Molecular and cellular strategies to enhance efficacy of T cell-based cancer therapy

Abstract

A combined approach of vaccination with β2-microglobulin (β2m)-deficient dendritic cells (DCs) and granulocyte-macrophage colony-stimulating factor (GM-CSF) as a potent adjuvant may link cellular and molecular strategies to further enhance anti-tumor T-cell responses. T lymphocytes can mediate a potent anti-tumor immune response. CD8+ T lymphocytes screen and recognize antigen in complex with a major histocompatibility complex (MHC) class I heavy chain (HQ and the β2m light chain. Metastatic cells commonly escape from "conventional" T lymphocyte-mediated recognition and elimination as a result of impaired cell surface expression of MHC class I antigen molecules. This impaired cell surface expression can be caused by loss or down-regulation of expression of different components of the MHC class I antigen processing machinery, such as MHC class I HCs, β2m, or the transporter associated with antigen processing (TAP). Such immune evasion poses a problem for autologous T cell-based cancer therapy.

In the first study we demonstrated protection against growth Of β2m-deficient tumor cells in syngeneic C57Bl/6 (B6) mice, following vaccination with β2m-deficient DCs. In vitro analysis of an effector cell population from vaccinated mice pointed to that CD3+ cells had been generated with the capability to induce apoptosis in syngeneic β2m-deficient tumor and nonmalignant cells. Further investigation of target cell recognition suggested that also tumor target cells lacking expression of classical MHC class I HCs and functional TAP were recognized by CD3+ effector cells from vaccinated mice. This study points to a new possible strategy to counteract the growth of metastatic cells.

The cytokine composition in the tumor microenvironment is a critical factor for an effective antitumor immune response. GM-CSF has been shown to be a very promising cytokine in anti-tumor immunomodulation. Continuously high concentrations of GM-CSF in the local tumor environment seem to be crucial to reach a therapeutic threshold. Such a favorable cytokine milieu can promote recruitment of DCs and augment DC activation with increased number of DCs expressing MHC and co-stimulatory molecules. GM-CSF can enhance tumor infiltration of T lymphocytes and their cross-priming. Furthermore, GM-CSF seems to trigger an increased and significantly more effective tumor lysis mediated by lymphocytes. GM-CSF has elicited anti-tumor immune responses in animal studies and clinical trials. However, the clinical efficacy has been limited, with local GM-CSF levels being therapeutically insufficient or systemic toxicity being a limiting factor.

In the second study, we developed and characterized a novel GM-CSF expression vector, pAD-HotAmp-GM-CSF, which can provide heat-inducible high-level expression of GM-CSF. In cytokine immunotherapy of cancer it is critical to deliver sufficiently high local cytokine concentrations in order to reach the therapeutic threshold needed for clinical efficacy. The novel vector, pAD-HotAmp-GM-CSF successfully integrates inducible and amplifying elements into a one-plasmid system. Moderate hyperthermia at 42°C for 30 min induced amplification of GM-CSF expression in pAD-HotAmp-GM-CSF that was over 2,8 fold higher than levels achieved with the prototypical human cytomegalovirus (CMV) promoter. Thus, the inducible amplifier vector, pAD-HotAmp-GM-CSF, represents a novel system for regulated and enhanced GM-CSF expression, which enables both greater efficacy and safety in cytokine immunotherapy of cancer.