One of major breakthroughs in immunotherapy against tumor is from blocking immune checkpoint molecules on tumor and reactive T cells. The development of CTLA-4 and PD-1 blockage antibodies has triggered to search for additional effective therapeutic strategies. This causes recent findings that blocking the interaction of checkpoint molecule NKG2A in NK and CD8 T cells with HLA-E in tumors is effective in defensing tumors. Interestingly, gut microbiota also affects this immune checkpoint immunotherapy against tumor. Gut microbiota such as bacteria can contribute to the regulation of host immune response and homeostasis. They not only promote the differentiation and function of immunosuppressive cells but also the inflammatory cells through the metabolites such as tryptophan (Trp) and bile acid (BA) metabolites as well as short chain fatty acids (SCFAs). These gut microbiota metabolites (GMMs) educated immune cells can affect the differentiation and function of effective CD8 and NK cells. Notably, these metabolites also directly affect the activity of CD8 and NK cells. Furthermore, the expression of CD94/NKG2A in the immune cells and/or their ligand HLA-E in the tumor cells is also regulated by gut microbiota associated immune factors. These findings offer new insights for the clinical application of gut microbiota in precise and/or personalized treatments of tumors. In this review, we will discuss the impacts of GMMs and GMM educated immune cells on the activity of effective CD8 and NK cells and the expression of CD94/NKG2A in immune cells and/or their ligand HLA-E in tumor cells.

Hematopoietic cell transplantation (HCT) represents a potentially curative therapeutic approach for various hematologic and non-hematologic malignancies. Human leukocyte antigen (HLA) matching is still the central selection criterion for HCT donors. Nevertheless, post-transplant complications, in particular graft-versus-host disease (GvHD), relapse of disease and infectious complications, represent a major challenge and contribute significantly to morbidity and mortality. Recently, non-classical HLA class I molecules, especially HLA-E, have gained increasing attention in the context of allogeneic HCT. This review aims to summarize the latest findings on the immunomodulatory role of HLA-E, which serves as a ligand for receptors of the innate and adaptive immune system. In particular, we aim to elucidate how (i) polymorphisms within HLA-E, (ii) the NKG2A/C axis and (iii) the repertoire of peptides presented by HLA-E jointly influence the functionality of immune effector cells. Understanding this intricate network of interactions is crucial as it significantly affects NK and T cell responses and thus clinical outcomes after HCT.

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We investigated the phenotypic characteristics of human leukocyte antigen (HLA)-E-expressing macrophages, NKG2A/CD94 expression in T and natural killer (NK) cells, and their interactions in patients with adult-onset Still\'s disease (AOSD). Peripheral blood mononuclear cells from 22 patients with AOSD and 22 healthy controls (HC) were used. Isolated monocytes were cultured first with macrophage colony-stimulating factor to differentiate into M0 macrophages and subsequently with lipopolysaccharide/interferon-γ or interleukin-4 to differentiate into M1 or M2 macrophages, respectively. HLA-E and NKG2A/CD94 expression levels were evaluated using quantitative RT-PCR and flow cytometry. HLA-E expression in M0 and M2 macrophages was significantly higher in patients with AOSD than in HC, and was positively correlated with serum C-reactive protein levels and erythrocyte sedimentation rate. NKG2A/CD94 expression in CD4 + and CD8 + T cells was significantly higher in patients with AOSD than in HC, but that in NK cells was not significantly different. In patients with AOSD, NKG2A expression in CD4 + T cells positively correlated with HLA-E expression in M0, M1, and M2 macrophages. CD94 expression in CD8 + T cells inversely correlated with HLA-E expression in M1 and M2 macrophages. NKG2A and CD94 expression in NK cells inversely correlated with HLA-E expression in M0, M1, and M2 macrophages. No significant correlation was observed between HLA-E and NKG2A/CD94 expression in HC. Increased expression of HLA-E in macrophages and NKG2A/CD94 in T cells can be observed in the inflammatory condition of AOSD. HLA-E-expressing macrophages may be associated with NKG2A/CD94 expression in T and NK cells with different correlations.

Natural killer (NK) cells eliminate infected or cancer cells via their cytotoxic capacity. NKG2A is an inhibitory receptor on NK cells and cancer cells often overexpress its ligand HLA-E to evade NK cell surveillance. Given the successes of immune checkpoint blockade in cancer therapy, NKG2A is an interesting novel target. However, anti-NKG2A antibodies have shown limited clinical response. In the pursuit of enhancing NK cell-mediated anti-tumor responses, we devised a Cas9-based strategy to delete KLRC1, encoding NKG2A, in human primary NK cells. Our approach involved electroporation of KLRC1-targeting Cas9 ribonucleoprotein resulting in effective ablation of NKG2A expression. Compared with anti-NKG2A antibody blockade, NKG2AKO NK cells exhibited enhanced activation, reduced suppressive signaling, and elevated expression of key transcription factors. NKG2AKO NK cells overcame inhibition from HLA-E, significantly boosting NK cell activity against solid and hematologic cancer cells. We validated this efficacy across multiple cell lines, a xenograft mouse model, and primary human leukemic cells. Combining NKG2A knockout with antibody coating of tumor cells further enhanced cytotoxicity through ADCC. Thus, we provide a comprehensive comparison of inhibition of the NKG2A pathway using genetic ablation and antibodies and provide novel insight in the observed differences in molecular mechanisms, which can be translated to enhance adoptive NK cell immunotherapy.

OBJECTIVE: The purpose was to detected features of the expression levels of NKG2A and its ligand HLA-E, a new member of the immune checkpoints, in advanced laryngeal carcinoma and their clinicopathologic significance.
METHODS: We analyzed the expression levels of HLA-E and NKG2A in multiple types of tumors utilizing the Tumor Immune Estimation Resource (TIMER) database and immunohistochemistry and qRT-PCR analysis of paraffin embedded tissue samples to reveal the correlations of the clinicopathological factors with the expression of these two proteins in advanced laryngeal carcinoma as well as their prognostic significance.
RESULTS: KLRC1 (the coding gene of NKG2A) and HLA-E are substantially overexpressed in various human cancers than normal tissues. HNSCC is also included. KLRC1 is differentially expressed in different HPV subgroups of patients, with higher expression in the HPV-positive group. Consistent with this, immunohistochemical results also revealed the high expression of these two proteins in tumor tissue. In addition, immunohistochemical staining also displayed a preference for the distribution of NKG2A-positive cells in tumor tissue. Clinicopathological analyses also displayed that the density of NKG2A-positive cells of the HPV-positive group infiltrating laryngeal carcinoma tissue was larger than that in the HPV-negative group. Prognostic analyses indicated that the expression of this immune checkpoint does not affect the overall survival length of patients, but the highly expressed HLA-E is significantly correlated with local recurrence in the patients.
CONCLUSIONS: The findings suggest that the expression levels of HLA-E and NKG2A is upregulated in advanced laryngeal carcinoma. The NKG2A-positive cells infiltrating the tumor are mainly distributed in the cancer nest, while infiltrating cell number may be regulated by HPV. The highly expressed HLA-E may promote local recurrence in patients with advanced laryngeal carcinoma.

The Class I MHC molecule (MHC-I) HLA-E presents peptides that are derived from the signal sequences, either those of other MHC-I products, or of viral type I membrane glycoproteins. Monoclonal antibodies with proven specificity for HLA-E, and with no cross-reactions with other MHC-I products, have yet to be described. To obtain anti-HLA-E-specific antibodies suitable for a range of applications, we generated monoclonal antibodies against a unique feature of HLA-E: its cytoplasmic tail. We created an immunogen by performing an enzymatically catalyzed transpeptidation reaction to obtain a fusion of the cytoplasmic tail of HLA-E with a nanobody that recognizes murine Class II MHC (MHC-II) products. We obtained a mouse monoclonal antibody that recognizes a 13-residue stretch in the HLA-E cytoplasmic tail. We cloned the genes that encode this antibody in expression vectors to place an LPETG sortase recognition motif at the C-terminus of the heavy and light chains. This arrangement allows the site-specific installation of fluorophores or biotin at these C-termini. The resulting immunoglobulin preparations, labeled with 4 equivalents of a fluorescent or biotinylated payload of choice, can then be used for direct immunofluorescence or detection of the tag by fluorescence or by streptavidin-based methods. We also show that the 13-residue sequence can serve as an epitope tag, independent of the site of its placement within a protein\'s sequence. The antibody can be used diagnostically to stain for HLA-E on patient tumor samples, it can be used as an antibody-epitope tag for extracellular proteins, and it enables research into the unique role of the cytoplasmic tail of HLA-E.

Peptides presented by HLA-E, a molecule with very limited polymorphism, represent attractive targets for T cell receptor (TCR)-based immunotherapies to circumvent the limitations imposed by the high polymorphism of classical HLA genes in the human population. Here, we describe a TCR-based bispecific molecule that potently and selectively binds HLA-E in complex with a peptide encoded by the inhA gene of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis in humans. We reveal the biophysical and structural bases underpinning the potency and specificity of this molecule and demonstrate its ability to redirect polyclonal T cells to target HLA-E-expressing cells transduced with mycobacterial inhA as well as primary cells infected with virulent Mtb. Additionally, we demonstrate elimination of Mtb-infected cells and reduction of intracellular Mtb growth. Our study suggests an approach to enhance host T cell immunity against Mtb and provides proof of principle for an innovative TCR-based therapeutic strategy overcoming HLA polymorphism and therefore applicable to a broader patient population.

During pregnancy, the maternal immune system must allow and support the growth of the developing placenta while maintaining the integrity of the mother\'s body. The trophoblast\'s unique HLA signature is a key factor in this physiological process. This study focuses on decidual γδT cell populations and examines their expression of receptors that bind to non-classical HLA molecules, HLA-E and HLA-G. We demonstrate that decidual γδT cell subsets, including Vδ1, Vδ2, and double-negative (DN) Vδ1-/Vδ2- cells express HLA-specific regulatory receptors, such as NKG2C, NKG2A, ILT2, and KIR2DL4, each with varying dominance. Furthermore, decidual γδT cells produce cytokines (G-CSF, FGF2) and cytotoxic mediators (Granulysin, IFN-γ), suggesting functions in placental growth and pathogen defense. However, these processes seem to be controlled by factors other than trophoblast-derived non-classical HLA molecules. These findings indicate that decidual γδT cells have the potential to actively contribute to the maintenance of healthy human pregnancy.

The commonly used antibodies 3D12 and 4D12 recognise the human leukocyte antigen E (HLA-E) protein. These antibodies bind distinct epitopes on HLA-E and differ in their ability to bind alleles of the major histocompatibility complex E (MHC-E) proteins of rhesus and cynomolgus macaques. We confirmed that neither antibody cross-reacts with classical HLA alleles, and used hybrids of different MHC-E alleles to map the regions that are critical for their binding. 3D12 recognises a region on the alpha 3 domain, with its specificity for HLA-E resulting from the amino acids present at three key positions (219, 223 and 224) that are unique to HLA-E, while 4D12 binds to the start of the alpha 2 domain, adjacent to the C terminus of the presented peptide. 3D12 staining is increased by incubation of cells at 27°C, and by addition of the canonical signal sequence peptide presented by HLA-E peptide (VL9, VMAPRTLVL). This suggests that 3D12 may bind peptide-free forms of HLA-E, which would be expected to accumulate at the cell surface when cells are incubated at lower temperatures, as well as HLA-E with peptide. Therefore, additional studies are required to determine exactly what forms of HLA-E can be recognised by 3D12. In contrast, while staining with 4D12 was also increased when cells were incubated at 27°C, it was decreased when the VL9 peptide was added. We conclude that 4D12 preferentially binds to peptide-free HLA-E, and, although not suitable for measuring the total cell surface levels of MHC-E, may putatively identify peptide-receptive forms.