Alzheimer's disease (AD) is a progressive neurodegenerative disorder. The generation of amyloid-beta from the amyloid precursor protein (APP) C-terminal fragment (C99) by gamma-secretase cleavage is one of the main pathological mechanisms of AD. Dendritic cell factor 1 (Dcf1) is a membrane protein that was previously found to play a role in the development of AD. Bioinformatic analysis of AD patients indicated that Dcf1 may affect gamma-secretase. In this study, we confirmed that Dcf1 attenuates the cleavage of C99 in vivo and in vitro. By using C99 transgenic AD drosophila, we found that Dcf1 reduces the cleavage of C99 by gamma-secretase using Dcf1 overexpression. The climbing ability and lifespan of C99 drosophila were significantly increased, while learning and memory were also enhanced with Dcf1 expression. Increased levels of C99 protein in Dcf1-AD drosophila reveals inhibition of C99 cleavage by Dcf1 in vivo. Dcf1 inhibition of gamma-secretase was further confirmed in vitro. These results provide a potential therapeutic target for the treatment of AD and also propose a new mechanism for understanding the occurrence of AD. (C) 2020 Elsevier Inc. All rights reserved.