Molecular characterization of post-transplant lymphoproliferative disorders of donor origin occurring in liver transplant recipients

Post-transplant lymphoproliferative disorders (PTLDs) represent a frequent complication of solid organ transplantation. Although most PTLDs arise from recipient lymphoid cells, a considerable fraction of cases may arise from donor B-cells. In an attempt to clarify the histogenesis and pathogenesis of PTLDs derived from donor B-cells, monoclonal PTLDs occurring in liver transplant recipients were chosen as a model to compare donor (D-PTLDs) versus recipient PTLDs (R-PTLDs). The tumour panel included nine D-PTLDs and six R-PTLDs. D-PTLDs were early-onset, EBV-infected lymphoproliferations classified as polymorphic PTLD (P-PTLD; n = 7) or diffuse large B-cell lymphoma (DLBCL; n = 2) with tumour localization confined to the hepatic hilum. All R-PTLDs were late-onset DLBCLs and showed extrahepatic localization. A BCL-6^-/MUM1 ⁺/CD138^+/- phenotype, consistent with a post-germinal centre (GC) stage of pre-terminal B-cell differentiation, was observed in all D-PTLDs and in 2/6 R-PTLDs, whereas a BCL6⁺/MUM1^-/ CD138^- profile, reminiscent of GC B-cells, was detected in 4/6 R-PTLDs. The presence of somatic IGHV hypermutation was observed in 6/9 D-PTLDs and in 4/6 R-PTLDs, suggesting derivation from antigen-experienced B-cells. IGHV4-39 was the IGHV gene most frequently encountered, being rearranged in 3/9 D-PTLDs. Among IGHV-mutated PTLDs, a mutational profile suggesting antigen stimulation and/or selection was observed in 4/6 D-s and in 2/4 R-PTLDs. The presence of ongoing IGHV mutations was detected in 2/4 D-PTLDs. Aberrant SHM was detected in 10/15 (66.7%) PTLDs, including 6/9 D-PTLDs and 4/6 R-PTLDs. Our findings suggest that (i) D-PTLDs show a clinical presentation distinct from R-PTLDs; (ii) immunophenotypic and genetic features of D-PTLDs are consistent with mature, GC-experienced B-cells; (iii) transformed donor-derived B-cells may experience antigen-driven stimulation and selection, and may acquire genetic lesions during neoplastic expansion in the recipient environment; and (iv) EBV infection and expression of viral oncoproteins may be relevant in the pathogenesis of D-PTLDs.