To the Editor: Most clinical HIV-1 isolates can infect CD4⁺ peripheral blood T lymphocytes, monocytes, and cultured macrophages (macrophage or M-tropic) but not transformed T-cell lines. In contrast, HIV-1 strains adapted for growth in transformed T-cell lines (T-cell line or T-tropic) do not infect primary monocytes or macrophages. This difference in tropism appears to be a consequence of specific amino acid changes in the env protein. Changes in env responsible for an M- to T-tropism shift often involve the acquisition of multiple positively charged residues in the hypervariable V3 loop domain (1). However, some non-V3 determinants are also important for viral tropism. Although both types of viruses use CD4 as receptor, the CXCR4 chemokine receptor (previously designated LESTR/fusin) is the unique cofactor for entry of T-tropic HIV-1 strains (2). The CCR5 chemokine receptor was subsequently demonstrated to be the cofactor for M-tropic HIV-1 isolates (3). Although some direct evidence for cell surface association of the CD4-env complex and the CXCR4 coreceptor was obtained (4), little detail is available on the molecular forces responsible for these protein-protein interactions. In particular, there is no direct evidence to indicate that the V3 loop binds to the chemokine receptor.

Figure

Figure. Amino acid alignment of the four chemokines was performed with the PC/Gene program. An overall identity and similarity of 12.4% and 40.2% was calculated, respectively. The initiator methionine contained within each...

Jiang (5) reported that the extracellular domain of the CXCR4 coreceptor for T-tropic HIV-1 isolates is more negatively charged than the CCR5 coreceptor. Because T-tropic isolates have evolved a positively charged V3 domain, it was suggested that coreceptor-env binding involves the interaction between oppositely charged residues. We now expand this analysis by showing that the chemokines corresponding to the different receptors have a similarly unbalanced composition of charged amino acids. So far, the CXCR4 receptor has been demonstrated to bind only the SDF-1 chemokine (6). The CCR5 receptor binds more than one chemokine, including RANTES, MIP-1α , and MIP-1ß (7). The amino acid sequence of these four chemokines is presented in the Figure. We listed the number of positive residues (arginine [R] and lysine [K]) and negative residues (aspartic acid [D] and glutamic acid [E]), and calculated the net charge. SDF-1 appears to have the highest number of positive residues and the lowest number of negative residues, resulting in a net charge of +11. All other chemokines have much less positively charged amino acids, resulting in a net charge for MIP-1α and MIP-1ß of -2 and -1, respectively. The RANTES chemokine has an intermediate charge of +7, which may correlate with the unique receptor use of this chemokine (e.g., RANTES, but not MIP-1α and MIP-1ß, binds the CCR3 receptor [7]). These results are consistent with the idea that positive charges in SDF-1 interact with negative charges in the CXCR4 receptor, and this binding may thus resemble the HIV-1 env-CXCR4 interaction.

Early evidence that both the chemokines and HIV-1 bind to the same domain of the chemokine receptor comes from virus inhibition studies. Several ß-chemokines suppress infection with M-tropic but not T-tropic HIV isolates (8), and SDF-1 specifically blocks entry of T-tropic isolates (6). Although direct blocking of the receptor may explain part of this chemokine-mediated inhibition, it has also been proposed that internalization of the receptor contributes to the antiviral effect (9). Irrespective of the precise antiviral mechanism, the combined results of this analysis and the one presented by Jiang (5) suggest that both the SDF-1 chemokine and the V3 loop of T-tropic HIV-1 viruses use positively charged amino acids for an electrostatic interaction with the negatively charged CXCR4 receptor. To examine whether the similarity between the chemokine and env V3 domain is also apparent at the primary sequence level, we performed an amino acid alignment; however, we found no conserved motifs (data not shown). A detailed mutational analysis is required to further our understanding of the env-coreceptor interaction.

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