Introduction

Colorectal cancer (CRC) is one of the most common malignant tumors of the digestive system all around the world [1]. Its incidence and mortality rate ranked third and second in the world, respectively [2]. In 2018, both new cases and deaths were close to 30% of the total number of CRC cases in the world [3, 4]. China’s cancer statistics indicated that the incidence and mortality of CRC ranked fifth among all malignant tumors in China, bringing about 380,000 new cases and 190,000 deaths annually [5]. Furthermore, most patients have already been in the severe stage when they were seeking the medical examination [6, 7]. Thus, it has become a major public health problem in many countries [8, 9].

Surgery, chemotherapy, and radiation therapy are the main treatments for cancer; unfortunately, the recurrence rate and metastasis rate (approximately 30% and 10%) in advanced CRC patients still remain high [10, 11]. In addition, some treatments showed only mild effects in reducing tumor load, such as cytokine therapy, toll-like receptors, and autologous cell therapy [12]. In recent years, immune card control point drugs have provided a new therapy for CRC, especially the programmed death 1 (PD-1)/programmed death ligand-1(PD-L1) monoclonal antibody as an immunodetection point inhibitor and an antibody-type tumor immune drug [13, 14]. PD-L1, also known as CD274 or B7-H1, is the ligand PD-1 and a sort of immune checkpoint inhibitors and belongs to the CD28 family and is expressed on the surface of activated T cells to regulate proliferation and activation [15]. The binding of PD-L1 on tumor cells to PD-1 on lymphocytes can lead to immune escape of tumor cells and ultimately promote the generation and development of tumors by inhibiting the release of cytokines, restricting lymphocyte function, and inducing lymphocyte apoptosis [16]. It was reported that PD-L1 correlated with the clinicopathological features and affected the prognosis of cancers (such as breast, gastric, and ovarian cancers) [17,18,19].

The correlation between PD-L1 expression and clinicopathological features of CRC was inconsistent, and the independent impacts of PD-L1 expression on CRC prognosis were unclear in the previous meta-analyses [20,21,22,23]. Additionally, some limitations reduced the reliability because of small sample sizes [21, 23] or the high heterogeneity [21, 23] or incorrect model selection [21, 23]. Thus, we aimed to update a meta-analysis of cohort studies to confirm the correlation between PD-L1 expression and clinicopathological features, and perform a meta-regression analysis to determine the independent role of PD-L1 on CRC prognosis after adjusting confounders.

Materials and method

Search strategy

Seven databases (PubMed, Cochrane Library, EMBASE, Web of Science, CBM, Wanfang, and CNKI) were searched through May 2020, and the search strategies were (“PD-L1” OR” B7-H1” OR “Programmed Cell Death Ligand 1” OR “CD274” OR “PD-1” OR “Programmed death 1”) AND (“Colorectal Cancer” OR “Colorectal Neoplasm” OR “Colorectal Tumor” OR “Colorectal Carcinoma” OR “Colorectal Cancer” OR “Rectal Cancer” OR “Colon Cancer” OR “Rectal Neoplasm” OR “Colon Neoplasm”). Furthermore, we reviewed the reference list of original and review articles to search for more studies. Only studies that were published as full articles and in Chinese and English were considered.

Inclusion and exclusion criteria

Inclusion criteria for study enrollment were (1) cohort studies; (2) patients had confirmed colorectal cancer; (3) PD-L1 expression detected method: immunohistochemistry (IHC); (4) the literature provides the relationship between PD-L1 expression and clinicopathological features, such as sex, age, lymphatic metastasis, differentiation, TNM stage, and tumor location; (5) studies that provided detailed pathological parameters and survival outcomes; and (6) studies that provided hazard ratios and 95% confidence interval (CI) to calculate survival outcomes. The exclusion criteria were (1) studies that were case reports, reviews, or conference papers; (2) republished literature, reviews, and case series; and (3) full text not available.

Data extraction

Two researchers (Shuxia Wang and Yun Wang) identified and classified the literature that met the inclusion criteria independently and excluded the study that obviously did not meet the inclusion criteria after reading the full text. For studies with insufficient information, we contacted the primary authors to acquire and verify data when possible. In cases of disagreement, the two researchers can make an attempt to reach a consensus. We extracted these objective data which were analyzed for aims of this study : (1) the basic information of the study including first author, year of publication, country, number of subjects, their demographic features, (2) type of study, (3) treatment method, (4) outcomes including the pathological parameters (sex, age, tumor location, TNM stage, lymphatic metastasis, differentiation, infiltration degree, tumor diameter, distant metastasis, and vascular invasion), and (5) prognostic values including overall survival (OS), disease-free survival (DFS) and progression-free survival (PFS).

Quality assessment

Study quality was assessed by using the Newcastle–Ottawa score [24], which consists of three factors: patients selection, comparability of study groups, and assessment of outcomes. A score of 0 to 9 was assigned to each study, and studies achieving a score of 6 or higher were considered high quality.

Statistical analysis

If the numbers of included studies were less than 3, the meta-analysis could not be used. All statistical analyses were conducted by using Review Manager 5.3. Odds ratios (OR) and 95% CI were analyzed for the relationship between PD-L1 expression and basic clinicopathological features including sex (male/female), age (≥ 60/< 60 years old), tumor location (right + rectum/left + colon), TNM stage (III–IV/I–II), lymphatic metastasis (yes/no), differentiation (high–middle/low), tumor diameter (≥ 5 cm/< 5 cm), vascular invasion (yes/no), infiltration degree (3–4/1–2), and distant metastasis (yes/no). Hazard ratio (HR) and its 95% CI were presented for PD-L1 on CRC prognosis. Subgroup analysis was used to find the source of heterogeneity according to treatment methods (surgery or surgery combined with chemoradiotherapy (CRT)). Moreover, meta-regression analysis was used to analyze the independent role of PD-L1 on the prognosis of CRC after adjusting for above clinicopathological features and treatment methods. If the numbers of included studies were less than 10, the meta-regression could not be used. Depending on the results from the tests of heterogeneity, a fixed effect model or a random effect model was chosen. The chi-square test and I2 were used to evaluate the heterogeneity of the included studies. Begg’s test was used to analyze publication bias by using the software Stata, version 15.1.

Results

Description of studies and quality assessment

Thirty-two eligible studies [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56] with Newcastle–Ottawa scale (NOS) score ≥ 6 were included in meta-analysis, including five in Chinese and twenty-seven in English, with a total of 8823 CRC patients. The follow-up duration was from 4 months to 7.3 years, and the sample size was from 65 to 1414. The selection process of literature is detailed in Fig. 1. Basic information and quality evaluation of included studies are presented in Table 1 and Table 2.

Fig. 1
figure 1

Flowchart of document retrieval

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Table 1 Basic study characteristics
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Table 2 Methodological quality evaluation of included studies by using the NOS
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Correlation between PD-L1 expression and clinicopathological features

The pooled OR indicated that there were significant positive correlations between PD-L1 expression and lymphatic metastasis (yes/no; n = 22; 3870 patients; OR = 1.24, 95% CI (1.11, 1.38), Z = 3.72, P < 0.05; I2 = 48%, P < 0.1) (Fig. 2a) and tumor diameter (≥ 5 cm/< 5 cm; n = 10; 1536 patients; OR = 1.34, 95% CI (1.06, 1.70), Z = 2.46, P < 0.05; I2 = 37%, P = 0.11) (Fig. 2b), but negative correlation with differentiation (high–middle/low; n = 21; 5319 patients; OR = 0.68, 95% CI (0.53, 0.87), Z = 3.10, P < 0.05; I2 = 47%, P < 0.1) (Fig. 3a) and vascular invasion (yes/no; n = 14; 4201 patients; OR = 0.80, 95% CI (0.69, 0.92), Z = 3.11, P < 0.05; I2 = 29%, P = 0.14) (Fig. 3b). However, there were no significant correlations found between PD-L1 expression and sex (male/female; n = 29; 8043 patients; OR = 0.94, 95% CI (0.85, 1.04), Z = 1.16, P > 0.05; I2 = 11%, P = 0.29) (Fig. S1A), age (≥ 60/<60 years old; n = 21; 4095 patients; OR = 0.96, 95% CI (0.84, 1.10), Z = 0.54, P > 0.05; I2 = 24%, P = 0.15) (Fig. S1B), TNM stage (III–IV/I–II; n = 23; 5108 patients; OR = 1.11, 95% CI (0.86, 1.43), Z = 0.81, P > 0.05; I2 = 57%, P < 0.1) (Fig. S2A), tumor location (right + rectal/left + colon; n = 16; 4421 patients; OR = 1.28, 95% CI (0.95, 1.74), Z = 1.60, P > 0.05; I2 = 65%, P < 0.1) (Fig. 3c), infiltration degree (3–4/1–2; n = 10; 1837 patients; OR = 0.82, 95% CI (0.64, 1.06), Z = 1.52, P > 0.05; I2 = 19%, P = 0.27) (Fig. S2B), and distant metastasis (yes/no; n = 10; 2486 patients; OR = 1.13, 95% CI (0.87, 1.47), Z = 0.91, P > 0.05; I2 = 30%, P = 0.18) (Fig. S2C).

Fig. 2
figure 2

Meta-analysis between PD-L1 expression and lymphatic metastasis (a) and tumor diameter (b)

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Fig. 3
figure 3

Meta-analysis between PD-L1 expression and differentiation (a) and vascular invasion (b) and tumor location (c)

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Correlation between PD-L1 expression and the prognostic parameters (OS, DFS, and PFS)

Twenty studies provided the OS parameters. As weak heterogeneity existed (I2 = 39%, P = 0.03), the random effects model was used. Meta-analysis showed that OS was significantly associated with PD-L1 expression in CRC patients (n = 21; HR = 1.93, 95% CI (1.66, 2.25), Z = 8.46, P < 0.05) (Fig. 4a).

Fig. 4
figure 4

Meta-analysis of PD-L1 expression on OS (a), DFS (b), and PFS (c)

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Sixteen studies provided the DFS parameters. Results showed that DFS was significantly associated with PD-L1 expression in CRC patients (n = 16; HR = 1.76, 95% CI (1.50, 2.07), Z = 6.97, P < 0.05; I2 = 23%, P = 0.20) (Fig. 4b).

Nine studies provided the PFS parameters. Results showed that PFS was significantly associated with PD-L1 expression in CRC patients (n = 9; HR = 1.82, 95% CI (1.60, 2.07), Z = 5.85, P < 0.05; I2 = 0%, P = 0.59) (Fig. 4c).

Subgroup analysis on OS under different treatment methods

Results were as follows: (1) surgery: PD-L1 expression was significantly associated with OS (n = 12; HR = 1.90, 95% CI (1.65, 2.20), Z = 8.70, P < 0.05; I2 = 23%, P = 0.22); (2) surgery + CRT: PD-L1 expression was significantly associated with OS (n = 9; HR = 1.69, 95% CI (1.39, 2.07), Z = 5.15, P < 0.05; I2 = 32%, P = 0.16) (Fig. 5).

Fig. 5
figure 5

Subgroup analysis of PD-L1 expression on OS by using different treatment methods

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Meta-regression analysis

Meta-regression analysis confirmed that PD-L1 expression was to be correlated with OS (HR = 1.95, 95% CI (1.92, 3.98)) and DFS (HR = 2.14, 95% CI (0.73, 4.52)). And the prognosis of patients with surgery treatment alone was worse than that of surgery combined with CRT. Patients with distant metastasis had a poor prognosis (Table 3).

Table 3 HR and 95% CI in meta-regression analysis for CRC prognosis
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Sensitivity analysis

Sensitivity analysis on OS, DFS, and PFS indicated that after excluding any single study individually, there was no separate study that significantly affected HR and 95% CI, suggesting that the results of this meta-analysis were stable (Fig. S3).

Publication bias

Results of Begg’s test suggested that there may be no publication bias among studies for OS, DFS, and PFS (all P > 0.05) (Fig. 6).

Fig. 6
figure 6

Begg’s funnel plot for OS (a), DFS (b), and PFS (c) publication bias in the included studies

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Discussion

Studies reported that the PD-1/PD-L1 pathway has become a promising therapeutic target for various human malignancies [17, 18, 57,58,59,60]. Nonetheless, the correlation between PD-L1 expression and clinicopathological features [26, 30] and the prognosis of CRC patients are still controversial [36, 51]. Therefore, this study comprehensively searched the literature to solve the above-existing controversies in order to draw more reliable conclusions.

Data of our meta-analysis from 32 studies (8823 CRC patients), the largest to date, indicated that PD-L1 expression was significantly positively correlated with lymphatic metastasis and tumor diameter, but negatively correlated with differentiation and vascular invasion. However previous meta-analysis found that PD-L1 expression was correlated with tumor stage [21] and gender [22] and tumor location [23], which results were unreliable due to high heterogeneity (all I2 > 70%) [21,22,23] and the incorrect analytical model (all selected the fixed effects model that is available for I2 < 50%) [21,22,23]. In this study, the random effects model was selected for TNM stage and tumor location because of mild heterogeneity (I2 = 57% for TNM stage and I2 = 65% for tumor location).

In univariate analysis, PD-L1 was correlated with poor prognosis of CRC in this study, which was similar to the results of previous meta-analysis [20,21,22,23]. However, high heterogeneity existed in our study and those meta-analyses [20,21,22,23]. Furthermore, in subgroup analysis based on treatment, we found that the degree of statistical heterogeneity reduced both in subgroup for OS (Fig. 5). It meant that the treatment method was the source of heterogeneity for OS. In order to control other confounders, meta-analysis should be necessary to analyze the independent role of PD-L1 on CRC prognosis. We found that PD-L1 expression independently predicted a poor prognostic outcome with meta-regression analysis. Previous meta-analysis made a contradictory conclusion by univariate analysis [20,21,22,23]. Meta-regression analysis can get a more reliable and accurate outcome after adjusting confounders including clinicopathological features and treatment methods that influence the CRC prognosis.

In our sensitivity analysis, none of the inclusions and exclusions of specific studies one by one materially changed the results of the primary meta-analysis; it suggested that the results of this meta-analysis were stable.

From the perspective of publication bias, Begg’s test on OS, DFS, and PFS found that there was no significant publication bias that existed among included studies, and the results of this study were relatively reliable.

Despite some positive findings from this meta-analysis, two limitations still existed to our study. Firstly, although Chinese and English studies were included in this meta-analysis, language bias still existed. Secondly, although the literature screening was carried out with a strict search strategy, a small number of literatures including gray literature and conference literature may still be missing.

Conclusions

In summary, PD-L1 expression was significant correlated with lymphatic metastasis, tumor diameter, differentiation, and vascular invasion, and could act as an independently poor prognostic factor for CRC.