Diagnostic Criteria for NK-Cell Large Granular Lymphocyte Leukemia: Validation Through a Multicentric International Study

: Natural killer large granular lymphocytic leukemia (NK-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 three national registries (USA, Italy, France). The revised framework integrates NK-score parameters with CCL22 mutations and bone marrow biopsy (BMB) findings. Four major criteria were defined: NK-cell count ≥1.0×10⁹/L, KIR 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 NK-LGLL by NK-score >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 compared to 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.