Diagnostic criteria for NK cell large granular lymphocyte leukemia: validation through a multicentric international study

Natural killer (NK) large granular lymphocytic leukemia (LGLL) is a rare lymphoproliferative disorder lacking definitive clonality markers, complicating diagnosis and distinction from reactive NK cell expansions. We previously proposed an NK clonality score with high diagnostic accuracy, but a subset of patients remained unclassified. In this multicenter international study, we refined and validated updated diagnostic criteria using independent training (n = 78) and validation (n = 57) cohorts from 3 national registries (United States, Italy, and France). The revised framework integrates NK score parameters with CC motif chemokine ligand 22 (CCL22) mutations and bone marrow biopsy (BMB) findings. Four major criteria were defined: NK cell count of ≥1.0 × 109/L, killer-cell immunoglobulin-like receptor restriction, CD94/NKG2A overexpression, and somatic mutations in STAT3, TET2, or CCL22, the latter newly introduced. In the training cohort, 50 patients were classified as having NK-LGLL by an NK score of >4, 18 had intermediate scores (2-3), and 10 were diagnosed as reactive proliferations. CCL22 mutations were identified in 16 patients (20%), including 5 with intermediate scores who were reclassified as NK-LGLL; BMB supported the diagnosis in 2 additional cases, resulting in 57 NK-LGLL overall. These patients exhibited more cytopenias, higher treatment needs, and greater transfusion requirements than patients with alternative diagnoses. In the validation cohort (25 NK-LGLL and 32 reactive cases), CCL22 mutations were detected in 5 NK-LGLL (20%). Altogether, incorporation of CCL22 mutations reduced the fraction of unclassified patients, improved diagnostic sensitivity without compromising specificity, and may decrease reliance on invasive procedures. These revised international criteria represent a step toward standardized, molecularly guided NK-LGLL diagnosis.