Before 1990 there was no effective, tolerated chemotherapy for mesothelioma. Some agents, such as anthracyclines, were used in selected patients but the response rates were so low and the side effects so disabling that many centers, including our own, opted solely for palliation for mesothelioma patients (Krarup-Hansen and Hansen, 1991). Indeed, there was substantial pessimism in the field of oncology that any chemotherapy would be effective in mesothelioma (Lee et al., 2002); it seemed to be a totally resistant cancer. With that in mind we used the murine mesothelioma model to test the efficacy of a number of, as yet, untrialled chemotherapies including the gemcitabine-cisplatin combination that was being used at that time in lung cancer (Nowak et al., 2002a; Nowak et al., 2003a), as well as cyclophosphamide (van der Most et al., 2009) and, over the years, a variety of other chemotherapeutic agents (Aston et al., 2017).

In our early study we showed that gemcitabine-cisplatin combination could successfully reduce the growth of both small and large mesothelioma tumors growing in mice (Nowak et al., 2002a) (Figure 1). These encouraging results led us to test these agents in a series of clinical trials.

In the original single centre study of 21 patients, an objective partial response rate of 48% was seen with symptom improvement in 57% of patients receiving intravenous cisplatin and gemcitabine (Byrne et al., 1999). A subsequent multicenter phase II study showed a partial response rate of 33% and stable disease in 60%, with a median survival of 17.3 months (Nowak et al., 2002b). This combination was established as standard clinical care practice (Nowak et al., 2004), one of the first chemotherapies for this disease to produce such good response rates. Not only did data generated in the mouse model lead to a change in clinical practice, they revealed previously poorly understood effects of chemotherapy on the immune system that continue to underpin current novel chemo-immunotherapy trials (Nowak et al., 2002a).

In order to understand if and how mesothelioma tumors engaged with the host immune system, something that was unknown at the time, we needed to create ways of tracking specific anti-mesothelioma immunity. As there were no mesothelioma antigens known at that time, we created mesothelioma cell lines that carried a stably transfected “nominal” antigen (a viral antigen that could be the target of immune attack target), and combined this with the adoptive transfer of T-cell receptor transgenic lymphocytes with specificity for this “neoantigen” (the equivalent to a T-cell monoclonal antibody) to track and explore if, and where, mesothelioma antigens were presented to the immune system (Marzo et al., 1999a).

We initially turned our attention to the lymph nodes that drain the tumor, where tumor-specific T cells are educated and activated. Somewhat to our surprise, mesothelioma antigens readily reached these tumor draining lymph nodes (dLN) and were presented to the immune system there (a process known as “cross presentation”) (Marzo et al., 1999b). This process proved to be highly efficient for all antigens tested (Kurts et al., 2010), occurred within 1–2 weeks of the commencement of tumor growth (Marzo et al., 1999a; Marzo et al., 1999b; Robinson et al., 1999) and was highly sensitive, requiring only 200 nmol of tumor neoantigen within the tumor mass to be “visible” to the host immune system (Anyaegbu et al., 2014) (Figure 2). These were the first studies to disprove the hypothesis that tumors grew in hosts because the immune system was ignorant of their presence. These studies also helped us to understand the role of CD4 help in the “post-licensing” of mesothelioma-specific CD8 T cells, a process that is critical in helping CD8 T cells enter the tumor and induce rejection (Marzo et al., 2000).

We then turned our attention to the tumor itself, where tumor-specific T cells need to enter tumors then attack and destroy the tumor cells (McDonnell et al., 2010). We showed that dendritic cells within the tumor, which may be required to restimulate T cells as they enter this site, were, in contrast, to those in tumor dLN, completely unable to present mesothelioma neoantigens to T cells (McDonnell et al., 2015a). Importantly, chemotherapy that destroys tumor cells delivers antigens to the dendritic cells within the tumor and reverses their inability to cross present neoantigen. This enhanced neoantigen presentation within the tumor allows for T-cell restimulation and enhanced anti-tumor effects (Kurts et al., 2010; McDonnell et al., 2015b). These and other ongoing studies of the relationship between mesothelioma and the immune system continue to drive us forward to test novel therapies for mesothelioma, and it has underpinned many of the novel clinical trials described below.

Having shown that the host immune system is not ignorant of the tumor, we began to test different immunotherapeutic strategies to bolster the anti-mesothelioma immune response with the goal of developing new treatment approaches (Robinson et al., 1993). These original immunotherapeutic strategies involved the administration to mesothelioma-bearing animals of systemic cytokines, such as recombinant interferon alpha (IFNα) (Bielefeldt-Ohmann et al., 1995) and also activating anti-CD40 antibodies (Stumbles et al., 2004; Khong et al., 2012). The latter agent being shown to ligate CD40 and activate dendritic cells. These studies confirmed that mesothelioma could be effectively controlled if the immune system was appropriately engaged, at least by the cytokine IFNα and by anti-CD40, which induced cures of established mesothelioma tumors in the majority of mice studied (van der Most et al., 2005).

One of the many attractive features of this mouse model is the capacity to genetically manipulate the tumor cells prior to their injection, in order to dissect the anti-mesothelioma immune response. We began by transfecting mesothelioma cell lines to express foreign transplantation (“allo”) antigens to make the mesothelioma cells look like a foreign organ transplant and be rejected, in the process stimulating a broader immune response against the untransfected parental tumor. This proved especially useful in mesothelioma lines that were otherwise ‘non-immunogenic’ (Leong et al., 1994). The expression of allogeneic MHC molecules in a highly immunosuppressive, non-immunogenic murine malignant mesothelioma cell line did led to tumor rejection, however this did not generate systemic immunity sufficient to protect from the parental cell line (Leong et al., 1994).

Transfecting these non-immunogenic mesothelioma cell lines with the immune co-stimulatory molecules B7.1 (CD80) but not B7.2 (CD86) induced and maintained a T-cell response to these mesothelioma tumors, but the response required ongoing expression of B7-1 and the upregulation of major histocompatibility complex class II expression and did not protect against the parental cell line (Leong et al., 1995; Leong et al., 1996; Marzo et al., 1997). Transfection of cytokines such as interleukin-2 (Leong et al., 1997) generated in vivo immunity to the mesothelioma tumor that was relatively weak and/or subject to down-regulation so that consistent rejection of unmodified parental tumor cells was not achieved. Transfection of both of the interleukin-12 genes generated a higher level of immunogencity which could control tumor growth (Caminschi et al., 1998). We showed that when cytokine genes were delivered using a vaccinia virus rather than gene transfection the growth of advanced mesothelioma tumors could also be controlled but not eradicated (Upham et al., 1995). These studies enabled us to gain an improved understanding of how a host responds, or fails to respond, to mesothelioma (Robinson et al., 1998; Nowak et al., 2002c; Nelson et al., 2005; van der Most et al., 2006a).

We have tried a number of loco-regional approaches to mesothelioma immunotherapy using these inbred mouse mesothelioma models. Because the immunosuppressive molecule TGF-β was known to be produced by mouse, as well as human mesothelioma tumor cells, we tested the efficacy of locally administered anti-sense oligonucleotides (ASONs) targeting TGF-β to reverse the immunosuppressive environment of mesothelioma. We showed that intratumoral delivery of these agents resulted in the accumulation of T cells in the tumor and the slowing of mesothelioma tumor growth (Fitzpatrick et al., 1994; Marzo et al., 1997). Intratumoral delivery of the cytokine interleukin-2, had little effect by itself (Jackaman et al., 2003) but when combined with anti-CD40, caused local regression and, importantly regression of a distal untreated tumor (Jackaman et al., 2012), possibly due to the induction of inflammation within the tumor (Jackaman et al., 2008). Similar results were observed when agents such as toll-like receptor (TLR) seven agonists were delivered intratumorally—they had little affect alone but were synergistic with anti-CD40 (Currie et al., 2008; Broomfield et al., 2009). These novel loco-regional therapies provide a foundation for novel approaches to mesothelioma because of the accessibility of the tumor and are summarized in (Nelson et al., 2006) and (Nelson et al., 2014).

One of our earliest observations was that the adoptive transfer of large numbers of mesothelioma-specific T cells, CD4 and/or CD8, did not result in eradication of an established tumor (Marzo et al., 1999a), raising concerns for us that any such therapy alone, such as modern CAR-T-cell therapy, may not be effective in solid tumors unless the obstacles of effector site resistance to immune attack could be overcome, something being studied in a number of centers.

Our first chemo-immunotherapy trial using Adriamycin and IFNα was disappointing, with the chemotherapy adding nothing to the immunotherapy except severe side effects (Upham et al., 1993). However, the results of subsequent preclinical experiments led directly to the establishment of a clinical trial using activating anti-CD40 with chemotherapy in patients with mesothelioma (Nowak et al., 2015). Anti-CD40 with cisplatin-pemetrexed chemotherapy was safe and tolerable (despite most patients experiencing cytokine release syndrome). Objective response rates were similar to chemotherapy alone however 20% of patients achieved long-term survival.

A subsequent trial, the DREAM trial, used the chemotherapies, cisplatin and pemetrexed plus an anti-PD-L1 antibody durvalumab and demonstrated promising activity and an acceptable safety profile as a platform for a current, on-going, phase III study (Nowak et al., 2020).