Aminopeptidase N (CD13) functionally interacts with FcγRs in human monocytes

Aminopeptidase N (E.C. 3.4.11.2) is a membrane-bound metalloproteinase expressed in many tissues. Although its cytoplasmic portion has only eight amino acids, cross-linking of CD13 by monoclonal antibodies (mAb) has been shown to trigger intracellular signaling. A functional association between CD13 and receptors for immunoglobulin G (Fc gamma Rs) has been proposed. In this work, we evaluated possible functional interactions between CD13 and Fc gamma Rs in human peripheral blood monocytes and in U-937 promonocytic cells. Our results show that during Fe gamma R-mediated phagocytosis, CD13 redistributes to the phagocytic cup and is internalized into the phagosomes. Moreover, modified erythrocytes that interact with the monocytic cell membrane through Fc gamma RI and CD13 are ingested simultaneously, more efficiently than those that interact through the Fc gamma RI only. Also, co-cross-linking of CD13 with Fc gamma RI by specific mAbs increases the level and duration of Syk phosphorylation induced by Fc gamma RI cross-linking. Finally, Fe gamma RI and CD13 colocalize in zones of cellular polarization and coredistribute after aggregation of either of them. These results demonstrate that CD13 and Fc gamma RI can functionally interact on the monocytic cell membrane and suggest that CD13 may act as a signal regulator of Fc gamma R function.