Prognostic significance of PD-L1 expression and CD8+ T cell infiltration in pulmonary neuroendocrine tumors

Abstract

Background: Recent research supports a significant role of immune checkpoint inhibitors in the treatment of solid tumors. However, relevant reports for programmed death-ligand 1 (PD-L1) and CD8+ tumor-infiltrating lymphocytes (TILs) in pulmonary neuroendocrine tumors (PNETs) have not been fully studied. Therefore, we investigated PNETs for the expression of PD-L1 and infiltration by CD8+ TILs as well as the prognostic value of both factors.

Methods: In total, 159 specimens of PNETs (35 TC, 2 AC, 28 LCNEC, 94 SCLC) were included in this study. Immunohistochemistry (IHC) was used to detect the expression of PD-L1 in these cases. Cases demonstrating ≥5% tumor cell expression or any expression (> 1%) of PD-L1 on immune cells were considered positive. CD8+ TILs both within stroma and tumor areas of invasive carcinoma were analyzed using whole-slide digital imaging. Manual regional annotation and machine cell counts were performed for each case.

Results: Positive expression of PD-L1 was observed in 72 cases (45.3%), including 9 cases (5.7%) with expression exclusively on tumor cells, 46 cases (28.9%) with expression exclusively on immune cells, and 17 cases (10.7%) with the expression on tumor cells and immune cells. PD-L1 expression was associated with necrosis (p < 0.001), high pathologic grade (p < 0.001) and histologic type (p < 0.001). No correlation was observed with overall survival (OS) (p = 0.158) or progression-free survival (PFS) (p = 0.315). In contrast, higher CD8+ T cell density was associated with the absence of vascular invasion (p = 0.004), histologic type (p = 0.005), negative lymph node metastasis (p = 0.005) and lower clinical staging (p = 0.007). Moreover, multivariate analysis revealed that CD8+ stromal TIL was an independent prognostic factor for improved OS (p = 0.009) and PFS (p = 0.002).

Conclusion: PD-L1 was expressed in approximately half of the PNETs. The majority of the expression was observed in immune cells. Positive expression of PD-L1 showed no correlation with OS or PFS, while higher CD8+ TILs within stroma was proved to be an independent prognostic factor for favorable OS and PFS of PNETs.

Keywords: CD8+ TILs; Pulmonary neuroendocrine tumor, PD-L1.

Fig. 1

Hemotoxylin and eosin (H&E), PD-L1 and CD8 stains, each performed on histologic sections of small cell lung cancer (a-c), large cell neuroendocrine carcinoma (d-f), atypical carcinoid (g-i) and typical carcinoid (j-l). a Small cell lung cancer was showing a scant cytoplasm, fine nuclear chromatin, absent or inconspicuous nucleoli, extensive necrosis. b PD-L1 was moderately expressed on the membrane of stromal immune cells in the desmoplastic stroma between clusters of tumor cells. c CD8+ TILs were observed in the stroma, while the intratumoral pattern of CD8 expression was not common. d Large cell neuroendocrine carcinoma with prominent nucleoli and abundant eosinophilic cytoplasm, necrosis was not shown. e, f PD-L1 was positively expressed on the membrane and cytoplasm of the immune cells, and a large number of CD8+ TILs could also be observed at the borderline (the 200× magnification for PD-L1 and CD8 was shown in the upper left, respectively). g Atypical carcinoid with vascularized stroma, focal necrosis and 6 mitosis/2 mm². h, i PD-L1 was negative expression either in tumor cells or stromal cells, while CD8+ TILs were exhibited in the interface of tumor and stroma. j Typical carcinoid with organoid growth pattern with intervening vascular stroma. k, l No PD-L1 can be detected, and only several CD8+ TILs could be found in the stroma. (The original magnification of e-f was 100×, magnification for remaining cases were 200×)

Fig. 2

Kaplan-Meier survival curves of OS and PFS for PD-L1 were performed. The PD-L1 expression has a trend with decreased OS (p = 0.158) (a) and PFS (p = 0.315) (b) but did not reach the significant level. Similarly, the PD-L1 positive target cells were further classified as tumor cells and immune cells. We found that expression on tumor cells seemed to have poorer OS (p = 0.459) (c) and PFS (p = 0.708) (d) than in immune cells

Fig. 3

In PNETs, Kaplan-Meier analysis of OS and PFS associations with CD8+ T cell density within stroma (a-b) and intratumor (c-d) were performed. Increasing stromal CD8+ T cell density is correlated with improved OS (p = 0.000) (a) and PFS (p = 0.000) (b), while the higher intratumoral CD8+ T cell density was associated with a trend toward improved prognosis (c and d)