Immunotherapy is revolutionizing cancer treatment and is the most promising bet we have at a long sought for cancer cure. By awakening and priming the immune system to fight cancer, such strategies have achieved impressive clinical successes. Cancer immunotherapies improve anti-tumor immune responses and have fewer off-target effects than chemotherapies and other cancer cytotoxic agents. In this article, I briefly describe the research advances for some of the most impactful approaches to cancer immunotherapy as well as the opportunities and challenges in their broad implementation for cancer treatment.

Emerging Approaches To Immunotherapy

Most relevant immunotherapies approved for cancer treatment fall into several classes, including checkpoint inhibitors, lymphocyte-activating cytokines, CAR T cells and other cellular therapies, cancer vaccines and oncolytic viruses.

Checkpoint Inhibitors

The most common checkpoint inhibitor immunotherapies are PD1/PDL1 blockade and CTLA4 inhibition. By expressing PDL1, tumor cells can bind to PD1 receptors on T cells rendering them inactive (1,2). Blocking this interaction with antibodies targeting either PD1 or PDL1 enables T cells to recognize and kill tumor cells. Similarly, blocking the interaction between CTLA4 and its ligands (CD80 and CD86) enables T cells to remain active and can recognize and kill tumor cells (3). Five PD1 or PDL1 inhibitors and one CTLA4 inhibitor have been shown to promote overall survival and have been approved to treat various types of cancer (4). Hundreds of clinical trials are currently testing checkpoint inhibitors alone or in combination with other targeted agents. The use of checkpoint inhibitors does have, however, its drawbacks. Beside having potential side-effects in numerous organs (5-8), many patients do not respond to treatment with checkpoint inhibitors. A lack of response to the treatment may be caused by a low number of tumor-infiltrating T cells, deregulated and alternate mechanisms for immunosuppression, or adapted resistance to checkpoint inhibition (9-11).

Cytokines

In the cytokine-based immunotherapy, injected cytokines directly stimulate the growth and activity of immune cells. The three main types of cytokines typically used in immunotherapy are interferons, interleukins and granulocyte-macrophage colony-stimulating factor (GM-CSF) (12). In response to microbial pathogens, immune cells express interferons which induce the maturation of numerous immune cells including macrophages, natural killer (NK) cells, lymphocytes and dendritic cells (13-16). Interferon-activated immune cells also hamper the blood supply for the tumor cells by inhibiting angiogenesis in the extracellular tumor space (12,14,17). Interleukins primarily stimulate the activity and growth of CD4+ T cells (T helper cells that regulate immune responses) and CD8+ T cells (cytotoxic T cells that kill abnormal cells) (18-21). Finally, GM-CSF promotes immune responses by improving T cell survival and supporting the differentiation of dendritic cells which in turn can express tumor-specific antigens (22). Three immune-activating recombinant cytokines are approved for cancer immunotherapy and several others, including IL-15 and IL-17, are being tested in clinical studies (23,24). However, owing to their somewhat short half-life, cytokines generally need to be injected at high dosages, that can cause vascular leakage and cytokine release syndrome (12). Furthermore, due to their effects on different T cell populations, cytokine therapy can also lead to an auto-immune attack against healthy tissues (25).

Engineered T Cells

In the chimeric antigen receptor (CAR) T cell approach, T cells are first collected from patient blood and then engineered to express CARs that recognize specific antigens on cancer cells. These engineered CAR T cells are then re-administered to the same patient where they can recognize and kill tumor cells expressing the targeted antigens (26). CAR T cell therapy has helped many cancer patients achieve remission and prolonged survival and, in contrast to other treatment options, CAR T cells are typically a one-time therapy and the cells can retain activity for over a decade after injection (27-30). Two CD19-targeting CAR T cell therapies are approved for clinical use against B cell leukemias and lymphomas (31,32). CAR T cells engineered to target different antigens are being developed for more generalized therapeutic approaches (26,33-35). One major drawback of CAR T cell therapy is that the production of CAR T cells remains an expensive, technically complex and time intensive process (36). A second drawback reveals itself when dealing with solid tumors, where the infused T cells do not persist and fail to effectively penetrate the tumor tissue (34,37,38).

T cell receptor (TCR) T cells are a second type of engineered T cells currently being tested in clinical trials for both hematological and solid cancers. TCR T cells can target tumor-specific intracellular antigens presented by major histocompatibility complexes (MHCs) (39). Optimal antigenic targets for TCR T cells are patient-specific neoantigens that result from tumor-associated mutations and are therefore not expressed by normal cells (40). Unlike CAR T cells, TCR T cells must be MHC-matched with the patient. The disadvantages of CAR T cell therapy are also hindering the widespread implementation of TCR T cell therapy. Furthermore, both CAR T and TCR T cells can cause cytokine release syndrome and neurotoxicity (41,42).

Cancer Vaccines

Leading cancer vaccines include dendritic cells, nucleic acids and neoantigens. Dendritic cells collected from patients that are engineered to express tumor-associated antigens are able to activate T cells to recognize and attack cancer cells (10). Sipuleucel-T is one such dendritic cell vaccine approved to treat prostate cancer (43,44). Although dendritic cell vaccines were shown to have high safety profiles, they have often failed in clinical trials due to lack of efficacy (45). Ongoing efforts to improve the efficacy of dendritic cell vaccines rely on better selection of specific dendritic cell populations and better delivery to relevant lymph nodes (10,46,47).

In nucleic acid therapeutics, DNA or mRNA is taken up by antigen presenting cells (APCs) and translated to stimulate antigen expression (48). Presentation of the expressed antigens to T cells induces their activation against tumor cells expressing the targeted antigens. Due to nuclear delivery barriers, mRNA-based vaccines are favored over DNA-based ones. Furthermore, mRNA half-life can be extended with modifications, it is non-infectious and does not integrate into the genome, as many DNA vaccines do (49,50). However, mRNA is quickly degraded by nucleases and requires intracellular delivery platforms (49,51).

A third category of cancer vaccines is based on neoantigens and can boost the immune response to cancer cells (52,53). Neoantigens are tumor-specific antigens that arise from somatic DNA alterations occurring in cancer cells and are therefore not expressed by normal cells. As they are present only in tumor cells, neoantigens can be used to boost the immune response against cancer cells without having any off-target adverse effects. Furthermore, neoantigen vaccines can encompass multiple tumor-specific antigens, which is ideal for treating heterogenous cancers. Both nucleic acid and neoantigen cancer vaccines can benefit from better delivery platforms that can incorporate serval mRNA or neoantigen molecules for a more complete approach to treat heterogeneous cancers (54,55).

Delivery Technologies Improving T Cell Immunotherapy

Molecular medicine technologies are overcoming biological barriers preventing the broad clinical application of T cell immunotherapy. Surface-conjugated nanoparticles have been designed to improve the viability and function of the transplanted cells. Adjuvant-loaded nanoparticles that are chemically conjugated to the surface of donor T cells maximize their efficacy and performance after transplantation while minimizing systemic side effects (56). Such nanoparticle-functionalized T cells have been used to achieve complete tumor clearance in a metastatic mouse model of melanoma (56).

A second technology relies on biomaterial-based implants to locally deliver engineered T cells to solid tumors (57,58). Polymeric scaffolds can localize T cells at tumor sites and act as reservoirs from which propagating T cells are released as the material degrades (57,58). The scaffolds can also be functionalized with immune-stimulating drugs and therefore enable both local and systemic anti-tumor immunity (58). In an aggressive ovarian cancer mouse model, local delivery of T cell implants eradicated or substantially shrunk the tumors in all tested animals (57).

A third technology further improving the clinical applicability of T cell therapy relies on synthetic artificial antigen-presenting cells (aAPCs). Having T cell-stimulating molecules (for example, targeting the CD28 receptor) conjugated to their surface, aAPCs activate T cells and trigger an anti-tumor response (59). aAPCs functionalized with MHC immunoglobulin G (IgG) and an anti-CD28 antibody have been developed to treat melanoma (60).

Oncolytic Viruses for Combination Immunotherapy

Oncolytic viruses (OVs) are therapeutics that have been engineered or selected to replicate within and destroy cancer cells (61). As replicating entities, OVs can be delivered locally or systemically and thereby act at both primary and metastatic tumor sites. One common design principle is to attenuate or remove viral virulence factors such that the OV is unable to replicate in normal cells but retains the ability to infect and kill cancer cells (61). Crosstalk between oncogenic signaling and antiviral pathways in tumor cells creates a permissive environment for OVs to propagate in and selectively kill cancer cells (62). It is now established that genes commonly mutated in cancer, such as Ras, p53, Rb1 and Pten can predispose cancer cells to virus infection (62).

A key antiviral pathway in mammalian cells is mediated by interferons which are cytokines that also regulate cell metabolism, apoptosis and antigen display (63). Thus, cells that become malignant due to aberrant interferon signaling often develop sensitivity to viral infection. Modified rhabdoviruses such as vesicular stomatitis virus (VSV) and Maraba virus are typical OVs depending on aberrant interferon signaling for their cancer selectivity (64-66). OVs are predicted to be a critical part of future immunotherapies, by functioning as multiplexed immune-modulating platforms expressing checkpoint inhibitors, tumor antigens, cytokines and T cell activators.

Engineering Macrophages to Fight Solid Tumors

Genetically modified lymphocytes such as CAR-T cells have had only modest success when directed against solid tumors. Startup companies, however, are now engineering phagocytes with chimeric antigen receptors (CARs) enabling them to infiltrate solid cancers and trigger a robust anti-tumor immune response. Carisma Therapeutics reported last year that their CAR-engineered macrophages can infiltrate solid tumors, ingest malignant cells and stimulate a strong immune response against the growing cancer (67). In the following months, Carisma aims to initiate human trials with an autologous CAR-macrophage therapy directed against tumors expressing epidermal growth factor receptor 2 (Her2). Two other companies, Thunder Biotech and Myeloid Therapeutics, are similarly developing CAR-based cellular therapies to eliminate cancer tissue. Physical barriers and the immunosuppressive environment created by solid cancers hinder CAR-engineered T cells and other lymphocytes to infiltrate and persist in the tumor tissue. In contrast, macrophages, monocytes and other myeloid cells are able to leave the bloodstream and infiltrate the tumor, making them ideal cell types to reprogram for tumor-targeting purposes (67,68).

Tumor associated macrophages (TAMs) are among the most abundant immune cells infiltrating solid tumors and are therefore the most promising target in CAR-based cellular therapies (67). TAMs can take on two different activation states: tumor-supportive or M2-like macrophages that inhibit inflammation and promote cancer progression; and tumor-suppressive or M1-like macrophages that produce proinflammatory cytokines and inhibit cancer growth (67,68). Unfortunately for patients, M2-like TAMs are the predominant type in most tumors. Over two dozen companies are now aiming to reprogram those TAMs, especially by targeting the ‘do-not-eat-me’ CD47 signaling pathway that protects tumor cells against macrophages.

Author: Sebastian I. Florescu, PhD

Published Online: 21 March 2021

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