PHORBOL ESTER-INDUCED DOWN-MODULATION OF TAILLESS CD4 RECEPTORS REQUIRES PRIOR BINDING OF GP120 AND SUGGESTS A ROLE FOR ACCESSORY MOLECULES

The entry of human immunodeficiency virus type 1 into cells proceeds v ia a fusion mechanism that is initiated by binding of the viral glycop rotein gp120-gp41 to its cellular receptor CD4. Species- and tissue-sp ecific restrictions to viral entry suggested the participation of addi tional membrane components in the postbinding fusion events. In a prev ious study (H. Golding, J. Manischewitz, L. Vujcic, R. Blumenthal, and D. Dimitrov, J. Virol. 68:1962-1968, 1994), it was found that phorbol myristate acetate (PMA) inhibits human immunodeficiency virus type 1 envelope-mediated cell fusion by inducing down modulation of an access ory component(s) in the CD4 expressing cells. The fusion inhibition wa s seen in a variety of cells, including T-cell transfectants expressin g engineered CD4 receptors (CD4.401 and CD4.CD8) which are not suscept ible to down modulation by PMA treatment. In the current study, it was found that preincubation of A2.01.CD4.401 cells with soluble monomeri c gp120 for 1 h at 37 degrees C primed them for PMA-induced down modul ation (up to 70%) of the tailless CD4 receptors. The gp120-priming eff ect was temperature dependent, and the down modulation may have occurr ed via clathrin-coated pits. Importantly, nonhuman cell lines expressi ng tailless CD4 molecules did not down modulate their CD4 receptors un der the same conditions. The gp120-dependent PMA-induced down modulati on of tailless CD4 receptors could be efficiently blocked by the human monoclonal antibodies 48D and 17B, which bind with increased avidity to gp120 that was previously bound to CD4 (M. Thali, J. P. Moore, C. F urman, M. Charles, D. D. Ho, J. Robinson, and J. Sodroski, J. Virol. 6 7:3978-3988, 1993). These findings suggest that gp120 binding to cellu lar CD4 receptors induces conformational changes leading to associatio n of the gp120-CD4 complexes with accessory transmembrane molecules th at are susceptible to PMA-induced down modulation and can target the v irions to clathrin-coated pits.