The
chemokine receptor CXCR4 is a co-
receptor for T-tropic strains of HIV-1. A number of
small molecule antagonists of CXCR4 are in development, but all are likely to lead to adverse effects due to the
physiological function of CXCR4. To prevent these complications, allosteric agonists may be therapeutically useful as
adjuvant therapy in combination with
small molecule antagonists. A synthetic
cDNA library coding for 160,000 different SDF-based peptides was screened for CXCR4
agonist activity in a
yeast strain expressing functional
receptor. Peptides that activated CXCR4 in an autocrine manner induced
colony formation. Two peptides, designated RSVM and ASLW, were identified as novel agonists that are insensitive to the CXCR4
antagonist AMD3100. In
chemotaxis assays using the acute
lymphoblastic leukemia cell line CCRF-CEM, RSVM behaves as a
partial agonist and ASLW as a superagonist. The superagonist activity of ASLW may be related to its inability to induce receptor
internalization. In CCRF-CEM cells, the two peptides are also not inhibited by another CXCR4
antagonist, T140, or the neutralizing
monoclonal antibodies 12G5 and 44717.111. These results suggest that alternative
agonist binding sites are present on CXCR4 that could be screened to develop molecules for therapeutic use.