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.