived suppressor cells (MDSCs), regulatory T cells (Tregs), and tumor-associated macrophages (TAMs). [23][24][25][26][27][28][29][30] It has been previously shown in human samples and experimental GBM models that MDSCs are powerful inhibitors of anti-tumor immune responses.…”

“…We previously developed a conditionally cytotoxic (TK+GCV)-immune stimulatory (FLT3L) gene therapy approach that is currently being tested in a phase I clinical trial at our institution (https://clinicaltrials.gov; Identifier: NCT01811992). [18][19][20][21] The success of this gene therapy approach relies on the generation of a robust anti-GBM cytotoxic and memory T cell response; thus, we wished to assess whether the presence of such large numbers of immunosuppressive MDSCs would interfere with the anti-GBM T cell response induced by the gene therapy. If our hypothesis was correct, then the efficacy of the gene therapy would be enhanced after depleting MDSCs.…”

HMGB1 mediates endogenous TLR2 activation and brain tumor regression.

“…We used first generation, recombinant Ads (serotype 5) expressing Herpes Simplex Virus Type I-TK (Ad-TK) [8,27], Flt3L (Ad-Flt3L) [7], and an Ad vector with no transgene (Ad0) in this study [8]. The methods for adenoviral generation, purification, characterization, and scale-up have been previously described by our lab [28].…”