Several reports including those from this laboratory have demonstrated that bone marrow cells (BMC) downregulate in vitro both mixed leukocyte reaction and cytotoxic T lymphocyte reactions. We consequently hypothesized that a general property of immature cells of hematopoietic organs is their ability to suppress immune reactivity. As one of these suppressive activities, the lack of costimulatory molecules was proposed as a mechanism by which immature antigen presenting cells of the bone marrow might be involved. In the present report, we used two culture environments, each of which would regulate a different maturation pattern of human bone marrow-derived enriched dendritic antigen presenting cells (DC or APC) to determine the respective effects on in vitro immune regulatory function. Human BMC depleted of CD3+ cells were cultured with either: interleukin-4 (IL-4) and granulocyte macrophage-colony stimulating factor (GM-CSF), to maintain DC-enriched populations in an immature state (iAPC); or an interferon-gamma (IFNgamma), tumor necrosis factor alpha (TNF-alpha), GM-CSF, LPS, and IL-6 cocktail to promote the maturation of DC-enriched APC (mAPC). These iAPC and mAPC were, respectively, phenotypically characterized and also tested in vitro for the following: (1) both direct and indirect-antigen presentation functions; (2) immune regulatory functions on the response of autologous and allogeneic peripheral blood lymphocytes (PBL); and (3) Western blot analysis determining the levels of both major histocompatibility complex (MHC) class I related cytoplasmic transporter molecules associated with antigen processing (TAP1) and as well as proteasome activator molecules (PA28alpha). The iAPC population expressed fewer dendritic cell markers (CD83 and DCsign), and costimulator molecules (CD86 and CD40) than the mAPC, such that there was an approximate threefold increase in expression of CD83, 2.5-fold increase in DCsign, and a threefold increase in CD40 and CD86 on mAPC than on iAPC (p=0.005 for CD83; p=0.001 for DCsign; p=0.001 for CD86; and p=0.001 for CD40). In lymphoproliferative assays, indirect and direct alloantigen presentation by iAPC was weaker than by mAPC (p=0.05 and 0.04). In addition, iAPC were able to downregulate allogeneic CTL responses. Also, after pulsing with Epstein-Barr virus (EBV) protein antigens, the iAPC were less efficient in their presentation to autologous EBV-specific T-cell lines, and caused an inhibition of EBV-CTL generation. The expression of TAP1 and PA28alpha was reduced in iAPC in comparison to mAPC. These findings support the notion that a maturation state of BMC-derived APC correlates with their capacity to present antigen. The observed in vitro deficiency of this function by immature bone marrow cells may therefore contribute to the immune downregulatory capacity seen in the BMC compartment.