The Potent Anti-HIV Activity of CXCL12γ Correlates with Efficient CXCR4 Binding and Internalization

Abstract

We previously demonstrated that the naturally occurring splice variant stromal cell-derived factor 1␥/ CXCL12␥ is the most potent CXCL12 isoform in blocking X4 HIV-1, with weak chemotactic activity. A conserved BBXB domain (B for basic and X for any residue) located in the N terminus ( 24 KHLK 27 ) is found in all six isoforms of CXCL12. To determine whether the potent antiviral activity of CXCL12␥ is due to the presence of the extra C-terminal BBXB domains, we mutated each domain individually as well as in combination. Although binding of CXCL12␥ to heparan sulfate proteoglycan (HSPG) was 10-fold higher than that observed with CXCL12␣, the results did not demonstrate a direct correlation between HSPG binding and the potent antiviral activity. CXCL12␥ mutants lacking the conserved BBXB domain (designated ␥B1) showed increased binding to HSPG but reduced anti-HIV activity. In contrast, the mutants lacking the C-terminal second and/or third BBXB domain but retaining the conserved domain (designated B2, B3, and B23) showed decreased binding to HSPG but increased anti-HIV activity. The B2, B3, and B23 mutants were associated with enhanced CXCR4 binding, receptor internalization, and restored chemotaxis. Internalization of CXCR4 was more potent with CXCL12␥ than with CXCL12␣ and was significantly reduced when the conserved BBXB domain was mutated. We concluded that the observed potent anti-HIV-1 activity of CXCL12␥ is due to increased affinity for CXCR4 and to efficient receptor internalization.Chemokines are small, structurally related chemoattractant cytokines characterized by conserved cysteine residues. Based on the positions of the first N-terminal cysteines, chemokines fall into four subfamilies. The CC and CXC subfamilies have been well characterized. The CC subfamily includes the following: regulated on activation, normal T-cell expressed and secreted (RANTES), monocyte chemoattractant protein 1 (MCP-1), and macrophage inflammatory peptides 1 (MIP-1). The prototype of the CXC subfamily is interleukin-8 (IL-8)/ CXCL8. The C chemokine (lymphotactine) and CX3C chemokine (fractalkine) subfamilies were recently identified (reviewed in reference 30). The physiological activities of chemokines are mediated by the selective recognition and activation of chemokine receptors belonging to the seven-membrane-domain G-protein-coupled receptor superfamily (GPCRs). In addition, chemokines also bind to glycosaminoglycans (GAGs) through distinct binding sites. Chemokine binding to GAGs on cells, particularly endothelial cells, results in chemotactic chemokine gradients that allow correct presentation of chemokines to leukocytes, therefore enabling target cells to cross the endothelial barrier and migrate into tissues (reviewed in reference 10).Stromal cell-derived factor 1 (SDF-1)/CXCL12 is a member of the CXC chemokine family and is a key regulator of B-cell lymphopoiesis, hematopoietic stem cell mobilization, and leukocyte migration (reviewed in reference 10). CXCL12 was originally thought to mediate these processes through...

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