Expression and coreceptor function of APJ for primate immunodeficiency viruses

APJ is a seven transmembrane domain G-protein-coupled receptor that functio ns as a coreceptor for some primate immunodeficiency virus strains. The in vivo significance of APJ coreceptor function remains to be elucidated, howe ver, due to the lack of an antibody that can be used to assess API expressi on, and because of the absence of an antibody or ligand that can block APJ coreceptor activity. Therefore, we produced a specific monoclonal antibody (MAb 856) to APJ and found that it detected this receptor in FAGS, immunofl uorescence, and immunohistochemistry studies. MAb 856 also recognized API b y Western blot, enabling us to determine that APJ is N-glycosylated. Using this antibody, we correlated APJ expression with coreceptor activity and fo und that APJ had coreceptor function even at low levels of expression. Howe ver, we found that API could not be detected by FAGS analysis on cell lines commonly used to propagate primate lentiviruses, nor was it expressed on h uman PBMC cultured under a variety of conditions. We also found that some v iral envelope proteins could mediate fusion with APJ-positive, CD4-negative cells, provided that CD4 was added in trans. These findings indicate that in some situations APJ use could render primary cell types susceptible to v irus infection, although we have not found any evidence that this occurs. F inally, the peptide ligand for APJ, apelin-13, efficiently blocked APJ core ceptor activity. (C) 2000 Academic Press.