Structural and Functional Basis for LILRB Immune Checkpoint Receptor Recognition of HLA-G Isoforms

Area of Science:

• Immunology
• Structural Biology
• Molecular Biology

Background:

• Human leukocyte Ig-like receptors (LILRs) are immune checkpoints regulating physiological responses.
• HLA-G, a ligand for LILRs, is expressed in placenta, immune cells, and tumors, with poorly understood isoform recognition.
• The β2-microglobulin (β2m)-free HLA-G1 isoform synthesized by placental cells dimerizes and multimerizes.

Purpose of the Study:

• To characterize the binding of LILRB1 and LILRB2 to the β2m-free HLA-G1 isoform.
• To elucidate the structural basis of LILR-HLA-G1 interactions.

Main Methods:

• Biochemical binding assays to assess LILR affinity for HLA-G1 isoforms.
• Crystal structure determination of the LILRB1-HLA-G1 complex.
• Analysis of LILR orientation and signaling potential.

Main Results:

• Multimerized β2m-free HLA-G1 dimers showed strong binding to LILRB2 but lacked detectable affinity for LILRB1.
• The crystal structure revealed LILRB1 binds HLA-G1 in a typical classical HLA class I complex conformation.
• LILRB1 exhibits flexible binding to the α3 domain but requires β2m for tight contacts, explaining β2m-dependent binding.
• Both LILRB1 and LILRB2 are positioned for efficient signaling upon binding HLA-G1 dimers.

Conclusions:

• LILRB2 exhibits strong affinity for multimerized β2m-free HLA-G1, suggesting a specific role in placental biology.
• LILRB1's β2m-dependent binding mechanism provides insights into its regulatory functions.
• These findings offer novel structural and functional understanding of LILR-ligand recognition in biological regulation.