Cytotoxicity against lymphoblastoid cells mediated by a T-cell clone from an aplastic anaemia patient: role of CD59 on target cells

In an attempt to elucidate the pathogenic role of a CD4(+) cytotoxic T-cell clone NT4.2 isolated from the bone marrow of a patient with cyclosporine-d ependent aplastic anaemia, we characterized the T-cell clone as well as its cytotoxicity against an autologous Epstein-Barr (EB) virus-transformed B-l ymphoblastoid cell line (LCL). NT4.2 expressed BV21(+) BJ2.7(+) with a comp lementarity-determining region (CDR) 3 motif of SQGQGEVEQY which was homolo gous to that of a T-cell clone isolated from a patient with connective tiss ue disease. NT4.2 started to lyse LCL cells within 2 h and exerted maximal cytotoxicity within 3 h of incubation. The cytotoxicity required the presen ce of divalent cations and was not associated with DNA fragmentation of the target cells. Anti-CD59 monoclonal antibodies (MoAb) blocked the cytotoxic ity to the same degree as anti-CD3, HLA-DR or CD2 mAb. Flow cytometric anal ysis of the peripheral blood of this patient during remission after cyclosp orine therapy revealed 1.7% of granulocytes to be deficient in CD59. These findings indicate that NT4.2 exerts its cytotoxicity through a perforin-med iated pathway, not a Fas/Fas ligand-dependent pathway, and that haemopoieti c stem cells lacking CD59 may evade cytotoxic T lymphocytes, leading to the in vivo expansion of a paroxysmal nocturnal haemoglobinuria clone.