Angioimmunoblastic T-Cell Lymphoma: Molecular Characterization of Clonal T and B-Cells and a Patient Derived Xenograft Model of Coexisting T and B-Cell Proliferations

Introduction: Angioimmunoblastic T-cell lymphoma (AITL) is a rare and aggressive type of lymphoma that accounts for about 20% of peripheral T-cell lymphomas with a 5 year overall survival rate of 30%. As most patients relapse after anthracycline-containing regimens and newer agents such as histone deacetylase inhibitors, other novel therapeutic approaches are needed. Signaling lymphocytic activation molecule F7 (SLAMF7), a molecule expressed on a subset of T-cells, activated B cells and myeloma cells, is an attractive target to explore based on our previous studies showing SLAMF7 expression in a subset of AITL cases. The association of AITL with Epstein Barr virus (EBV) positive B-cells is nearly always present and the efficacy of treatment in such patients with significant EBV viral load is not well-understood. In this study, we performed the molecular characterization of an aggressive EBV+ AITL case, established a patient-derived xenograft (PDX) AITL model of coexisting T and B-cell proliferations and evaluated novel therapeutic strategies.

Methods: Primary tumor cells were injected into a NSG mouse. Flow cytometry, immunohistochemistry (IHC), CISH-EBER and BIOMED 2 PCR based clonality studies were used to confirm the engraftment and compared the consistency of engrafted tumor cells with the primary sample. Genomic DNA extracted from sorted T and B cells and from paired normal neutrophils of the original patient were subjected to Whole Exome sequencing (WES). In vivo AITL PDX model trials were tested for the efficacy of romidepsin (Rom), elotuzumab (Elo), rituximab (Rit) and in combinations of these drugs.

Results: A 53 year old woman with AITL was treated with 6 cycles of CHOEP followed by autologous stem cell transplantation. 3 months after transplantation (9 months after diagnosis) she developed progressive fatigue and arthralgias. PET-CT scan showed new cervical, thoracic, abdominal and pelvic lymphadenopathy. A cervical lymph node biopsy was performed to confirm relapse. IHC staining showed atypical T cells expressing CD2, CD3, CD4, CD5, CD7, CD10, BCL6, PD1, SLAMF7 and CXCL13. Scattered CD20+/EBER+ B-immunoblasts were present with focal large clusters/small sheets. Primary tumor cells engrafted in NSG mouse via tail vein injection caused splenomegaly. Flow cytometry assay demonstrated the engraftment of tumor cells in peripheral blood, bone marrow and spleen tissue. CD3+CD19- cells dominated the engrafted cells in all three tissues. Histologic examination and immunophenotyping (IHC and EBER staining) of spleen were consistent with primary tumor tissue. Engrafted tumor cells were capable of serial passage in NSG mice with an increasing malignant B cell percentage that mimics the situation in which the B-cell component masks an underlying T-cell lymphoma in humans. T-cell receptor gene rearrangement assay confirmed the clonal identity of engrafted T-cells matched the primary relapsed tumor. A clonal IGH rearrangement of engrafted B-cells was also detected, while no monoclonal B-cell population was detected in the relapsed AITL sample, possibly due to the low number of EBV+ B-cells present in that biopsy. WES of sorted malignant T-cells showed 33 mutants in 31genes, including RhoA G17V, TET2,STAT3 and VAV1, previously described in AITL or other T-cell lymphomas. In parallel WES assay, 9 mutations were found in 9 genes from sorted EBV+ B immunoblasts.

A PDX model using cells harvested from the second passage showed single agent, Elo or Rit, extended the survival of mice compared to the control group (p < 0.05). Rom alone had no such effect (p = 0.27). Combination of Rit with either Elo or Rom further improved survival compared to each single agent exposed cohort (p < 0.05). There was no significant difference between Rit/Elo and Rit/Rom (p = 0.067). PARP cleavage by IHC was higher in the Rit/Rom and Rit/Elo groups compared to other cohorts. Expression of SLAMF7 in a subset of engrafted T and B cells of the control mouse were confirmed via flow cytometry assay.

Conclusions: To date, this is the first molecular characterization of AITL tumor cells in comparison with associated EBV+ B cells and use of such a PDX model for therapeutic evaluation of agents targeting both malignant T and B cells simultaneously. The in vivo data support further clinical investigation of applying elotuzumab or romidepsin in combination with rituximab in AITL containing EBV-positive B-cell proliferations.