Nodal Marginal Zone Lymphoma with Prominent Expansion of PD-1+ T-Follicular Helper Cells: A Persistent Diagnostic Challenge with a Heterogeneous Mutational Architecture.

Nodal marginal zone lymphoma (NMZL) is an indolent B-cell lymphoma that may pose diagnostic challenges due to the absence of distinct markers. In rare atypical cases, an overabundance of PD1+ T follicular helper (TFH) cells in tumor tissue may mimic peripheral T-cell lymphoma (PTCL) of TFH origin, further complicating the diagnosis. A 72-year-old woman with progressive lymphadenopathy had a cervical lymph node biopsy showing a disrupted architecture with monomorphic nodules of CD20+/MNDA+ B-cells and a prominent central population of proliferating CD4+/PD1+ T-cells, initially suggestive of a PTCL-TFH. The bone marrow contained aggregates of CD20+ B-cells intermixed with CD3+/CD4+/PD1+ T-cells. Next-generation sequencing (NGS) revealed clonal immunoglobulin heavy-chain rearrangements in the lymph node and bone marrow, with T-cell receptor genes displaying a polyclonal pattern. Targeted NGS showed no PTCL-related alterations but identified NMZL-associated mutations with different distributions across lymph node and bone marrow compartments. NOTCH2 mutations (c.6418C>T; p.Gln2140*) were found in both tissues, while the (c.69+2T>A; p.?) TNFRSF14 gene mutation was only detected in the lymph node. The KMT2D gene displayed a frameshift variant in the lymph node (c.4801_4802delinsT; p.Arg1601Leufs*3) and an in-frame deletion (c.11756_11758del; p.Gln3919del) in the bone marrow. Notably, NGS and digital droplet PCR confirmed a TP53 frameshift mutation (c.902del; p.Pro301Glnfs*44) with a fractional abundance of 0.31% in the lymph node and a (c.742C>T; p.Arg248Trp) mutation (0.309%) in the bone marrow. Results underscore the importance of NGS-based clonality to diagnose NMZL with prominent PD1+ T-cell hyperplasia, and prompt further investigation into tissue-specific mutational signatures in these unusual cases.