What Are PD-1 and PD-L1?
Programmed death 1 (PD-1) is an immune inhibitory receptor expressed on several immune cells, particularly cytotoxic T cells. It interacts with two ligands, programmed death ligand 1 (PD-L1) (B7-H1, CD274) and PD-L2 (B7-DC). While PD-L2 is expressed primarily on macrophages and dendritic cells, PD-L1 is expressed on tumor cells, as well as other immune cells. The interaction of these ligands with PD-1 inhibits T-cell activation and cytokine production. Their ligation with PD-1 during infection or inflammation in normal tissue is critically important in maintaining homeostasis of immune response to prevent autoimmunity. Their interaction in tumor microenvironments, however, provides an immune escape for tumor cells by turning off cytotoxic T cells. Thus, blocking these interactions may subject the tumor cells to attack from cytotoxic T cells.
How Is PD-L1 Expression Determined? What Is Its Significance?
PD-L1 expression is measured most commonly by immunohistochemistry (IHC). Tumoral PD-L1 expression status has been shown to be prognostic in multiple tumor types, including melanoma (MEL), renal cell carcinoma (RCC), and nonâ€“small-cell lung cancer (NSCLC). In addition, tumoral PD-L1 expression appears to correlate closely with response to antiâ€“PD-1 antibodies. However, no test is uniformly accepted as the standard for quantitating PD-L1 expression. The IHC tests used in clinical trials are proprietary; data on similarities between and among the antibodies used and the assay conditions, staining pattern, threshold for signal detection, and assessment of positivity are not published. The protein expression patterns of PD-L1 on tumor cells, dendritic cells, and tumor-infiltrating immune cells differ, and exact cell type and degree of expression vary between assays. A different methodology for evaluating PD-L1 messenger RNA (mRNA) expression, using an antibody-independent in situ hybridization assay coupled with quantitative fluorescence, showed that increased PD-L1 mRNA transcript was associated with elevated tumor-infiltrating lymphocytes and better clinical outcomes in patients with breast cancer and NSCLC. The role of PD-L1 expression as a biomarker is discussed in this supplement to ONCOLOGY, in the review “Prognostic and Predictive Markers for the New Immunotherapies,” by Drs. Kathleen M. Mahoney and Michael B. Atkins.
Antiâ€“PD-1 and Antiâ€“PD-L1 Antibodies
Several PD-1 and PD-L1 antibodies are in clinical development (Table 1). Overall, they are very well tolerated; most did not reach dose-limiting toxicity in their phase I studies. As listed in Table 2, no clinically significant difference in adverse event profiles has been seen between antiâ€“PD-1 and antiâ€“PD-L1 antibodies. Slightly higher rates of infusion reactions (11%) were observed with BMS-936559 (antiâ€“PD-L1) than with BMS-96558 (nivolumab). In an early stage of a nivolumab phase I study, there was concern about fatal pneumonitis. It has been hypothesized that PD-1 interaction with PD-L2 expressed on the normal parenchymal cells of lung and kidney provides unique negative signaling that prevents autoimmunity. Thus, antiâ€“PD-1 antibody blockage of such an interaction may remove this inhibition, allowing autoimmune pneumonitis or nephritis. Antiâ€“PD-L1 antibody, however, would theoretically leave PD-1â€“PD-L2 interaction intact, preventing the autoimmunity caused by PD-L2 blockade. With implementation of an algorithm to detect early signs of pneumonitis and other immune-related adverse events, many of these side effects have become manageable. However, it does require discerning clinical attention to detect potentially fatal side effects. In terms of antitumor activity, both antiâ€“PD-1 and antiâ€“PD-L1 antibodies have shown responses in overlapping multiple tumor types. Although limited to a fraction of patients, most responses, when observed, were rapid and durable.