Inflammatory processes involving reactive microglia, e.g., those associated with /1-amyloid containing neuritic and core plaques in Alzheimer's disease, appear to contribute to neuronal degeneration in the CNS. The fact that increased nerve growth factor (NGF) protein levels were found throughout brains of Alzheimer's disease patients led us to investigate neurotrophin synthesis in a human microglial cell line showing typical properties of human microglial cells, including expression of neurotrophins such as NGF, as well as the NGF receptor trkA and the low-affinity neurotrophin receptor p75. We found that the cytokines interleukin-1/3 and tumor necrosis factor-a synergistically stimulate microglial NGF transcription and protein release. Moreover, exposure of microglial cells to complement factor C3a induces NGF expression. To assess the role of the transcription factor nuclear factor-KB (NF-KB) in inflammatory mediator-induced microglial NGF expression, the effect of the NF-KB inhibitor pyrrolidine dithiocarbamate (PDTC) was analyzed. In the presence of PDTC, a dose-dependent inhibition of cytokine-activated NGF expression occurred. In contrast, the C3a-dependent stimulation of NGF synthesis was not influenced by PDTC. In addition, microglial neurotoxicitymediating /3-amyloid peptides A/1(1-40) and A~3(1-42) failed to alter NGF synthesis, whereas A/9(25-35) specifically induced NF-KB-dependent microglial NGF expression. In conclusion, inflammatory signals (cytokines and complement factors), as well as A/3(25-35), are potent stimulators of human microglial NGF synthesis involving NF-KB-dependent and -independent mechanisms. Microglial secretion of neurotrophins appears to be involved in early processes of neuronal regeneration. Key Words: Nerve growth factor-Cytokines-Complement factors-Alzheimer's disease. J. Neurochem. 70, 699-707 (1998).ing free oxygen intermediates, proteases, excitatory amino acids, arachidonic acid derivatives, and cytokines, and have been implicated in the destruction of neurons in trauma (Banati et al., 1993;Thanos et al., 1993), Alzheimer's disease (McGeer et al., 1987(McGeer et al., , 1993 Giulian et al., 1995Giulian et al., , 1996McGeer and McGeer, 1995;Kaltschmidt et al., 1997), and human immunodeficiency virus infection (Giulian et al., 1990;Dickson et al., 1993). Moreover, in vitro studies have revealed that microglia/brain macrophages can secrete growth factors [e.g., basic fibroblast growth factor and nerve growth factor (NGF) 1~known to enhance survival and fiber outgrowth of different types of CNS neurons Mallat et al., 1989;Shimojo et al., 1991;Araujo and Cotman, 1992;Elkabes et al., 1996). For their survival and maintenance of function, several classes of neurons rely on the continuous supply of neurotrophic factors, including NGF. After injury, increased synthesis of NGF occurs in glial cells and in target tissues (Thoenen and Barde, 1980;Levi-Montalcini and Calissano, 1986; Levi-Montalcmi, 1987). In response to CNS injury, both microglia and astrocytes undergo str...