T-Cell Immunotherapies Targeting Histocompatibility and Tumor Antigens in Hematological Malignancies.

Over the last decades, T-cell immunotherapy has revealed itself as a powerful, and often curative, strategy to treat blood cancers. In hematopoietic cell transplantation, most of the so-called graft-vs.-leukemia (GVL) effect hinges on the recognition of histocompatibility antigens that reflect immunologically relevant genetic variants between donors and recipients. Whether other variants acquired during the neoplastic transformation, or the aberrant expression of gene products can yield antigenic targets of similar relevance as the minor histocompatibility antigens is actively being pursued. Modern genomics and proteomics have enabled the high throughput identification of candidate antigens for immunotherapy in both autologous and allogeneic settings. As such, these major histocompatibility complex-associated tumor-specific (TSA) and tumor-associated antigens (TAA) can allow for the targeting of multiple blood neoplasms, which is a limitation for other immunotherapeutic approaches, such as chimeric antigen receptor (CAR)-modified T cells. We review the current strategies taken to translate these discoveries into T-cell therapies and propose how these could be introduced in clinical practice. Specifically, we discuss the criteria that are used to select the antigens with the greatest therapeutic value and we review the various T-cell manufacturing approaches in place to either expand antigen-specific T cells from the native repertoire or genetically engineer T cells with minor histocompatibility antigen or TSA/TAA-specific recombinant T-cell receptors. Finally, we elaborate on the current and future incorporation of these therapeutic T-cell products into the treatment of hematological malignancies.

The expansion of targetable biomarkers for CAR T cell therapy.

Biomarkers are an integral part of cancer management due to their use in risk assessment, screening, differential diagnosis, prognosis, prediction of response to treatment, and monitoring progress of disease. Recently, with the advent of Chimeric Antigen Receptor (CAR) T cell therapy, a new category of targetable biomarkers has emerged. These biomarkers are associated with the surface of malignant cells and serve as targets for directing cytotoxic T cells. The first biomarker target used for CAR T cell therapy was CD19, a B cell marker expressed highly on malignant B cells. With the success of CD19, the last decade has shown an explosion of new targetable biomarkers on a range of human malignancies. These surface targets have made it possible to provide directed, specific therapy that reduces healthy tissue destruction and preserves the patient's immune system during treatment. As of May 2018, there are over 100 clinical trials underway that target over 25 different surface biomarkers in almost every human tissue. This expansion has led to not only promising results in terms of patient outcome, but has also led to an exponential growth in the investigation of new biomarkers that could potentially be utilized in CAR T cell therapy for treating patients. In this review, we discuss the biomarkers currently under investigation and point out several promising biomarkers in the preclinical stage of development that may be useful as targets.

Optimized tumor cryptic peptides: the basis for universal neo-antigen-like tumor vaccines.

The very impressive clinical results recently obtained in cancer patients treated with immune response checkpoint inhibitors boosted the interest in immunotherapy as a therapeutic choice in cancer treatment. However, these inhibitors require a pre-existing tumor specific immune response and the presence of tumor infiltrating T cells to be efficient. This immune response can be triggered by cancer vaccines. One of the main issues in tumor vaccination is the choice of the right antigen to target. All vaccines tested to date targeted tumor associated antigens (TAA) that are self-antigens and failed to show a clinical efficacy because of the immune self-tolerance to TAA. A new class of tumor antigens has recently been described, the neo-antigens that are created by point mutations of tumor expressing proteins and are recognized by the immune system as non-self. Neo-antigens exhibit two main properties: they are not involved in the immune self-tolerance process and are immunogenic. However, the majority of the neo-antigens are patient specific and their use as cancer vaccines requires their previous identification in each patient individualy that can be done only in highly specialized research centers. It is therefore evident that neo-antigens cannot be used for patient vaccination worldwide. This raises the question of whether we can find neo-antigen like vaccines, which would not be patient specific. In this review we show that optimized cryptic peptides from TAA are neo-antigen like peptides. Optimized cryptic peptides are recognized by the immune system as non-self because they target self-cryptic peptides that escape self-tolerance; in addition they are strongly immunogenic because their sequence is modified in order to enhance their affinity for the HLA molecule. The first vaccine based on the optimized cryptic peptide approach, Vx-001, which targets the widely expressed tumor antigen telomerase reverse transcriptase (TERT), has completed a large phase I clinical study and is currently being tested in a randomized phase II trial in non-small cell lung cancer (NSCLC) patients.

The therapeutic value of monoclonal antibodies directed against immunogenic tumor glycoproteins.

Monoclonal antibodies developed against immunogenic proteins (Tumor Specific Antigens/TSA's) that are expressed in human cancers, display a unique behavioral pattern. They appear to serve in a dual role. This includes the early recognition of these immunogenic membrane proteins that can serve as diagnostic markers, and the targeting of such markers for the destruction of the tumor, primarily thru ADCC.The monoclonals (mAbs) that we have developed against specific immunogenic tumor membrane proteins have been studied in detail. These tumor proteins, when first defined, were referred to as tumor associated antigens. With the ability of the mAbs to demonstrate therapeutic antitumor activity in those patients with relatively advanced malignancies, the term tumor specific was introduced. Monoclonals that we were able to develop from tumor specific proteins derived from colon and pancreas cancer were found capable of targeting those tumors to induce apoptosis. We were also able to define immunogenic membrane proteins from lung (squamous and adenoCa) as well as prostate neoplasms. Monoclonals developed from these tumor antigens are in the initial phases of investigation with regard to their specificity and antitumor activity.Mabs capable of targeting the malignancies noted above were produced following immunization of BALBc mice with the Tumor Specific Antigens. The hybridomas that were screened and found to express the antibodies of interest appeared for the most part as IgG2a's. It became apparent after a short period of time that stability of the Fab CDR loops as well as the therapeutic efficacy of the hybridoma mAbs could be lost. Stability was achieved by chimerization and or humanization. The resulting mAbs were found to switch their isotypes to an IgG1 subsequent to chimerization and or humanization, when expressed in CHO cells. The monoclonals, so produced, were not only more efficient in controlling tumor growth but minimized the development of a HAMA response.Because of 1) the specificity of this group of monoclonal antibodies in targeting well defined immunogenic proteins that were expressed on the tumor cell membrane,2) their lack of cross reactivity to normal tissue, 3) relatively low toxicity when delivered intravenously, 4) rapid targeting of tumor cell populations (4-6 hrs in vitro) and their 5) ability to destroy xenograft transplants (in vivo) within days of delivery, these mAbs were felt to be ideal for possible use in the treatment of patients with recurrent and or metastatic tumors.Initial clinical studies have been planned for following the filing of an IND. It is required by FDA that the potential effects of tumor control and toxicity be defined using the naked antibodies produced under GMP conditions, In those situations where patients with recurrent malignancies are to be studied we have come to realize that a number of factors can influence the response to monoclonal therapy. This includes the amount of shed antigen in the serum at the time of treatment that could initiate immune complex formation as well as the shedding of inhibitory material into the serum possibly effecting an immune response. As such we plan to eventually employ the therapeutic mAbs in combination with chemotherapy as a means of enhancing the immunogenicity of the tumor system being treated and to possibly weaken the malignant growth for easier destruction by the mAb. We will also look at the combination of mAbs with immunostimulants such as GMCSF and IL-2 (fusion proteins) and eventual conjugation of the mAbs with alpha and possibly B-emitters to help in targeting bystander cells. The present paper reviews the potential therapeutic value of such mAbs in the treatment of recurrent malignancies, especially those having failed chemotherapy in established clinical trials.