The PD-1/PD-L1 Axis Contributes to T Cell Dysfunction in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a progressive accumulation of mature B lymphocytes and it is marked by profound defects in T cell function. The mechanisms responsible for T cell dysfunction remain unclear, even if several observations show that T cells from CLL patients express markers of chronic activation. One of this marker is Programmed death-1 (PD-1), a cell surface molecule that inhibits activation of immune cells and it is involved in tumor escape mechanisms through binding of the specific PD-L1 ligand. The aim of this work is to evaluate the expression and function of the PD-1/PD-L1 axis in the CLL context. Using multiparameter flow cytometry, we showed that CD4+ and CD8+ T lymphocytes from CLL patients (n=117) express significantly higher levels of the PD-1 receptor, as compared to the same cell subpopulations purified from age- and sex-matched normal donors (n=33; 52% vs 34%, p <0.001). In keeping with the notion that PD-1 is a marker of cell exhaustion, CD4+ and CD8+ T lymphocytes from CLL patients displayed increased numbers of effector memory and terminally differentiated cells, respectively, with a concomitant decrease in naïve and central memory cells, when compared to controls. The number of effector memory and terminally differentiated cells positively associated with a more advanced stage of disease, treatment requirements and unfavorable genomic aberrations. Moreover, leukemic lymphocytes expressed higher levels of PD-L1 than circulating B lymphocytes from normal donors. PD-1 and PD-L1 expression significantly increased when T or B lymphocytes were treated with mitogenic signals, suggesting that this interaction might work efficiently in an activated environment. This hypothesis was tested by immunohistochemical analyses determining PD-1 and PD-L1 expression in the proliferation centers of lymph nodes sections from CLL patients. The results obtained indicate that PD-L1+ proliferating CLL cells are in close contact with CD4+/PD-1+ T lymphocytes. Lastly, functional experiments performed using anti-PD-1 antibodies or recombinant soluble PD-L1 clearly indicate that the PD-1/PD-L1 axis contributes to driving IL-4 secretion and to the inhibition of IFN-g production by CD8+ T cells. In conclusion, these results show that CD4+ and CD8+ T lymphocytes from CLL patients express high levels of the surface marker PD-1 and exhibit an exhausted phenotype, while B leukemic cells express the PD-L1 ligand. Functional data suggest that PD-1/PD-L1 interactions are critical in skewing the T cell compartment towards a Th2 phenotype, by impairing IFN-g secretion by CD8+ cells. Taken together, these observations suggest that pharmacological manipulation of the PD-1/PD-L1 axis might be relevant in restoring T cell functions in the CLL microenvironment.