Emerging roles of KIR2DL4 in cancer immunotherapy.

KIR2DL4, also known as CD158d, exhibits predominant expression on NK cells, with evidence suggesting its presence on select subsets of T cells. The expression levels of KIR2DL4 are subject to interindividual variation and can be modulated by various factors such as genetic polymorphisms, and cytokines. Accumulated evidence has shown KIRs exhibit high levels of polymorphism and contribute to the wide range of diversity within the human NK cell repertoire through three key attributes [ 17 ]. Firstly, individuals have the potential to inherit distinct haplotypes of the 14 KIR family member genes, resulting in diverse gene content that may consist predominantly of inhibitory KIRs (haplotype A) or a more heterogeneous repertoire with a higher proportion of activating KIRs (haplotype B) [ 18 ]. Furthermore, individual KIRs have the ability to identify specific subsets of classical MHC-I ligands. Additionally, the expression of KIR genes on NK cells is random, resulting in the generation of a varied repertoire. In contrast to other KIR2DL receptors, KIR2DL4 is unique in being expressed by all NK cells and present in all individuals and comprises the framework genes of the locus and is grouped into all KIR haplotypes [ 19 – 21 ]. Goodridge et.al found that the 10 A KIR2DL4 allele encodes a full-length receptor with significant membrane expression, while the 9 A KIR2DL4 allele results in a truncated protein that lacks the inhibitory motif. The study demonstrates that individuals with at least one 10 A allele show cell surface expression of KIR2DL4, primarily on the CD56 bright NK cell subset [ 22 ]. Additionally, Goodridge et.al reported that individuals with the 10 A KIR2DL4 allele may lack detectable KIR2DL4 on resting CD56 bright NK cells due to the excision of the D0 domain and the 9 A KIR2DL4 allele can produce a secreted KIR2DL4 receptor [ 23 ]. All these studies indicated that KIR2DL4 can be detected on all NK cell subset. Our previous study has shown that KIR2DL4 facilitates antibody-dependent cell-mediated cytotoxicity (ADCC) and establishes a regulatory relationship with the interferon-γ (IFN-γ) production pathway. This pathway involves IFN-γ inducing the upregulation of KIR2DL4 through JAK2/STAT1 signaling, followed by KIR2DL4 working in conjunction with the Fcγ receptor to enhance IFN-γ secretion by NK cells [ 13 ]. Akiko et.al reported that KIR2DL4 expressed on the cell surface can be up-regulated by IL-2 [ 24 ]. HLA-G, a non-classical MHC class I molecule, is the only known ligand of KIR2DL4 [ 25 ]. The HLA-G gene consists of 8 exons and 7 introns, with most full-length HLA-G transcripts typically containing 7 exons due to the removal of exon 7 through splicing. In comparison to classical HLA class I molecules, HLA-G is relatively short, with a total length of approximately 340 amino acids. The signal peptide is encoded by exon 1, while the three extracellular domains (α1-α3) are translated from exons 2, 3, and 4, respectively. The transmembrane region is generated by exon 5, and the intracellular region is generated by exon 6. Accumulating evidence has demonstrated the existence of seven isoforms of HLA-G, each characterized by a unique molecular structure resulting from distinct transcript splicing events [ 9 ]. Among these isoforms, HLA-G1 exhibits both membrane-binding and soluble forms, featuring three extracellular domains (α1-α3), while HLA-G2, HLA-G3, and HLA-G4 are exclusively membrane-binding isoforms. Specifically, HLA-G2 possesses two extracellular domains (α1 and α3), HLA-G3 contains only the α1 domain, and HLA-G4 displays two extracellular domains (α1 and α2). Conversely, HLA-G5, HLA-G6, and HLA-G7 are classified as soluble isoforms, with HLA-G5 presenting three extracellular domains (α1-α3), HLA-G6 featuring two extracellular domains (α1 and α3), and HLA-G7 having an α1 domain. In contrast to the classical MHC class I molecule, The D0 domain of KIR2DL4 interacts with the α1 domain of HLA-G within the peptide binding cleft. Nevertheless, the peptide is situated at a greater depth within the binding cleft compared to the location where KIR2DL4 binds, indicating that the peptide does not play a role in the binding interaction between KIR2DL4 and HLA-G [ 26 – 28 ]. Despite the predominant expression of HLA-G in the fetal cytotrophoblast, its physiological presence has been documented in adult stem cells, progenitor cells, somatic cells in immune-privileged tissues, and certain immune cells [ 29 – 31 ]. Additionally, ectopic expression of HLA-G has been observed in various pathological states, such as tumor [ 32 ].

KIR2DL4 stands out among KIRs for its distinct capacity to interact with HLA-G, a ligand frequently overexpressed on tumor cells as a means of evading immune surveillance [ 33 , 34 ]. This interaction has the potential to impede NK cell-mediated cytotoxicity, thereby facilitating uncontrolled tumor progression [ 35 ]. Consequently, the investigation of the targeting KIR2DL4 therapy has garnered significant attention in the realm of research, as it presents promising opportunities for the advancement of novel immunotherapeutic approaches aimed at bolstering anti-tumor immune responses. He et.al found that 10 candidate hub genes including KIR2DL4 exhibit a significant correlation with immune infiltration, which has successfully established a comprehensive immune network and systematically elucidated the molecular mechanisms associated with endometriosis [ 36 ]. Recent study has found a novel comprehensive index of aging and immune (CIAI) signature comprising 7 genes, including KIR2DL4, in which patients with melanoma in the high-CIAI group exhibited significantly shorter overall survival (OS), disease-specific survival (DSS), and progression-free interval (PFI), suggesting that the CIAI model can serve as an independent prognostic index. Additionally, their analysis revealed potential correlations between the CIAI score and immune scores, estimate score, immune cell infiltration levels, tumor microenvironment characteristics, immunotherapy response, and drug sensitivity [ 37 ]. Multiple studies have demonstrated that the expression levels of KIR2DL4 may serve as a potential prognostic marker for melanoma, indicating the likelihood of disease aggressiveness [ 38 – 41 ]. Breast cancer frequently exploits immune checkpoints to evade surveillance. Ozturk et.al found that the frame genes of KIR2DL4 were found in all breast cancer patients [ 42 ]. In 2015, Ueshima and their colleagues found that the KIR2DL4 on human mast cells facilitates HLA-G-expressing breast cancer invasion and the subsequent metastasis [ 43 ]. Our previous study has shown that the interaction between HLA-G and KIR2DL4 on NK cells inhibits trastuzumab-induced ADCC, but blocking this interaction can enhance the effectiveness of trastuzumab in combating tumors in vivo. Conversely, when not bound by HLA-G, KIR2DL4 can stimulate ADCC and establish a regulatory loop with the IFN-γ production pathway through JAK2/STAT1 signaling in NK cells. Furthermore, the presence of paracrine TGF-β and IFN-γ in the breast cancer microenvironment can induce the expression of PD-1/PD-L1 on NK cells and tumor cells, potentially exacerbating immunosuppression through enhanced intercellular signaling. These findings underscore the potential clinical implications of targeting HLA-G/KIR2DL4 interactions to enhance antitumor immune responses [ 13 ]. All these studies showed that KIR2DL4 plays an important role in cancer immunotherapy and targeting KIR2DL4 might be an effective approach. Due to the special structure, KIR2DL4 can deliver both activating and inhibitory signals. Based on this, there are few effective targeting KIR2DL4 treatment regimens have been developed. However, there are a series of KIRs based immunotherapy strategies that can provide new insights into future cancer immunotherapies targeting KIR2DL4, such as immune checkpoint blockade (ICB) therapy, adoptive cell transfer (ACT) therapy and small-molecule inhibitors [ 44 ]. The cytotoxic activity of NK cells against healthy host cells is primarily inhibited by the interaction between inhibitory KIRs and MHC class I molecules. Previous findings suggest that tumor cells evade NK cell-mediated killing by increasing the expression of MHC class I molecules. Consequently, researchers have focused on inhibitory KIRs as a key regulatory mechanism in NK cell function and have explored the use of specific antibodies to block these receptors in clinical trials [ 44 ]. IPH2101 and Lirilumab (IPH2102), which are considered the most suitable antibodies, along with anti-KIR 1-7F9 IPH2101 (previously known as 1-7F9), the initial agent in this category, have been utilized to target KIR2DL1/2/3 to enhance NK cell cytotoxicity [ 45 , 46 ]. Ren et.al elucidated the functional mechanisms by which KIR2DL5 mediates NK cell immune evasion, demonstrated the potential therapeutic efficacy of blocking the KIR2DL5/PVR axis in human cancers, and offered insight into the underlying mechanism for the lack of clinical success of anti-TIGIT therapies [ 47 ]. All these studies suggested that blockade of KIR2DL4-HLA-G interaction by using an agonistic antibody targeting KIR2DL4 might have therapeutic potential. Series studies have found that KIRs play a critical role in controlling NK cell function. Strategies to improve CAR-NK cell function include targeting inhibitory KIR to prevent trogocytosis-mediated fratricide and activating KIR-rich NK cell donors may be beneficial for allogeneic NK cell therapy, which indicates that targeting KIR has the potential to enhance patient outcomes and efficacy of CAR-NK cell therapy [ 48 , 49 ]. These studies suggested that targeting KIR2DL4 by CAR-NK might switch inhibitory signals to ablate HLA-G positive tumor cells. As previously stated, SHP-1 and SHP-2 play crucial roles as primary mediators in the inhibitory signaling cascade. The interaction between inhibitory KIR and MHC class I molecules can result in the dephosphorylation of vav1 downstream of activating receptors by SHP-1. Subsequently, this process leads to the dephosphorylation of ERK and STAT3, ultimately suppressing NK cell cytotoxicity against normal cells [ 50 , 51 ]. Hence, targeting SHP-1 and SHP-2 as pivotal checkpoints in cancer immunotherapy to enhance NK cell activity within tumor microenvironments is a viable strategy. Experimental studies utilizing SHP-1 and SHP-2 inhibitor have shown promising results in the realm of cancer therapeutics [ 52 – 54 ]. KIR2DL4 can also recruit SHP-1 and SHP-2 to antagonize kinase-dependent activation cascades via ITIM signaling motif, which indicates that targeting SHP-1 and SHP-2 might be an efficient approach to retain the active potential of KIR2DL4. Recent studies have found that KIR2DL4 might be a tumor suppressor via its activating potential in breast cancer [ 55 ], which is consistent with the results in our previous study [ 13 ]. In general, the development of therapies could involve the use of monoclonal antibodies that specifically target KIR2DL4 or its interaction with HLA-G. In the case of therapies that target the interaction between KIR2DL4 and HLA-G, there is a risk of disrupting normal immune functions that rely on this interaction. Since KIR2DL4 is also involved in normal physiological processes, such as in pregnancy related immune tolerance, targeting it could potentially lead to unintended consequences in the immune system. Additionally, if the therapy affects NK cell function too profoundly, it could compromise the body’s natural defense against infections and other malignancies, highlighting the need for careful evaluation of potential side effects in the development of KIR2DL4 targeted therapies.

The immune system has been identified as a significant factor in the development and advancement of cancer. Various immunotherapy methods, such as targeting tumor antigens with antibodies, immune checkpoint blockade, and adoptive cell transfer (ACT) therapy, have been explored for the treatment of cancer patients. Recent research has indicated that combining traditional chemotherapy or radiotherapy with immunotherapy can lead to better outcomes for cancer patients [ 56 , 57 ]. The treatment approaches for cancer have evolved from cytotoxic therapy to enhancing anti-tumor immune responses. However, a limited proportion of cancer patients are able to derive advantages from immunotherapy at present. The efficacy of immunotherapy is hindered by the comparatively low response rate and the presence of an immunosuppressive tumor microenvironment even among those who do respond. The dysregulated expression of immune checkpoint molecules is identified as the primary factor contributing to immune evasion. Consequently, immune checkpoint blockade emerges as a crucial strategy for counteracting the immunosuppressive conditions within the tumor microenvironment. Combining KIR2DL4 targeted therapies with immune checkpoint inhibitors that target other molecules, such as PD-1/PD-L1, could also have synergistic effects. By simultaneously targeting different immune regulatory pathways, it may be possible to enhance the antitumor immune response and improve patient outcomes. Prospective studies should carefully evaluate the safety and efficacy of these combination therapies, taking into account factors such as potential drug-drug interactions and additive side effects. This review provides an overview of the distinctive structural characteristics of KIR2DL4, followed by an examination of its expression within the immune microenvironment of tumor and the potential utility of targeting KIR2DL4 in cancer immunotherapy. KIR2DL4 is the only KIR receptor known to interact with HLA-G. It possesses an ITIM domain within its cytoplasmic tail and a positively charged arginine in its transmembrane region, facilitating the recruitment of activation adaptors containing an ITAM domain. Due to its unique structural features, KIR2DL4 plays a crucial role in orchestrating a complex interplay between immune checkpoint signaling and cytokine responses within the tumor microenvironment. The engagement of HLA-G by KIR2DL4 ultimately determines the efficacy and outcome of immunotherapeutic interventions. The long-term outlook for KIR2DL4 targeted cancer immunotherapy is promising but also faces challenges. If the challenges related to specificity, efficacy, and side effects can be addressed, KIR2DL4 targeted therapies could become an important part of the cancer treatment arsenal. In the long term, these therapies could potentially be used in combination with other emerging immunotherapies and targeted drugs to provide more personalized treatment options for cancer patients. As our understanding of the role of KIR2DL4 in the tumor immune microenvironment continues to grow, more precise and effective therapies can be developed. Additionally, with the advancement of technologies for KIR2DL4 modulation, such as gene editing and novel imaging techniques, the future of KIR2DL4-targeted cancer immunotherapy looks promising. However, continued research, well-designed clinical trials, and careful consideration of ethical and safety aspects are essential for realizing the full potential of these therapies in improving cancer patient survival and quality of life.

Cancer immunotherapy has emerged as a transformative approach in oncology, offering a new paradigm for treating various malignancies [ 1 ]. It encompasses a range of strategies designed to stimulate the immune system to target and eliminate cancer cells [ 2 ]. This approach offers targeted and personalized therapy with durable responses and fewer side effects than traditional chemotherapy or radiotherapy. Cancer immunotherapy consists of four categories, including immune checkpoint blockade, tumor vaccines, adoptive cell immunotherapy, and oncolytic viruses [ 3 ]. Immunotherapy has led to remarkable improvements in survival for patients with melanoma, lung cancer, and several hematological malignancies [ 4 – 6 ]. It is also used in combination with other therapies, such as chemotherapy or radiation, to enhance their effectiveness [ 7 , 8 ]. While immunotherapy has transformed cancer treatment, it is not effective in all patients. Predicting which patients will respond to immunotherapy remains a challenge. Future research will focus on identifying biomarkers that can predict immunotherapy response and on developing new immunotherapy strategies to target a broader range of cancers. KIR2DL4, also known as killer cell immunoglobulin-like receptor 2DL4, is a member of the inhibitory killer cell immunoglobulin-like receptors (KIRs) family, which plays a crucial role in regulating the immune system [ 9 ]. HLA-G, a non-classical MHC class I molecule, is the only known ligand of KIR2DL4 [ 10 ]. By binding to HLA-G, KIR2DL4 can transmit inhibitory signal to immune cells, suppressing their activation and proliferation [ 9 ]. This interaction is crucial for maintaining immune homeostasis. KIR2DL4 plays a vital role in protecting the fetus from maternal immune attack. KIR2DL4 on maternal NK cells can recognize HLA-G expressed by the trophoblasts, which allows the fetus to evade immune surveillance via resulting in the suppression of NK cell activation and the promotion of a tolerogenic immune environment [ 10 ]. In addition to its role in pregnancy, KIR2DL4 also plays a role in anti-infective immune response. It has been shown to regulate NK cell and T cell responses against viral infections [ 11 ]. By understanding the function and regulation of KIR2DL4, we can gain insights into the immune system and develop more effective strategies for treating immune-related diseases and disorders. Accumulated evidences have shown that KIR2DL4 plays an important role in tumor immune escape [ 12 ]. KIR2DL4 can interact with its ligand HLA-G, which is often overexpressed in tumor cells. This interaction further inhibits the antitumor activity of immune cells, providing a mechanism for tumor evasion from immune surveillance. Our prior research demonstrated that inhibiting the interaction of KIR2DL4 in NK cells can enhance the efficacy of trastuzumab treatment in breast cancer, suggesting that KIR2DL4 plays a crucial role as an immune checkpoint in NK cells [ 13 , 14 ]. According to this study, blockade of KIR2DL4/HLA-G signaling using specific antibodies or other immunomodulatory agents, might be possible to enhance the antitumor activity of immune cells and improve the efficacy of tumor immunotherapy. In this review, we summarized and discussed the pivotal role of KIR2DL4 in tumor immunotherapy by regulating the immune response against tumor cells. Deeply understanding the functions and interactions of KIR2DL4 may lead to the development of more effective immunotherapy strategies that harness the immune system against cancer cells.