Immune reconstitution after bone marrow transplantation for combined immunodeficiencies: down-modulation of Bcl-2 and high expression of CD95/Fas account for increased susceptibility to spontaneous and activation-induced lymphocyte cell death

We have studied the regeneration of T cell subsets and function after BMT i n 21 children affected by combined immunodeficiency after BMT, Zn the first months, the striking predominance of CD4(+) cells displayed the primed CD4 5R0(+) phenotype and a high number of activated (HLA-DR+)T cells were obser ved, Regeneration of naive CD4(+)CD45RA(+) cells correlated with the recove ry of proliferative responses to mitogens (r = 0.64, P < 0.001). Peripheral blood lymphocytes circulating after BMT undergo an increased process of in vitro cell death, resulting from two mechanisms: spontaneous apoptosis (SA ), a consequence of defective production of IL-2 and down-regulation of Bcl -2 (P = 0.02 vs healthy controls), and high susceptibility to activation-in duced cell death (AICD) after restimulation with mitogens, In accordance wi th the role of CD95/Fas in this latter process, we have observed a high lev el of CD95 expression (P < 0.001 vs healthy controls), correlated with AICD (P < 0.001) but not with SA, and decreasing with time after BMT (P < 0.001 ). Both SA and AICD levels correlated with the presence of activated T cell s and decreased with the progressive recovery of T cell proliferative respo nse. Therefore, the lymphocyte hyperactivated status might explain their su sceptibility to apoptosis and contribute to the genesis of immunodeficiency that follows BMT.