All data were double entered and then exported to tab-delimited text files. Probability values of less than 0.05 were considered statistically significant.
Table 1 shows the clinicopathologic characteristics of the 433 study participants who received curative surgery. In the training cohort, 57, 63, and 85 patients had stage I, II, and III disease, respectively. Through the end of the study, 79 of the 205 patients had no evidence of NSCLC recurrence, whereas the remaining 126 patients had documented evidence of lung cancer recurrence with a median follow-up of 58.7 months (range 8–89 mo). In the validation cohort, 85, 60, and 83 patients had stage I, II, and III disease, respectively. Total 147 of 228 patients presented with tumor recurrence and 81 patients were still recurrence-free with a median follow-up of 55.6 months (range 11–83 mo). The clinicopathologic characteristics were similar between the 2 cohorts, except for smoking history. The validation cohort included more patients with smoking history than those in the training cohort.
The results of univariate analysis suggested that TNM stage and the TII were associated with both OS and RFS, whereas age, sex, comorbidities, smoking history, type of surgery, and radiotherapy had no prognostic significance for OS and RFS, in the training group (Table 2). In addition, we also found that patients with commodities, especially diabetes, were more likely to have higher TII values (P = 0.008, see Table S1, Supplemental Content, http://links.lww.com/MD/A538, which illustrates the correlation between TII and clinicopathological characteristics). After adjusting for commodities and potential confounders found in the univariate analysis, a multivariate regression analysis was performed to estimate the independent prognostic factors for OS and RFS (Table 3). The results showed the TII was an independent prognostic factor for both OS (hazard ratio [HR] 3.5, 95% confidence interval [CI] 2.2–5.6, P < 0.001) and RFS (HR 3.1, 95% CI 1.9–4.9, P < 0.001). A lower TII was significantly associated with both higher OS and RFS rates. The prognostic value of the TII was further confirmed in another independent validation cohort of 228 patients. These results from univariate and multivariate analyses were similar to those obtained from the previous training cohort. The high TII remained decreased both OS (HR 2.7, 95% CI 1.8–4.2, P < 0.001) and RFS (HR 2.4, 95% CI 1.6–3.8, P < 0.001) rates (Table 3). In addition, TNM stage and tumor history also had prognostic significance in predicting both OS and RFS in the validation cohort.
The discriminative ability of the TII and clinical indices was compared using the AUCs for recurrence and survival (Fig. 1). TII was considered as an indicator of survival, with AUC of 0.66 (95% CI 0.61–0.69), and the optimal cut-off point was 578 × 109, with a sensitivity of 43.3% and a specificity of 86.0% (Fig. 1A). In addition, TII was also considered as an indicator of recurrence, with AUC of 0.67 (95% CI 0.61–0.70). The optimal cut-off point had a sensitivity of 42.8% and a specificity of 86.6%. (Fig. 1C). TNM yielded the AUC values of 0.59 (95 % CI 0.54–0.63) and 0.58 (95 % CI 0.53–0.63) in discriminating survival and recurrence, whereas CEA yield the AUC values of 0.56 (95% CI 0.52–0.61) and 0.57 (95% CI 0.52–0.62). Among these markers, TII was the strongest predictor of survival (Fig. 1B) and recurrence (Fig. 1D).
In all the studied patients, the OS in the high TII group was significantly lower than that in the low TII group (Fig. 2A; P = 0.001). The cumulative 1, 3, and 5-year RFS rates were 89.4%, 26.1%, and 16.9%, respectively, in the high TII group, and 96.9%, 56.1%, and 47.5%, respectively, in the low TII group. Similarly, the high TII group also had a lower RFS rate than the low TII group (Fig. 2B; P < 0.001). The 1, 3, and 5-year OS rates were 95.7%, 55.6%, and 26.0%, respectively, in the high TII group, and 99.6%, 81.6%, and 60.4%, respectively, in the low TII group.
We investigated the prognostic significance of the TII in the different TNM stage subgroups in greater detail. The results showed the TII was significantly correlated with RFS (P = 0.001, P = 0.009, and P = 0.007 in the TNM I, II, and IIIa subgroups, respectively; Figure 3A, C, and E) and OS (P < 0.001, P = 0.006, and P = 0.014 in the TNM I, II, and IIIa subgroups, respectively; Figure 3B, D, and F).
Exploratory subgroup analyses were conducted to evaluate the consistency of the results in patients with N2-positive lesions. Through the end of follow-up, patients in the high TII group had an OS rate of 16.7%, compared with 26.4% for patients in the low TII group. The Kaplan–Meier curve identified significance between the 2 groups (P = 0.026; Figure 4A). The RFS rates were 16.7% and 25.3% in the high and low TII groups, respectively, at the end of follow-up. Similarly, the RFS rate in the high TII group was also significantly lower than that in the low TII group (P = 0.007; Figure 4B).
Several studies revealed the prognostic significance of tumor-associated antigen or immune-related biomarkers in peripheral blood in postsurgery patients with lung cancer.7–17,24,25 In the present study, a novel tumor immune-based prognostic index (TII) was constructed based on CEA levels, and lymphocyte and platelet counts; then it was proved to be an independent predictor of recurrence and survival for patients with early-stage NSCLC after surgery. On the one hand, the predictive ability of the TII was found to be as strong as that of TNM stage for total patients. On the other hand, it was also possible to distinguish patients with similar TMN stages into high and low-risk categories based on the probability of recurrence according to a convenient blood-based test. Thus, there is potential for the TII to be used as a marker for tumor recurrence and treatment response surveillance or combined with TNM stage to provide more accurate guidance of postoperative adjuvant therapy in patients with NSCLC.
As an integrated index based on peripheral CEA levels and lymphocyte and platelets counts, the predictive value of the TII for tumor recurrence and metastasis might be explained by the function of the 3 biomarkers. Many previous studies showed the prognostic value of CEA in serum/plasma in early-stage NSCLC.22,24 In addition, some studies also evaluated the use of consecutive measurements of serum CEA during treatment and follow-up. They observed increases in the serum CEA level to be significant as prognostic factors for early recurrence,7 progression,9 or progression-free survival.26 It should also be noted that a limited number of studies also reported no association between serum CEA levels and the prognosis of NSCLC.27–29 Likewise, we also found that CEA was not an independent prognostic factor for either RFS or OS in this study. These different conclusions cast doubt on the use of CEA itself as a strong enough indicator to guide treatment decisions, although it does provide prognostic information as a tumor antigen. Lymphocytes play crucial roles in surveillance and destroying metastatic embolic cells.17,30,31 The lungs have the largest concentration of natural killer cells of any peripheral organ.32 Previously published data suggested that inhibition of natural killer cell-mediated immunity might increase the likelihood of successful tumor metastasis.33,34 Evidence is also emerging that platelets can facilitate tumor cell survival within the vasculature (immune evasion), which enables tumor cell survival and proliferation within target tissues of metastasis.35–39 In the clinic, thrombocytosis (high platelet count) was reported to be associated with poor prognosis in many cancers, including lung cancer.40,41 To overcome the limits of using CEA alone, we integrated lymphocyte and platelet counts as cancer immunomodulation factors to develop the TII index.
Results from our study paralleled the well established association between tumor cells and the host immune system. As we know, cancer immunotherapy was recently selected as the breakthrough of the year in 2013 by editors of the journal Science. It focused on the immune microenvironment as well as the tumor itself. In light of therapeutic cancer vaccines, cell-to-cell interactions triggered by tumor antigens and resulting in proper activation of the immune system have been considered for predicting clinical responses.42 Interestingly, the TII also could reflect the ability of the host immune system to survey and eliminate detached metastatic tumor cells and thus play important roles in the outcomes of patients who underwent surgical resection for early-stage NSCLC. This suggests that patients with NSCLC who have a higher TII might benefit more from targeted immunotherapy after surgery.
Currently, TNM classification is still one of the most important prognostic factors in NSCLC. However, such classification struggles to explain why some people do not experience relapse despite having the same TNM stage as other patients. In the clinic, another marker that enables accurate stratification of recurrence risk beyond that provided by TNM stage is necessary for more accurate prognostication. We found the TII could effectively predict patients’ RFS and OS in different TNM subgroups. For patients with N2-positive NSCLC in particular, the optimal management strategy remains controversial.2 Surgical resection has been favored for these patients in some centers, whereas other centers assumed that surgical resection was not indicated for these patients because of the high recurrence rate and the low OS after the operation.6 The results of our study demonstrated the prognostic significance of the TII remained strong in patients with N2-positive lesions. If this finding can be further verified, it may largely affect our treatment decisions for patients with N2-positive NSCLC, thus effectively improving their long-term survival.
Recent findings for hepatocellular carcinoma recurrence illustrated that immune-related factors in peripheral blood might be related to higher circulating tumor cell (CTC) counts.43 Previous studies of lung cancer also reached a positive conclusion toward the role of CTCs in the prognostic prediction of recurrence after curative resection.44 Therefore, the finding that patients with higher TII values had poorer prognosis could be reasonably explained by the following mechanisms regarding CTC. Patients with higher CEA levels have greater numbers of CTCs invading the peripheral blood, platelets can protect CTCs from different potential damage, and lymphocytes then eliminate CTCs to prevent tumor metastasis and recurrence. However, these specific underlying mechanisms need further experimental confirmation.
There are a few limitations of this study. As noted in previous studies,43,45 we also used lymphocyte and platelet counts to represent protumor and antitumor powers. But this quantitative method might ignore the disparity of their capabilities between different patients. In addition, we have to admit that CEA and platelets may affect tumor biology through nonimmunological mechanisms, like induce epithelial–mesenchymal transition in tumor cells.14 Nevertheless, the endogenous immunological response during the natural course of cancer constitutes the concept of cancer immunomodulation and has been accepted by many researchers.46 Actually, emerging evidence suggests that efficiently stimulating endogenous anticancer immunity is a prerequisite for the successful outcome of conventional cancer therapies.47 Because of the limitations of retrospective studies, it is difficult to find out these mechanisms underlying the association between the TII and tumor recurrence. Future investigations are needed to elucidate this by clarifying the immunological and nonimmunological mechanisms among peripheral lymphocytes, platelets, and vascular invasion.
Taken together, our data suggested TII can be used to distinguish patients with similar TNM stages into high and low-risk categories based on the probability of recurrence according to a convenient blood-based test. To the best of our knowledge, this is the first study to show the prognostic value of the TII for patients with early-stage NSCLC after surgery. Because the TII has the advantages of simplicity, convenience, and reproducibility, this approach merits further investigations exploring its potential applications in preventing NSCLC recurrence.
DTL and BBL conceived and designed the experiments. BBL, RF, and JL performed the experiments. SGL and BBL analyzed the data. HBG contributed reagents/materials/analysis tools. DTL and BBL wrote the article.
All the authors read and approved the final manuscript.
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