THE PD-1/PD-L1 AXIS CONTRIBUTES TO T CELL DYSFUNCTION IN CHRONIC LYMPHOCYTIC LEUKEMIA

Introduction. Chronic Lymphocytic Leukemia (CLL) is marked by profound defects in T-cell function. Programmed death-1 (PD-1) is a cell surface molecule that inhibits activation and is involved in tumor escape mechanisms through binding of the specific PD-L1 ligand. Methods.We compared T cell subpopulations of CLL patients (n=120) to age- and sex-matched healthy donors (HD, n=30) using multiparameter flow cytometry. Immunohistochemical analyses were used to study PD-1 and PD-L1 expression in the lymph node microenvironment. Functional assays were used to determine the involvement of the PD-1/PD-L1 axis in shaping T cell responses. Results. The first finding of this work is that CD4+ T lymphocytes from CLL patients express significantly higher levels of the PD-1 receptor, as compared to the same cells purified from age- and sex-matched donors (52% vs 34%, P<.001). In keeping with the notion that PD-1 is a marker of cell exhaustion, we found that CD4+ T lymphocytes from CLL patients display increased numbers of effector memory cells with a concomitant decrease in naïve and central memory cells, when compared to age- and sex-matched donors. As expected, the number of effector memory cells positively associated with a more advanced stage of disease, treatment requirements and unfavorable genetic aberrations. On the other side, leukemic lymphocytes expressed higher levels of PD-L1 than circulating B lymphocytes from normal donors. PD-1 and PD-L1 increased dramatically when T or B lymphocytes were treated with mitogenic signals (e.g., PHA or PMA, respectively), suggesting that this interaction might work efficiently in an activated environment. This hypothesis was tested by determining PD-1 and PD-L1 expression in the proliferation centers located in the lymph nodes of CLL patients. Results 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 PD-L1 ligands clearly indicate that this axis contributes to driving IL-4 secretion and to the inhibition of IFN-gamma production by CD8+ T cells. Conclusions. CD4+ T lymphocytes from CLL patients express high levels of the surface marker PD-1 and exhibit an exhausted phenotype, while CLL cells express the PD-L1 ligand. Functional data suggest that the PD-1/PD-L1 interactions are critical in skewing the T cell compartment towards a Th2 phenotype, by impairing IFN-gamma secretion by CD8+ cells. These observations imply that pharmacological manipulation of the PD-1/PD-L1 axis might be relevant in restoring T cell functions.