Abstract
Immune checkpoint blockades (ICBs) have revolutionized cancer therapy through unleashing anti-tumor adaptive immunity. Despite that, they are usually effective only in a small subset of patients and relapse can occur in patients who initially respond to the treatment. Recent breakthroughs in this field have identified innate immune checkpoints harnessed by cancer cells to escape immunosurveillance from innate immunity. MHC1 appears to be such a molecule expressed on cancer cells which can transmit a negative signal to innate immune cells through interaction with leukocyte immunoglobulin like receptor B1 (LILRB1). The review aims to summarize the current understanding of MHC1/LILRB1 axis on mediating cancer immune evasion with an emphasis on the therapeutic potential to block this axis for cancer therapy. Nevertheless, one should note that this field is still in its infancy and more studies are warranted to further verify the effectiveness and safety in clinical as well as the potential to combine with existing immune checkpoints.
1 Introduction
Immune checkpoint blockades (ICBs) are the most advanced methods for cancer treatment (, ). Through harnessing adaptive immunity, ICBs lead to long-lasting anti-tumor responses for certain types of cancers even at advanced stages (, ). Inspired by this tremendous success, recent efforts in cancer immunology have also recognized immune surveillance mediated by innate immune system. The MHC1/LILRB1 axis represents such a critical facet of the intricate interplay between cancer cells and the innate immune system, offering a unique perspective in the field of cancer immunotherapy (, ).
MHC1 molecules are expressed on the cell surface of all nucleated cells and primarily present peptides derived from intracellular proteins, including both self-proteins and viral proteins (). These peptides are typically 8 to 10 amino acids in length and are bound within a peptide-binding groove formed by the α1 and α2 domains of the MHC1 molecule (). The binding specificity of MHC1 molecules is influenced by the amino acid residues within the peptide-binding groove, which interact with specific amino acids of the presented peptide. This interaction contributes to the stability of the peptide-MHC complex and determines the recognition by cytotoxic T lymphocytes (CTLs), leading to the mobilization of adaptive immunity (). In the context of cancer immunology, orchestrating immune recognition of MHC1 molecules is responsible for presenting fragments of tumor-specific proteins (antigens) to CTLs which serves as the fundamental step for immune system to identify and eliminate cancer cells (, ).
Leukocyte immunoglobulin like receptor B1 (LILRB1), also known as ILT2 (Immunoglobulin-Like Transcript 2) or CD85j, is a cell surface receptor that belongs to the immunoglobulin superfamily (, ). It is primarily expressed on various innate immune cells, including natural killer (NK) cells, monocytes, macrophages, and dendritic cells (). LILRB1 functions as an immune inhibitory receptor and is involved in immune regulation and modulation of immune responses (). It interacts with specific ligands on target cells, which can be either classical or non-classical MHC1 molecules expressed on the cell surface. Through engaging MHC1, LILRB1 transmits inhibitory signals that dampen immune responses (). As a result, activation of LILRB1 can impact the clearance of pathogens and/or tumor cells through inhibition of innate immune system. Correspondingly, disrupting MHC1/LILRB1 has shown both preclinical and clinical therapeutic activity against several types of cancers, behaving like the established immune checkpoints such as PD-1/PD-L1 and CD47/Sirpα which have been extensively reviewed elsewhere (, ).
This review focuses on this newly emerged immune checkpoint and summarizes the current understanding of MHC1/LILRB1 axis on mediating cancer evasion from innate immune cells and the therapeutic potential to block this axis for cancer therapy. Additionally, it proposes that further studies are warranted to verify the effectiveness and safety in clinical as well as the synergistic potential to combine with existing immune checkpoint blockades.
2 Structure and mechanism of MHC1/LILRB1 interaction
MHC1 molecules consist of ternary complexes containing α chain (the MHC heavy chain), soluble serum protein β2 microglobulin (β2m), and bound peptide (, ). The α1 and α2 domains on the α chain form the peptide binding groove and contact the T-cell receptor, while α3 domain engages with the T cell co-receptor CD8 during T cell recognition. Differently, MHC1 binds to LILRB1 via α3 domain and β2m (, ). The β2m subunit is essential for the binding of MHC1 to the LILRB1 receptor, as evidence clearly shows the incapability of LILRB1 to recognize β2m-free forms of MHC1 molecules (). On top of this, β2m is also necessary for the proper folding and stability of the MHC I molecule (Figure 1) ().
Figure 1
The binding site on LILRB1 localizes on the two membrane-distal Ig domains while the other two Ig domains are not involved as disclosed by the crystal structure analysis (
3 Regulatory effect of the MHC1/LILRB1 Axis on innate immune cells
LILRB1 is the most widely expressed LILRB family member and its expression is demonstrated on various immune cell populations, including monocytes/macrophages, dendritic cells, and subsets of NK cells and T cells (
3.1 MHC1–LILRB1 signaling in macrophages
Macrophages, being major components in innate immune systems, exhibit notable plasticity and functional diversity (
The rapid advancements in cancer immunotherapy have underscored the critical need for fundamental investigations into immunoregulation across both adaptive and innate cell lineages (
Monocyte-lineage cells have been observed to express two MHC1-binding proteins namely LILRB1 and LILRB2, both of which not only bind to MHC1 but also possess ITIMs responsible for transmitting inhibitory signals intracellularly (
While macrophage-mediated immunotherapy has been extensively studied and has shown promising results in clinical practice (
Figure 2

The role of MHC1/LILRB1 axis on innate immune cells and cancer therapy. (A) MHC1 on cancer cells transmits a negative signal through LILRB1 to inhibit the phagocytic ability of TAMs (top panel). NK cells express markedly high levels of LILRB1 molecules, which, upon engagement with MHC1 on tumor cells, leads to suppression of NK cell activity (middle panel). Cancer cell MHC1 induces tolerogenic DCs through LILRB1 (bottom panel). (B) The interaction between MHC1 and LILRB1 primarily involves the β2m subunit. β2m levels are typically low in serum and synovial fluid under normal conditions but are significantly upregulated in various cancers, including cervical, ovarian, and gallbladder cancer, serving as a significant prognostic marker. (C) Blocking the MHC1/LILRB1 axis with monoclonal antibody or targeted drug has shown remarkable anti-tumor activity through harnessing tumoricidal activity of macrophages and NK cells.
3.2 MHC1–LILRB1 signaling in NK cells
NK cells play crucial roles in mediating anticancer immunity, and their effectiveness against cancer cells can be enhanced by engaging activating receptors or blocking inhibitory receptors (
Unlike macrophages, the expression of LILRB1 is only restricted to a subpopulation of NK cells and at lower level (
It is worth mentioning that LILRB1 is not the sole receptor for MHC1 engagement on NK cells. Killer inhibitory receptors (KIR), which are ubiquitously expressed on all types of NK cells, are the main MHC1 binding partners on NK cells (
3.3 MHC1–LILRB1 signaling in DC cells
Dendritic cells (DCs) are the most effective antigen presentation cells in the immune system, bridging the activation of innate and adaptive immunity (
DCs have been implicated in the induction of immune tolerance through their interaction with MHC1-LILRB1. This interaction transmits inhibitory signals that contribute to the suppression of immune responses, thereby preventing excessive activation and promoting tolerance. For instance, in the presence of a LILRB1 ligand, DCs cannot be fully activated by LPS and are unable to secrete inflammatory cytokines (
4 Emerging role of MHC1/LILRB1 axis as an innate immune checkpoint
Immune checkpoint proteins are regulatory molecules that act as gatekeepers of immune responses (
In recent years, the concept of immune checkpoints has transformed cancer immunotherapy, allowing for remarkable advances in the field. In contrast, innate immune checkpoints are a relatively new concept, involving receptors and ligands regulating innate immune cell activity (
4.1 Role of MHC1/LILRB1 axis in cancer
MHC1 is a fundamental component of the immune system, responsible for presenting antigenic peptides to T cells (
Numerous studies have revealed that cancer cells can actively exploit this axis to evade immune surveillance by the innate immune system. As mentioned above, the main subunit on MHC1 interacting with LILRB1 is β2m, which remains at a low level in serum and synovial fluid under normal physiological conditions. In contrast, the elevation of β2m has been shown to be a significant prognostic marker for various types of cancers such as cervical cancer, ovarian cancer, gallbladder cancer and others (Figure 2B) (
In Tumor microenvironment (TME), the expression of LILRB1 is mainly restricted in tumor stroma with tumor-associated macrophages as the major immune cell populations. This has been demonstrated in various types of cancers (
4.2 Targeting MHC1/LILRB1 axis in cancer therapy
Due to the important inhibitory function of MHC1/LILRB1 axis on innate immune cells, therapeutic disruption of this interaction may offer beneficial effects through unleashing anti-tumor innate immunity. Barkal et al. initially used GHI/75, a monoclonal antibody derived from murine hybridoma, to block MHC1/LILRB1 interaction. They showed that this antibody enhances macrophage-mediated phagocytosis of solid tumor cells which can be largely boosted when combining with CD47-targeted antibody magrolimab. Additionally, LILRB1 antibody can increase CD20 antibody-induced phagocytosis and EGFR antibody-induced phagocytosis in the presence or absence of an Fc-silent CD47 antibody (
Emerging evidence also shows that LILRB1 blockade leads to anti-tumor effects through NK cells. For example, in chronic lymphocytic leukimia, combination of LILRB1 antibody with lenalidomide promotes lysis of tumor cells through activation of NK cells (
The above promising results in preclinical models have paved the way for clinical trials for LILRB1 antagonists in patients with cancer. BND-22 is a promising therapeutic agent that specifically targets the LILRB1 receptor. It is an antibody antagonist designed to disrupt the interactions between LILRB1 and its ligands, thereby modulating the immune response against solid tumors (
5 Perspectives
The cancer-immune axis serves as a crucial cellular checkpoint, by which the immune system controls immune responses against cancer. In contrast, cancer cells often hijack these mechanisms to evade immune surveillance and progress. MHC1/LILRB1 seems to be such an axis to regulate innate immune responses for various types of cancer. Intriguingly, therapeutic blockades disrupting MHC1/LILRB1 interaction have been advanced in clinical trials, displaying a great promise for the therapy of cancer. Nevertheless, one should note that this field is still in its infancy and more studies are warranted to further investigate research directions including but not limited to (1) identification of possible alternative ligands on cancer cells to interact with LILRB1; (2) assess the function of immune cells according to induvial patients upon LILRB1 blockade treatment; (3) evaluate alteration of cytokine profiles of immune cells when inhibiting LILRB1; (4) demonstrate to what extent the expression of other inhibitory receptors may impact the efficacy of LILRB1 targeting agents and (5) therefore discover the rational combination for synergism to test in preclinical and clinical studies. Ultimately, it is hopeful that completion of these studies will help turn this promising strategy into a real therapy for human cancer.
Statements
Author contributions
ZHu: Writing – original draft, Writing – review & editing, Conceptualization, Investigation, Project administration, Visualization. QZ: Writing – original draft, Writing – review & editing, Software, Visualization. ZHe: Writing – original draft, Writing – review & editing, Visualization. XJ: Writing – original draft, Writing – review & editing, Investigation. WZ: Writing – original draft, Writing – review & editing, Investigation. XC: Writing – original draft, Writing – review & editing, Conceptualization, Funding acquisition, Investigation, Project administration, Supervision.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was funded by National Natural Science Foundation of China (82373892) to XC and the startup funding of Shanghai Jiao Tong University to XC.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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Summary
Keywords
MHC1, LILRB1, innate immunity, cancer immunotherapy, immune regulation
Citation
Hu Z, Zhang Q, He Z, Jia X, Zhang W and Cao X (2024) MHC1/LILRB1 axis as an innate immune checkpoint for cancer therapy. Front. Immunol. 15:1421092. doi: 10.3389/fimmu.2024.1421092
Received
21 April 2024
Accepted
27 May 2024
Published
07 June 2024
Volume
15 - 2024
Edited by
Jochen Mattner, University of Erlangen Nuremberg, Germany
Reviewed by
Benyamin Rosental, Ben-Gurion University of the Negev, Israel
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Copyright
© 2024 Hu, Zhang, He, Jia, Zhang and Cao.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Xu Cao, xcao@sjtu.edu.cn
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