Novel immunotherapies for hematologic malignancies

Abstract

The immune system is designed to discriminate between self and tumor tissue. Through genetic recombination, there is fundamentally no limit to the number of tumor antigens that immune cells can recognize. Yet, tumors use a variety of immunosuppressive mechanisms to evade immunity. Insight into how the immune system interacts with tumors is expanding rapidly and has accelerated the translation of immunotherapies into medical breakthroughs. Herein, we appraise novel strategies that exploit the patient's immune system to kill cancer. We review various forms of immune-based therapies, which have shown significant promise in patients with hematologic malignancies, including (i) conventional monoclonal therapies like rituximab; (ii) engineered monoclonal antibodies called bispecific T-cell engagers; (iii) monoclonal antibodies and pharmaceutical drugs that block inhibitory T-cell pathways (i.e. PD-1, CTLA-4, and IDO); and (iv) adoptive cell transfer therapy with T cells engineered to express chimeric antigen receptors or T-cell receptors. We also assess the idea of using these therapies in combination and conclude by suggesting multi-prong approaches to improve treatment outcomes and curative responses in patients.

Keywords: adoptive T-cell transfer therapy; allogeneic hematopoietic stem cell transplantation; bispecific T-cell engagers; gene transfer; immune checkpoint modulators; monoclonal antibodies.

Fig. 1. Bispecific antibodies

Bispecific antibodies were generated by genetic or chemical cross-linking of monoclonal antibodies (mAbs) or by F(ab’)₂ fragments (upper row). Antibody engineering permitted the generation of small bispecific antibodies comprising the variable heavy (VH) and light (VL) domains of the original mAbs (lower row). Single-chain fragment variable (scFv); single-chain bi-specific tandem variable domain (BiTEs) and dock-and-lock trivalent Fab (DNL-(F(ab)₃)). Modified figure from Stamova et al. (35).

Fig. 2. Mechanisms of action of antibody-based immunotherapy in cancer

Mechanisms of antitumor antibody therapies are distinct and based on the antibody structure and function. Displayed are various strategies for antibody mediated tumor death. Upper left: direct cytotoxicity, in which mAbs induce cytotoxicity directly to the tumors by trumping signaling pathways or in which immune-conjugates kill targeted cells. Lower left: FcReceptor-mediated immune effector engagement, in which the Fc portion of mAbs engage immune effector functions, including soluble CMC (through the membrane attack complex MAC) as well as NK cells, macrophages, and dendritic cells, through FcRs, allowing for ADCC, ADCP, and IC uptake. Upper right: Non-restricted activation of cytotoxic T cells, in which tumor-infiltrating CTLs can be activated against tumor cells—independent of T cell receptor (TCR) specificity—by engaging co-receptors on the T cells and tumor antigens. Lower right: blockade of inhibitory signaling, in which cytotoxic lymphocytes, including NK cells and CTLs, express inhibitory receptors for various ligands (such as PD-L1) that may be expressed by tumor cells. Antagonistic antibodies that target these inhibitory receptors can block ligand-receptor interactions so that targeted cytotoxicity can ensue (such as the FDA approved CTLA-4 and PD-1 blocking antibodies). These four strategies enhance tumor cell death, which can promote phagocytosis of tumor cell antigens, and induction of adaptive immune responses (bottom right) in two ways: MHC class I cross-presentation and priming of cytotoxic T cells and MHC class II presentation and priming of helper T cells. These adaptive immune responses can enhance antitumor immunity. Figure reprinted from Cell, Volume 128, pages 1081-1084, Weiner LM, Murray JC, Shuptrine CW, 'Antibody-based immunotherapy of cancer'. Copyright 2012, with permission from Elsevier (38).

Fig. 3. Construction of a BiTE

The variable domains of two monoclonal antibodies recognizing either tumor or T cell are genetically linked, as indicated by dotted lines. A single polypeptide chain is produced in which two single-chain antibodies are flexibly linked (BiTE). This small molecule tethers the tumor cells to the T cells by the tumor-associated antigen (TAA-in this cased CD19 expressed on hematologic malignancies) to the CD3 antigen on T cells.

Fig. 4. Confocal imaging of T cell to target cell via BiTEs

Staining of CD45 (blue), granzyme A (red) and CD3 (green). Results on synapse formation are depicted in this picture. The upper image depict synapses induced by erbB2 peptide while the lower rows show synapses formed in the presence of BiTE. The left columns show the cell conjugates in the differential interference contrast mode; the middle columns show confocal images of immunofluorescence staining of the same cell conjugates. The white bars represent 10 μm. The right columns show the synapse interface. Image reproduced from Molecular Immunology, Volume 43, pages 763-771, Offner S, Hofmeister R, Romaniuk A, Kufer P, Baeuerle PA, 'Induction of regular cytolytic T cell synapses by bispecific single-chain antibody constructs on MHC class I-negative tumor cells,' Copyright 2006, with permission from Elsevier (69).

Fig. 5. T cells redirected to recognize and kill tumors

Gene transfer can be used to engineer T cells to express CARs that target antigens in an MHC-independent manner. CARs are fusion proteins composed of an extracellular portion that is usually derived from an antibody and intracellular signaling modules derived from T cell signaling proteins. First-generation CARs contain CD3ζ, whereas second-generation CARs possess a costimulatory endodomain (e.g. CD28 or 4-1BB) fused to CD3ζ. Third-generation CARs consist of two costimulatory endodomains linked to CD3ζ. Abbreviations: CAR, chimeric antigen receptor; MHC, major histocompatibility complex; scFv, single-chain variable fragment; TCR, T-cell receptor.