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T cell-mediated suppression of haematopoiesis is believed to play an important role in the pathophysiology of aplastic anaemia (AA) and in the pancytopenia of some myelodysplastic syndromes (MDS). Natural-killer T (NKT) cells belong to a unique lymphocyte subset that expresses an invariant T-cell receptor (TCR), consisting of Valpha24JalphaQ, and common NK cell surface markers. NKT cells have been hypothesized to play a role in immune regulation, and many human autoimmune conditions are associated with NKT cell deficiency. Here we investigate the role of NKT cells in AA and MDS patients. Flow cytometry demonstrated that NKT cells, unlike other T-lymphocyte subpopulations, were disproportionally decreased in AA and MDS marrow. When we compared variability within the CDR3 region of Valpha24 in CD4-CD8- T cells derived from AA and healthy individuals, the CDR3 size of Valpha24 cells showed a polyclonal distribution in AA patients, while in control subjects a typical oligoclonal or monoclonal pattern was found. Southern blot and sequence analysis of Valpha24 polymerase chain reaction products revealed that the NKT cell-specific JalphaQ region was predominant in control subjects, whereas it was not, or only very weakly, detected in AA and MDS patients. These results show that NKT cells are profoundly decreased in AA and MDS, and their deficiency may, as in other human autoimmune diseases, play a role in the local immune dysregulation in AA and MDS.

Summary. We studied 18 patients with myelodysplastic syndrome (MDS), measuring clonality and T-cell receptor Vbeta (TCRBV) expression of CD4 and CD8 T cells by polymerase chain reaction and by flow cytometric analysis of TCRBV families. The CD4 and CD8 T-cell repertoire in most MDS patients is characterized by an abnormal TCRBV-restricted expansion of T cells in CD4 and CD8 cells, and increased expression of the CD8 effector marker CD57 of multiple TCRBV in CD8 cells. Clonality analysis of CD4 and CD8 cells showed that seven of 10 patients analysed had a major clone in the CD8 cells but not in CD4 cells. Furthermore, in one patient we found that both the CD57– and CD57+ fraction contained the clone (which was absent from the TCRBV-negative fraction). These data suggest that, in MDS, multiple T-cell expansions can be found in both helper and cytotoxic T cells, and that, in the CD8 cells, T cells functionally differentiate in vivo from memory to effector T cells. Together, these data support the hypothesis of the involvement of T cells in the pathogenesis of MDS.

Large granular lymphocyte (LGL) leukaemia is a disease with increased numbers of circulating granular lymphocytes and an increased percentage of clonally rearranged CD8(+)CD57(+) cells. To determine whether LGL cells are also found in other lymphocyte subsets, CD8(+) cells from 10 LGL patients were sorted into CD57(+) and CD57(-) fractions and analysed for clonality using a T-cell receptor gamma (TCR gamma) polymerase chain reaction (PCR). In nine patients, a clonal TCR rearrangement was identified in the CD8(+)CD57(+) cells, and in one patient, the TCR rearrangement was oligoclonal in the CD8(+)CD57(+) fraction. In eight out of nine of the clonally rearranged patients, the same band was also present in the CD8(+)CD57(-) fraction. To define the relationship between CD57(-) and CD57(+) LGL populations, CD8(+)CD57(-) and CD8(+)CD57(+) cells were sorted from five patients and cultured in the presence of anti-CD3 plus CD28 antibodies. The CD57(+) cells died of apoptosis before d 7, while the CD57(-) cells proliferated and differentiated into CD57(+) cells. Clonal analysis identified the same band in both cultured subpopulations and in the uncultured CD8(+) cells. Immunophenotypical analysis showed that CD8(+)CD57(-) cells expressed memory cell markers, while the CD8(+)CD57(+) cells exhibited effector characteristics. These results suggest that LGL disease originates in a CD57(-) memory T-cell compartment that continually generates CD57(+) (effector cell) progeny.

To investigate mechanisms of stem cell graft rejection we studied the allo-stimulatory potential of G-CSF mobilized peripheral blood progenitor cells (PBPC). CD34+ cells were purified (> 95%) in a two-step procedure using immunoaffinity columns for CD34 selection and T-depletion. The capacity of CD34+ cells to stimulate allogeneic T-cell responses was compared with other cells from the same individual. CD34+ cells induced potent proliferative responses at stimulator:responder ratios of 1:20, but were approximately 50-fold less efficient compared to dendritic cells. Furthermore, CD34+ cells primed responses from partially matched allogeneic T cells in bulk cultures. Dual-colour flow cytometry showed that the co-stimulatory molecules B7.1, CD40 and ICAM-1 were absent on resting CD34-positive progenitor cells, but were induced during incubation with allogeneic lymphocytes due to a cytokine-mediated effect. Up-regulation of accessory molecules on CD34+ cells was reproduced by incubation with interferon-γ or GM-CSF which enhanced the allo-stimulatory activity of CD34+ cells. Blocking studies with inhibitory antibodies suggested co-stimulatory functions for B7.2, ICAM-3, CD40 and LFA-3. CD34+ cells were more efficient in inducing allogeneic T-cell responses when compared to the unprocessed leukapheresis products. The reduced allo-stimulatory ability of G-CSF mobilized PBPC could be explained by the presence of CD3+4+ and CD3+8+ lymphocytes with suppressor activity. We conclude that current methods of stem cell selection for transplantation do not avoid allo-sensitization of the recipient and that further graft manipulation with add-back of lymphocytes or selection of subsets of CD34+ cells with reduced allo-stimulatory ability may reduce graft rejection.