Man-made superantigens: Tumor-selective agents for T-cell-based therapy

Endogenous HLA-DQ8αβ programs superantigens (SEG/SEI) to silence toxicity and unleash a tumoricidal network with long-term melanoma survival

Background

As the most powerful T cell agonists known, superantigens (SAgs) have enormous potential for cancer immunotherapy. Their development has languished due to high incidence (60%–80%) of seroreactive neutralizing antibodies in humans and tumor necrosis factor-α (TNFα)-mediated cardiopulmonary toxicity. Such toxicity has narrowed their therapeutic index while neutralizing antibodies have nullified their therapeutic effects.

Methods

Female HLA-DQ8 (DQA*0301/DQB*0302) tg mice expressing the human major histocompatibility complex II (MHCII) HLA-DQ8 allele on a high proportion of PBL, spleen and lymph node cells were used. In the established tumor model, staphylococcal enterotoxin G and staphylococcal enterotoxin I (SEG/ SEI) (50 µg each) were injected on days 6 and 9 following tumor inoculation. Lymphoid, myeloid cells and tumor cell digests from tumor tissue were assayed using flow cytometry or quantitated using a cytometric bead array. Tumor density, necrotic and viable areas were quantitated using the ImageJ software.

Results

In a discovery-driven effort to address these problems we introduce a heretofore unrecognized binary complex comprizing SEG/SEI SAgs linked to the endogenous human MHCII HLA-DQ8 allele in humanized mice. By contrast to staphylococcal enterotoxin A (SEA) and staphylococcal enterotoxin B (SEB) deployed previously in clinical trials, SEG and SEI does not exhibit neutralizing antibodies in humans or TNFα-mediated toxicity in humanized HLA-DQ8 mice. In the latter model wherein SAg behavior is known to be ‘human-like’, SEG/SEI induced a powerful tumoricidal response and long-term survival against established melanoma in 82% of mice. Other SAgs deployed in the same model displayed toxic shock. Initially, HLA-DQ8 mediated melanoma antigen priming, after which SEG/SEI unleashed a broad CD4+ and CD8+ antitumor network marked by expansion of melanoma reactive T cells and interferon-γ (IFNy) in the tumor microenvironment (TME). SEG/SEI further initiated chemotactic recruitment of tumor reactive T cells to the TME converting the tumor from ‘cold’ to a ‘hot’. Long-term survivors displayed remarkable resistance to parental tumor rechallenge along with the appearance of tumor specific memory and tumor reactive T memory cells.

Conclusions

Collectively, these findings show for the first time that the SEG/SEI-(HLA-DQ8) empowers priming, expansion and recruitment of a population of tumor reactive T cells culminating in tumor specific memory and long-term survival devoid of toxicity. These properties distinguish SEG/SEI from other SAgs used previously in human tumor immunotherapy. Consolidation of these principles within the SEG/SEI-(HLA-DQ8) complex constitutes a conceptually new therapeutic weapon with compelling translational potential.

Targeting the MHC Class II antigen presentation pathway in cancer immunotherapy

The success of immunotherapy relies on the participation of all arms of the immune system and the role of CD4+ T lymphocytes in preventing tumor growth is now well established. Understanding how tumors evade immune responses holds the key to the development of cancer immunotherapies. In this review, we discuss how MHC Class II expression varies in cancer cells and how this influences antitumor immune responses. We also discuss the means that are currently available for harnessing the MHC Class II antigen presentation pathway for the development of efficient vaccines to activate the immune system against cancer.

Design of split superantigen fusion proteins for cancer immunotherapy

Several antibody-targeting cancer immunotherapies have been developed based on T cell activation at the target cells. One of the most potent activators of T cells are bacterial superantigens, which bind to major histocompatibility complex class II on antigen-presenting cells and activate T cells through T cell receptor. Strong T cell activation is also one of the main weaknesses of this strategy as it may lead to systemic T cell activation. To overcome the limitation of conventional antibody-superantigen fusion proteins, we have split a superantigen into two fragments, individually inactive, until both fragments came into close proximity and reassembled into a biologically active form capable of activating T cell response. A screening method based on fusion between SEA and coiled-coil heterodimers was developed that enabled detection of functional split SEA designs. The split SEA design that demonstrated efficacy in fusion with coiled-coil dimer forming polypeptides was fused to a single chain antibody specific for tumor antigen CD20. This design selectively activated T cells by split SEA-scFv fusion binding to target cells.

Naptumomab estafenatox: a new immunoconjugate

IMPORTANCE OF THE FIELD

New agents that specifically engage the immune system are being tested in a variety of malignancies. This review provides an overview of naptumomab, an immunotoxin, with encouraging clinical activity in Phase I trials.

AREAS COVERED IN THIS REVIEW

This review examines the preclinical and the published clinical data with regards to naptumomab.

WHAT THE READER WILL GAIN

This review provides the reader with an understanding of the mechanism of action, immunology, pharmacokinetics and clinical activity of this agent.

TAKE HOME MESSAGE

Naptumomab has a unique mechanism of action and appears to be an active agent in the treatment of refractory solid tumors such as renal cell carcinoma.

A phase II study of a 5T4 oncofoetal antigen tumour-targeted superantigen (ABR-214936) therapy in patients with advanced renal cell carcinoma

In a phase II study, 43 renal cell carcinoma patients were treated with individualised doses of ABR-214936; a fusion of a Fab recognising the antigen 5T4, and Staphylococcal enterotoxin A. Drug was given intravenously on 4 consecutive days, treatment was repeated 1 month later. Treatment was associated with moderate fever and nausea, but well tolerated. Of 40 evaluable patients, 28 had disease control at 2 months, and at 4 months, one patient showed partial response (PR) and 16 patients stable disease. Median survival, with minimum follow-up of 26 months was 19.7 months with 13 patients alive to date. Stratification by the Motzer's prognostic criteria highlights prolonged survival compared to published expectation. Patients receiving higher drug exposure had greater disease control and lived almost twice as long as expected, whereas the low-exposure patients survived as expected. Sustained interleukin-2 (IL-2) production after a repeated injection appears to be a biomarker for clinical effect, as the induced-IL-2 level on the day 2 of treatment correlated with survival. The high degree of disease control and the prolonged survival suggest that this treatment can be effective. These findings will be used in the trial design for the next generation of drug, with reduced antigenicity and toxicity.

Superantigens: structure-function relationships

Superantigens are a class of highly potent immuno-stimulatory molecules produced by Staphylococcus aureus and Streptococcus pyogenes. These toxins possess the unique ability to interact simultaneously with MHC class II molecules and T-cell receptors, forming a trimolecular complex that induces profound T-cell proliferation. The resultant massive cytokine release causes epithelial damage and leads to capillary leak and hypotension. The staphylococcal superantigens are designated staphylococcal enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, and toxic shock syndrome toxin-1. The streptococcal superantigens include the pyrogenic exotoxins A (and antigenic variants), C, G-J, SMEZ, and SSA. Superantigens are implicated in several diseases including toxic shock syndrome, scarlet fever and food poisoning; and their function appears primarily to debilitate the host sufficiently to permit the causation of disease. Structural studies over the last 10 years have provided a great deal of information regarding the complex interactions of these molecules with their receptors. This, combined with the wealth of new information from genomics initiatives, have shown that, despite their common molecular architecture, superantigens are able to crosslink MHC class II molecules and T-cell receptors by a variety of subtly different ways through the use of various structural regions within each toxin.

Identification of the antigenic epitopes in staphylococcal enterotoxins A and E and design of a superantigen for human cancer therapy

Monoclonal antibodies have a potential for cancer therapy that may be further improved by linking them to effector molecules such as superantigens. Tumor targeting of a superantigen leads to a powerful T cell attack against the tumour tissue. Encouraging results have been observed preclinically and in patients using the superantigen staphylococcal enterotoxin A, SEA. To further improve the concept, we have reduced the reactivity to antibodies against superantigens, which is found in all individuals. Using epitope mapping, antibody binding sites in SEA and SEE were found around their MHC class II binding sites. These epitopes were removed genetically and a large number of synthetic superantigens were produced in an iterative engineering procedure. Properties such as decreased binding to anti-SEA as well as higher selectivity to induce killing of tumour cells compared to MHC class II expressing cells, were sequentially improved. The lysine residues 79, 81, 83 and 84 are all part of major antigenic epitopes, Gln204, Lys74, Asp75 and Asn78 are important for optimal killing of tumour cells while Asp45 affects binding to MHC class II. The production properties were optimised by further engineering and a novel synthetic superantigen, SEA/E-120, was designed. It is recognised by approximately 15% of human anti-SEA antibodies and have more potent tumour cell killing properties than SEA. SEA/E-120 is likely to have a low toxicity due to its reduced capacity to mediate killing of MHC class II expressing cells. It is produced as a Fab fusion protein at approximately 35 mg/l in Escherichia coli.

A new screening model for safety evaluation of superantigen-antibody recombinant fusion proteins (mAb Fab-SEA/E) using telemetric monitoring in conscious rabbits

INTRODUCTION

Carcinoma recognising monoclonal antibodies (mAb) and mutated forms of the T-cell-activating bacterial staphylococcal enterotoxin A/E (SEA/E) have been combined in single hybrid constructs (mAb Fab-SEA/E). By introducing substitutions in an MHC class II binding site, these harmful toxins can be converted into tolerable immunotoxins. Rabbits and humans are sensitive to SE toxins, and cardiovascular effects in rabbits are similar to those seen in septic shock in man. A new screening model using telemetry in conscious rabbits was applied in the safety evaluation of different mAb Fab-SEA/E constructs administered intravenously.

METHODS

Telemetry transmitters were implanted in the peritoneal cavity of animals with a pressure catheter in the aorta and electrodes for ECG recording subcutaneously following administration of mAb Fab-SEA/E constructs intravenously.

RESULTS

The responses in body temperature, heart rate, and blood pressure varied depending on the treatment regimen and the mutations of the drug given. For example, 25 micro g/kg of C215 Fab-SEAmut9 were given as a first treatment cycle on days 1, 5, and 7 and as a second treatment cycle on days 13-15. The first dose induced high fever, whereas the second and third doses induced fever responses more rapidly and were of lower and shorter duration. The second treatment cycle, starting on day 13, did not induce any responses probably due to anti-SEA antibodies formed because of the treatment. Another construct, 5T4 Fab-SEA/E-11 at 50 micro g/kg, induced a similar response as C215 Fab-SEAmut9 on days 1, 5, and 7. In this case, the pharmacologic response was still present on days 13-15, though no clinical signs developed or no formation of anti-SEA antibodies occurred. When 50 micro g/kg of 5T4 Fab-SEA/E-11 was administered once daily for 4 days, body temperature after the first dose increased slowly during the first 24 h, whereas the second to fourth doses induced more rapid and higher responses. The fourth dose of another compound, K305 Fab-SEA/E-11 (50 micro g/kg), induced an even more pronounced response both in magnitude and in duration as well as in adverse clinical signs.

DISCUSSION

By using continuous telemetric registration in the rabbit as a tool in superantigen-antibody (mAb Fab-SEA/E) drug selection, it has been possible to evaluate the dynamics of drug-induced immune effects (fever) and concomitant engagement of the cardiovascular system, conditions that are essential before clinical trials can be initiated.

Therapy of human non-small-cell lung carcinoma using antibody targeting of a modified superantigen

Superantigens activate T-cells by linking the T-cell receptor to MHC class II on antigen-presenting cells, and novel reactivity can be introduced by fusing the superantigen to a targeting molecule. Thus, an antibody-targeted superantigen, which activates T cells to destroy tumour cells, might be used as cancer therapy. A suitable target is the 5T4 oncofetal antigen, which is expressed on many carcinomas. We constructed a fusion protein from a Fab of a monoclonal antibody recognizing the 5T4 antigen, and an engineered superantigen. The recombinant product 5T4FabV13-SEA(D227A)bound the 5T4 antigen expressed on the human non-small-cell lung cancer cell line Calu-1 with a Kd of 1.2 nM while the substitution of Asp227 to Ala in the superantigen moiety reduced binding activity to MHC class II. 5T4FabV13-SEA(D227A)tumour reactivity was demonstrated in 7/7 NSCLC samples by immunohistochemistry, while normal tissue reactivity was low to moderate. 5T4FabV13-SEA(D227A)induced significant T-cell-dependent in vitro killing of sensitive 5T4 bearing Calu-1 cells, with maximum lysis at 10(-10)M, while the capacity to lyse MHC class II expressing cells was approximately 1000 times less effective. Immunotherapy of 5T4FabV13-SEA(D227A)against human NSCLC was investigated in SCID mice reconstituted with human peripheral blood mononuclear cells. Mice carrying intreperitoneally growing Calu-1 cells showed significant reduction in tumour mass and number after intravenous therapy with 5T4FabV13-SEA(D227A). Thus, 5T4FabV13-SEA(D227A)has highly attractive properties for therapy of human NSCLC.

The use of phage display for the development of tumour targeting agents

One way to improve the selectivity of therapeutic molecules in clinical oncology would be to target them on the tumour site, thereby sparing normal tissues. The development of targeted therapeutic methodologies relies in most cases on the availability of binding molecules specific for tumour-associated markers. The display of repertoires of polypeptides on the surface of filamentous phage, together with the efficient selection-amplification of the desired binding specificities using affinity capture, represents an efficient route towards the isolation of specific peptides and proteins that could act as vehicles for tumour targeting applications. Most investigations in this area of research have so far been performed with phage derived recombinant antibodies, which have been shown to selectively target tumour-associated markers both in preclinical animal models and in the clinic. However, future developments with other classes of polypeptides (small constrained peptides, small globular proteins) promise to be important for the selective delivery of therapeutic agents to the tumour site.

A3--a novel colon and pancreatic cancer reactive antibody from a primate phage library selected using intact tumour cells

The identification of novel tumour-associated antigens (TAAs) is pivotal for progression in the fields of tumour immunotherapy and diagnosis. In the present study, we have developed, based on flow cytometric evaluation and use of a mini-library composed of specific antibody clones linked to different antibiotic resistance markers, methods for positive and subtractive selection of phage antibodies employing intact cells as the antigen source. An scFv phage library (2.7 x 10(7)) was constructed from a primate (Macaca fascicularis) immunised with pooled human colon carcinomas. This library was selected for 3 rounds by binding to Colo 205 colon adenocarcinoma cells and proteolytic elution followed by phage amplification. Several antibodies reactive with colon carcinomas and with restricted reactivity to a few epithelial normal tissues were identified by immunohistochemistry. One clone, A3 scFv, recognised an epitope that was homogeneously expressed in 11/11 of colon and 4/4 pancreatic carcinomas studied and in normal tissue restricted to subtypes of epithelia in the gastrointestinal tract. The A3 scFv had an apparent overall affinity approximately 100-fold higher than an A3 Fab, suggesting binding of scFv homodimers. The cell surface density of the A3 epitope, calculated on the basis of Fab binding, was exceptionally high, approaching 3 million per cell. We also demonstrate efficient T-cell-mediated killing of colon cancer cells coated with A3 scFv fused to the low MHC class II binding superantigen mutant SEA(D227A). The identified A3 molecule thus represents a TAA with properties that suggest its use for immunotherapy of colon and pancreatic cancer.