Recent highlights of cancer immunotherapy

Cancer immunotherapy represents a groundbreaking paradigm shift in the field of cancer treatment, harnessing the power of the immune system to combat cancer cells. As an innovative approach, it has shown immense promise and has revolutionized the way we perceive and treat cancer. This commentary aims to highlight the recent important advances in cancer immunotherapy, including immune checkpoint blockade therapy, chimeric antigen receptor T cell therapy, T cell receptor-gene engineered T cell therapy, and tumor vaccines.


Introduction
Immunotherapy has rapidly evolved in recent decades, emerging as a viable option for numerous advanced cancer patients and delivering unprecedented therapeutic benefits.Immunotherapy is based on the interaction between immune cells and tumor cells in the tumor microenvironment.It achieves tumor killing by reactivating depleted immune cells and restoring or enhancing their effector functions through immune checkpoint blockade (ICB) or engineering of T cells.Notably, the past few months have witnessed significant advances in the field of cancer immunotherapy.This article tends to provide a brief review of noteworthy breakthroughs.

CB therapy
Over the past decade, ICB therapy, including anti-PD-1, PD-L1, and CTLA-4, has marked a significant breakthrough in treating advanced cancer patients.It has shown substantial efficacy across a range of cancers such as melanoma and non-small cell lung cancer.However, some patients do not respond to ICB treatment, and some develop hyperprogressive disease (HPD), resulting in rapid cancer progression.Thus, additional research is imperative to understand the intricate interplay between cancer and immunity.

Dendritic cells (DCs) in ICB
Conventionally, the mechanism by which ICB therapy activates the immune system to combat cancer involves the direct activation of CD8 + T cells within the tumor microenvironment (TME), thereby enabling them to perform their cytotoxic functions.A recent study has highlighted the involvement of CD5 + DCs in the initiation of CD8 + T cells responses during ICB therapy.During this process, the presence of low levels of IL-6 promotes an increased proportion of CD5 + DCs, which subsequently interact with T cells and induce T cell activation.This finding suggests that CD5 could serve as a potential target for enhancing the effectiveness of ICB therapy.In addition, CD5 expression on DCs can function as a biomarker for predicting patient responses to ICB therapy [1].

γ-δ T cells in ICB
As the primary effector cells of ICB, CD8 + T cells rely on binding to antigenic epitopes presented by HLA Class I molecules to exert their function.However, defective DNA mismatch repair in colorectal cancer often leads to the loss of HLA Class I-mediated antigen delivery due to impaired antigen processing mechanisms.Despite this, certain patients with these cancers can still maintain a response to PD-1 blocking, with some even exhibiting lasting response [2].It suggests that immune cell subsets beyond CD8 + T cells are involved in antitumor immunity.Research revealed that γ-δ T cells serve as the primary effector cells responsible for post-ICB activation in cancers exhibiting HLA Class I molecular defects, with their antitumor effects partly dependent on NKG2D/ NKG2D-ligand interactions [2].This finding further expands the understanding of the mechanism of ICB treatment response.

B cells in ICB
The role of B-cell responses in cancer immunotherapy has also been further elucidated.The researchers found that levels of antibodies associated with endogenous retroviruses (ERV) are significantly elevated in the blood and tumor tissue of lung cancer patients.Despite ERV-derived antigens being autoantigens, their substantial upregulation in cancer results in immune tolerance.Further studies revealed that ERV-associated antibodies exert anti-tumor activity, activate tumor immune response and enhance the effect of anti-PD-1/ PD-L1 immunotherapy [3].This study provides new ideas for the development of new immunotherapeutic strategies for lung cancer.Moreover, ERV antibodies hold the potential to emerge as a novel immunotherapeutic strategy for lung cancer, with the prospect of enhancing the effectiveness of PD-1/PD-L1 immune checkpoint inhibitor therapy.

Tumor draining lymph nodes in ICB
In addition to tumor environment, a team has found that tumor lymph nodes play an important role in the response to ICBs.CD8 + T cells in tumor draining lymph nodes are crucial in ICB therapy.Alterations in the proportion of CD8 + T cell subtypes in unaffected areas were noted after ICB treatment.Specifically, progenitor exhausted CD8 + T cells (Tpex cells) in lymph nodes differentiate into proliferating intermediate-exhausted CD8 + T cells (Tex-int), which are transported through the blood to the site of the cancer.Within the lymph nodes housing the tumor, changes in the composition of immune cells impair this critical process.Consistent with this finding, the proportion of Tex-int in the blood was also elevated after ICB treatment, and a high degree of elevation predicted a better prognosis [4].This suggests that Tex-int may function as a biomarker for assessing the efficacy and prognosis of ICB therapy.

Bacteria in ICB
Bacterial presence has been detected in a variety of tumors.Lactobacillus royale (Lr) is a commonly used probiotic, and a recent preclinical study found that Lr translocates and colonizes melanoma from the intestine via the vascular and lymphovascular pathways.Lr metabolizes tryptophan to produce indole-3-aldehyde (I3A), which activates the Aryl hydrocarbon receptor (AhR) pathway.This activation promotes CD8 + T cell proliferation and enhances their functionality within the tumor immune microenvironment, resulting in increased IFN-γ production and heightened responsiveness to ICB therapy.Furthermore, patients with high serum I3A concentrations have a better prognosis than those with low I3A concentrations [5].This emphasizes the important role of microbial metabolism in restraining tumor growth, and Lr therapy is expected to be combined with ICB therapy.However, the mechanisms governing the translocation of Lr from the intestine to the TME and its precise localization within the TME remain unelucidated, warranting further investigation in these directions.It is important to note that prior research has revealed the capacity of tumor cells to produce kynurenine through the metabolism of tryptophan, thereby activating the AhR pathway and suppressing antitumor immunity.This implies the complexity of the impact of AhR on cancer.Variations in cancer types and the TME can result in divergent or even contradictory outcomes following AhR pathway activation, necessitating consideration in future investigations.

Circadian rhythms in ICB
Interestingly, circadian rhythms have emerged as an essential element in immunotherapy.A study comparing afternoon and early morning times revealed that immune cell delivery was notably more active in the afternoon, resulting in a more substantial reduction in cancer volume with anti-immunotherapy.Mechanically, circadian rhythms regulate the expression of the co-stimulatory molecule CD80 of DCs and the rhythmicity of antigenspecific CD8 + T cells transported by DCs to cancer sites [6].These new findings suggest that the duration of treatment as a variable may affect the effectiveness of immunotherapy in patients, and that synchronizing treatment with the function of DCs may improve the effectiveness of cancer immunotherapy.Appropriate intervention timing is expected to maximize the therapeutic effect and improve the safety of treatment.

Mechanisms of ICB tolerance
PD-L1 is an immunosuppressive factor expressed in a variety of cancer cells and immune cells.A recent research discovered that a large amount of PD-L1 is distributed in the nucleus of uveal melanoma samples, which is associated with unfavorable clinical outcomes.They further revealed the angiogenic function of nPD-L1 by enhancing the binding of phosphorylated signal transducer and activator of transcription 3 (p-STAT3) to the early growth response 1 (EGR1) promoter, leading to the acitivation of EGR1-mediated angiogenesis.In addition, the use of HDAC2 inhibitors could restore the acetylation level of PD-L1 in UM and prevent its nuclear translocation.Thus, it decreases EGR1 expression levels, which would inhibit UM angiogenesis [7].Therefore, the combination of anti-PD-L1 immunotherapy and HDAC2 inhibitors represents a potential treatment strategy for UM patients.A separate study has pinpointed the innate immune kinase, TANK-binding kinase 1 (TBK1), as a gene associated with tumor immune evasion.TBK1 inhibits the cell death signaling downstream of the TNF receptor, which can be targeted to enhance tumor sensitivity to TNF and IFN-γ cytotoxicity, thus sensitizing PD-1 blocking therapy [8].Nevertheless, clinical-grade TBK1 inhibitors are currently unavailable.The experiments are currently limited to the animal stage and further studies are still needed.
On the other hand, the underlying mechanism of the HPD occurrence has been elucidated.IFN-γ from CD8 + T cells can induce the upregulation of fibroblast growth factor 2 (FGF2) in cancer cells, initiating a metabolic reprogramming that suppresses PKM2 activity and reduces NAD levels.This further leads to increased acetylation of β-actin and ultimately increased stemness of tumor cells [9].This process can be summarized as immune alterations triggering metabolic reprogramming in cancer cells, subsequently driving tumor progression following ICB treatment.Patients with a "triple high" profile of IFN-γ, FGF2 and β-catenin are more likely to experience hyperprogression after ICB treatment.This crucial discovery not only aids in the pre-treatment assessment of patients to avoid direct ICB treatment for those with the "triple high" profile, potentially preventing HPD, but also offers insights into a new target and theoretical foundation for combining immunotherapy with metabolic targeted therapy.

The new generation of checkpoint inhibitors
Some patients may develop resistance to current immunotherapy regimens, necessitating additional co-blockade targeting new inhibitory receptors (IRs) and ligands.A new generation of checkpoint inhibitors is presently undergoing clinical trials, with early study results showing promise.Emerging clinical immunotherapeutic targets include IRs on T cells such as LAG-3, TIM-3, and TIGIT, as well as inhibitory ligands within the B7 family like B7-H3, B7-H4, and B7-H5.While many of these targets involve complex and as-yet-unrevealed mechanisms, they have demonstrated excellent therapeutic efficacy.Recent research suggests that LAG3 (CD223) accumulates at the immunological synapse, creating acidic conditions that dissociate the tyrosine kinase Lck from the CD4 or CD8 co-receptor, thereby impeding a crucial requirement for T-cell activation and signaling [10].This challenges the previous consensus that LAG3 functions as a signal disruptor in a major histocompatibility complex class II-dependent manner.These novel findings may boost the efficacy of IRs, providing new theoretical indication for the development of new generation of checkpoint inhibitors.

Chimeric Antigen Receptor (CAR) T cell therapy
CAR T cell therapy is another important approach of immunotherapy.It can produce high complete response rates in hematologic tumors such as acute lymphoblastic leukemia and lymphoma.Nevertheless, disease recurrence is common in a substantial proportion of patients.In addition, CAR T therapy is less effective in solid tumors and is currently restricted to hematologic tumors.Therefore, the focus of research is to improve the therapeutic response rate of CAR T and reduce relapse and adverse reactions.Overcoming these clinical limitations of CAR T therapy could help expand the use of CAR T in solid tumors.
Ark313 is a variant of adeno-associated virus vectors (AAV6) that exhibits high transduction efficiency in mice T cells and can be genetically modified to enhance CAR T cells' cytotoxicity [11].Ark313 allows for efficient transgene delivery of large DNA fragments.Moreover, Ark313 extends the means of T-cell genetic engineering studies to mice and is not limited to immunodeficient murine and human T cells.This discovery facilitates experimental T-cell immunology research and indicates that Ark313 has great potential in the field of tumor immunotherapy.
Epigenetic regulation is associated with T cell differentiation and functional status.A new study demonstrated that reducing 5-methylcytosine oxidation in DNA by biallelic TET2 editing promoted antigen independent cloning amplification and efficacy of CAR T cells, thereby enhancing antitumor effect [12].This finding suggests epigenetic intervention has clinical translational potential in CAR T therapy.However, it is essential to note that this technique also exposes the risk of overexpansion of CAR T cells in vivo, which may cause secondary mutations.
Antigenic variability and antigen loss are pivotal factors contributing to immune evasion and resistance in CAR T therapies.Recent research suggests the concept of vaccine-augmented CAR T as a novel approach to address this challenge [13].The investigators devised a vaccineboosted CAR T strategy aimed at stimulating CAR T cell proliferation and augmenting their anti-tumor potential via CAR ligands.Surprisingly, they observed that this approach also triggers antigen spreading and initiates anti-tumor responses in native T cells, leading to a substantial enhancement of the anti-cancer effect and a reduction in tumor recurrence following CAR T therapy.
The application of CAR T to solid tumors is limited by the overexpression of antigens found in both solid tumors and normal tissues.Applying Boolean logic to CAR T cells can enable T cells to distinguish between normal tissue and tumor tissue, thus playing an important role in advancing the application of CAR T in solid tumors.The researchers designed Boolean-logic AND-gate CAR T cells by replacing the CD3ζ domain in CAR with the pairing of LAT and SLP-76.The designed CAR T cells attack only cancer cells that are double positive for antigens, without killing single antigen-positive normal cells, thus avoiding systemic toxicity [14].This AND-gated CAR can more precisely regulate the activation of CAR T cells, thereby increasing the specificity of killing and reducing off-target effects.In addition, the finding could have broad implications beyond cancer immunotherapy, potentially extending the application of CAR T to the study of autoimmune diseases.However, it is worth noting that this AND-gated mediated double positive killing may increase the risk of immune evasion of tumor cells.

T Cell Receptor-Gene Engineered (TCR) T cell therapy
TCR T-cell therapy is performed by engineering TCR from autologous T cells in vitro and then transfusing them back to the patient so that these T cells can specifically target tumor antigens.In contrast to CAR T-cell therapy, the recognition of antigens by TCR T cells remains dependent on the delivery of MHC molecules.This property determines that the target antigens of TCR T cells are not limited to those expressed on the cell surface, making them more effective than CAR T therapies in treating solid tumors.However, there are also many obstacles in TCR T cell therapy.Affinity is a major obstacle to the success of TCR T therapy.Insufficient affinity can lead to off-target effects, prompting T cells to mistakenly target normal tissues expressing tumor-associated antigens or similar ligands.Conversely, excessively high affinity may induce abnormal immune activation, elevating the risk of inciting a cytokine storm.Therefore, achieving optimal affinity is pivotal for ensuring the safety and efficacy of TCR T therapy.In addition, challenges related to T cell depletion, dysfunction during application, tumor immune evasion, and the scarcity of effective tumor-specific antigens for targeting in the majority of cancer patients remain unresolved.Overcoming these challenges will be key to achieving greater clinical success in the future.
A rencent breakthrough was the use of neoantigen specific TCR (neoTCR), which involves non-viral CRISPR-Cas9 gene editing of the patients's autologous T cells TCR chain to express neoTCR.In this process, multiple neoTCRs recognizing the patient's tumor antigens were first isolated and cloned, and the endogenous TCRs of T cells were knocked out and knocked into the neoTCRs in vitro using non-viral CRISPR-Cas9 gene editing.Finally, the engineered T cells were transfused back to the patient.The safety and feasibility of such personalized engineered T cells was demonstrated in a Phase I clinical trial [15].And a follow-up study by the same team found that reconstruction of neoTCR in T cells by gene editing showed tumor specific recognition and cytotoxicity in patients who did not respond to PD-1 blocking therapy [16].This technology facilitates the development of TCR T therapies and provides an alternative treatment option for patients with advanced solid tumors that do not respond to ICB therapy.Based on this technology, it may be possible in the future to knock out or insert certain genes into T cells to prevent T cell exhaustion and improve the durability of immunotherapy.

Tumor vaccines
Therapeutic cancer vaccines evoke endogenous T cell immune responses against tumor antigens through uptake, processing, and presentation by dendritic cells.Tumor antigens may either be shared across various cancer types, such as common mutant variants of KRAS or p53, or they can be personalized neoantigens designed to target specific somatic mutations unique to individual tumors [17].Merck and Moderna conducted a randomized trial involving patients diagnosed with advanced melanoma who had previously undergone surgery to remove their melanomas.All patients received ICB therapy, while two-thirds of them also received the mRNA tumor vaccine, instructing cells to produce tumor-specific antigens.Marvelously, the reported data showed a 44 percent reduction in the mortality and recurrence rates among vaccinated patients [18].This represents an inspiring milestone in clinical trials of tumor vaccines, demonstrating the therapeutic potential of individualized immunotherapeutic drugs and tumor vaccines.