PS1136 THE PI3KD‐SELECTIVE INHIBITOR IDELALISIB INDUCES T AND NK CELL DYSFUNCTION INDEPENDENT OF B‐CELL MALIGNANCY‐ASSOCIATED IMMUNOSUPPRESION
Authors: Rohrbacher L. , Wagner A. Ogrinc , Lohner A. , Bücklein V. , von Bergwelt‐Baildon M. , Subklewe M. 
Published: June 2019

Background:In addition to B‐cell receptor (BCR) signaling, multiple receptor tyrosine kinases (RTKs) and downstream effectors have been reported to be constitutively active in chronic lymphocytic leukemia (CLL) B cells. Activation of these pathways results in resistance to apoptosis and enhanced survival of the leukemic cells. The phosphoinositide 3‐kinase (PI3K) pathway is one of the most commonly activated signaling pathways. Idelalisib, a highly selective inhibitor of the PI3K p110∂ isoform, received FDA approval for the treatment of CLL in patients with relapsed/refractory disease or in CLLs harboring 17p deletions or TP53 mutations. However, the therapeutic success of Idelalisib is hampered by severe opportunistic infections.Aims:We hypothesize that Idelalisib interferes with T‐ and NK‐cell function, thereby contributing to an increased rate of opportunistic infections independent of underlying B‐cell malignancies. We aimed to characterize the modulation of functional properties of lymphoid effector cells by Idelalisib.Methods:All experiments were conducted with different concentrations of Idelalisib (0.05 μM, 0.5 μM, 1 μM) and DMSO as control. The maximum peak concentration (cmax) in patients was mimicked by using 0.5 μM Idelalisib. T‐cell proliferation was evaluated by CFSE dilution. Expression of checkpoint molecules (PD‐ 1, CTLA‐4) on healthy donor (HD) CD4+ and CD8+ T cells and regulatory T cells (Tregs) was assessed by multiparameter flow cytometry (MPFC). Cytometric bead arrays were performed to quantify secreted cytokines after 3 days of stimulation with CD3/CD28 beads. To test the effect of Idelalisib on NK‐cell proliferation, HD NK cells were incubated with IL‐2 in presence of Idelalisib or DMSO for 10 days. NK‐cell cytotoxicity was assessed using Calcein AM+ labeled K562 target cells. Additionally, the cytotoxic capacity of T and NK cells was evaluated by MPFC, utilizing the degranulation marker CD107a and the effector molecules Perforin and Granzyme B.Results:Idelalisib induced a dose‐dependent inhibition of T‐cell proliferation from healthy donors. However, significant effects on the number of CD4+ and CD8+ dividing T cells were only seen at concentrations higher than 5 μM, indicating that T‐cell proliferation is not affected at therapeutic concentrations of the agent. CD4+ T cells, as well as CD8+ T cells, showed reduced expression of CTLA‐4 and PD‐1 after treatment with 0.5 μM Idelalisib upon CD3/CD28 stimulation. Similar effects were detected for the respective Treg subsets. Interestingly, cytokine secretion of T cells was significantly impaired for IL‐10, TNF and IFNγ for all tested Idelalisib concentrations (p = 0.0008; p = 0.03; p = 0.03; n = 11–23). We also found Idelalisib to significantly decrease upregulation of Granzyme B and Perforin in CD8+ and CD4+ T cells (CD8+: p = 0.0002; p = 0.0215; CD4+: p = 0.0002; p = 0.015, n = 12–13) upon CD3/CD28 stimulation.NK cells treated with 0.5 μM Idelalisib showed decreased overall proliferation (p = 0.02; n = 12). NK cell cytotoxicity against K562 target cells was significantly reduced in a concentration dependent manner (0.5 μM, p = 0.0031, n = 12).Summary/Conclusion:Idelalisib impaired the cytotoxicity of T and NK cells at therapeutic concentrations in our in vitro studies. In addition, secretion of pro‐inflammatory and immune‐mediating cytokines like IL‐10,TNF and IFNγ was significantly reduced for T cells by Idelalisib. These observations support the hypothesis that the T‐ and NK‐cell function is directly impaired by Idelalisib, and not by B‐cell malignancy‐inflicted immunosuppression.