Background: The programmed death 1 (PD-1) receptor pathway is responsible for the negative regulation of both T and B lymphocytes upon activation of these cells. There is growing evidence that chronic lymphocytic leukemia (CLL) cells exploit the PD-1 ligand (PD-L1) to resist antitumor immune reactions and maintain their survival by shaping their own microenvironment.
Methods: We used a quantitative RT-PCR method to analyze PD-L1 gene expression in bone marrow and peripheral blood mononuclear cells, representing the proliferation and accumulation compartments of CLL.
Results: PD-L1 expression was found to be significantly higher in 112 CLL patients than in controls. Levels of PD-L1 expression in bone marrow and peripheral blood were comparable and showed a positive correlation. Furthermore, expression of PDL1 strongly correlated with expression of PD-1 receptor in mononuclear cells from the same compartment, and was not affected by incubation with immunomodulatory drug thalidomide.
Conclusion: PD-L1 expression is shared between CLL cells localized in distinct disease compartments, demonstrating that PD-1/PD-L1 a universal target for therapy.