Genetic Inhibition of PD-L1 on Myeloma Cells Improves Survival in Contrast to the Treatment with Checkpoint Inihibitors

Checkpoint inhibition (CPI) has shown dramatic improvements in overall survival in many malignant diseases. However, in multiple myeloma (MM) the results were disappointing resulting in an early termination of clinical trials. Despite the advantages in therapy the disease remains incurable.

Methods: We analyzed the efficiency and immunological mechanisms of PD-1/PD-L1 blockade using KaLwRij mice that develop MM upon injection of 5T33 myeloma cells. Treatment of mice started d19 post inoculation. Bone marrow (BM) and spleen cells were analyzed by flow cytometry for the phenotype of immune cells.

Results: Comprehensive immunophenotyping including the analysis of T and NK cell subpopulations revealed no differences of early MM disease stage compared to healthy control groups. Treatment of mice with mAbs blocking PD-1 or PD-L1 had no effect on tumor growth and survival. It was demonstrated that HDAC inhibitors beside their direct effect on malignant cells may increase the immunogenicity of malignant cells by improving the presentation of tumor antigens and modulate the immunological composition of the tumor microenvironment (TME). The pan-HDAC inhibitor panobinostat that is approved for the treatment of myeloma patients inhibited the development of myeloma in treated mice. Surprisingly, the combined application of the anti-PD-1 blocking antibody with panobinostat reduced the anti-myeloma effect of the compound and resulted in decreased survival.

By analyzing the phenotype of immune cells in the different populations, we found in the panobinostat treated animal group an increase in the CXCR4 expressing CD4+ NKT cells. Additionally, the CD8+ T cells expressing CD1d and CXCR4 decreased compared to the other groups in the spleen. CD1d is a MHC like receptor for glycolipids activating NKT cells, whereas CXCR4 is a BM homing receptor and linked to metastasis and tumor aggressiveness. We found a gradually increase of CXCR4+ NKT cells in the BM corresponding to MM disease progression. Interestingly, we observed a shift in the CD4+/CD4- NKT cell ratio during disease progression, whereby the CXCR4+ CD4- NKT cells seem to be associated with advanced tumor growth, while the increase of CXCR4+ CD4+ NKT are associated with prolonged survival as observed in in the panobinostat treated group.

To further analyze the role of PD-L1 expression on myeloma cells we generated a PD-L1 KO of the 5T33 cell line using the CRISPR/Cas9 technology. We found no differences in the expression of surface molecules such as MHC class I and II, co-stimulatory or adhesion molecules, proliferation and migration of the genetically engineered cells in comparison to the mock control. Interestingly, mice inoculated with the 5T33 PD-L1 KO cells showed a significant longer survival compared with the 5T33 mock injected, indicating that blocking of the PD-L1 molecule on myeloma cells plays an important role in the pathogenesis of MM and its direct blocking on malignant cells rather than in the TME might have an impact on the clinical efficiency. When analyzing the spleen of the mock vs PD-L1 KO myeloma inoculated mice, we found the same downregulation of CXCR4 and CD1d on CD8+ T cells in the PD-L1 KO myeloma group as observed in the panobinostat treated group with extended survival.

In addition, we used NOD. scid. Il2Rγc ^null (NSG) mice to proof that the survival prolongation is a result of the immunological response to PD-L1 and that the myeloma cells are not otherwise effected in their tumor cell properties in vivo. NSG mice experience the same tumor burden post 5T33 mock and PD-L1 KO challenge, assuming that the previous observed survival prolongation is exclusively dependent on the PD-L1 tumor- immune cell interaction.

Conclusion: We found that PD-1 blockade might negatively affect and inhibit the therapeutic efficacy of HDAC inhibitors such as panobinostat. Genetic down regulation of PD-L1 on the myeloma cells enables a significant improvement and longer survival. These results give new insights into the complexity of the action of CPI in the treatment of malignant diseases which might help to develop combinatorial approaches of checkpoint inhibitors in clinical trials. Furthermore, the increase of CD4- CXCR4 expressing NKT cells in the BM might be used as biomarker to monitor MM disease progression, whereas the increase of the CD4+/CD4- NKT cell ratio in the BM might be associated with the shrinkage of MM tumor burden.