HLA-G turns off erythropoietin receptor signaling through JAK2 and JAK2 V617F dephosphorylation: clinical relevance in polycythemia vera

The LILRB family in hematologic malignancies: prognostic associations, mechanistic considerations, and therapeutic implications

The leukocyte immunoglobulin-like receptor B (LILRB) proteins, characterized by their transmembrane nature and canonical immunoreceptor tyrosine-based inhibitory motifs (ITIM) signaling, play a pivotal role in maintaining immune homeostasis and are implicated in the pathogenesis of various disease states. This comprehensive review will focus on the intricate involvement of the LILRB family in hematologic malignancies. These receptors have emerged as valuable diagnostic and prognostic biomarkers in leukemia, lymphoma, and myeloma. Beyond their prognostic implications, LILRBs actively shape the immune microenvironment and directly influence the disease pathogenesis of hematologic malignancies. Furthermore, their identification as potential therapeutic targets offer a promising avenue for precision medicine strategies in the treatment of these disorders. Currently, multiple LILRB directed therapies are in the preclinical and clinical trial pipelines. This review underscores the multifaceted role of the LILRB family in hematologic malignancies, highlighting their significance from diagnostic and prognostic perspectives to their broader impact on disease pathophysiology and as valuable therapeutic targets.

HLA-G Neo-Expression on Tumors

HLA-G is known to modulate the immune system activity in tissues where physiological immune-tolerance is necessary (i.e., maternal-fetal interface, thymus, and cornea). However, the frequent neo-expression of HLA-G in many cancer types has been previously and extensively described and is correlated with a bad prognosis. Despite being an MHC class I molecule, HLA-G is highly present in tumor context and shows unique characteristics of tissue restriction of a Tumor Associated Antigen (TAA), and potent immunosuppressive activity of an Immune CheckPoint (ICP). Consequently, HLA-G appears to be an excellent molecular target for immunotherapy. Although the relevance of HLA-G in cancer incidence and development has been proven in numerous tumors, its neo-expression pattern is still difficult to determine. Indeed, the estimation of HLA-G's actual expression in tumor tissue is limited, particularly concerning the presence and percentage of the new non-canonical isoforms, for which detection antibodies are scarce or inexistent. Here, we summarize the current knowledge about HLA-G neo-expression and implication in various tumor types, pointing out the need for the development of new tools to analyze in-depth the HLA-G neo-expression patterns, opening the way for the generation of new monoclonal antibodies and cell-based immunotherapies.

Novel landscape of HLA-G isoforms expressed in clear cell renal cell carcinoma patients

Immune checkpoints are powerful inhibitory molecules that promote tumor survival. Their blockade is now recognized as providing effective therapeutic benefit against cancer. Human leukocyte antigen G (HLA-G), a recently identified immune checkpoint, has been detected in many types of primary tumors and metastases, in malignant effusions as well as on tumor-infiltrating cells, particularly in patients with clear cell renal cell carcinoma (ccRCC). Here, in order to define a possible anticancer therapy, we used a molecular approach based on an unbiased strategy that combines transcriptome determination and immunohistochemical labeling, to analyze in-depth the HLA-G isoforms expressed in these tumors. We found that the expression of HLA-G is highly variable among tumors and distinct areas of the same tumor, testifying a marked inter- and intratumor heterogeneity. Moreover, our results generate an inventory of novel HLA-G isoforms which includes spliced forms that have an extended 5'-region and lack the transmembrane and alpha-1 domains. So far, these isoforms could not be detected by any method available and their assessment may improve the procedure by which tumors are analyzed. Collectively, our approach provides the first extensive portrait of HLA-G in ccRCC and reveals data that should prove suitable for the tailoring of future clinical applications.

Natural killer cells in patients with polycythemia vera

Natural killer cells (NK) are pivotal cells of innate immunity. They are potent antileukemic cytotoxic effectors. A defect in their cytotoxicity has been described in some hematopoietic malignancies such as acute myeloid leukemia, multiple myeloma and myelodysplastic syndromes. This defect is at least partially linked to a decreased or absent expression of some activating NK cells molecules, more particularly the so-called natural cytotoxicity receptors. In the present study, we more particularly focused our attention on NK cells of polycythemia vera, a myeloproliferative disease characterized by the presence of mutated JAK2 tyrosine kinase. The polymerase chain reaction analysis of NK cells from patients showed that they expressed the mutated form of JAK2. In polycythemia vera the proportion of NK was increased compared to healthy donors. The proliferative and cytotoxic abilities of NK cells from patients were similar to healthy donors. Expression of activating or inhibitory receptors was comparable in patients and donors, with nonetheless an imbalance for the inhibitory form of the CD158a,h couple of receptors in patients. Finally, the transcriptomic profile analysis clearly identified a discriminant signature between NK cells from patients and donors that could putatively be the consequence of abnormal continuous activation of mutated JAK2.

The dual role of HLA-G in cancer

We here review the current data on the role of HLA-G in cancer based on recent findings of an unexpected antitumor activity of HLA-G in hematological malignancies. For the past decade, HLA-G has been described as a tumor-escape mechanism favoring cancer progression, and blocking strategies have been proposed to counteract it. Aside from these numerous studies on solid tumors, recent data showed that HLA-G inhibits the proliferation of malignant B cells due to the interaction between HLA-G and its receptor ILT2, which mediates negative signaling on B cell proliferation. These results led to the conjecture that, according to the malignant cell type, HLA-G should be blocked or conversely induced to counteract tumor progression. In this context, we will here present (i) the dual role of HLA-G in solid and liquid tumors with special emphasis on (ii) the HLA-G active structures and their related ILT2 and ILT4 receptors and (iii) the current knowledge on regulatory mechanisms of HLA-G expression in tumors.

HLA-G1 increases the radiosensitivity of human tumoral cells

Different molecules regulate the response of tumoral tissues to ionizing radiation. The objective of this work was to determine if HLA-G1 expression modulates the radiosensitivity of human tumoral cell lines. To this end, human melanoma M8 and human erythroleukemia K562 cell lines, with their correspondent HLA-G1 negative and positive variants, were gamma irradiated and the survival frequency was determined by clonogenic assay. The survival fraction of HLA-G1 expressing cells was around 60% of HLA-G1 negative cells. The generation of acidic vesicular organelles was higher in HLA-G1 positive cells. Apoptosis levels showed statistically significant differences only in K562 cells, whereas the variation in G2/M cycle progression was only significant in M8 cells. In addition, irradiation diminished cell-surface HLA-G1 and increased soluble HLA-G1 levels. Soluble HLA-G1 has no influence on cell survival in any cell line. In summary, we could demonstrate that HLA-G1 confers higher radiosensitivity to HLA-G1 expressing cells.

HLA-G1 and HLA-G5 active dimers are present in malignant cells and effusions: the influence of the tumor microenvironment

Dimers of the nonclassical HLA-G class I molecule have recently been shown to be active structures that mediate inhibition of NK-cell cytotoxic activity through interaction with the immunoglobulin-like transcript (ILT)-2 inhibitory receptor. However, this has only been proven in trophoblasts and HLA-G transfectants. Here, we document for the first time the existence of HLA-G dimers in cancer. Indeed, we identified both surface and soluble HLA-G dimers in tumor cells and malignant ascites respectively. Interestingly, factors from the tumor microenvironment, such as interferons, enhanced the formation of HLA-G dimers and increased the protection of tumors from NK cell-mediated lysis. These data emphasize the impact of HLA-G conformation on its efficiency at inhibiting the antitumor response and thus favoring tumor progression. In view of these results, the effect of the tumor microenvironment on upregulation of HLA-G function deserves particular attention when designing cancer immunotherapy protocols.

Soluble HLA-G dampens CD94/NKG2A expression and function and differentially modulates chemotaxis and cytokine and chemokine secretion in CD56bright and CD56dim NK cells

Soluble HLA-G (sHLA-G) inhibits natural killer (NK) cell functions. Here, we investigated sHLA-G-mediated modulation of (1) chemokine receptor and NK receptor expression and function and (2) cytokine and chemokine secretion in CD56bright and CD56dim NK cells. sHLA-G-treated or untreated peripheral blood (PB) and tonsil NK cells were analyzed for chemokine receptor and NK receptor expression by flow cytometry. sHLA-G down-modulated (1) CXCR3 on PB and tonsil CD56bright and CD56dim, (2) CCR2 on PB and tonsil CD56bright, (3) CX3CR1 on PB CD56dim, (4) CXCR5 on tonsil CD56dim, and (5) CD94/NKG2A on PB and tonsil CD56brigh) and CD56dim NK cells. Such sHLA-G-mediated down-modulations were reverted by adding anti-HLA-G or anti-ILT2 mAbs. sHLA-G inhibited chemotaxis of (1) PB NK cells toward CXCL10, CXCL11, and CX3CL1 and (2) PB CD56bright NK cells toward CCL2 and CXCL10. IFN-γ secretion induced by NKp46 engagement was inhibited by NKG2A engagement in untreated but not in sHLA-G-treated NK cells. sHLA-G up-regulated secretion of (1) CCL22 in CD56bright and CD56dim and (2) CCL2, CCL8, and CXCL2-CXCL3 in CD56dim PB NK cells. Signal transduction experiments showed sHLA-G-mediated down-modulation of Stat5 phosphorylation in PB NK cells. In conclusion, our data delineated novel mechanisms of sHLA-G-mediated inhibition of NK-cell functions.

HLA-G and pregnancy adverse outcomes

There is growing evidence that pregnancy complications such as preeclampsia, recurrent pregnancy loss (RPL), IUGR, and premature birth could be associated with abnormal immunologic interactions at the fetal-maternal interface. The restricted expression of HLA-G to the subpopulation of trophoblast cells which invade the uterus has generated much interest. The alternative splicing of HLA-G primary transcript, gives origin to seven isoforms, including both membrane-bound forms (HLA-G1, G2, G3, G4) and soluble forms (sHLA-G: sHLA-G5, G6, G7). sHLA-G consists predominantly of sHLA-G1 after its shedding by metalloproteinases, and secreted sHLA-G5 representing the quantitatively dominating and full-length isoforms. HLA-G expression and HLA-G genetic variations in both the mother and the embryo/fetus may be important for pregnancy outcome. It is also intuitively apparent that a gene with putative immunosuppressive and immunotolerant potential might be functional in both the mother and the embryo/fetus/placenta. Reduced or aberrant HLA-G expression seems to be associated with certain complications of pregnancy, among which preeclampsia and possibly the risk of miscarriage, and that this may be further linked to HLA-G polymorphisms. Most of the studies aimed at assessing the role of HLA-G in pregnant diseases have considered only the maternal genotype and ignored the contribution of the fetus. In this regard, the mother, placenta and the fetus form a synthesis. Therefore, studies on placental diseases should address HLA-G expression and genetic variations also to the fetus/placenta.

Role of HLA-G in tumor escape through expansion of myeloid-derived suppressor cells and cytokinic balance in favor of Th2 versus Th1/Th17

The expression of HLA-G by malignant cells has been proposed as a tumor escape mechanism from immunosurveillance. However, although the inhibitory effect of HLA-G on antitumoral immune effectors has been documented in vitro, it remains to be resolved in vivo. In this context, the development of an animal model is now a priority to establish the proof of concept that an HLA-G(+) tumor cell develops and tolerizes the host antitumor immune response in vivo. In the present study, we provide the first in vivo evidence of such a role by a xenotumor model in mice based on the interactions between human HLA-G and the murine paired immunoglobulin-like receptor-B (PIR-B). We demonstrate that human tumor cells expressing HLA-G grow in an immunocompetent host by affecting both innate and adaptive immunity. Expansion of blood myeloid-derived CD11b(+)Gr1(+)PIR-B(+) suppressor cells, loss of peripheral T cells, and cytokinic balance in favor of Th2 versus Th1/Th17 constitute the main mechanisms by which HLA-G promotes tumor expansion. These data demonstrate for the first time that HLA-G plays a crucial role in in vivo tumor evasion. Finally, blocking HLA-G function by a specific Ab inhibits the in vivo development of the tumor, offering a new innovative therapeutic strategy in cancer.

The role of HLA-G in immunity and hematopoiesis

The non-classical HLA class I molecule HLA-G was initially shown to play a major role in feto-maternal tolerance. Since this discovery, it has been established that HLA-G is a tolerogenic molecule which participates to the control of the immune response. In this review, we summarize the recent advances on (1) the multiple structures of HLA-G, which are closely associated with their role in the inhibition of NK cell cytotoxicity, (2) the factors that regulate the expression of HLA-G and its receptors, (3) the mechanism of action of HLA-G at the immunological synapse and through trogocytosis, and (4) the generation of suppressive cells through HLA-G. Moreover, we also review recent findings on the non-immunological functions of HLA-G in erythropoiesis and angiogenesis.

STAT3 inhibitors attenuate interferon-γ-induced neurotoxicity and inflammatory molecule production by human astrocytes

Activation of signal transducer and activator of transcription (STAT) 3 is observable in reactive astrocytes under certain neuropathological conditions. Interferon (IFN)-γ is shown to activate STAT3 in cultured rodent astrocytes. Here we investigated the effects of inhibiting STAT3 signaling on IFNγ-activated human astrocytes since we have recently demonstrated that human astrocytes become neurotoxic when stimulated by IFNγ. We found that 5'-deoxy-5'-(methylthio)adenosine (MTA) (300 μM), S3I-201 (10 μM), STAT3 inhibitor VII (3 μM) and JAK-inhibitor I (0.3 μM) had anti-neurotoxic effects on IFN-γ (50 U/ml)-activated astrocytes and U373-MG astrocytoma cells. Another inhibitor, AG490 (30 μM) had no significant effect. The active inhibitors also attenuated IFN-γ-induced phosphorylation of Tyr(705)-STAT3 and astrocytic expression of intercellular adhesion molecule-1 (ICAM-1). They also decreased astrocytic production of IFN-γ-inducible T cell α chemoattractant (I-TAC). AG490, which did not affect the Tyr(705)-STAT3 phosphorylation or ICAM-1 expression, nevertheless reduced the I-TAC secretion. Because these results indicate that pharmacological inhibition of STAT3 signaling correlates with reduced astrocytic neurotoxicity and ICAM-1 expression, but not that of I-TAC secretion, we consider that STAT3 activation mediates, at least in part, the IFN-γ-induced neurotoxicity and ICAM-1 expression by human astrocytes.

Increased soluble human leukocyte antigen-G levels in peripheral blood from climbers on Mount Everest

Soluble human leukocyte antigen-G (HLA-G) is involved in maternal-fetal tolerance, transplant acceptance, and tumor escape from immunosurveillance, operating by inhibiting activity of T, antigen presenting cells (APC), and natural killer (NK) cells. HLA-G gene expression is modulated in vitro after hypoxic conditions, a situation evidenced during pregnancy and tumor progression. In extreme altitude, mountaineers are in hypoxic conditions that generate physiologic adaptative responses, some of them giving rise to pathologic signs. We performed measurements of plasma soluble HLA-G in six climbers before departure of the expedition and during their ascent to and descent from summit of Mount Everest, and in 3 Sherpas at 5300-6400 m. We found that HLA-G levels are upregulated during the ascent with a unique pattern in comparison with angiogenic/lymphangiogenic factors. Our data suggest that HLA-G has to be taken into account in the mechanisms participating in adaptation to high altitudes and reinforce hypoxia as an important factor in the regulation of HLA-G expression.

A novel mechanism of soluble HLA-G mediated immune modulation: downregulation of T cell chemokine receptor expression and impairment of chemotaxis

Background

In recent years, many immunoregulatory functions have been ascribed to soluble HLA-G (sHLA-G). Since chemotaxis is crucial for an efficient immune response, we have investigated for the first time the effects of sHLA-G on chemokine receptor expression and function in different human T cell populations.

Methodology/Principal Findings

T cell populations isolated from peripheral blood were stimulated in the presence or absence of sHLA-G. Chemokine receptors expression was evaluated by flow cytometry. sHLA-G downregulated expression of i) CCR2, CXCR3 and CXCR5 in CD4⁺ T cells, ii) CXCR3 in CD8⁺ T cells, iii) CXCR3 in Th1 clones iv) CXCR3 in TCR Vδ2γ9 T cells, and upregulated CXCR4 expression in TCR Vδ2γ9 T cells. sHLA-G inhibited in vitro chemotaxis of i) CD4⁺ T cells towards CCL2, CCL8, CXCL10 and CXCL11, ii) CD8⁺ T cells towards CXCL10 and CXCL11, iii) Th1 clones towards CXCL10, and iv) TCR Vδ2γ9 T cells towards CXCL10 and CXCL11. Downregulation of CXCR3 expression on CD4+ T cells by sHLA-G was partially reverted by adding a blocking antibody against ILT2/CD85j, a receptor for sHLA-G, suggesting that sHLA-G downregulated chemokine receptor expression mainly through the interaction with ILT2/CD85j. Follicular helper T cells (T_FH) were isolated from human tonsils and stimulated as described above. sHLA-G impaired CXCR5 expression in T_FH and chemotaxis of the latter cells towards CXCL13. Moreover, sHLA-G expression was detected in tonsils by immunohistochemistry, suggesting a role of sHLA-G in local control of T_FH cell chemotaxis. Intracellular pathways were investigated by Western Blot analysis on total extracts from CD4+ T cells. Phosphorylation of Stat5, p70 s6k, β-arrestin and SHP2 was modulated by sHLA-G treatment.

Conclusions/Significance

Our data demonstrated that sHLA-G impairs expression and functionality of different chemokine receptors in T cells. These findings delineate a novel mechanism whereby sHLA-G modulates T cell recruitment in physiological and pathological conditions.

Recent advances on the non-classical major histocompatibility complex class I HLA-G molecule

The human leukocyte antigen (HLA)-G non-classical major histocompatibility complex (MHC) class I molecule was originally described in first-trimester trophoblasts at the fetal-maternal interface in 1990. Eight years later, the First International Conference on this molecule was inaugurated by Prof Jean Dausset, recipient of the Nobel Prize in Medicine. The Fifth International Conference on HLA-G, held in Paris on July 2009, began with a tribute to Prof Jean Dausset who left us recently. This conference was co-chaired by Dr Edgardo D. Carosella and Prof Hans Grosse-Wilde, included 57 oral presentations and was attended by approximately 140 delegates from 16 countries. We summarize here the major advances on the HLA-G molecule that were reported, including findings on its biological activity and characterization of new mechanisms of action, notably through mesenchymal stem cells and regulatory cells, and the previously unexplored role of HLA-G on immune cells such as gammadelta T-cells and B lymphocytes. Furthermore, the role of HLA-G during pregnancy was revisited and its impact in pathologies such as cancer, autoimmune disorders and transplantation was further extended.

Immunosuppressive HLA-G molecule is upregulated in alveolar epithelial cells after influenza A virus infection

Influenza virus type A (IAV) infections constitute an important economic burden and raise health-care problems. Host defense mechanisms usually clear IAV infections after a few days by exploiting a variety of cellular immune responses. However, increasing the production of immunosubversive molecules is a mechanism by which viruses escape host surveillance. In this regard, the nonclassical HLA class I molecule HLA-G displays strong tolerogenic properties. We show here that several strains of IAV differently upregulate HLA-G expression, at both the mRNA and protein levels, in alveolar epithelial cells. Thus the virulence of IAV may be caused by the capability of different strains to upregulate HLA-G allowing their escape from host immune responses.

Human leukocyte antigen-G is expressed in advanced-stage ovarian carcinoma of high-grade histology

HLA-G is a nonclassical MHC class I molecule with restricted normal tissue distribution, but whose expression is induced in pathologic situations such as cancer. In regard to the its immunosuppressive properties, it has been suggested that HLA-G could be a way for tumors to escape immunosurveillance. HLA-G has been described in almost all types of cancer, whatever their origins. Ovarian cancer is the second leading cause of death among women with gynecologic cancers, after breast cancer. The high mortality rate of this pathology is linked to its late discovery resulting from discreet symptomatology at early stages and the current paucity of highly sensitive and specific biomarkers. The aim of the present study was to analyze, in a French, retrospectively based study, whether HLA-G could be a marker for the detection of ovarian cancer and the prediction of clinical evolution. For this purpose, we looked for HLA-G expression in ovarian carcinoma lesions from low to high grade and stage, and we showed that HLA-G is selectively expressed in advanced-stage disease of high-grade histology.