T cells expressing the activating NK-cell receptors KIR2DS4, NKG2C and NKG2D are elevated in paroxysmal nocturnal hemoglobinuria and cytotoxic toward hematopoietic progenitor cell lines

OBJECTIVE: To investigate the presence of T cells with natural killer cell receptors (NKR) in paroxysmal nocturnal hemoglobinuria (PNH), and their potential involvement in clonal expansion of glycosylphosphatidylinositol (GPI)-deficient hematopoietic stem cells by selective immune attack to normal and not GPI-deficient hematopoietic stem cells. MATERIALS AND METHODS: By flow cytometry, the frequency and number of T cells expressing NKR was evaluated in 39 PNH patients and compared to healthy controls. Elevated T cell subsets in PNH were assessed for differential cytotoxic lysis of GPI(+) and GPI(-) CD34(+) hematopoietic progenitor cell lines. RESULTS: In PNH patients, the frequency (p < 0.001) and absolute number of T cells expressing the NKR CD56 (p = 0.01) were significantly increased. Furthermore, a higher percentage of T cells expressed the activating NKR NKG2D (p < 0.01), NKG2C (p < 0.01), and KIR2DS4 (p = 0.01). Further characterization showed that these populations predominantly consist of CD8(+) effector memory CD45RA(+) T cells (T(EMRA)). NKR(+) cytotoxic T-lymphocyte lines isolated from PNH patient peripheral blood and bone marrow displayed high cytotoxicity towards CD34(+) hematopoietic progenitor cell lines and K562 cells, suggesting major histocompatibility complex class I-independent cytotoxicity. These cytotoxic T lymphocyte (CTL) lines are capable of differential lysis of GPI(+) and GPI(-) hematopoietic cell lines, however, not in all cases. This suggests that multiple factors, such as the highly activated status of in vitro cultured CTLs, influence whether GPI-dependent lysis occurs. CONCLUSIONS: The increased frequency of CD8(+) effector-memory T cells with activating NKR and cytotoxicity toward hematopoietic cell lines suggests involvement in bone marrow failure and clonal expansion in PNH