Increased tumour cell PD-L1 expression, macrophage and dendritic cell infiltration characterise the tumour microenvironment of ulcerated primary melanomas

Abstract

Background: Primary melanoma ulceration is an unfavourable prognostic factor included in current staging systems. Yet, the immunological and molecular alterations responsible for this adverse outcome have not been fully elucidated.

Objectives: We aimed to identify immunological differences between ulcerated and non-ulcerated primary melanomas concerning both innate and adaptive immunity and to correlate these with clinical outcome.

Methods: Formalin-fixed paraffin-embedded primary melanomas from 112 patients (pts) were analysed by immunohistochemistry. The expression of various markers identifying tumour-infiltrating lymphocytes, macrophages and dendritic cells was evaluated semi-quantitatively by three independent investigators. Tumour cell expression of programmed death-ligand 1 (PD-L1), transporter of antigen processing 1 and the MxA protein was also analysed.

Results: Recurrence occurred in 21/56 pts (37.5%) with ulcerated vs. 14/56 pts (25.0%) with non-ulcerated tumours (P = 0.15). Tumour ulceration was associated with more frequent development of brain metastasis (17.6 vs. 3.6% of pts, P = 0.015). Immunohistochemistry showed an association of ulceration with the presence of intratumoural CD68⁺ macrophages (P = 0.028) as well as with increased numbers of intratumoural CD11c⁺ dendritic cells (P = 0.014) and CD163⁺ macrophages (P = 0.001). PD-L1 positivity (expression in >1% of tumour cells) was more frequent in ulcerated than non-ulcerated tumours [40 (72.7%) vs. 25 (44.6%), P = 0.003]. A positive correlation between intratumoural CD11c⁺ (Spearman's correlation coefficient ρ: 0.42) and CD163⁺ (ρ: 0.31) cell count and frequency of tumour cell PD-L1 expression was detected.

Conclusions: Our results confirm the adverse clinical outcome associated with primary melanoma ulceration, particularly concerning the risk of recurrence and subsequent development of brain metastases. The observed immunological differences suggest a conceivable role of increased intratumoural macrophage and dendritic cell counts associated with enhanced tumour cell PD-L1 expression potentially contributing to the immunosuppressive, growth-promoting microenvironment of ulcerated primary melanomas.