We initially identified a total of 547 papers from database search and 1054 ASCO abstracts. 170 papers were excluded due to duplication. 89 papers and 43 ASCO abstracts fulfilled our inclusion criteria after reading the titles and abstracts. The papers were further assessed for eligibility by reading the full-texts. Information of 43 ASCO abstracts were collected by their clinical trial numbers through Clinicaltrials.gov or Google Scholar. Finally, we included 20 clinical trials17,20,21,22,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42, 17 from database search and 331,35,38 from ASCO abstracts (Fig. 1).

Figure 1 Study flow diagram. Full size image

The characteristics of included 20 trials were listed in Table 1. Four trials32,34,39,42 involved patients with stage IIIA NSCLC, but were restricted to unresectable NSCLC patients. Survival data were extracted from all eligible trials. HRs of overall survival (OS) were reported directly in sixteen trials, but were estimated by median survival time of treatment group and control group for three trials27,40,42. The remaining one trial only provided survival curve41, so HR was calculated through curve data.

Table 1 Characteristics of included trials. Full size table

Therapeutic Vaccines

Etiologically, the infiltration of Treg cells leaded to the immune evasion of tumor cells, which explained the weak immunogenicity of NSCLC43. Tumor vaccines were designed to prompt an immune response to tumor-associated antigens through active immunization with either whole-cell or antigen-specific vaccines44.

Belagenpumatucel-L was an allogeneic tumor cell vaccine that were transfected with a TGF-β2 antisense plasmid. Giaccone et al.’s study34 found no differences on overall survival between belagenpumatucel-L and placebo in the ITT analysis. But a prespecified COX regression analysis suggested that early enrollment after first-line therapy and prior chemoradiation were positive prognostic factors that favored belagenpumatucel-L. In this trial, injection site reaction, induration and erythema were more frequent in belagenpumatucel-L group, but they were all classified as grade 1 or 2 AEs. These safety results were consistent with previous study45 and indicated the well-tolerance of belagenpumatucel-L.

Tecemotide (L-BLP25) and TG4010 were antigen-specific vaccines designed for inducing a T-cell response to aberrant MUC1 protein. Several RCTs have reported that the vaccines improved overall survival compared with control group29,30,32,35, but neither of them reached significant level in the ITT analyses. Subgroup analyses stratified by patients’ biological status (pretreatment or histopathology) provided information for the patients’ screening for immunotherapy. Safety results were considerable that grade 3 or 4 AEs or SAEs were similar between experimental group and control group. Pneumonia and dyspnea were slightly more common in patients with L-BLP25 than placebo32. In patients with a high percentage of CD16 + CD56 + CD69 + lymphocytes at baseline, there was a significantly higher incidence of serious adverse events in the TG4010 group (15 of 21) than in the chemotherapy alone group (5 of 16)30.

Epidermal Growth Factor (EGF) promoted tumor cell proliferation and survival upon binding to its receptor46. The EGF vaccine (CIMAVax) can simulate an antibody-mediated immune response against EGF ligand. A phase II randomized controlled trial27 involving 80 patients demonstrated a remarkable improvement in survival for patients with substantial immunological response when compared with BSC group (11.7 months vs. 5.33 months, p = 0.002). But in the ITT population, the difference was not significant which may result from the small sample size. The vaccine was very well tolerated that no grade 3 or 4 AEs or SAEs were attributed to the study drug among three trials27,46,47. The most common AEs included chills, fever, injection-sit pain, nausea and vomiting.

There were also RCTs exploring other types of vaccines for advanced NSCLC. Racotumomab-alum was an anti-idiotype vaccine mimicking the NeuGcGM3 tumor-associated ganglioside33. Bavituximab was a novel monoclonal antibody that targetd phosphatidylserine (PS), binding PS to simulate an immune response31. Both of the drugs were reported to be safe and well-tolerated. PF-3512676 was a synthetic TLR9-activating oligo deoxy nucleotide that mimic the natural ligand of TLR9, thereby inducing a cascade of immune reactions and potentially promoting an antitumor immune response28. However, Manegold et al.’s study suggested that grade 3 or 4 hematological AEs were more frequent in the PF-3512676 plus chemotherapy arm than chemotherapy-alone arm. But these issues did not result in any clinically significant sequelae. Efficacy results of these trials were showed in Fig. 2.

Figure 2 Forest plot of overall survival in advanced NSCLC patients who received therapeutic vaccines with or without chemotherapy compared to control therapies. I: immunotherapy; C: chemotherapy; P: placebo. Full size image

In pooled analyses, therapeutic vaccines significantly improved survival as compared with placebo, with an HR of 0.81 (95%CI, 0.71 to 0.91). Results were similar for chemotherapy combined vaccines versus chemotherapy alone (Fig. 2; HR, 0.76; 95%CI, 0.60 to 0.92). Neutropenia and thrombocytopenia were more common in the experimental group, while other types of AEs were similar between experimental group and control group (Table 2). There were no differences in incidence of grade ≥3 AEs or SAEs between two groups.

Table 2 Comparative adverse events (grade ≥3) of experimental group versus control group. Full size table

Immune Checkpoint Inhibitors

Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) was a molecular that had the same ligands as CD28. It inhibited the activation of cytotoxic T-cells by interfering the action of CD28 after antigen presentation48. Ipilimumab was a monoclonal antibody targeting CTLA-4, thus enhanced the T cells response. Lynch et al.’s study36 demonstrated a trend of survival improvement among phased ipilimumab arm (two doses of placebo plus paclitaxel and carboplatin followed by four doses of ipilimumab plus paclitaxel and carboplatin) when compared with control arm (six cycles of paclitaxel, carboplatin and placebo). Subgroup analyses indicated that phased ipilimumab appeared to show improved efficacy for squamous histology (HR, 0.48; 95%CI, 0.22 to 1.03). Grade 3 rash, diarrhea and colitis were noted in phased ipilimumab arm or concurrent ipilimumab arm. The incidence of grade 3 or 4 immune-related adverse events was higher in patients receiving ipilimumab (15%, 20% and 6% for phased ipilimumab, concurrent ipilimumab and the control, respectively).

Programmed cell death 1(PD-1) was a co-inhibitory molecule receptor expressed by activated T cells and its ligands PD-L1 was expressed on tumor cells as well as stromal cells. PD-1/PD-L1 pathway played an important role in immune-mediated tolerance of NSCLC49. Nivolumab (also known as BMS-936558) was a human IgG4 anti-PD-1 antibody. It can enhance the cytotoxic activity of T lymphocytes by blocking the ligand activation of PD-1. Two phase III trials17,37 had demonstrated a remarkable efficacy of nivolumab as compared with docetaxel in patients with advanced squamous or non-squamous NSCLC (Fig. 3). Fatigue, decreased appetite and asthenia were the most frequently reported treatment-related AEs in both studies. Nivolumab had a better tolerance than docetaxel. In Brahmer et al.’s study, grade 3 or 4 treatment-related AEs occurred in 7% of patients received nivolumab and in 55% of patients received docetaxel. Similarly, 10% and 54% of patients experienced grade 3 or 4 treatment-related AEs in Borghaei et al.’s study respectively. Drug-related adverse events of special interest such as hypothyroidism and pneumonitis were observed in both trials. As mentioned in a prior phase I study, these adverse events were regarded to have potential immune-related causes50. Notably, the correlation between PD-L1 expression and survival were inconsistent within two studies, which may due to the different histopathology of NSCLC37.

Figure 3 Forest plot of overall survival in advanced NSCLC patients who received immune checkpoint inhibitors with or without chemotherapy compared to control therapies. I: immunotherapy; C: chemotherapy; P: placebo. Full size image

MPDL3280A was another engineered monoclonal antibody of IgG1 isotype against PD-1. Results of an interim analysis of a randomized phase II study of MPDL3280A compared with docetaxel in patients with locally advanced or metastatic NSCLC had been presented at ASCO annual meeting in 201538. In this study, improved efficacy of MPDL3280A was observed among patients with increasing PD-L1 expression (HR, 0.63; 95%CI, 0.42–0.95), but not for patients with the lowest PD-L1 levels (HR, 1.22; 95%CI, 0.69–2.14). This agent was more tolerable than docetaxel in this trial. 43% of patients in MPDL3280A arm and 54% of patients in docetaxel arm experienced grade ≥3 AEs.

In pooled analyses, anti-PD-1 antibodies achieved inspiring improvement on survival (HR, 0.69; 95%CI, 0.59 to 0.80) in patients with advanced NSCLC when compared with docetaxel (Fig. 3). Immune checkpoint inhibitors were well-tolerated. Grade 3 or 4 hematological adverse events were less frequent in experimental group while there were no differences among diarrhea, nausea and vomiting (Table 2).

Other Immunomodulators and Cellular Therapy

Talactoferrin was an orally immunomodulatory protein that interacted with gut-associated lymphoid tissue, prompting the maturation of dendritic cells and thus simulated a strong anti-tumor immune response20. Two randomized phase II study had showed a promising efficacy of talactoferrin in advanced NSCLC patients20,21. But in the phase III FORTIS-M trial, no differences on overall survival were identified between talactoferrin and placebo22. Ramalingam et al. pointed out that the variety of patients’ pretreatment and population could potentially have impacted the inconsistent outcomes. The safety and tolerability of talactoferrin were verified by these trials. No drug-related SAEs were reported.

Interleukin 2 (IL-2) was a type of cytokine that regulated the activities of lymphocytes. Tumor-induced immunosuppressive phenomena were reversible in vitro by the addition of exogenous IL-240. A phase III randomized multicenter trial40 comparing chemotherapy with or without low dose IL-2 in patients with advanced NSCLC failed to demonstrate any survival benefits of IL-2. In this study, more patients experienced grade 4 AEs in chemotherapy plus IL-2 group than chemotherapy alone group (50 vs. 27).

SRL172 was a suspension of killed Mycobacterium vaccae. It can activate antigen-presenting cells and natural killer cells as well as suppress the activation of Treg cells. O’Brien et al.’s study39 found no statistical difference in overall survival between the chemotherapy plus SRL172 group and the chemotherapy alone group. But quality of life was higher in patients received chemotherapy plus SRL172. Treatment-related SAEs were more frequent in the chemotherapy plus SRL172 group (106/210 patients) than the chemotherapy alone group (80/209 patients).

Pooled analysis suggested that there were no significant improvements in overall survival for these immunomodulators, either combined with chemotherapy or used as monotherapy (Fig. 4). In general, Episodes of grade ≥3 AEs were similar between experimental groups and control groups except that thrombocytopenia were more common in patients received immunomodulators (Table 2).

Figure 4 Forest plot of overall survival in advanced NSCLC patients who received other immunomodulators with or without chemotherapy compared to control therapies. I: immunotherapy; C: chemotherapy; P: placebo. Full size image

Cytokine-induced killer (CIK) cells were a group of immune effector cells that can recognize malignant cells in the absence of major histocompatibility complex (MHC), allowing for a fast and unbiased immune reaction. Dendritic cells co-cultured with CIK cells (DC-CIK) can cause changes in the surface molecule expression of both population, thus leading to an improved cytotoxic activity51. Zhong et al.’s study41 demonstrated a trend of improved overall survival in chemotherapy plus DC-CIK group than chemotherapy alone group (HR≈0.87; log-rank p value = 0.18), while Wu et al.42 had indicated that the addition of CIK cells to chemotherapy statistically prolonged patients’ survival (median OS, 15 months versus 11 months; log-rank p value = 0.029). Non-infectious fever was mentioned in these two trials to be more frequent in the experimental groups. No treatment-related SAEs were reported.

Sensitivity analysis

To assess the robustness of our results, sensitivity analysis was conducted to evaluate the influence of uncertain factors. When trials with high risk of bias were excluded in pooled analysis, the survival benefits of therapeutic vaccines and immune checkpoint inhibitors were still detected (Table 3).

Table 3 Sensitivity analyses of efficacy of immunotherapy in patients with advanced NSCLC. Full size table

Two phase III study of PF-3512676 (NCT00254904 and NCT00254891) had been terminated because of safety issues potentially related to the investigational drug. When Manegold et al.’s study was excluded, the differences of incidences of AEs tended to be non-significant.