Association of Combined PD-L1 Expression and Tumour-infiltrating Lymphocyte Features with Survival and Treatment Outcomes in Patients with Metastatic Melanoma

Background: Recent advances obtained with immune checkpoint inhibitors (ICIs) targeting the programmed cell death-1 (PD-1) protein have significantly improved the outcome of patients with metastatic melanoma. The PD-L1 expression in tumour cells as detected by immunohistochemistry is a predictive biomarker in some solid tumours, but appears insufficient as prognostic or predictive factor of response to ICIs in metastatic melanomas.

Objectives: We investigated whether the presence and the features of pretreatment CD8 tumour-infiltrating T lymphocytes (TILs) could be a complementary prognostic or predictive biomarker in patients with metastatic melanoma.

Methods: In this retrospective study, we evaluated the association of PD-L1 expression ≥5% of tumour cells combined with TIL features (CD8, CD28, Ki67) with the overall survival (OS) among 51 patients treated with ICIs and 54 patients treated with other treatment options (non-ICIs).

Results: PD-L1 positivity was observed in 33% and 39% of primary melanomas and matched metastases, respectively, with, however, poor concordance between the primary and the matched metastatic site (κ = 0.283). No significant association was noted between PD-L1 expression and CD8 TIL profile analysed as single markers and OS or response to immunotherapy. Instead, their combined analysis in primary melanoma samples showed that the PD-L1-/CD8 status was significantly associated with prolonged OS in the whole population (P = 0.04) and in the subgroup treated with non-ICIs (P = 0.009). Conversely, the PD-L1+/CD8 status was a good prognostic factor in patients treated with ICIs (P = 0.022), whereas was significantly associated with poor prognosis in patients treated with non-ICIs (P = 0.014). While the expression of CD28 was not related to outcome, the Ki67 expression was significantly associated with poor OS in the subgroup CD8 TIL+/PD-L1- (P = 0.02).

Conclusions: The pretreatment combination of PD-L1 expression with the level of CD8 TILs could better assess OS and predict therapeutic response of patients with metastatic melanoma treated by either immunotherapy or other treatment regimens.