Potential immune escape mechanisms underlying the distinct clinical outcome of immune checkpoint blockades in small cell lung cancer

First, Reck et al. conducted a randomized phase II trial to investigate ipilimumab in combination with chemotherapy in previously untreated patients with lung cancer (ED-SCLC, n = 130; NSCLC, n = 204) [48, 49]. For both the NSCLC and SCLC cohorts, phased ipilimumab with carboplatin and paclitaxel but not a concurrent regimen showed improved immune-related (ir) progression-free survival (PFS) and a numeric, though not statistically significant, increase in median overall survival (OS) over chemotherapy alone. Overall grade 3-4 (G3-4) immune-related adverse events (irAEs) were more common for phased ipilimumab. Later, the phase III CA184-156 study further evaluated the efficacy of phased ipilimumab with etoposide and platinum as the first-line regimen for ED-SCLC [51]. Disappointingly, phased ipilimumab did not significantly prolong PFS and OS over the placebo and produced more irAEs. The rate of treatment-related discontinuation was even higher (18% v 2%). Similar results were observed in patients with advanced squamous NSCLC [50]. Another phase II trial involving phased ipilimumab to first-line chemotherapy even reported a G3-5 irAE rate as high as 69.2% with 5 of 42 patients dying [67]. The unfavorable benefit to risk profile limits the first-line application of ipilimamab in lung cancer.

The addition of atezolizumab to chemotherapy as the first-line treatment significantly improved the ORR and PFS among patients with metastatic nonsquamous and squamous NSCLC in IMpower 131 and IMpower 132 [52, 54]. Among patients with metastatic nonsquamous NSCLC, atezolizumab plus bevacizumab and chemotherapy significantly improved PFS and OS in IMpower 150, which led to FDA approval of the combined regimen for the first-line treatment of nonsquamous NSCLC [55]. For ED-SCLC, the clinical efficacy and safety of atezolizumab plus chemotherapy was evaluated in IMpower133 [53]. The interim analysis of the intention-to-treat population in this phase III trial showed that the addition of atezolizumab to standard chemotherapy significantly prolonged OS and PFS over the placebo (median OS, 12.3 v 10.3 months and median PFS, 5.2 v 4.3 months) and increased the 1-year OS by 13.5% (51.7% v 38.2%). The safety profile was consistent with previous observations. In March 2019, this combined regimen was approved as a first-line therapy for ED-SCLC. Overall, atezolizumab in combination with chemotherapy as a first-line treatment could be a novel option for people with advanced NSCLC and ED-SCLC.

Pembrolizumab in combination with standard chemotherapy resulted in significantly prolonged OS and PFS compared with chemotherapy alone among patients with squamous and nonsquamous NSCLC in KEYNOTE-407 and KEYNOTE-189 [68, 69]. However, the attempt to use pembrolizumab in the first-line setting for SCLC is limited. Only the phase Ib PembroPlus trial indicated that the standard dose pembrolizumab could be safely combined with many chemotherapy regimens across 6 advanced solid tumors, including relapsed SCLC [70]. A randomized phase III trial, KEYNOTE-604, is underway to assess the clinical benefit of pembrolizumab in combination with etoposide and platinum as a first-line treatment for ES-SCLC (Table 2).

As shown in Table 1, the second-line nivolumab monotherapy significantly improved ORR, PFS, and OS compared with docetaxel among patients with advanced squamous and nonsquamous NSCLC in CheckMate 017 and CheckMate 057 [56, 58]. The response rate to nivolumab monotherapy was approximately twice that of docetaxel (20% v 10%), and nivolumab extended OS by approximately 3 months over chemotherapy. For SCLC, in the nonrandomized cohort in CheckMate 032 [57], the ORR was 10% (10 of 98) and 23% (14 of 61), and the median OS was 4.4 and 7.7 months for patients receiving nivolumab 3 mg/kg and nivolumab 1 mg/kg plus ipilimumab 3 mg/kg, respectively. One-year OS was 33% and 43% for the two groups, respectively. Based on this trial, nivolumab and nivolumab plus ipilimumab were added as category 2A recommendations to the NCCN guidelines [11]. In August 2018, under accelerated approval, FDA approved nivolumab for treating patients with relapsed SCLC after the failure of platinum-based chemotherapy and one or more other lines of treatment. Unfortunately, CheckMate 331, a randomized phase III trial, demonstrated that nivolumab was inferior to topotecan or amrubicin in improving ORR, PFS, and OS among patients with relapsed SCLC [59].

Based on KEYNOTE-010, pembrolizumab was approved as a second-line treatment for advanced NSCLC patients with PD-L1 expression on ≥ 1% of tumor cells [60]. The phase Ib KEYNOTE-028 trial showed favorable efficacy and tolerable safety of pembrolizumab in treating patients with relapsed ED-SCLC and PD-L1 expression on ≥ 1% of tumor and stromal cells [71]. Further, the phase II KEYNOTE-158 trial confirmed the beneficial role of pembrolizumab in treating SCLC [72]. The latest results of KEYNOTE-028 and KEYNOTE-158 from 2019 from the American Association for Cancer Research (AACR) showed that pembrolizumab produced a durable response with tolerable toxicity for advanced SCLC patients after ≥ 2 lines of prior therapy. The ORR was 19.6% (16 of 83), with 2 patients having a complete response (CR) and 14 having a partial response (PR). More than half (9 of 16) had a response duration of ≥ 18 months. The median PFS was 2.0 months, and the median OS was 7.7 months, with a 1-year OS rate of 20.7%. The toxicity was manageable, with a G3-5 AE incidence of 9% [61]. Despite the encouraging results of single-arm studies, large randomized controlled studies are needed.

Atezolizumab also significantly improved OS by 3 to 4 months over docetaxel in patients with previously treated NSCLC in POPLAR and OAK [62, 64]. Unfortunately, the randomized phase II IFCT-1603 trial indicated that atezolizumab was not superior to chemotherapy as a second-line treatment in SCLC [63].

Durvalumab, an anti-PD-L1 antibody, has significantly prolonged PFS by more than twofold compared with the placebo (17.2 v 5.6 months, HR = 0.51) among patients with unresectable stage III NSCLC without disease progression after concurrent chemoradiotherapy in PACIFIC [65]. For patients with SCLC, the clinical outcome of ICBs as a maintenance regimen is quite dissatisfactory. A single-group, phase II study showed that maintenance pembrolizumab did not prolong OS compared with historical chemotherapy after first-line chemotherapy in patients with ED-SCLC [66]. In November 2018, Bristol-Myers Squibb announced that the phase III CheckMate 451 trial did not meet the primary endpoint of OS with nivolumab and nivolumab plus ipilimumab versus the placebo as maintenance therapy in patients with ED-SCLC.

In conclusion, anti-PD-1/PD-L1 blockades have revolutionized the standard treatment for NSCLC across the first-line, second-line, and maintenance settings based on multiple large randomized controlled trials (Table 1). However, for SCLC, despite moderate benefit in phase I/II trials, ICBs did not outcompete traditional chemotherapy in large randomized trials. Only atezolizumab in combination with chemotherapy conferred a survival benefit in CheckMate 331. Table 2 lists the ongoing trials involving ICBs or combination regimens in SCLC.

PD-L1 expression reflects pre-existing antitumor immunity in the TME and has been associated with a clinical benefit from anti-PD-1/PD-L1 blockade across multiple cancer types, including NSCLC [73‐75]. PD-L1 expression in NSCLC is approximately 50–70% according to previous reports [55, 56, 58, 62, 64, 68, 69, 76]. In stark contrast to NSCLC, PD-L1 expression in SCLC has been reported to be relatively low in most studies, as listed in Table 3. Most studies showed less than 50% PD-L1 expression in SCLC. The reason for the disparity in PD-L1 expression in these findings is not well understood and may be explained by the differences in staining antibodies, scoring algorithms, tissue biopsy types (archival or fresh), and detection platforms (Table 3). Notably, substantial PD-L1 expression occurs on stroma cells, including tumor-infiltrating immune cells (TILs and TIMs), in SCLC and less in tumor cells. Some of these studies demonstrated a positive correlation between PD-L1 expression on TICs and a favorable clinical outcome for SCLC patients [41, 66, 77, 80, 81]. Emerging data indicated that infiltrating stroma cells, such as dendritic cells and macrophages, could have protumorigenic functions by shaping antitumor immunity and the response to immunotherapy [84‐86]. Perhaps PD-L1 expression in immune cells may play a exceptional role in the pathophysiological process in SCLC. Overall, the relatively low PD-L1 expression may be at least one reason that the efficacy of ICBs in SCLC is not as good as that in NSCLC.

The downregulation of MHC molecules is an immune escape mechanism. In contrast to NSCLC cells, which readily express MHC class I molecules, most SCLC cell lines and tissues showed marked deficient expression of MHC class I molecules, thus directly preventing tumor cells from presenting neoantigens to CD8+ T cells in the lymph nodes and inhibiting CTL recognition in the TME [87, 88]. Additionally, due to the lack of IFN-γ inducible expression of class II transactivator (cIITA), MHC class II molecule expression is absent in SCLC cells and significantly lower in SCLC TILs compared with in NSCLC, resulting in reduced presentation of tumor neoantigens to CD4+ T cells. In addition, mainly induced by IFN-γ-producing T cells, MHC class II molecules also reflect a less immunogenic environment in SCLC compared with NSCLC [89‐91].

The interaction between SCLC cells and the immune system seems to be more complicated than previously thought. Tumors may destroy the host immune system in a variety of ways. The functions of the immune system, including lymphocyte reactivity to lectins and cytokines and the production of cytokines, were even more impaired in patients with SCLC compared with patients with NSCLC. The peripheral blood lymphocytes (PBLs) of SCLC patients showed significantly lower proliferative responses to phytohemagglutinin and human recombinant interleukin 2 (IL-2) than those of the NSCLC and noncancer groups. The ability of PBL to produce lymphokines (IL-2 and macrophage-activating factor) was evidently impaired in the SCLC group but not in the NSCLC group [92]. In contrast to NSCLC, the suppression of cytokine secretion in SCLC was dependent on tumor load and could be improved upon the reduction of the tumor load [93]. In addition, Wang et al. found that IL-15 secreted by SCLC cells could contribute to local and systemic immune escape and poor prognosis by inhibiting CD4+ T cell proliferation and supporting Treg cell induction [47]. Moreover, CD47 mainly functions to inhibit the activation and phagocytosis of macrophages through CD47/SIRPα signaling[94]. The remarkable upregulation of CD47 on the SCLC cell surface may be another important immune escape mechanism by inhibiting the activation and phagocytosis of macrophages. Preliminary experiments suggested that blocking CD47 induced macrophage-mediated phagocytosis in SCLC cell lines and mouse xenograft models [95]. Dysregulation of the Fas/FasL signaling pathway in tumors by reducing Fas expression and increasing FasL expression could participate in tumor development and immune escape [96, 97]. Fas expression is markedly decreased and even completely lost in lung tumors. However, FasL expression is decreased in most NSCLCs but is upregulated in 91% of SCLCs. FasL overexpression in the context of Fas downregulation in SCLC confers the ability on SCLC cells of inducing paracrine killing of Fas-expressing cytotoxic T cells and inhibiting self-apoptosis. These results indicated that the heavy tumor burden in SCLC makes the immune system less functional than in NSCLC through IL-15 secretion and the CD47/SIRPα and Fas/FasL pathways.

A distinguishing feature of SCLC is the substantial autocrine and paracrine stimulation by growth factor receptors and chemokine receptors. Bombesin/gastrin-releasing peptide (BN/GRP) is a relevant growth factor in SCLC, and its receptor in overexpressed in SCLC [98]. Compared with NSCLC, which does not secrete autocrine growth factor granulocyte chemotactic protein (GCP-2), SCLC constantly secretes GCP-2 and expresses its receptors, CXCR1 and CXCR2 [99]. Stem cell factor (SCF) and its receptor c-kit, expressed in 40–70% SCLC specimens, are important regulators of SCLC viability [100]. In contrast to NSCLC cell lines, SCLC cell lines do not express intercellular adhesion molecule-1 (ICAM-1), which is involved in the interaction with lymphokine-activated killer cells [101]. IL-8 acts as an autocrine and/or paracrine growth factor for lung cancer, mediated by CXCR1 and CXCR2 receptors on the tumor cell surface. However, NSCLC cells produce much higher levels of IL-8 than SCLC cells. As a result, IL-8 may attract neutrophils and induce the immune response in NSCLC but mainly promote autocrine growth in SCLC [102, 103]. Lopez-Gonzalez et al. found that NSCLC and SCLC cell lines had diverse expression levels of TGF-β and its receptor [104]. NSCLC synthesizes both TGF-β isoforms and TGF-β RII. In stark contrast, malignant transformation in SCLC alters the synthesis of TGF-β isoforms and TGF-β RII, thereby avoiding autocrine and paracrine growth inhibition by TGF-β in SCLC cells. Another study found that four to eight SCLC cell lines could constantly secrete biologically active TGF-β1 to suppress IL-2-dependent T cell growth, and a specific anti-TGF-β1 antibody blocked the immunosuppressive activity induced by the SCLC cell lines [105]. These studies suggested that SCLC cells could markedly promote self-growth via autocrine and paracrine regulation but had less immune stimulatory function in adjacent immune cells compared with NSCLC cells.

These comparative analyses of immune regulation patterns between SCLC and NSCLC partially explain the immunodeficiency observed in patients with SCLC and the worse immune response in SCLC patients. Detailed investigations are essential to determine the exact immune escape mechanisms in SCLC to support the development of immunotherapy for SCLC.

The intrinsic nature of ED-SCLC is so aggressive that a great proportion of patients dropped out or discontinued without completing the whole treatment course [51]. As illustrated in part 3, SCLC cells can induce immunosuppression and promote autocrine and paracrine regulation. Immune suppression seems to be associated with tumor load and could improve upon the reduction of the tumor load [93]. Chemotherapy and radiotherapy can not only reduce the heavy tumor burden of SCLC and further reinvigorate immune function but can also elevate PD-L1 expression and tumor antigen presentation by MHC molecules, thus favoring subsequent immunotherapy [106, 107]. We speculate that chemotherapy and radiotherapy prior to ICBs could be effective in improving the clinical outcome of immunotherapy for SCLC.

Angiogenesis is one of the hallmarks of cancer [108]. Anlotinib, an oral anti-angiogenesis tyrosine multikinase inhibitor, is an approved third-line or later therapy for advanced NSCLC according to the Chinese Food and Drug Administration (CFDA), based on the ALTER 0303 study [109]. ALTER 1202, a phase II trial, has also demonstrated that anlotinib significantly improves OS as a third-line or later treatment [110]. Combination therapy with antiangiogenic therapy is a new paradigm in the treatment of advanced-stage malignancies via normalizing tumor vasculature and increasing drug delivery [111, 112]. Vascular endothelial growth factor (VEGF), the major regulator in tumor angiogenesis, plays an important role in immune modulation by blocking dendritic cell (DC) differentiation, reducing T cell infiltration, inducing inhibitory immune cells (Tregs and MDSCs) and inhibiting T cell development [112, 113]. Theoretically, combining immunotherapy with antiangiogenic treatment may have a synergistic effect, enhance the efficacy of both. Atezolizumab in combination with bevacizumab and chemotherapy significantly improved OS among patients with metastatic nonsquamous NSCLC in IMpower 150 [55]. The combination of ICBs with antiangiogenic therapy could also be promising in SCLC.

Compared with NSCLC, PD-L1 expression is relatively low in SCLC. In NSCLC, multiple clinical trials have defined PD-L1 as a biomarker for pembrolizumab, and the FDA-approved IHC assay utilized cutoffs of a 50% tumor proportion score for first-line and 1% for second-line pembrolizumab therapy [114]. Although PD-L1 expression was associated with a survival benefit in SCLC as indicated in early phase studies [66, 71, 72], it is not a perfect biomarker in SCLC. TMB, another biomarker of immunotherapy, could also predict the clinical outcome of ICBs based on exploratory analyses of CheckMate 026 [39] and CheckMate 227 [76] in NSCLC. In SCLC, Hellmann et al. retrospectively evaluated 211 TMB-evaluable patients from the total nonrandomized and randomized cohorts of CheckMate 032 [33]. For patients receiving nivolumab monotherapy and nivolumab plus ipilizumab, the ORR was higher in patients with a high TMB (21.3% and 46.2%) than in patients with a medium (6.8% and 16.0%) or low (4.8% and 22.2%) TMB. The OS for SCLC patients with a high TMB treated with nivolumab plus ipilimumab (22 months) was nearly three times that (6–8 months) achieved by the historical topotecan chemotherapy [16, 17]. Additionally, there was no association between PD-L1 expression and TMB in lung cancer [33, 39, 115]. Patients receiving nivolumab with a high level of these two biomarkers had a higher ORR than those with only one of these factors in CheckMate 026 [39]. Up to now, there are no exact biomarkers for immunotherapy in SCLC, TMB seemed to be more important than PD-L1 expression and perhaps the combination of the two biomarkers will be more valuable for selecting patients who will benefit from ICBs.