Chemokine ligands and their interactions with GPCR
Exclusive to vertebrates (1), chemokines (also called intercrines, or chemotactic cytokines) are a family of highly conserved secreted proteins of 8-15 kDa size, whose main role is to control immune cell trafficking, mainly leucocytes. An important attribute of chemokines is that they can be produced by the very cells they attract to inflammatory sites.
Chemokine ligands interact with 7-transmembrane-spanning family of 19 canonical specific receptors (cCKRs) and 4 atypical chemokine receptors (ACKRs). Together, these 23 chemokines receptors form the class A G-protein-coupled receptors (GPCRs) family. The atypical receptors, expressed by non-leukocyte cell types, have the particularity of being able to trigger signals through non-G protein-coupled mechanisms.
The 46 human chemokines have been classified into four classes depending of the number and positioning of four conserved cysteine residues (Oppenheim et al., 1991; Schall, 1991), giving birth to C, CC, CXC and CX3C families (2). If you check the previous figures again, you’ll realize that the number of ligands and receptors are not identical. In fact, chemokines are able to bind to several receptors, but receptors rarely bind chemokines from multiple families.
Since 1996, and thanks to mutational studies (3), the mechanism of ligand binding and receptor activation has traditionally been described as a two steps process occurring in two different locations. First, the core of the ligand (chemokine N-loop) specifically binds CKR N-terminus (chemokine recognition site 1, CRS1), and then the unstructured and flexible N-terminus of the chemokine (CS2) interacts with receptor transmembrane (TM) domain to induce a conformational change that is translated into intracellular signals. Extracellular loops of the GPCR may or may not intervene during step 1 or step 2. In 2019, an extension of the conventional “two-site, two-step model” was proposed by Sanchez et al. conferring a crucial importance to chemokine N-terminal region (4).
Similar to their ligands, chemokine receptors (a.k.a. beta chemokine receptors) are classified into 4 groups, namely:
- and CX3CRn.
Chemokine receptors targeting: a potential for new therapeutics
Along with the accelerated rate of discovery of chemokines has come the realization that these molecules not only control hemopoietic cell migration, but also are involved in a number of other physiological and pathological processes like wound healing, Th1/Th2 development, or inflammation. These findings have supported therapeutic development in the field of allergy, infectious diseases and autoimmune diseases by specifically targeting chemokine receptors. CCR2 has so been targeted in the context of rheumatoid arthritis (5), CCR3 for allergy (6), CCR5 for HIV (7) and CCR7 for pulmonary fibrosis (8).
But the subsequent discovery of the involvement of chemokines in the processes of metastasis, angiogenesis and angiostasis has extended the therapeutic field of GPCR CKR targeting to oncology. Famous humanized defucosylated IgG1 mAb-targeting CCR4 developed by Kyowa Kirin - for patients with relapsed or refractory CCR4+ adult T-cell leukemia (ATL) - is one example of major breakthrough in cancer immunotherapy.
But generating therapeutics targeting CKRs is not a long quiet river.
First, is the problem of species cross-reactivity. Preclinical data obtained on rodents - like described for BX471 anti-CCR1 (9) - don’t permit to extrapolate data to humans. A second issue exists where the chemokine receptor expression pattern differs between the two species. To avoid major potency differences, SYnAbs recommends solutions like the development of surrogate molecules and the use of transgenic animals in which the murine chemokine receptor is replaced with its human counterpart.
Another concern is the multiplicity of molecules involved in the immune response. It is rare, if not unlikely, that cell recruitment is limited to a single chemokine receptor. Therefore, an effector drug that interacts simultaneously with multiple receptors, or a combination therapy of multiple molecules, would potentially be more effective.
We can also mention the side effect of increased susceptibility to infection as a consequence to reduced chemokine function (10), suboptimal pharmacokinetic, toxicity profile of some of chemokine receptor antagonists, or the lack of selectivity towards the receptor (like MK-0812 for CCR2).
Small molecules or chemokine receptor-specific antibodies?
While small molecules were rapidly developed to target the chemokine receptors as a first approach - and despite the success of the CCR5 antagonist (Pfizer's Maraviroc, for the treatment of HIV) and their lower production cost compared to monoclonal antibodies -, small molecules still face several hurdles:
- in-vivo distribution, with potential penetration of the central nervous system (CNS), whereas antibodies have limited exposure to the CNS due to the blood-brain barrier and can target peripheral receptors,
- short half-life,
- inter-patient variability in plasma concentration at a given dose,
- restrictive mechanisms of action, where antibodies may offer Fc effector function, T-cell engagement, antibody-drug conjugation or even CAR-T modalities.
Consequently, more and more antibodies approaches are currently in development and has led to push drug pipelines with promising antibodies like Cytodyn’s PRO-140 Leronlimab (anti-CCR5 for graft-vs-host disease, human immunodeficiency virus-1 and COVID-19) or Bristol Myers Squibb’s BMS-936564/MDX-1338 Ulocuplumab (anti-CXCR4 for lymphoma and multiple myeloma).
(1) Zlotnik A, Yoshie O, Nomiyama H. The chemokine and chemokine receptor superfamilies and their molecular evolution. Genome Biol. 2006; 7(12):243.
(2) Miller MC, Mayo KH. Chemokines from a Structural Perspective. Int J Mol Sci. 2017;18(10):2088. Published 2017 Oct 2. doi:10.3390/ijms18102088
(3) Monteclaro FS, Charo IF. The amino-terminal extracellular domain of the MCP-1 receptor, but not the RANTES/MIP-1alpha receptor, confers chemokine selectivity. Evidence for a two-step mechanism for MCP-1 receptor activation. J Biol Chem. 1996 Aug 9; 271(32):19084-92.
(4) Sanchez J, E Huma Z, Lane JR, et al. Evaluation and extension of the two-site, two-step model for binding and activation of the chemokine receptor CCR1. J Biol Chem. 2019;294(10):3464-3475. doi:10.1074/jbc.RA118.006535
(5) Vergunst CE, Gerlag DM, Lopatinskaya L, Klareskog L, Smith MD, van den Bosch F, et al. Modulation of CCR2 in rheumatoid arthritis: a double-blind, randomized, placebo-controlled clinical trial. Arthritis Rheum (2008) 58(7):1931–9. doi:10.1002/art.23591
(6) Ben S, Li X, Xu F, Xu W, Li W, Wu Z, et al. Treatment with anti-CC chemokine receptor 3 monoclonal antibody or dexamethasone inhibits the migration and differentiation of bone marrow CD34 progenitor cells in an allergic mouse model. Allergy (2008) 63(9):1164–76. doi:10.1111/j.1398-9995.2008.01747
(7) Olson WC, Jacobson JM. CCR5 monoclonal antibodies for HIV-1 therapy. Curr Opin HIV AIDS (2009) 4(2):104–11. doi:10.1097/COH.0b013e3283224015
(8) Pierce EM, Carpenter K, Jakubzick C, Kunkel SL, Flaherty KR, Martinez FJ, et al. Therapeutic targeting of CC ligand 21 or CC chemokine receptor 7 abrogates pulmonary fibrosis induced by the adoptive transfer of human pulmonary fibroblasts to immunodeficient mice. Am J Pathol (2007) 170(4):1152–64. doi:10.2353/ajpath.2007.060649
(9) James Onuffer, Margaret A. McCarrick, Laura Dunning, Meina Liang, Mary Rosser, Guo-Ping Wei, Howard Ng and Richard Horuk. Structure Function Differences in Nonpeptide CCR1 Antagonists for Human and Mouse CCR1. Immunol February 15, 2003, 170 (4) 1910-1916; DOI: https://doi.org/10.4049/jimmunol.170.4.1910
(10) Schroff, R., Touvay, C., Culler, M., Dong, J., Taylor, J., Thurieau, C., & McKilligin, E. (2005). The Toxicology of Chemokine Inhibition. Mini-Reviews in Medicinal Chemistry, 5(9), 849–855. doi:10.2174/1389557054867093