ImmTACs

Abstract

Our expanding knowledge of the immune system is guiding a new era of targeted anticancer therapies. Here, we describe our recent work on a novel class of anticancer agents termed ImmTACs. These molecules combine the power of picomolar-affinity TCR-based antigen recognition with the immune-activating potential of an anti-CD3 antibody fragment, to potently redirect T-cell killing to tumor cells.

Keywords: :

• CD3
• ImmTAC
• T cell receptor
• immunotherapy
• thymic selection

The constant demand for targeted and efficacious anticancer therapies has driven researchers to strive for a greater understanding of the immune system and its potential for tumor eradication. These efforts are now beginning to yield fruit as new knowledge brings about new methods and new reagents that are designed to stimulate the defense mechanisms of the host and block the loopholes exploited by tumors. Strategies with arguably the greatest potential for success exploit the inherent ability of the adaptive arm of the immune system to distinguish the antigenic markers which form the signature of tumor cells. Monoclonal antibodies were the first agents to come to prominence. However, despite considerable success, antibodies are generally restricted to targeting membrane-bound cell surface proteins, limiting the number of potential targets. Attention has now refocused on T cells and, in particular, the unique properties of the T-cell receptor (TCR).

T Cells and Tolerance

The isolation of cancer-specific T cells from patients first indicated a central role for T cells in mediating antitumor responses in vivo.¹ TCRs expressed by CD8⁺ T cells specialize in recognizing peptides derived from intracellular proteins and presented on the cell surface in complex with MHC molecules. Since the majority of tumor antigens are of intracellular origin, T cells are ideally placed to drive an antitumor response to a wide range of targets. Nevertheless, tumors can and do evade T-cell attacks, often by exploiting low-affinity antigen recognition. The natural affinity of TCRs for their cognate antigen is much weaker than that of antibodies, which is typically in the low micromolar range, and tumor-specific T cells appear to be affected by an especially weak antigen binding. In our recent work published in The European Journal of Immunology,² we compared the binding affinity of 24 isolated TCRs to their cognate virus- or cancer-specific antigen. This work represents the largest study of its type and provides clear evidence for the comparatively weaker affinity of cancer-specific TCRs. The reason for such a weak affinity lies with the mechanism of central tolerance and T-cell selection in the thymus.³ Since many tumor-associated antigens are derived from self-proteins, T cells bearing the corresponding high affinity TCRs are deleted from the circulating repertoire. Often, a low affinity for antigens is further compounded by the low amounts of antigens typically presented on the surface of tumor cells (our unpublished observations).⁴

High Affinity TCRs and ImmTACs

To overcome the inherent recognition problems of the natural T-cell repertoire, our new immune mobilizing monoclonal T-cell receptors against cancer (ImmTAC) reagents are based on engineered, soluble, affinity-enhanced and monoclonal TCRs (mTCRs).⁵ Engineered TCRs are generating encouraging results in adoptive T-cell therapy,⁶ and a relatively small number of mutations is required to improve affinity down to the picomolar range.⁵ In addition, the removal of the transmembrane domain and the addition of a non-native disulphide bond generate a readily soluble protein with exceptional stability.⁷ Altogether, these properties make mTCRs potentially useful diagnostic tools.⁴ ImmTACs are engineered by fusing an mTCR to a humanized anti-CD3 single chain antibody fragment to activate a potent and target-specific redirected T-cell response. The creation of bi-specific molecules for targeted immunotherapy is not without precedence: bi-specific T cell engaging antibodies (BITEs)⁸ and chimeric antigen receptors (CARs)⁹ are notable examples. Nevertheless, ImmTACs are the first of such agents to combine high-affinity recognition of MHC-presented tumor antigens with the simultaneous redirection and activation of non-tumor-specific T cells (Fig. 1).

Figure 1. Schematic representation of the mechanism of action of ImmTACs. The soluble monoclonal T-cell receptor (mTCR) component (in dark blue, with the non-native disulphide bond in white) binds, with high affinity, to peptide antigens (in red) presented on the surface of cancer cells in the context of HLA molecules (in light blue). The anti-CD3 component (in gray) engages CD3 molecules (in red) on non-cancer-specific T cells, leading to a potent redirected T-cell response and tumor-cell destruction.

ImmTACs Mediate Potent Killing of Tumor Cells

Our initial investigations with ImmTACs have been recently published in Nature Medicine.¹⁰ Each of the four ImmTACs studied was shown to generate a potent redirected T-cell response to tumor cell lines presenting the corresponding tumor-associated peptide antigen. We used time-lapse videomicroscopy to directly visualize the targeted destruction of cancer cells. The elevated affinity of the interaction resulted in the fact that very little reagent was required to produce a response (in the region of 100 pM), and, within this range, stringent specificity was maintained. The affinity of the mTCR component correlates closely with enhanced T-cell activation and, importantly, provides greater sensitivity to low numbers of antigens.

The success of immunotherapeutic agents in the clinic depends to a large extent on their ability to drive a poly-functional T-cell response. Redirected T cells generate multiple effector functions including the production of various cytokines. ImmTAC-activated CD8⁺ T cells include various subsets of memory cells. Moreover, we have recently found evidence for an ImmTAC-mediated cross presentation by dendritic cells (manuscript in preparation). All these observations point toward the potential for inducing a durable and self-sustaining antitumor immune response in vivo.

ImmTACs in Vivo

For a therapeutic reagent to be successful, in vitro functionality must translate into in vivo efficacy. We have tested the effects of ImmTACs in vivo using mouse xenograft models. We observed an ImmTAC-dependent reduction in tumor burden over a 40-day period even with low doses of the reagent. Importantly, in the absence of specific target cells, the anti-CD3 component did not lead to any adverse effects. Increased survival and reduced tumor burden were associated with T-cell localization at the tumor site. These results supported the entry of ImmTACs in early Phase clinical trials and preliminary results are promising.

Outlook

The ability to generate ImmTAC reagents to any one of a vast number of cancer epitopes provides a novel opportunity to produce tailored off-the-shelf therapeutics possessing exceptionally high specificity and efficiently mediating cancer-cell killing.

Abbreviations:
ImmTAC	=	immune mobilizing monoclonal T-cell receptors against cancer
TCR	=	T-cell receptor

Disclosure of Potential Conflicts of Interest

J.O and B.K.J. are employees of Immunocore Ltd. The ImmTAC reagents discussed in this manuscript were developed by Immunocore Ltd.