The rapidity of ECP induction of costimulatory molecule CD86 by monocytes, along with the homogeneity of its expression, was compared in parallel with the efficiency of the conventional method. (A) This typical flow cytometric histogram of a normal subject's ECP-processed monocyte population reveals the rapidity with which homogeneous expression of the costimulatory molecule CD86 (B7.2) was induced. After 18-hour incubation (Day 1), 94% of ECP-processed monocytes uniformly increased their expression of the cell surface CD86 (red), above the level of the isotype control (blue). Gating of leukocytes into the Monocyte/DC fraction was accomplished by forward and side scatter. Mean fluorescence intensity (MFI) of the induced CD86⁺ cells was 15. With such a large fraction of processed monocytes positive, the overall MFI was nearly as high (14.5). (B) ECP induction of CD86 was more rapid and extensive than that observed with cytokine-stimulated conventional monocyte-to-DC conversion. Even after the usual 6 days of culture with GMCSF and IL4, cytometric analysis of monocytes from a typical normal subject revealed less uniform induction of this costimulatory molecule. Because, for immunotherapeutic protocols, antigen loading of DC produced in this manner occurs at this juncture, prior to a second maturation step, this time point is most appropriate for comparison with ECP-induced DCs. The percentage of monocytes expressing CD86 (64 %) was lower than that observed with ECP-processed monocytes after only a single day of incubation. Although the MFI of the positive fraction was moderately higher at 6 days (MFI = 20) than that of the ECP-induced DCs at 1 day, the overall MFI was lower (MFI = 10.8), with nearly one-third of monocytes remaining negative for this marker.