Immunohistochemical Expression of Programmed Cell Death Ligand 1 (PD-L1) in Human Cutaneous Malignant Melanoma: A Narrative Review with Historical Perspectives

Programmed death-ligand 1 (PD-L1) is the primary ligand of the receptor programmed death-1 (PD-1) which is constitutively expressed or activated in myeloid, lymphoid (T, B and NK), normal epithelial cells, and cancer. The PD-1/PD-L1 interaction is crucial for the physiological development of immunological tolerance but also in the development of the cancer. Among these, malignant melanoma represents a tumour in which the immunohistochemical expression of PD-L1 is important to guide future therapeutic choices based on the presence/absence of expression. Various clones have been used over time for immunohistochemical determination, and different results and heterogeneity remain among the various studies in the literature. We perform a narrative review of the present studies in order to discuss and take stock of what certain achievements have been made in this field, what challenges remain, and what possible solutions can be found.