Targeting T cells against brain tumors with a bispecific ligand-antibody conjugate

Redirecting cytotoxic T cells with chemically programmed antibodies

T-cell engaging bispecific antibodies (T-biAbs) mediate potent and selective cytotoxicity by combining specificities for target and effector cells in one molecule. Chemically programmed T-biAbs (cp-T-biAbs) are precisely assembled compositions of (i) small molecules that govern cancer cell surface targeting with high affinity and specificity and (ii) antibodies that recruit and activate T cells and equip the small molecule with confined biodistribution and longer circulatory half-life. Conceptually similar to cp-T-biAbs, switchable chimeric antigen receptor T cells (sCAR-Ts) can also be put under the control of small molecules by using a chemically programmed antibody as a bispecific adaptor molecule. As such, cp-T-biAbs and cp-sCAR-Ts can endow small molecules with the power of cancer immunotherapy. We here review the concept of chemically programmed antibodies for recruiting and activating T cells as a promising strategy for broadening the utility of small molecules in cancer therapy.

IFNγ-induced Chemokines Are Required for CXCR3-mediated T-Cell Recruitment and Antitumor Efficacy of Anti-HER2/CD3 Bispecific Antibody

PURPOSE

The response to cancer immune therapy is dependent on endogenous tumor-reactive T cells. To bypass this requirement, CD3-bispecific antibodies have been developed to induce a polyclonal T-cell response against the tumor. Anti-HER2/CD3 T-cell-dependent bispecific (TDB) antibody is highly efficacious in the treatment of HER2-overexpressing tumors in mice. Efficacy and immunologic effects of anti-HER2/CD3 TDB were investigated in mammary tumor model with very few T cells prior treatment. We further describe the mechanism for TDB-induced T-cell recruitment to tumors.

EXPERIMENTAL DESIGN

The immunologic effects and the mechanism of CD3-bispecific antibody-induced T-cell recruitment into spontaneous HER2-overexpressing mammary tumors was studied using human HER2 transgenic, immunocompetent mouse models.

RESULTS

Anti-HER2/CD3 TDB treatment induced an inflammatory response in tumors converting them from poorly infiltrated to an inflamed, T-cell abundant, phenotype. Multiple mechanisms accounted for the TDB-induced increase in T cells within tumors. TDB treatment induced CD8⁺ T-cell proliferation. T cells were also actively recruited post-TDB treatment by IFNγ-dependent T-cell chemokines mediated via CXCR3. This active T-cell recruitment by TDB-induced chemokine signaling was the dominant mechanism and necessary for the therapeutic activity of anti-HER2/CD3 TDB.

CONCLUSIONS

In summary, we demonstrate that the activity of anti-HER2/CD3 TDB was not dependent on high-level baseline T-cell infiltration. Our results suggest that anti-HER2/CD3 TDB may be efficacious in patients and indications that respond poorly to checkpoint inhibitors. An active T-cell recruitment mediated by TDB-induced chemokine signaling was the major mechanism for T-cell recruitment.

Protection from tumor recurrence following adoptive immunotherapy varies with host conditioning regimen despite initial regression of autochthonous murine brain tumors

Adoptive T cell transfer (ACT) has achieved clinical success in treating established cancer, particularly in combination with lymphodepleting regimens. Our group previously demonstrated that ACT following whole-body irradiation (WBI) promotes high-level T cell accumulation, regression of established brain tumors, and long-term protection from tumor recurrence in a mouse model of SV40 T antigen-induced choroid plexus tumors. Here we asked whether an approach that can promote strong donor T-cell responses in the absence of WBI might also produce this dramatic and durable tumor elimination following ACT. Agonist anti-CD40 antibody can enhance antigen-specific CD8(+) T-cell responses and has shown clinical efficacy as a monotherapy in the setting of cancer. We show that anti-CD40 conditioning promotes rapid accumulation of tumor-specific donor CD8(+) T cells in the brain and regression of autochthonous T antigen-induced choroid plexus tumors, similar to WBI. Despite a significant increase in the lifespan, tumors eventually recurred in anti-CD40-conditioned mice coincident with loss of T-cell persistence from both the brain and lymphoid organs. Depletion of CD8(+) T cells from the peripheral lymphoid organs of WBI-conditioned recipients failed to promote tumor recurrence, but donor cells persisted in the brains long-term in CD8-depleted mice. These results demonstrate that anti-CD40 conditioning effectively enhances ACT-mediated acute elimination of autochthonous tumors, but suggest that mechanisms associated with WBI conditioning, such as the induction of long-lived T cells, may be critical for protection from tumor recurrence.

Mutation of herpesvirus Saimiri ORF51 glycoprotein specifically targets infectivity to hepatocellular carcinoma cell lines

Herpesvirus saimiri (HVS) is a gamma herpesvirus with several properties that make it an amenable gene therapy vector; namely its large packaging capacity, its ability to persist as a nonintegrated episome, and its ability to infect numerous human cell types. We used RecA-mediated recombination to develop an HVS vector with a mutated virion protein. The heparan sulphate-binding region of HVS ORF51 was substituted for a peptide sequence which interacts with somatostatin receptors (SSTRs), overexpressed on hepatocellular carcinoma (HCC) cells. HVS mORF51 showed reduced infectivity in non-HCC human cell lines compared to wild-type virus. Strikingly, HVS mORF51 retained its ability to infect HCC cell lines efficiently. However, neutralisation assays suggest that HVS mORF51 has no enhanced binding to SSTRs. Therefore, mutation of the ORF51 glycoprotein has specifically targeted HVS to HCC cell lines by reducing the infectivity of other cell types; however, the mechanism for this targeting is unknown.

Folate-immunoglobulin G as an anticancer therapeutic antibody

Folate receptor-alpha (FR) is a promising cellular marker for tumor-specific drug delivery. Conjugation of folic acid to therapeutic and imaging agents has been shown to enhance their delivery to FR (+) cancer cells in vitro and in tumor-bearing mice via an FR-mediated cellular uptake mechanism. In this study, immunoglobulin G (IgG) was conjugated to folate and evaluated as a therapeutic antibody against folate receptor (FR)-positive tumors. Murine IgG (mIgG) was conjugated to folate via an amide bond to yield folate-conjugated mIgG (f-mIgG) that contained an average of approximately 2.6 folates per molecule. Selective uptake of f-IgG by FR (+) tumor cells was determined by fluorescence microscopy and by flow cytometry. Lysis of L1210JF cells by NK cells from murine donors was increased 1.4-9.0-fold at the effector:target (E:T) ratio of 25:1, relative to control mIgG. In mice bearing L1210JF tumors, f-mIgG was found to significantly inhibit tumor growth and to have prolonged the median survival time (MeST). Significantly, the antitumor efficacy of f-mIgG was greatly increased when combined with liposomal G3139, an 18-mer phosphorothioate oligonucleotide. In fact, the combination resulted in a 100% cure rate among the tumor-bearing mice. Injection of f-mIgG significantly increased serum INF-gamma and IL-6 level in mice compared with mIgG and dramatically increased serum INF-gamma and IL-6 level when combined with liposomal G3139. These results suggested that f-IgG, a novel immunotherapy agent, has potent activity as a therapeutic antibody to the FR-positive cancer, and the therapeutic activity is enhanced by immunomodulatory agents.

Targeted drug delivery via folate receptors

Targeted delivery via selective cellular markers can potentially increase the efficacy and reduce the toxicity of therapeutic agents. The folate receptor (FR) has two glycosyl phosphatidylinositol (GPI)-anchored isoforms, alpha and beta. FR-alpha expression is frequently amplified in epithelial cancers, whereas FR-beta expression is found in myeloid leukemia and activated macrophages associated with chronic inflammatory diseases. Conjugates of folic acid and anti-FR antibodies can be taken up by cancer cells via receptor-mediated endocytosis, thus providing a mechanism for targeted delivery to FR+ cells. The aim of this article is to provide a brief overview of applications of FR targeting in drug delivery, with an emphasis on the strategy of using folate as a targeting ligand. In order to do this, recent literature is surveyed on targeted delivery via both FR sub-types, as well as new findings on selective receptor upregulation in the targeted cells. A wide variety of molecules and drug carriers, including imaging agents, chemotherapeutic agents, oligonucleotides, proteins, haptens, liposomes, nanoparticles and gene transfer vectors have been conjugated to folate and evaluated for FR-targeted delivery. Substantial targeting efficacy has been found both in vitro and in vivo. In addition, mechanisms and methods for selective FR upregulation have been uncovered, which might enhance the effectiveness of the FR-targeted delivery strategy. FR-alpha serves as a useful marker for cancer, whereas FR-beta serves as a marker for myeloid leukemia and chronic inflammatory diseases. FR-targeted agents have shown promising efficacy in preclinical models and significant potential for future clinical application in a wide range of diseases.

The folate receptor: what does it promise in tissue-targeted therapeutics?

For over a decade the folate receptor has been intensively investigated as a means for tumor-specific delivery of a broad range of experimental therapies including several conceptually new treatments. Despite a few set backs in clinical trials, the literature is replete with encouraging in vitro and pre-clinical studies of gynecological and other tumors and more therapeutic approaches are ready for clinical testing. Recent studies have added myelogenous leukemias to the list of candidate cancers for FR-targeted therapies. Each approach faces unique challenges in translation that could be addressed through a mechanistic understanding of the function and expression of the receptor in the appropriate experimental systems and by improvements in the technology. This review discusses FR in the context of positive recent developments in broad areas of FR-targeted therapy and attempts to highlight its potential and the anticipated challenges.

Folate Receptor-Mediated Drug Targeting: From Therapeutics to Diagnostics

Folate targeted drug delivery has emerged as an alternative therapy for the treatment and imaging of many cancers and inflammatory diseases. Due to its small molecular size and high binding affinity for cell surface folate receptors (FR), folate conjugates have the ability to deliver a variety of molecular complexes to pathologic cells without causing harm to normal tissues. Complexes that have been successfully delivered to FR expressing cells, to date, include protein toxins, immune stimulants, chemotherapeutic agents, liposomes, nanoparticles, and imaging agents. This review will summarize the applications of folic acid as a targeting ligand and highlight the various methods being developed for delivery of therapeutic and imaging agents to FR-expressing cells.

Folate-mediated delivery of macromolecular anticancer therapeutic agents

The receptor for folic acid constitutes a useful target for tumor-specific drug delivery, primarily because: (1) it is upregulated in many human cancers, including malignancies of the ovary, brain, kidney, breast, myeloid cells and lung, (2) access to the folate receptor in those normal tissues that express it can be severely limited due to its location on the apical (externally-facing) membrane of polarized epithelia, and (3) folate receptor density appears to increase as the stage/grade of the cancer worsens. Thus, cancers that are most difficult to treat by classical methods may be most easily targeted with folate-linked therapeutics. To exploit these peculiarities of folate receptor expression, folic acid has been linked to both low molecular weight drugs and macromolecular complexes as a means of targeting the attached molecules to malignant cells. Conjugation of folic acid to macromolecules has been shown to enhance their delivery to folate receptor-expressing cancer cells in vitro in almost all situations tested. Folate-mediated macromolecular targeting in vivo has, however, yielded only mixed results, largely because of problems with macromolecule penetration of solid tumors. Nevertheless, prominent examples do exist where folate targeting has significantly improved the outcome of a macromolecule-based therapy, leading to complete cures of established tumors in many cases. This review presents a brief mechanistic background of folate-targeted macromolecular therapeutics and then summarizes the successes and failures observed with each major application of the technology.

IL-12 treatment of endogenously arising murine brain tumors

A number of recent studies have indicated that T cells can be stimulated to attack transplanted brain tumors in rodent models. As IL-12 has been shown to activate cytotoxic T cell responses, we tested the idea that it might stimulate a T cell response against endogenous brain tumors that arise in SV40 large T Ag transgenic mice (SV11). SV11 mice develop tumors of the choroid plexus, a specialization of the ependymal lining of the brain ventricles. They are a particularly relevant model of human disease, because they are immunocompetent but immunologically tolerant of the tumors. SV11 mice were treated with recombinant murine IL-12 for 10 days. Tumors grew more slowly than in control treated mice, and in some cases were reduced in size, as assessed by magnetic resonance imaging before and after treatment. At the end of treatment, tumors, but not brain parenchyma, exhibited extensive infiltration of activated CD8(+) and CD4(+) T cells. Tumors also showed a reduction in vascular density. Mice treated with IL-12 lived significantly longer than control mice. Tumors that progressed were nearly devoid of T cells, indicating that the T cell response was not sustained. In addition, some mice that had a substantial tumor burden at the beginning of treatment displayed evidence of immunosuppression, which might be related to TGF-ss2 detected in tumors. We conclude that IL-12 treatment can initiate an anti-tumor response even against endogenously arising brain tumors, but factors that will allow a sustained and more effective anti-tumor response need to be determined.

Optimization of folate-conjugated liposomal vectors for folate receptor-mediated gene therapy

A folate-targeted transfection complex that is internalized by certain cancer cells and displays several properties reminiscent of enveloped viruses has been developed. These liposomal vectors are comprised of a polycation-condensed DNA plasmid associated with a mixture of neutral and anionic lipids supplemented with folate-poly(ethylene glycol)-dioleylphosphatidylethanolamine for tumor cell-specific targeting. N-Citraconyl-dioleylphosphatidylethanolamine is also included for pH-dependent release of endosome-entrapped DNA into the cytoplasm, and a novel plasmid containing a 366-bp segment from SV40 DNA has been employed to facilitate transport of the plasmid into the nucleus. Because formation of the DNA core is an important step in the assembly of liposomal vectors, considerable effort was devoted to comparing the transfection efficiencies of various DNA condensing agents. It was found that complexation of plasmid DNA with high molecular weight polymers such as acylated-polylysine and cationic dendrimers leads to higher folate-mediated transfection efficiency than DNA complexed with unmodified polylysine. In contrast, compaction of plasmid DNA with small cationic molecules such as spermine, spermidine, or gramicidin S yields only weakly active folate-targeted liposomal vectors. Compared to analogous liposomal vector preparations lacking an optimally compacted DNA core, a cell-specific targeting ligand, a caged fusogenic lipid, and a nucleotide sequence that facilitates nuclear uptake, these modified liposomal vectors display greatly improved transfection efficiencies and target cell specificity.

Bispecific agents target endogenous murine T cells against human tumor xenografts

A variety of immunological approaches to cancer treatment are currently being explored. These include strategies designed to enhance or redirect the activity of T cells against tumors. Bispecific antibodies comprise a class of agents capable of redirecting T cells by binding to a tumor antigen and the T-cell receptor (TCR). In vivo pre-clinical testing of bispecific antibodies against human tumors has to date been limited to the use of immunodeficient mice that receive the bispecific agent, activated human effector T cells, and human tumor cells. In this report, we show that TCR transgenic/RAG-1 knockout mice (TCR/RAG) serve as a unique model allowing endogenous T cells to be redirected against transplanted human tumors. The findings show that TCR/RAG mice (i) accepted transplants of human tumors, including the folate-receptor-positive tumor line KB; (ii) contained endogenous cytotoxic T lymphocytes that could be activated in vivo with an antigenic peptide recognized by the transgenic TCR; (iii) rejected human tumors after treatment with the activating peptide and bispecific agents that contained folic acid co-valently linked to an anti-TCR antibody. Successful rejection was achieved with folate conjugates of Fab or scFv fragments. Treatment with activating agents and bispecific conjugates resulted in the complete eradication of freshly transplanted tumors as well as significantly prolonging the survival of mice bearing established solid tumors. Our results highlight the importance of including T-cell-activating modalities in combination with bispecific antibodies. Additionally, we introduce a system that allows endogenous T cells to be redirected against human tumor xenografts and in which the T cells may be followed in vivo by use of a clonotypic marker.

Innovative therapies for pediatric brain tumors

Success in the treatment of pediatric brain tumors has lagged behind that of other pediatric cancers. This paper highlights many of the advances that have taken place over the past few years in the surgical, radiotherapeutic, and chemotherapeutic approaches to central nervous system lesions that we hope will lead to a dramatic improvement in outcome. Innovations in neurosurgical and radiotherapeutic techniques have resulted in decreasing toxicity although substantial improvement in cure rates has not been observed. Many new techniques such as gene therapy, angiogenesis inhibitors, immunotherapy, and others that have not been part of the classic approach to these lesions are now in clinical trials in the hope that they will impact on the survival of these patients. The scientific basis for these new treatment modalities and preliminary clinical results are discussed.

Intracerebral bispecific ligand-antibody conjugate increases survival of animals bearing endogenously arising brain tumors

Bispecific antibodies capable of simultaneously binding a tumor surface antigen and the T-cell receptor/CD3 complex are capable of inducing polyclonal immune effector cells to destroy targeted tumor cells. Bispecific antibody immunotherapies have shown some promise against tumors of hematopoietic origin such as lymphomas, but use of bispecific antibodies for the treatment of solid tumors has been less fully explored. To test the preclinical potential of bispecific antibody therapy against an endogenously arising solid brain tumor, we have utilized a novel variation of conventional bispecific antibodies, referred to as bispecific ligand-antibody conjugates, to target choroid plexus tumors. The bispecific ligand-antibody conjugate described in this study is a chemical conjugate between an anti-CD3 monoclonal antibody (MAb) and folic acid, the ligand for a high-affinity surface receptor expressed on the surface of choroid plexus tumors. SV11 mice transgenic for SV40 large T antigen and its promoter develop solid choroid plexus tumors in the brain. We demonstrate that choroid plexus tumor cells are susceptible in vitro to cytolysis mediated by cytotoxic T cells in the presence of the bispecific ligand-antibody conjugate in a folate-inhibitable manner. Adoptive immunotherapy studies demonstrate the potential benefits of the bispecific ligand-antibody conjugate in vivo. The bispecific conjugate is capable of retaining adoptively transferred T lymphocytes specifically within tumor tissue for periods of up to at least 1 week. Further, following intracerebro-ventricular injection of bispecific conjugate and splenocytes containing activated cytotoxic T cells, T cells were observed to penetrate to interior regions of the tumor. A single treatment of adoptively delivered activated effectors and bispecific conjugate into the brain ventricles was insufficient to produce significant increases in survival of SV11 mice, but repeated treatment through indwelling cannulas prolonged survival of animals treated with activated effectors and bispecific ligand-antibody conjugate compared to animals treated with activated effectors or saline alone. Our results demonstrate that the SV11 model may be useful for preclinical evaluation and optimization of bispecific ligand-antibody conjugate treatments of solid tumors.