Immunotherapeutic interventions of Triple Negative Breast Cancer

Tumours can be controlled by the immune system. This has been the subject of research for over a century, from the existence of tumour antigens and the cancer immunosurveillance hypothesis to the immunoediting hypothesis [1]. According to the cancer immunoediting hypothesis, tumour fate is shaped by the host immune system through three phases: the elimination, equilibrium and escape phases. The immune balance is first tilted to anti-tumour immunity in the elimination phase, and an intact and competent immune system detects and then destroys the developing tumour during immunosurveillance. Sporadic tumour cells may survive this editing phase and progress to the equilibrium phase, where the balance lies between anti-tumour and tumour-promoting factors, resulting in a functionally suppressed state of the tumour. Finally, the tumour cells acquire the ability to circumvent immune surveillance and destruction, and these immunologically sculpted tumours emerge with a progressively outgrowing status, establishing an immunosuppressive tumour microenvironment (TME) in the escape phase [1, 2].

It is not only infection-derived immunity, immune deregulation and autoimmunity preceding tumour development but also the intrinsic inflammation triggered by malignancies following tumour development that promotes cancer development and progression. As a result of these different forms of inflammation, the TME contains innate immune cells [macrophages, neutrophils, mast cells, myeloid-derived suppressor cells (MDSC), dendritic cells (DCs), and natural killer (NK) cells] and adaptive immune cells (T and B lymphocytes), in addition to the cancer cells and the surrounding stroma (fibroblasts, endothelial cells, pericytes, and mesenchymal cells) [3]. At the same time, inflammation also influences the host immune response to tumours and can be used in cancer immunotherapy and chemotherapy [3]. The immune response in tumours mainly relies on adaptive immunity, usually focusing on T cell-mediated cellular immunity [4]. CD8⁺ T cells evolve and kill tumour cells by excreting perforin, granzymes and IFN-γ [5]. There is evidence that some immune cells [DCs, MDSC, B cells, CD8⁺, CD4⁺ Th1, CD4⁺ Th17, CD4⁺ Tregs (regulatory T cells), macrophages, and neutrophils] exert both anti-tumourigenic and pro-tumourigenic effects and that others exert only pro-tumourigenic effects (mast cells, CD4⁺ Th2 cells) but that NK cells lack a protumourigenic effect [3]. DCs found in the TME play an important role in the induction of anti-tumour responses by cross-presenting antigens to CD4⁺ and CD8⁺ T cells [6]. While Tregs normally act against autoimmune diseases by suppressing self-reactive T cells, in the TME, they block anti-tumour responses by suppressing immune cells, such as CD8⁺ T cells, NK cells and DCs, and even participating in metastasis [7]. The depletion of Tregs in tumours by intratumoural NK cells, macrophages and neutrophils swings the immune balance towards a CD8⁺ T cell effector function, resulting in tumour suppression and regression [8]. Therefore, augmenting the anti-tumourigenic effect of CD8⁺ T cells, DCs and NK cells and minimizing the protumourigenic effect from Tregs may serve as potential immunotherapies similar to adoptive cell therapy (ACT). Moreover, the contents of the extracellular matrix (ECM), such as MMPs, prevalently change their activity and show an association with cancer progression and thus serve as potential immunotherapeutic targets [9]. Tumour antigens comprise tumour-associated antigens (TAA) and tumour-specific antigens (TSA), which can be used to specifically detect neoplasms [4]. These antigens, especially TSA, can be harnessed as candidates for tumour-specific antibody treatments, chimeric antigen receptor cell therapies or antibody–drug conjugates to accurately target tumours. Still, there are many sophisticated mechanisms that regulate this process, such as the autocrine effect of T cells, and we should concentrate on the aspect that is helpful to tumours by way of immunotherapy. Initial theories suggested that breast cancer (BC) is a non-immunogenic disease with fewer immunogenic tumour antigens [10], so BC has not been considered a cancer amenable to immunotherapeutic approaches for a long time; however, recent studies have shown evidence of significant immune cell infiltration of tumour-infiltrating lymphocytes (TILs) in a subset of patient tumours and a consolidated understanding that Triple Negative Breast Cancer (TNBC) is a highly heterogeneous breast cancer subtype, with higher expression levels of PD-L1 and more TILs. The TIL score can be a prognostic and predictive marker in standard therapies. High numbers of TILs correlate with increased pathological complete responses to neoadjuvant chemotherapy in TNBC, which demonstrates that the immune system plays an active role in the subgroup of breast cancer [11].

Breast cancer is categorized into two types: non-invasive and invasive. Clinically, BC is also divided into three types: hormone receptor positive BC, human epidermal growth factor receptor 2 (HER2) positive BC and TNBC. As an invasive breast carcinoma, TNBC is defined by the absence of the three main breast cancer biomarkers, namely, the lack of the expression of oestrogen receptors (ERs), progesterone receptors (PRs) and HER2. According to its molecular heterogeneity, TNBC is divided into six subclasses, including basal-like (BL1 and BL2 of basal or myoepithelial origin), mesenchymal-like (M), mesenchymal stem-like (MSL), luminal androgen receptor expression (LAR), immunomodulatory (IM) and an unstable type [12]. Representing 10–20% of breast carcinomas, TNBC has a greater heterogeneity, for which specific therapies have not long been available, and it has a worse survival rate compared to other subtypes [13].

The lack of the identification of driver alterations that can be targeted, such as for traditional anti-Her2 therapy and endocrine therapy, leads to TNBC having a poorer prognosis than other invasive breast cancers [14]. Treatment options for TNBC are at the forefront of clinical research on breast cancer. Currently, TNBC patients undergo combination therapies, consisting of surgery, radiation, chemotherapy, newly developed targeted therapy and immunotherapy. The locoregional treatment of TNBC includes lumpectomy, breast-conserving surgery, total mastectomy and radiation therapy to the whole breast with or without a boost [15]. While some scientists maintain that TNBC requires an aggressive locoregional surgical option, necessarily removing all the breast tissue, emerging studies show that conservation therapy might improve locoregional outcome [16, 17]. Adjuvant/neoadjuvant chemotherapy is the mainstay of treatment for TNBC; the regimens include administering anthracyclines, taxanes, and/or platinum compounds, dose-dense AC (doxorubicin/cyclophosphamide) and TC (docetaxel/cyclophosphamide). Although TNBC is sensitive to chemotherapy, conventional chemotherapy options are far from satisfactory. However, the addition of platinum to standard chemotherapy could increase the proportion of patients achieving a pathologic complete response [18, 19]. Radiation therapy should follow chemotherapy when indicated; this is composed of whole breast radiation, chest wall radiation, regional nodal radiation and accelerated partial breast irradiation [20]. For years, TNBC was not typically considered as a cancer amenable for immunotherapy, until recent studies demonstrated several promising immunotherapeutic agents and the immune signature [21, 22]. TNBC is divided into five immune subtypes based on four distinct expression signatures, including T/B cell, interferon gamma (IFN-γ), transforming growth factor beta (TGF-β), and core-serum response, DCs, and/or macrophages (CSR), namely, T/B-cell/IFN high, IFN/CSR high, CSR high, TGFβ high, and immune low [22], which are the specific molecular signatures for the immune heterogeneity of TNBC and put immunotherapeutic interventions on the table for TNBC.

Currently, immunotherapy is emerging as an exciting treatment option for TNBC patients. In general, cancer immunotherapy encompasses immune checkpoint inhibitors and cytokines, adoptive cell therapy, and cancer vaccines.

Despite the remarkable benefits from the use of checkpoint inhibitors, clinical trials evaluating the use of combinations of checkpoint inhibitors to improve the response rate are now demonstrating that dual application of immune checkpoints inhibitors (blockade of CTLA-4 and PD-1/PD-L1 pathways) is a promising approach in TNBC. Additionally, combination therapy of checkpoint inhibitors with targeted treatments has shown the ability to increase the efficacy of immunotherapy and to slow down primary tumour outgrowth and metastasis, especially in the neoadjuvant background, and may simultaneously improve the tumour-specific T lymphocyte response, which is observed in multiple syngeneic TNBC models [38].

Adoptive cell therapy is a promising and likely potent approach to inducing anti-tumour immune responses via the isolation of highly active and tumour-specific lymphocytes, including TILs, cytotoxic T lymphocytes (CTLs), Th cells, NK and DC cells, large-scale ex vivo expansion and the activation of these lymphocytes for autologous therapy [109, 110]. As the ultimate effector cells, CTLs express a unique T-cell-antigen receptor (TCR) that confers specificity for the particular target antigen. The productive engagement of TCR/MHC/antigen complexes on the target-cell surface triggers the CTL’s effector functions and induces the destruction of the target cell through the release of inflammatory cytokines, including tumour-necrosis factor-α (TNF-α), IFN-γ, FAS ligand (FASL), TNF-related apoptosis-inducing ligand (TRAIL) and cytotoxic degranulation [109]. NK cells play a critical role in cancer immunosurveillance. Therapies utilizing NK cells have shown great promise due to advances in NK cell expansion protocols [111, 112], which are classified as follows: (1) harnessing endogenous responses by NK stimulants or targeting agents and (2) using exogenous NK cells via haematopoietic stem cell transplant (HSCT) or ACT [113]. Clinical studies show that NK cells from BC patients can be expanded and have a high cytotoxic ability to kill breast cancer cells [113, 114]. Until now, ACT has been successfully used for the treatment of patients with metastatic melanoma, leukaemia, and neuroblastoma [115, 116]. Therefore, with host immune environment manipulation, including the pre-administration host immunosuppression and concurrent cytokine administration, with the transferred cells, more novel adoptive cell therapies can be applied in patients with TNBC in the future.

Sipuleucel-T, a personalized treatment working by programming each patient’s immune system, is a therapeutic vaccine for prostate cancer approved by the FDA in April 2010, showing an ability to improve overall survival in patients with castrate-resistant prostate cancer. The MAGE-3 protein-based vaccine is also undergoing phase III clinical trial testing in patients with melanoma and NSCLC [144]. To date, a tremendous number of cancer vaccines, from peptide vaccines such as PPV, to DNA vaccines such as the hDR5 DNA vaccine, and from cytokine vaccines such as combined GM-CSF to Lymphocyte vaccines such as a DC-related vaccine, are carving the way for TNBC treatment.

Overall, it is evident from these studies that immunotherapy is emerging as a novel promising option for TNBC, and there is the possibility of developing exosomes as potential multifunctional therapeutic agents for TNBC. In the future, modern therapeutics needs hold a huge guarantee for the development of affordable, novel and safe antibodies, antibody–drug conjugates, adoptive cell therapies, and cancer vaccines. We expect that exosomes from the tumour microenvironment are likely to become the most effective vaccines for TNBC. With the promising outcomes of immunotherapies, several immunotherapies are being evaluated in phase III trials, which means there would be more therapeutic choices, other than chemotherapy, for TNBC. Immunotherapeutic interventions in TNBC possibly exert a noteworthy effect based on the ongoing clinical trials in the neoadjuvant setting, adjuvant setting and metastatic setting. The synergistic effects and safety concerns of immunotherapy and chemotherapy need be addressed. As it moves towards phase III trials in this field, with a growing interest from pharmaceutical agencies, the final treatment pattern of Triple Negative Breast Cancer will be determined by these ongoing clinical trials, which will lead to a more refined immunotherapy. However, in-depth scientific investigations are required to completely determine the safety and effectiveness of these immunotherapies and open new avenues for the better management of patients with TNBC.