DETERMINANTS OF HIV-1 CORECEPTOR FUNCTION ON CC-CHEMOKINE-RECEPTOR-3 - IMPORTANCE OF BOTH EXTRACELLULAR AND TRANSMEMBRANE CYTOPLASMIC REGIONS/

The chemokine receptors CXCR4, CCR2b, CCR3, and CCR5 are cell entry co receptors for HIV-1. Using an HIV-1 envelope (Env) dependent cell-cell fusion model of entry, we show that CCR3 can interact with Envs from certain macrophage (M)-tropic strains (which also use CCR5), T cell li ne (TCL)-tropic laboratory-adapted strains (which also use CXCR4), and a dual tropic primary isolate (which also uses CCR2b, CCR5, and CXCR4 ). Paradoxically, CCR1 is the closest homologue to CCR3 (63% amino aci d identity), but lacked HIV-1 coreceptor activity, These results confi rm and extend previous reports, Replacing the N-terminal segment of CC R3 with that of CCR1 abolished activity of the resulting chimera for M -tropic and TCL-tropic Envs, but not for the dual-tropic Env, Replacin g extracellular loop 2 of CCR3 with that of CCR1 abolished activity fo r TCL-tropic Envs, but not for M- and dual-tropic Envs, A chimera cont aining all four extracellular regions of CCR3 on a backbone of CCR1 la cked any activity, Env-CCR3 interactions were strongly inhibited by th e major CCR3 ligand eotaxin, but weakly or not at all by other CCR3 li gands, With primary macrophages, eotaxin induced transient calcium flu x and partially inhibited fusion with cells expressing M-tropic Envs. We conclude that specificity determinants for different Envs are locat ed in shared and distinct extracellular regions of CCR3, the transmemb rane/cytoplasmic domains make major contributions to coreceptor functi on, and CCR3 may be used by certain HIV-1 strains as a cell fusion fac tor on macrophages.