A search for T lymphocyte-specific chemokine receptors led to the discovery of the CXCR3 receptor in 1996. A novel cDNA was isolated from a human CD4 þ T cell library and the encoded GPCR proved to have affinity for chemokines [1]. Further work revealed that CXCR3 consists of 368 amino acids which assemble in a typical seven-transmembrane a-helical architecture. Chapter 2 has described in detail the structural characteristics of the chemokine receptor family and CXCR3 fits this general picture. CXCR3 exhibits typical structural motifs for GPCRs, such as the conserved DRY motif, the NPxxYx 5,6 F motif and cysteine residues in the first and second extracellular loops [2]. Also, like most chemokine receptors, CXCR3 has additional cysteine residues in the N-terminus and third extracellular loop. The threonine and serine residues in the intracellular C-terminal tail are potential sites for phosphorylation by receptor-or second messenger-regulated kinases [1, 3].CXCR3 receptors are found on activated Th1 lymphocytes, blood T cells and on a small proportion of B cells and natural killer cells [1,4,5]. CXCR3 signaling occurs preferably through pertussis toxin-sensitive Ga i proteins and involves several downstream processes such as mediation of chemotaxis, induction of calcium flux and activation of kinases such as p44/p42 MAPK and Akt [1, 3,6]. The endogenous agonists for CXCR3 are CXC chemokines CXCL9, CXCL10 and CXCL11, which were traditionally called Mig, IP-10 and I-TAC/IP-9, respectively [1, 7-10]. Of these, CXCL11 has the highest potency and efficacy on CXCR3 [1,[7][8][9][10][11]. Several reports exist on the interaction of other CXCL chemokines with CXCR3, most notably CXCL13 [12] and CXCL4 [13]. Generally, a chemokine is considered to bind to the N-terminus and extracellular loops of a chemokine receptor. Activation is thought to occur through interaction of the N-terminus of the chemokine with the transmembrane domains of the receptor [14,15]. Indeed, manipulations at the N-terminal amino acids of CXCL10 or CXCL11 can change it from a CXCR3 agonist into a CXCR3 antagonist [16][17][18][19].