Direct measurement of soluble CD4 binding to human immunodeficiency virus type 1 virions: gp120 dissociation and its implications for virus-cell binding and fusion reactions and their neutralization by soluble CD4.
We have analyzed the binding of soluble CD4 (sCD4) to human immunodeficiency virus type 1 (HIV-1) virions (isolates IIIB and RF) at 4 and 37 degrees C by using a combination of gel exclusion chromatography and enzyme-linked immunosorbent assay detection systems. The sCD4 binding curve at 37 degrees C indicates that the affinity of the interaction of sCD4 with gp120 on the virion surface is indistinguishable from the affinity of sCD4 for the equivalent concentration of soluble gp120. At 4 degrees C, however, the affinity of sCD4 for virion-bound gp120 but not for soluble gp120 is reduced by about 20-fold. Binding of sCD4 (greater than 0.2 microgram/ml) to virions at 37 degrees C but not 4 degrees C induces the rapid dissociation of a major proportion of gp120 from gp41 on the virion surface. This dissociation requires occupancy by sCD4 of multiple (probably two) binding sites on a gp120-gp41 oligomer. At 37 degrees C there are two components to the neutralizing action of sCD4 on HIV-1; reversible, competitive inhibition at low sCD4 concentrations (less than 0.2 microgram/ml) and essentially irreversible inhibition due to gp120 loss at higher sCD4 concentrations. At 4 degrees C, sCD4 neutralizes HIV infectivity by competitive inhibition alone. These findings may have implications for the HIV-CD4+ cell binding and fusion reactions and the mechanism by which sCD4 blocks infectivity.
Moore
J P
JP
Chester Beatty Laboratories, Institute of Cancer Research, London, United Kingdom.
McKeating
J A
JA
Norton
W A
WA
Sattentau
Q J
QJ
D015704:CD4 Antigens / Q000276:immunology*; D002460:Cell Line; D004797:Enzyme-Linked Immunosorbent Assay; D015699:HIV Envelope Protein gp120 / Q000276:immunology*; D015497:HIV-1 / Q000276:immunology*; D006801:Humans; D007700:Kinetics; D009500:Neutralization Tests; D011485:Protein Binding; D011994:Recombinant Proteins / Q000276:immunology; D014771:Virion / Q000276:immunology*
D015704:CD4 Antigens; D015699:HIV Envelope Protein gp120; D011994:Recombinant Proteins