Subset-specific and tissue-specific expression of NK-cell markers and killer immunoglobulin-like receptors: Implications for NK-cell clonality assessment.

Accurate immunophenotypic evaluation of natural killer (NK)-cell neoplasms is challenging because NK-cells lack somatically rearranged antigen receptors, are comprised of phenotypically distinct subsets, and exhibit tissue-specific subset distribution. We developed a flow cytometry (FC) panel for NK-cell immunophenotypic analysis and clonality assessment and applied it to 120 specimens, including normal peripheral blood (PB), bone marrow (BM), lymph node (LN), and body fluids, as well as a series of NK-cell neoplasms and NK-cell clones of uncertain significance (NK-CUS). The expression ranges for NK-cell receptors (NKR) including killer immunoglobulin-like receptors (KIR, CD158) in the PB were consistent with those reported previously by other groups. The distribution of NK-cell subsets differed by the specimen type. While PB and BM NK-cells showed predominance of the CD56dim subset, LN and fluid samples were enriched for the CD56bright subset, resulting in higher CD94/NKG2A and lower KIR expression compared to PB. Epstein-Barr-virus (EBV)-positive NK/T-cell neoplasms exhibited a relatively consistent CD56bright CD16- CD94/NKG2A+ CD57- KIR- immunophenotype that resembled normal CD56bright NK-cells, emphasizing the importance of immunophenotypic aberrancies and increased scatter properties rather than restricted KIR expression in diagnosing such neoplasms. NK-large granular lymphocytic leukemia (NK-LGLL) demonstrated a mature CD56dim phenotype with restricted KIR expression. Twelve cases fulfilled the proposed criteria for NK-CUS, comprising both putative cytomegalovirus (CMV)-related NK-cell clones and phenotypically distinct (likely) CMV-unrelated clones with more pronounced immunophenotypic abnormalities. Our findings suggest that interpretation of NK-cell clonality should take into account NK-cell subset composition and tissue-specific receptor expression. These results provide practical considerations for NK-cell immunophenotypic analysis and contribute to a better understanding of the role of FC in diagnosing such neoplasms.