21 Sep Melanoma: PD-L1 As Negative Prognostic Marker
Medical Research: What are the main findings of the study?
Dr. Mandalà: We evaluated PD-L1 expression by IHC in 81 consecutive metastatic melanoma patients, with well-defined demographic and clinical characteristics. Protein expression levels were correlated with clinical outcome. PD-L1+ and PD-L1- subsets of the A375 cell line were stabilized in vitro and compared using gene expression profiling and functional assays. Results were confirmed using xenograft models. In our study PD-L1 membrane positivity was an independent negative prognostic marker. Furthermore PD-L1 expression defined a subset of the BRAF-mutated A375 cell line characterized by a highly invasive phenotype and by enhanced ability to grow in xenograft models. If confirmed, our clinical and experimental data suggest that PD-L1+ melanomas should be considered a disease subset with distinct genetic and morpho-phenotypic features, leading to enhanced aggressiveness and invasiveness.
Medical Research: What was most surprising about the results?
Dr. Mandalà: Besides its role as an immunomodulatory molecule, recent data in ovarian tumor models suggest that PD-L1 might per se determine a more aggressive clinical course. To clarify whether this may be confirmed in melanoma models, we explored the functional significance of PD-L1 expression by melanoma cells. To do so we used the A375 line, which constitutively presents distinct PD-L1+ and a PD-L1- subpopulations, a unique situation in the melanoma lines tested. Stable PD-L1+ and PD-L1- variants of the A375 cell line were thus stabilized and comparatively studied. Gene profiling indicated that the PD-L1+ cell subset is characterized by a unique genetic signature, with enhanced expression of genes connected to growth, activation and invasion. A functional comparison confirmed that:
- i) the PD-L1+ variant showed signs of increased growth and invasion in vitro,
- ii) these features were enhanced when using three-dimensional cultures and maintained after xenografting in immunocompromised mice. Forced expression of PD-L1 molecules in the PD-L1- variant of the A375 line however, was not followed by the acquisition of increased growth and motility properties. This observation suggests that PD-L1 expression might be a downstream marker of the activation of an oncogenic pathway characterizing a genetically different cell subset, which shows enrichment in genes that control cell differentiation and movement. It also argues against a mechanistic involvement of PD-L1 in determining increased aggressiveness of the disease.Medical Research: What should clinicians and patients take away from your report?Dr. Mandalà:
1) 5H1 is a reliable antibody to detect PD-L1 in FFPE melanoma samples.
2) The membrane immunostaining in our series is absolutely clear and cells can be evaluated without background. This is very important considering that this is one of the biggest issue in clinical research.
3) By evaluating paired melanoma samples our study shows that metastatic melanoma tissues express PD-L1 in significantly higher proportions than primary lesions (40.3% vs. 14%). In 17/22 patients the metastatic site resulted positive while primary melanomas were negative, suggesting that PD-L1 expression is acquired during disease progression.
4) The present results suggest PD-L1 expression as a negative prognostic factor in melanoma patients. Nevertheless in our series most of patients received chemotherapy or BRAF inhibitors, while in other studies evaluating PD-L1 by IHC patients were receiving immunotherapy, including PD-1 antibodies. This suggests that the type of treatment received should also be considered when correlating PD-L1 expression with survival outcomes.
5) PD-L1 in melanoma samples should be still considered as a field of investigation and not a routine test to be performed in the daily clinical practice.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Mandalà: Our group is now focusing attention on the role of PD-L1 overexpression in melanoma patients treated with targeted oriented therapies. Another field of investigation is to understand whether PD-L1 is a predictive marker of PD-1 antibodies or it simply correlates with response. A third field of research is to understand the biological meaning of PD-L1 overexpression in melanoma tissues with or without T cells infiltration in the microenvironment.
PD-L1 marks a subset of melanomas with a shorter overall survival and distinct genetic and morphological characteristics
D. Massi, D. Brusa, B. Merelli, M. Ciano, V. Audrito, S. Serra, R. Buonincontri, . Baroni, R. Nassini, D. Minocci, L. Cattaneo, E. Tamborini, A. Carobbio, E. Rulli, S. Deaglio, and M. Mandalà