Introduction

Cancer is one of the major health problems worldwide (Siegel, Miller & Jemal, 2019). Each year the number of cancer cases and associated deaths increases worldwide. Nonetheless, despite the increase in cancer incidence over the past 30 years, the number of deaths has fallen short of the estimated values. Several milestones in cancer therapy are at the origin of the steadily increasing survival rates, and of particular interest are the interleukin 2 (IL-2)-based immunotherapy regimens (DeVita & Rosenberg, 2012).
Immunotherapy was recently added as a major cancer treatment approach, along with surgery, radiotherapy, and chemotherapy. In the early 1960s, it became evident that the immune system was able to prevent the onset of several cancers, as pointed out by some ground-breaking experiments (Brondz, 1964; Granger & Weiser, 1964; Hellstrom, Hellstrom, Pierce & Bill, 1968). The discovery of the T cell growth factor in 1976, later renamed IL-2, further stimulated the studies of the immune response of lymphocytes to cancer cells. The seminal work of Rosenberg et al . (Rosenberg et al., 1985) showed that immune modulation could be used to cause the regression of invasive metastases in patients suffering from various types of cancers, including malignant melanoma, colorectal cancer, sarcoma, renal cell cancer adenocarcinoma and oesophageal cancer. This led to the approval of IL-2 for the treatment of renal carcinoma and metastatic melanoma in 1992 and 1998, respectively (Jiang, Zhou & Ren, 2016).
The notion that the immune system could play an important role in cancer treatment led to the development of new antibodies for use as immunomodulatory agents. Patients treated with ipilimumab or rituximab, two monoclonal antibodies designed to promote the immune system response against melanoma and lymphoma, respectively, showed an improvement in overall survival(Hodi et al., 2010; Molina, 2008). These rates increase significantly when these immunomodulators are used in combination with conventional chemotherapeutic regimens (Castro, Sandoval-Sus, Bole, Rassenti & Kipps, 2008) and other immunomodulators (Shanafelt et al., 2019).
The first application of immunomodulators in cancer therapy was the use of IL-2 and interferon-α (IFN-α). IL-2, as mentioned above, was first used in the treatment of metastatic melanoma in 1984. In this pioneering work, one patient was treated with a high dose of Escherichia coli -derived recombinant human IL-2 (rhIL-2). Within two months, all tumours showed extensive necrosis, and were completely eliminated a few months later (Rosenberg, 2014). Following this success, 409 patients with metastatic melanoma and metastatic renal cancer were subject to high dose rhIL-2 regimens. Complete regression was observed in 7% of the melanoma and 9% of the renal carcinoma cases, granting this therapy its approval by the FDA (McDermott & Atkins, 2006). However, it became clear that IL-2 alone was not enough to improve patient survival(Jiang, Zhou & Ren, 2016). Nevertheless, rhIL-2 is currently used, either alone or in combination with other therapeutic agents, in the treatment of a variety of malignancies that include melanoma and renal carcinoma, leukaemia, neuroblastoma and non-small cell lung cancer (Mortara, Balza, Bruno, Poggi, Orecchia & Carnemolla, 2018).
IFN-α, first approved for clinical use in the US in 1986, is another cytokine widely used in cancer treatment. Recombinant human IFN-α type 2b (rhIFN-α2b) is used as a monotherapy or in combination with antineoplastic drugs (Asmana Ningrum, 2014). Like other cytokines, interferons display pleomorphic activities. In the case of IFN-α2b, the anti-cancer activity can be divided into direct and indirect effects. Cancer cell growth is inhibited directly by the induction of cell cycle arrest and apoptosis(Gutterman, 1994). On the other hand, type I interferons, such as IFN-α, have multiple effects on the overall state of the immune system, promoting its activity against cancer cells(Gutterman et al., 1980; Tompkins, 1999).
These discoveries led to the development of new treatments based on the immune system potential to fight cancer. The field of immuno-oncology grew significantly in the last 30 years and today it can be divided into two major strategies: passive and active immunotherapy. Passive immunotherapy comprises the use of antibodies or immunomodulatory molecules, as well as the adoptive transfer of activated immune cells. This last approach includes the isolation and expansion of patient lymphocyte-activated killer (LAK) cells that are primed ex vivowith IL-2 to increase their anti-tumour activities (Nagasawa et al., 2012). This ex vivo priming of lymphocytes results in a population of highly active T cells that can be expanded and then reinfused back into the patient, with significant results in tumour regression (Nagasawa et al., 2012; Tsurushima et al., 1999), and have also been used in a few trials with somewhat encouraging results (Quattrocchi et al., 1999). The concept of isolating tumour-infiltrating lymphocytes (TIL) or circulating T cells for in vitrocultivation, activation and expansion, and subsequent reinfusion, has demonstrated promising results in the treatment of several malignancies, supporting the therapeutic potential of tumour-specific T cells (Sharpe & Mount, 2015). However, TIL isolation requires surgical techniques that may not be possible in patients with visceral tumours in later stages (Fan, Shang, Han, Song, Chen & Yang, 2018), and TAA recognition is MHC-restricted. This means that the anti-tumour activity is only developed against cells presenting foreign antigens bound to MHC molecules. As tumours use immune escape mechanisms based on MHC expression alteration and TAA processing, a fair part of these adoptive cell transfer (ACT) therapies fail to produce significant results. To overcome these problems, genetically engineered T cells have emerged as an alternative (Fousek & Ahmed, 2015).
The branch of active immunotherapy comprises the methodologies that aim to induce a response by stimulating the immune system. This differs from the passive immunotherapy as the molecules/cells used in this case are not produced by the host. In active immunotherapy the immune system is boosted, with either cytokines, immunoadjuvants, or vaccines, to elicit generic or specific responses (Baxter, 2014).
All these techniques have been proven effective in cancer treatment. However, none of these is free from pitfalls. All the immunotherapy regimens currently approved or under study perform extremely well in some cancers and patients but fail completely in others. This was observed early in the immuno-oncology era with IL-2-based treatments that have a response of only ca . 7% (Rosenberg et al., 1985). One drawback is the fact that some patients present little or no immune response to their tumours, with the tumour microenvironment being devoid of infiltrating lymphocytes, rendering cytokine-, antibody-, CTL- and TIL-based therapies useless. This is nowadays recognised as the next big challenge in cancer immunotherapy (Gajewski, 2015).
It is important to mention the economic impact of the available treatments per patient. The cost of standard treatment with one checkpoint inhibitor antibody is around \euro100 000 per year (Fellner, 2012), while a single CAR-T treatment can reach \euro430 000 (Hay & Cheung, 2019). For these reasons, new cost-effective immuno-oncology treatments are necessary to increase the number of patients receiving such regimens and their efficacy. Many authors have focused recently on a subset of lymphocytes – natural killer (NK) cells – that possess the innate ability to detect transformed cells, proposing them as the next “major target in cancer immunotherapy” (Lorenzo-Herrero, Lopez-Soto, Sordo-Bahamonde, Gonzalez-Rodriguez, Vitale & Gonzalez, 2018; Souza-Fonseca-Guimarães, Cursons & Huntington, 2019).