The chemokine receptor CCR5 is constitutively associated with the T cell co-receptor CD4 in plasma cell membranes. The CD4-CCR5 complex exhibits distinct binding properties for macrophage inflammatory protein 1beta (MIP-1beta) and enhanced G-protein signaling as compared with those of CCR5 alone. Here we report that recombinant soluble CD4, when refolded into its dimeric form, allosterically modulates CCR5 and decreases the affinity for its natural ligand MIP-1beta. Monomeric soluble CD4 had little inhibitory effect on CCR5. In contrast, the two-domain amino-terminal fragment of soluble CD4 was able to completely inhibit the interaction of CCR5 with MIP-1beta. Thus, we suggest that various conformational states of CD4 exist, which differ markedly with regard to inhibiting the interaction of CCR5 with its ligand MIP-1beta. R5-tropic HIV-1 glycoprotein 120, but not interleukin-16, the natural agonist, or X4-tropic glycoprotein 120, inhibited MIP-1beta binding to CCR5 in the presence of monomeric and dimeric soluble CD4. (C) 2003 Elsevier Inc. All rights reserved.