DENDRITIC CELLS AS THE TERMINAL STAGE OF MONOCYTE DIFFERENTIATION

Monocytes (MO) cultured for greater than or equal to 5 days with eithe r macrophage-CSF (M-CSF) or granulocyte macrophage (GM)-CSF and IL-4 d ifferentiated without concomitant proliferation into CD14(+) macrophag es (M phi) or CD1a(+) dendritic cells (DC), respectively. When adheren t and nonadherent CD14(high) M phi from M-CSF cultures were separated and cultured further in cytokine-free medium or with GM-CSF/IL-4, most cells from both fractions that were exposed to GM-CSF/IL-4 acquired C D1a expression and DC morphology and function. Conversely, GM-CSF/IL-4 withdrawal at day 5 and additional culture of sorted CD1a(+) DC for 2 to 7 days in cytokine-free medium led to cells rapidly becoming adher ent CD1a(-)CD14(+) M phi. Replacing GM-CSF/IL-4 with M-CSF hastened th e conversion of DC to M phi without increasing cell numbers. CD1a(+)CD 14(-)CD83(+) mature DC were induced by a greater than or equal to 2-da y exposure to MO-conditioned medium, LPS, or TNF-alpha/IL-1 beta. Upon cytokine removal or culture with M-CSF, DC that had been pushed to ma turation by conditioned medium or LPS remained stable or died in the n ew environment. TNF-alpha/IL-1 beta-driven DC displayed heterogeneous CD83 expression and could thus be sorted into CD83(high) and CD83(low/ -) cells; in cytokine-free medium or in M-CSP, most CD83(low/-) cells converted to M phi, whereas most CD83(high) cells remained nonadherent CD1a(+)CD14(-) or died and thus appeared truly terminally differentia ted. Hence, MO are precursors of M phi as well as of DC, with each cel l type having the capability to convert into the other until late in t he differentiation/maturation process. Accordingly, the cytokine envir onment and the presence of differentiation and/or other stimulatory si gnals may be the ''final decision-making factors'' determining whether these cells will acquire DC or M phi characteristics and function.