PS1306 JAB1/CSN5 STABILIZES PD‐L1 PROTEIN AND CORRELATES WITH STAT3 ACTIVATION IN T‐CELL LYMPHOMAS
Authors: E. Drakos, P. Farrajota Neves da Silva, G. Kokaraki, K. Stathopoulou, V. Atsaves, L.J. Medeiros, A. Österborg, F.X. Claret, G. Rassidakis
Journal: HemaSphere (2019)

Background: Peripheral T‐cell lymphomas (PTCL) represent 8–10% of all non‐Hodgkin lymphomas and are generally associated with poor clinical outcome with the exception of ALK+ anaplastic large cell lymphomas (ALCL) The genetic basis and transcriptional regulation of PD‐L1 has been recently investigated by our group in various PTCL types including ALK+ and ALK‐ ALCL (ASH 2018 poster). However, PD‐L1 can be regulated at the post‐translational level as well. The Jab1 (c‐Jun activation domain‐binding protein‐1), initially discovered as a c‐Jun co‐activator, represents the fifth component of an evolutionary highly conserved 8‐subunit protein complex, named COP9 signalosome (CSN). The Jab1/CSN5 gene operates as an oncogene in cancer through multiple mechanisms and recent evidence suggests that it is also involved in immune checkpoint regulation through PD‐L1 stabilization and decreased degradation. Moreover, the recent discovery of a novel Jab1/CSN5 inhibitor (CSN5i3) designed for clinical use may allow for novel investigational therapies combining targeted therapy and immunotherapy approaches. Aims: This study aimed to investigate the expression patterns of Jab1/CSN5 in PTCL tumors as well as its biologic effects on PD‐L1 protein stabilization. Methods: The PTCL study group included a cohort of 80 patients diagnosed and treated at Karolinska University Hospital (Sweden) and MD Anderson Cancer Center (USA). All tissue samples were formalin‐fixed, paraffin‐embedded (FFPE) diagnostic biopsies obtained prior to therapy. Jab1/CSN5, PDL1 and T705‐pSTAT3 expression were assessed by immunohistochemistry performed on tissue microarrays or full tissue sections using specific antibodies. For all proteins, the percentage of positive neoplastic cells was evaluated with counting of at least 500 lymphoma cells in representative microscopic fields. The in vitro study model included five T‐cell lymphoma cell lines. A T‐cell lymphoma (Karpas 299) and leukemia (Jurkat) cell lines were used in an ex vivo animal model (xenografts in SCID mice), following Jab1/CSN5 gene silencing using shRNA constructs. In addition, the cell lines were treated with the novel Jab1/CSN5 inhibitor, CSN5i3, or with the selective STAT3 inhibitor XIII and the levels of the relevant proteins were assessed by Western blot analysis. Results: The percentage of Jab1/CSN5+ lymphoma cells was significantly higher in ALK+ ALCL and ALK‐ ALCL as compared to angioimmunoblastic T‐cell lymphoma (AITL), PTCL‐NOS, and other histologic types of PTCL ( p &lt; 0.001, Kruskal‐Wallis). As continuous variables, the percentage of Jab1/CSN5+ tumor cells significantly correlated with that of PDL1 (Spearman R = 0.33, p = 0.03) and T705‐pSTAT3 (Spearman R = 0.42, p = 0.01) in the entire cohort of PTCL. Knocking down Jab1/CSN5 using shRNA in Karpas 299 xenograft tumors resulted in longer survival of SCID mice as compared to control mice, which was associated with significantly decreased PD‐L1 protein levels in tumor cells. Treatment of T‐cell lymphoma cell lines with the STAT3 inhibitor XIII resulted in downregulation of Jab1/CSN5. Similarly, treatment of T‐cell lymphoma and leukemia cells with the novel inhibitor of Jab1/CSN5 activity, CSN5i3, decreased the protein level of PD‐L1 in vitro at a concentration‐dependent manner. Summary/Conclusion: Jab1/CSN5 regulates PD‐L1 through stabilization of the protein and can be efficiently targeted by CSN5i3 in preclinical models of T‐cell lymphomas and leukemias. Jab1/CSN5 expression significantly correlates with PD‐L1 protein levels and STAT3 activation in PTCL and its clinical significance is currently under investigation.