Targeting PD-L1/ PD-1-mediated inhibitory signaling with BTK inhibitors in Chronic Lymphocytic Leukemia (CLL).

Abstract

Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in adults in the West. An unmet need for equally curative and tolerable treatment approaches exists. Our group has previously shown that the PD-1/PD-L1 immune checkpoint pathway is pivotal in mediating CLL-associated T-cell dysfunction. Bruton’s tyrosine kinase inhibitors such as Ibrutinib have been shown to be able to modulate the function of T-cells and myeloid cells. Using the Eμ-TCL1 mouse model of CLL, we have aimed to analyse the effect of BTK inhibitors on expression of immune checkpoint molecules, immune phenotype and T-cell function as well as develop a combination approach of BTK inhibitors and anti-PD-L1 immune checkpoint blockade. We detected a modest increase in PD-L1 expression among CLL B-cells and a decrease among myelomonocytic cells with both Ibrutinib and Acalabrutinib treatment. We have demonstrated an amelioration of the exhaustion phenotype of CD4+ and CD8+ T-cells with BTK-inhibitor treatment with downregulation of CD69, PD-1, LAG-3 and KLRG-1. We also found downregulation of inhibitory receptor 2B4, LAG-3 and KLRG-1 on NK cells. On myeloid cells we observed downregulation of PD-1 and 2B4 as well as a differential effect on expression of TIM-3 with upregulation among myelomonocytic cells and downregulation among classical dendritic cells. Immunophenotypes of BTK inhibitor and BTK inhibitor/anti-PD-L1 combination treated animals were similar with a slightly higher expression level of PD-1 among combination treated animals. Both substances improved helper cell cytokine profiles, degranulation capacity of cytotoxic T-cells and T-cell synapse formation to a similar extent. The combination of BTK inhibitor treatment and PD-L1 blockade failed to achieve improved correction of CLL-associated T-cell exhaustion phenotype and Ibrutinib/anti-PD-L1 combination treatment achieved only a very modest improvement of T-cell function over single agent treatments. Suprisingly, the combination of Acalabrutinib and anti-PD-L1 immune checkpoint blockade was detrimental regarding both helper cell and cytotoxic T-cell function. These findings would caution against the use of Acalabrutinib/anti-PD1 or anti-PD-L1 combinations in the clinical setting.