Human peripheral blood contains a small subpopulation of immature dendritic cells (iDC) distinguished from circulating monocytes by their low expression of CD14. We utilized leukapheresis and countercurrent centrifugal elutriation to obtain myeloid origin mononuclear cell (
MOMC) fractions of monocytes and iDC for study. These subpopulations were ultrastructurally and immunophenotypically similar before culture. After a 20- to 96-h culture either alone, with recombinant human granulocyte-monocyte CSF, or with
endotoxin, greater up-regulation of costimulatory molecule expression was observed among iDC than among monocytes, and only iDC expressed the activation molecule CD83. Treatment with rhIL-4 caused many
MOMC to develop morphologic properties of dendritic cells within 96 h, but costimulatory molecule up-regulation and CD14 down-regulation were heterogeneous, and CD83 expression was infrequent. In contrast,
calcium ionophore (CI) treatment induced rapid and consistent effects in
MOMC from both healthy volunteers and
cancer patients, including down-regulated CD14 expression, acquisition of dendritic cell morphologic properties, up-regulated MHC and costimulatory molecule expression, and de novo CD83 expression. Many such effects occurred within 20 h of treatment. CI treatment activated purified CD14+ monocytes and also enhanced the spontaneous activation of purified CD14-/dim iDC in culture. Unfractionated
MOMC, purified monocytes, and purified iDC displayed equivalently enhanced T cell-sensitizing efficiency following CI treatment. CD4+ T cell sensitization to
keyhole limpet hemocyanin and CD8+ T cell sensitization to MART-1
melanoma-associated
peptide were achieved in a single culture stimulation. Therefore, circulating monocytes and iDC can be induced by CI to manifest properties of activated DC, providing large numbers of efficient, nontransformed autologous APC for T cell sensitization strategies.