DARRELL J. IRVINE
Massachusetts Institute of Technology
Immunotherapy aims to promote an immune response to disease. Pursued for more than 30 years as a potential treatment for cancer, it is based on the capacity of the immune system to safely distinguish healthy cells from tumor cells and to be resistant to mutational escape by tumors, and on the possibility of establishing immune memory to prevent recurrence.
THE NEW AGE OF IMMUNOTHERAPY
For many years treatments targeting the immune system showed only anecdotal efficacy in clinical trials, leading many researchers to become disillusioned with the field by the late 1990s. Yet the 1990s were a period when many critical elements of fundamental biology regulating the immune response were identified or characterized: the first tumor antigens, Toll-like receptors and related signaling pathways that govern inflammation and the immune system’s ability to identify “danger,” regulatory receptors that promote or block T cell activation, and specific mechanisms used by tumor cells to avoid immune destruction.
These discoveries led to a transformation in the field of immuno-oncology, which was most prominently impacted by clinical studies, in the early 2000s, of an antibody that blocks a key negative regulatory receptor on T cells, cytotoxic T lymphocyte antigen-4 (CTLA-4). Treatment of melanoma patients with this antibody enabled endogenous antitumor immune responses that led to tumor regressions in a small proportion of heavily pretreated patients with metastatic disease. About 20 percent of the patients survived more than 5 years, well beyond the expected lifespan for advanced disease (Hodi et al. 2010; Lebbé et al. 2014). This “tail of the curve” effect in overall survival reflects a dramatic change in
outcome from the best modern “targeted” therapies, where early tumor regression is generally followed by drug resistance, relapse, and death.
Following these early findings, a second class of antibodies blocking another negative regulator axis in T cells, antibodies to PD-1 on T cells (or to its ligand, PD-L1 expressed on tumor cells), showed even more dramatic effects in large clinical trials. Among patients with melanoma, renal cell carcinoma, and lung cancer, 30–50 percent showed tumor regressions (Topalian et al. 2012). These drugs, although acting by distinct mechanisms, are collectively referred to as “checkpoint blockade” therapies, as they disrupt regulatory checkpoints that restrain the immune response to cancer.
In parallel to these advances, a second type of immunotherapy approach has been developed: adoptive cell therapy (ACT), based on the transfer of autologous tumor-specific T cells into patients. In ACT, T cells are isolated from the peripheral blood or from tumor biopsies, cultured with the patient’s own tumor cells to identify tumor-reactive clones, and then expanded to large numbers for reinfusion into the patient (Rosenberg and Restifo 2015). The creation ex vivo of an army of tumor-specific T cells has been shown to elicit objective tumor regressions when combined with appropriate adjuvant treatments that promote the functionality of the transferred T cells (e.g., administration of adjuvant drugs such as interleukin-2).
Other strategies genetically modify T cells for patients by introducing a synthetic T cell receptor (chimeric antigen receptor, or CAR) that allows any T cell to become a tumor-specific T cell. These have shown particular promise in treating certain leukemias: greater than 75 percent of patients have experienced complete remissions (Maude et al. 2014).
Thus, in the space of a few short years the field of cancer immunotherapy has been revolutionized in the clinic, from a peripheral approach notorious for high toxicity and low efficacy, to a frontline treatment with the prospect of eliciting durable responses—and perhaps cures—in some patients.
ROLE OF ENGINEERING IN THE FUTURE OF CANCER IMMUNOTHERAPY
Immunology has advanced by embracing new technologies, from the early days of monoclonal antibody technology to the recent inventions of powerful mass spectrometry–based cellular analysis tools.
The field has also recently attracted the attention of a growing number of interdisciplinary scientists, who bring to bear a unique mindset and new approaches to problems in immunology and immunotherapy. Some of these techniques are rooted in engineering, leading to exciting advances in basic science and new approaches to vaccines and immunotherapies.
Engineers excel at creating model systems that break complex problems into manageable hurdles, and at drawing on applied chemistry, physics, and mathemat-
ics to create new technologies that solve practical problems. Engineering contributions to the evolution of cancer immunotherapy can be illustrated by recent examples in the areas of cancer vaccines and ACT. These by no means represent all the areas where engineers are actively working on cancer immunotherapy, but rather are two representative examples.
Enhancing Cancer Vaccines
As mentioned, checkpoint blockade with anti-CTLA-4 or anti-PD-1 has elicited objective tumor regressions in a small proportion of patients. This incomplete response rate has motivated a strong interest in finding additional treatments that can be combined with these drugs to expand the responding population.
Because these drugs act to enhance T cell responses against tumors, one obvious strategy is to combine checkpoint blockade with therapeutic cancer vaccines, for patients whose spontaneous T cell responses to tumors may be too weak to be rescued by checkpoint blockade alone. To this end, a renewed interest in cancer vaccines has been kindled in both preclinical and clinical studies. However, cancer vaccines to date have generally been perceived as a failure, both because of their lack of objective responses in patients and their inability to elicit the kind of robust T cell priming that is believed to be necessary for tumor regression (i.e., T cell responses more like those to live infectious agents).
How can the efficacy of cancer vaccines be improved?
Vaccines are generally based on the delivery of antigens (the protein, peptide, or polysaccharide target of the immune response) together with inflammatory cues that stimulate the immune system to respond to the antigens.
One of the simplest approaches that has been most extensively explored in the clinic is the use of peptide antigens combined with adjuvants as T cell–focused vaccines. But short peptides injected in vivo have several significant limitations: they are quickly degraded, they largely flush into the bloodstream rather than traffick to lymphatics and lymph nodes, and they can be presented by any nucleated cell to T cells. The latter phenomenon, in which T cells are stimulated by random tissue cells rather than professional antigen-presenting cells (APCs) in lymph nodes, leads to tolerance or deletion of tumor-specific cells.
One way to deal with all of these challenges at once is to conjugate so-called “long” peptide antigens (that can be presented only by professional APCs) to an albumin-binding lipid tail through a water-soluble polymer spacer. Albumin constitutively traffics from blood to lymph, and, thus linking antigens to an albumin-binding lipid “tail,” redirects these molecules efficiently to lymph nodes instead of the bloodstream after parenteral injection. In addition, the polymer/lipid linkage
protects the peptide from degradation. A similar strategy can be used to create “albumin hitchhiking” adjuvants.
These simple chemical modifications lead to 15- to 30-fold increases in vaccine accumulation in lymph nodes, both enhancing the safety of the vaccine and dramatically increasing vaccine potency (Liu et al. 2014).
Engineers have also used methods developed in the regenerative medicine field to create implantable vaccine “centers” that coordinate multiple steps in an anticancer vaccine response. A common strategy in regenerative medicine is to create biodegradable polymeric scaffolds as artificial environments that can protect and nurture therapeutic cells on implantation in vivo.
Mooney, Dranoff, and colleagues demonstrated that a similar approach can be used to regulate the response to a vaccine (Ali et al. 2009). By loading polymeric sponges with tumor antigens, chemoattractants for APCs, and adjuvants, they coordinated a 3-step process of (1) APC attraction to the implanted scaffold, (2) uptake of antigen and adjuvant by the APCs, and (3) migration of the now activated APCs to draining lymph nodes, where they could initiate a potent antitumor immune response. This approach is currently being tested in a phase I clinical trial.
Thus chemistry and biomaterials approaches offer a number of ways to create enhanced cancer vaccines.
Engineering Adoptive Cell Therapy
As noted above, adoptive transfer of tumor antigen-specific T cells is one of the two classes of immunotherapies to demonstrate significant durable responses in the clinic so far, but strategies to improve this treatment for elimination of solid tumors are still sought.
Engineers are contributing to the evolution of ACT treatments through the application of synthetic biology principles for the creation of novel genetically engineered T cells. Recently, for example, bioengineers have generated completely artificial ligand-receptor-transcription factor systems, which enable the introduction of a synthetic receptor and transcription factor pair into T cells to enable T cell recognition of a tumor-associated ligand to be transduced into production of an arbitrary biological response (Morsut et al. 2016; Roybal et al. 2016).
Another strategy introduces synthetic fragmented antigen receptors that are activated only when a small molecule drug is present, to allow precise control over the activity of therapeutic T cells in vivo (Wu et al. 2015). These are only a few representative examples of a rapidly moving and exciting area of research.
A third strategy chemically engineers T cells using an approach from the nanotechnology and drug delivery communities to “adjuvant” T cells with supporting drugs, such as cytokines that promote T cell function and proliferation.
One promising approach is to attach drug-releasing nanoparticles directly to the plasma membrane of ACT T cells so that the modified cells carry supporting drugs on their surface wherever they home in vivo. This approach has been shown to greatly augment the expansion and antitumor activity of T cells when used to deliver supporting cytokines to the donor cells (Stephan et al. 2010). This basic demonstration also opens the potential to target supporting drugs directly to T cells in vivo, through targeted nanoparticle formulations (Zheng et al. 2013). Such studies show promise in preclinical models and are entering the early stages of translation into clinical testing.
Cancer therapy is being revolutionized by the first successful immunotherapy treatments. It has also created exciting new opportunities for engineers to impact the field of cancer immunotherapy, by solving challenging problems to safely enhance the immune response to tumors.
The marriage of cutting-edge tools from engineering with the latest understanding of the immune response to tumors offers the promise of further advances toward the goal of curing cancer or rendering many cancers a manageable, chronic condition.
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