CCR8
C-C motif chemokine receptor 8 (CCR8) is a class A G protein-coupled receptor (GPCR) implicated in regulating the immune system’s response to cancer. It is highly expressed on the surface of immune cells within the tumor microenvironment, specifically T regulatory cells (Tregs), and has emerged as a promising target in cancer.
Antibodies targeting CCR8 have proven effective in generating robust anti-tumor responses in preclinical studies, making them an appealing therapeutic approach. Several companies, including Bristol Myers Squibb, Bayer, and Amgen, have developed anti-CCR8 antibodies that are currently being tested in clinical trials.
However, the precise molecular mechanism of CCR8-mediated activation of immune response and the antibody-mediated inhibition of CCR8 are not fully understood.
In a recent study by Genentech, the 3D structure of CCR8 in a complex with an antibody was determined and elucidated the molecular basis of interaction, providing valuable insights for the generation of future therapeutic antibodies targeting chemokine GPCRs.
The scientists produced and characterized an antibody, mAb1, that binds CCR8. MAb1 inhibits the protein function and is selective for binding CCR8 over other C-C chemokine receptors (CCR). They solved the 3D structure of the fragment-antigen binding region of mAb1 (Fab1) bound to CCR8 by cryo-EM. The Fab1 binds the extracellular region of CCR8, forming extensive interactions with the extracellular loop 1 (ECL1) and ECL2, but not with the N-terminus or ECL3, which are not resolved in the structure (Image 1). This is particularly notable because many previously studied anti-GPCR antibodies bind to the N-terminal of the receptors.
Image 1. Cryo-EM model of the Fab1-CCR8 complex (PDB: 8TLM; CCR8 in red, Fab1 heavy variable chain (VH) in orange, Fab1 light variable chain (VL) in purple). ELC1 is highlighted in green, and ECL2 is highlighted in cyan, using the 3decision® Annotation Browser feature. The location of the N-terminus and ECL3 are indicated by the light grey arrows. Zoomed-in panels show details of protein-protein interactions (PPIs) between the ECL2 and the CDRH3 (a) and CDRH1 (b). PPIs are calculated by the software 3decision®. Measurement distances between the ECL2 and CDRH3 β-strands are reported (a). In the bottom-right box the protein is rotated to show the PPIs between the CDRH1 and ECL2. All pictures are produced with the 3decision® software.
The Fab1-CCR8 interactions are mainly electrostatic and are primarily mediated by Fab1's heavy chain complementarity-determining region 3 (CDRH3), especially with the ECL2 region (Image 1, zoomed-in view). ECL2 is stabilized in a β-hairpin fold and forms β-strand/β-strand interactions with CDRH3. CDRH1 and CDRH2 also form additional interactions with the side of ECL2 facing ECL1 (Image 1, rotated view), making ECL2 the main region of CCR8 that mediates the interaction with the antibody. Since ECL2 is not highly conserved across the CCR family, this provides a structural rationale for the observed selectivity of the antibody.
A few other ECL-binding antibodies have been previously reported (e.g., mAbs targeting 5-HT2B), and they have long and structured CDRH3, which form β-strand/β-strand interactions with ECL2, similar to CCR8 (Image 2). This structural insight suggests that long and structured CDRH3 may be a key feature for developing effective ECL-targeting antibodies.
Image 2. 3D structure of the complex between 5-HT2B and an antibody Fab fragment (PDB: 5TUD; 5-HT2B in green, Fab light chain in dark red, Fab heavy chain in pink). ELC2 is highlighted in orange, using the 3decision® Annotation Browser feature. The CDRH3 is indicated by the arrow. PPIs between the ECL2 and the CDRH3 are represented. PPIs are calculated by the software 3decision®.
This study opens novel opportunities for designing therapeutic antibodies with improved binding properties that target chemokine receptors and - more broadly - class A GPCRs.
Reference:
Sun D, Sun Y, Janezic E, Zhou T, Johnson M, Azumaya C, Noreng S, Chiu C, Seki A, Arenzana TL, Nicoludis JM, Shi Y, Wang B, Ho H, Joshi P, Tam C, Payandeh J, Comps-Agrar L, Wang J, Rutz S, Koerber JT, Masureel M. Structural basis of antibody inhibition and chemokine activation of the human CC chemokine receptor 8. Nat Commun. 2023 Dec 1;14(1):7940. doi: 10.1038/s41467-023-43601-8. PMID: 38040762; PMCID: PMC10692165.
Check out the 3decision annotation browser feature in the video below: