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Traditional CAR-T cells, while effective against liquid cancers, face challenges in solid tumors: the cells wear out and ultimately fail to destroy the cancer completely. Ground-breaking research is providing an innovative approach to this challenge. Researchers are introducing CAR-T cells that excrete the IL-10 molecule. In other words, the cell has been designed to produce its own "drug" to stay healthy in the tumor's hostile environment. In the laboratory, this innovative CAR-T therapy systematically eradicated cancerous tumors in mouse models. What's more, in ongoing clinical trials, eleven patients have appeared to achieve complete remission with this treatment, representing a 100% success rate to date. Notably, the evidence from the laboratory study suggests the long-term efficacy of the therapy, and indicates that its manufacture could be both faster and more cost-effective than current methods 

One of the challenges of CAR T cell therapy in solid tumors is a phenomenon known as T cell exhaustion. Previous studies have alluded to the inflammatory regulator Regnase-1 as a potential target to indirectly overcome the effects of T-cell exhaustion, as it can cause hyperinflammation when disrupted in T cells, reviving them to produce an antitumor response. The research team hypothesized that targeting the related but independent Roquin-1 regulator at the same time could boost responses further. The team used CRISPR-Cas9 gene editing to knock out Regnase-1 and Roquin-1 individually and together in healthy donor T cells with two different immune receptors that are currently being studied in Phase I clinical trials: the mesothelin-targeting M5 CAR (mesoCAR) and the NY-ESO-1-targeting 8F TCR (NYESO TCR). Following CRISPR editing, the T cells were expanded and infused into solid tumor mouse models, where the researchers observed that the double knockout resulted in at least a 10-fold increase in modified T cells compared to knocking down Regnase-1 alone, as well as increased anti-tumor immune activity and longevity of modified T cells. In some mice, this also led to an overproduction of lymphocytes, causing toxicity.

Researchers have bolstered the power of chimeric antigen receptor (CAR)-T cancer therapies, which use genetically altered T cells to seek out tumours and mark them for destruction. Now scientists have further engineered the cells to contain switches that allow control over when and where the cells are active. This helps them to infiltrate tumours and dodge immune-suppressing defences.

CAR T cells currently in clinical use are designed to recognize cancer cells directly and have successfully treated several blood cancers. But there have been challenges that prevent their effective use in many solid tumors. Most solid tumors are heavily infiltrated by a type of immune cell called macrophages. Macrophages help tumors grow by blocking the entry of T cells into the tumor tissue, which prevents CAR T cells and the patient's own T cells from destroying the cancer cells. To address this immune suppression at the source, the researchers engineered T cells to make a chimeric antigen receptor that recognizes a molecule on the surface of macrophages. When these CAR T cells encountered a tumor macrophage, the CAR T cell became activated and killed the tumor macrophage. Treating mice with ovarian, lung and pancreatic tumors with these macrophage-targeting CAR T cells reduced the number of tumor macrophages, shrank the tumors and prolonged their survival. The destruction of tumor macrophages allowed the mice's own T cells to access and kill the cancer cells. The researchers further demonstrated that this anti-tumor immunity was induced by the release of the cytokine interferon-gamma by CAR T cells.

Currently, too few CAR T cells become memory cells that persist and create more T cells in the long term. However, a group of researchers recently showed that the distribution of the c-Myc protein in a parental T cell may be important for this process and published this work in the journal Nature. The researchers knew that a daughter cell with more c-Myc became an effector cell. In this study, the team found that the protein complex cBAF (canonical Brg1/Brg-associated factor) interacted with c-Myc. Daughter cells with high concentrations of cBAF and c-Myc became effector T cells. The cBAF binds certain regions of chromatin, proteins on DNA. The discovery suggests that it can guide the fate of cells, what type of T cells they become, by controlling the expression of effector cell-related genes. The distribution of cBAF occurs in the first activated T cell that begins the adaptive immune response; therefore, the researchers realized that cell fate is decided early in the immune response. The researchers used the molecular information they discovered. They applied a cBAF inhibitor during CAR T cell activation to generate more memory T cells. In a preclinical model, T cells treated with an inhibitor-controlled tumor growth better than untreated cells. The treated cells also survived longer and in greater numbers.

CAR T cells have proven to be a powerful weapon against blood cancers, but against solid tumors they are much less effective, in part because of a process called T cell exhaustion. However, Penn Medicine researchers have uncovered key molecular details of this exhaustion process that point to a specific strategy to overcome it. In this study, the scientists developed a labmodel  allowing them to thoroughly study the exhaustion process of CAR T cells designed to attack pancreatic tumors. They observed that the process of T cell exhaustion in the model closely resembled that observed in patient T cells. The model also revealed new facets of the exhaustion process, including the role of 2 genetic regulators of exhaustion, ID3 and SOX4, whose silencing allowed CAR T cells to retain much of their effectiveness against tumor cells. The scientists observed that CAR T cell exhaustion was accompanied by increases in the levels of two proteins, ID3 and SOX4, which function as master switches for large sets of genes in immune cells. Silencing these apparent T-cell exhaustion switches allowed the depleted CAR T cells to retain much of their antitumor efficacy even after prolonged exposure to tumor cells.

The study thus points to a specific strategy - inhibition of ID3 and/or SOX4 - that could help CAR T cells function better against solid tumors.

A chimeric antigen receptor (CAR) T-cell therapy that targets the protein mesothelin showed no evidence of major toxicity and had antitumor activity in patients with malignant pleural disease from mesothelioma, according to new results.

Solid tumors generate anti-immune signals that deactivate CAR T cells, making treatment less effective. To solve this problem, scientists have combined CAR T cells with cytokine injection, which can cause significant unintended toxicities. Researchers replaced the extracellular domain of various cytokine receptors with leucine zippers to create constitutively active receptors. CAR T cells expressing one of these chimeric cytokine receptors had superior antitumor activity against several types of cancer in cell lines and mouse models compared with conventional CAR T cells. Although chimeric cytokine receptors give a constant "on" signal to CAR T cells, they do not induce non-specific proliferation of CAR T cells. The system thus limits the effect of cytokine signaling to modified cells only, reduces the risk of cytokine-related toxicity, and provides a signal that these CAR T cells should function effectively in a suppressive tumor microenvironment. 

T-cell transfer therapies have not yet been successfully applied to solid tumors because T cells do not readily penetrate and persist in solid tumor masses for long periods of time, and because their activity is attenuated by an immunosuppressive tumor microenvironment. One way to overcome these limitations could be to couple T cell transfer therapies with cytokine therapy. However, a serious drawback of this approach is the significant side effects resulting from cytokines circulating freely in the body, leading to toxicity and potentially fatal inflammatory syndromes. Now, researchers have developed a nanotechnology-based solution to these problems. The method uses an unnatural sugar that is absorbed and embedded in the outer coating of T cells, which can then be used to anchor cytokines. The concentrated cytokines improve T-cell function locally without producing unwanted systemic side effects. In mice with melanoma, the approach also stimulated the host immune system against tumor cells, which inhibited tumor growth. As an adjunct to CAR-T cell therapy, it resulted in complete regression of lymphoma tumors at otherwise non-curative cell doses.

In most cases, the number of naturally occurring cancer-targeting T cells will be far too low to trigger an immune response capable of eradicating a patient's tumor(s). PACT Pharma is a privately held biopharmaceutical company developing personalized, neo-antigen-specific T-cell receptor (TCR)-T therapies to treat a range of solid tumors.  Patient T cells are isolated and CRISPR-modified by electroporation with Cas9 protein, guide RNAs to knock out endogenous TCR genes (TCRα ( TRAC) and TCRβ ( TRBC)) and an HR template plasmid encoding the transgenic neoTCR. The results of a phase 1 clinical trial that were published in Nature. The researchers report that CRISPR-modified T cells were preferentially directed to the tumor and could be recovered from post-infusion biopsies in all patients for whom biopsies were available. They also note that CRISPR-edited T cells frequently accounted for the top 2-20% of immune cells in the tumor, and a reduction in tumor size was observed in some lesions in a single lung cancer patient.

T cells used in immunotherapy treatments can become exhausted by the task of fighting cancer cells or shut down when they enter tumours. However, a set of CRISPR screens allowed researchers to deactivate each gene in the genome, one at a time, in a pool of human T cells and the team found a handful of candidates that could make T cells resistant to key aspects of the immunosuppressive microenvironment often present in tumours. The researchers were particularly intrigued by a gene called RASA2, as it had never been associated with immune cell function before. The team created T cells with the RASA2 gene knocked out. They then subjected these T cells to various "stress tests" by repeatedly exposing them to cancer cells and models of the tumour microenvironment. They compared the performance of these cells to the original therapeutic T cells that still contained a functional RASA2 gene. Long after the original cells had lost their cancer-fighting abilities, the cells with knocked-out RASA2 remained remarkably tireless. The researchers thus made the therapeutic cells more resistant. This discovery could help overcome a major factor limiting the success of these promising therapies in the fight against solid and liquid tumours.

Researchers have shown that a synthetic IL-9 receptor allows anti-cancer T cells to do their job without the need for chemo or radiation. T cells modified with the synthetic IL-9 receptor were potent against tumours in mice, as published in Nature. This group of researchers were interested in testing modified versions of the synthetic receptor that transmit other cytokine signals from the common gamma chain family: IL-4, -7, -9 and -21. Of the synthetic common gamma chain signals, the IL-9 signal was worth studying and unlike other cytokines, IL-9 signalling is not generally active in naturally ocurring T cells. The synthetic IL-9 signal gave the T cells a unique blend of stem cell and killer cell qualities that made them more robust in fighting tumours. In particular, the researchers targeted two types of difficult-to-treat cancer models in mice: pancreatic cancer and melanoma. They used T cells targeted to the cancer cells via the natural T cell receptor or a chimeric antigen receptor (CAR). In all cases, T cells engineered with synthetic IL-9 receptor signalling were superior and helped cure some tumours in mice when they could not do otherwise. 

A boost for CAR–T cells

Chimeric antigen receptor (CAR)–T cell immunotherapy has been highly successful for treating certain blood cancers. Yet this approach has been a challenge for solid tumors, in part because it is difficult to target functional engineered T cells to the tumor site. Ma et al.designed a vaccine strategy to improve the efficacy of CAR–T cells by restimulating the CAR directly within the native lymph node microenvironment (see the Perspective by Singh and June). Injected “amph-ligand” vaccines promoted synthetic antigen presentation and led to CAR–T cell activation, expansion, and increased tumor killing. The system could potentially be applied to boost any CAR–T cell.