PB1882 DECONSTRUCTING RICHTER SYNDROME TUMOR NICHE: JAGGED‐1 LIGAND LEADS THE WAY
Authors: A. Marra, M.E. Laurenti, S. Baldoni, D. Sorcini, S. Pasquino, M. Giansanti, B. Bigerna, C. Rompietti, F. De Falco, B. Del Papa, G. Martino, S. Ascani, C. Mecucci, E. Rosati, P. Sportoletti
Journal: HemaSphere (2019)

Background: Richter Syndrome (RS) is defined as the occurrence of a high‐grade lymphoma (usually diffuse large B‐cell lymphoma ‐ DLBCL) in patients with a previous/concomitant diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL).RS usually occurs in lymph‐nodes, but sometimes may infiltrate extranodal sites.Isolated cardiac involvement is very unique.Most DLBCL‐type RS does not share lesions with de novo subtypes.NOTCH1 mutation was reported in almost 30% of cases,but NOTCH1 signaling activation independent of its mutational status,as NOTCH ligands mapping in tumor niche, remain scarcely estimated, as opposed to CLL.This has directed our attention to the role of cell non‐autonomous NOTCH1 activation at the cross‐roads between other pathways coming from the niche (i.e. immune checkpoint axis). CLL cells also express the NOTCH ligand JAGGED1 (JAG1), that contributes to CLL pathogenesis by activating its own signaling.Experimental proofs in other settings were made, linking the NOTCH1‐JAG1 signaling to the immunological niche.Strikingly, DLBCL‐type RS exhibits PD‐1/PD‐L1 axis upregulation in up to 80% of cases.Taking advantage of these findings, we decided to perform immunohistochemistry (IHC), using validated antibodies,to dissect the precise anatomy of the NOTCH/ligand interface in RS,in vivo. Aims: 1) To investigate context‐dependent, cell‐autonomous/cell non‐autonomous NOTCH1 network activation, using IHC stains. Specifically, we focused on NOTCH1 and its ligand JAG1, a well‐known pivotal player in embryonic heart and cardiac diseases. 2) To zoom into the PD‐1/PDL‐1 axis activation in immune niche, and crosslink the immune‐evasion pathway to tumor oncogenic promotion. Methods: A 69‐year old female was admitted to the Cardiology Intensive Care Unit (ICU), for tachy‐/bradycardia. A cardiac computed tomography identified a voluminous rounded mass at the interatrial septum.The pathologic diagnosis was consistent with non‐GC DLBCL.Bone marrow (BM) biopsy detected CLL/SLL.Molecular analysis did not reveal SF3B1,NOTCH1 and TP53 alterations but evidenced the same IgVH mutation in both mass and BM specimens,proving a clonally‐related RS. Results: IHC stains for the cleaved NOTCH1‐intracellular domain (ICD) protein was negative on both RS lymphoma cells and the surrounding stroma, as in BM specimen. On the contrary, JAG1 was markedly positive both on large neoplastic lymphocytes and in the stromal compartment (Figure 1A). Conversely, JAG1 staining was moderate‐to‐weak on CLL lymphocytes in the BM, as well as in the stromal scaffold (Figure 1B), indicating differential expressions. Unexpectedly, JAG1 upregulation occurred independently of NOTCH1 receptor co‐activation. According to this, JAG1 may stimulate other pro‐survival cascades in the neoplastic B cells, transducing microenvironmental cues, coming from TME, where JAG1 itself was found highly expressed. In addition, we performed IHC stain for PD‐1, proving strong positivity on tumor‐infiltrating lymphocytes (TILs) (Figure 1C); moreover, PD‐L1 marked both neoplastic cells and stromal meshwork in the cardiac TME (Figure 1D). Summary/Conclusion: Our report proved the importance of IHC in characterizing JAG1‐NOTCH1 ligand‐receptor interactions in the RS pathogenesis, in order to anatomically characterize cell‐extrinsic pathways in vivo . We traced JAG1 upregulation, independently of NOTCH1 signaling activation and described PD‐1/PD‐L1 axis functional switching in the context of a very unique extra‐nodal RS localization, thus identifying novel potential actionable therapeutic targets. image