Aims Pulmonary squamous cell carcinoma (SqCC) represents the second most common non-small cell lung carcinoma type. The mechanisms which regulate programmed death ligand 1 (PD-L1) expression in this form of lung cancer are not fully elucidated yet.

Methods We immunohistochemically determined the level of PD-L1 expression using the Tumour Proportion Score system in surgical resections of 133 patients with pulmonary SqCC. The results from PD-L1 immunohistochemistry were analysed in relation to tumour differentiation and the presence of necrotic areas comprising at least 20% of the tumour mass.

Results No significant differences in terms of PD-L1 expression were found between SqCC subtypes as defined by the current WHO classification: better differentiated, keratinising tumours (12/24, 50.0 %) compared with less differentiated, non-keratinising and basaloid forms (62/109, 56.9 %) were PD-L1 positive in a comparable proportion of cases (p=0.1903). Contrary to that, SqCCs with the presence of necrosis (51/61, 83.6 %) had significantly more PD-L1-positive cases (p<0.001) compared with SqCCs without necrotic areas (23/72, 32.0 %)

Conclusions We demonstrated that PD-L1 expression in pulmonary SqCCs does not correlate with the traditionally defined degree of differentiation of these tumours. On the other hand, we found a significant association between the positive result of PD-L1 immunohistochemistry and tumour necrosis. Further investigation regarding the role of hypoxic pathways as presumable inducers of PD-L1 expression in pulmonary SqCCs might contribute to the understanding of this phenomenon.