Soluble factors also play an important role in mucosal transmission of HIV. Indeed, lymphokines RANTES, MIP-1a and MIP-1b, SDF-1 natural ligands of HIV co-receptors are found in varying concentrations in vaginal secretions. More recently, it has been shown that the CCL20/MIP3alpha, which is produced by epithelial cells genital HIV could interact with X4-and R5-tropic to inhibit infection.