Immune regulation by pretenders: cell-to-cell transfers of HLA-G make effector T cells act as regulatory cells

Trogocytosis-mediated immune evasion in the tumor microenvironment

Trogocytosis is a dynamic cellular process characterized by the exchange of the plasma membrane and associated cytosol during cell-to-cell interactions. Unlike phagocytosis, this transfer maintains the surface localization of transferred membrane molecules. For example, CD4 T cells engaging with antigen-presenting cells undergo trogocytosis, which facilitates the transfer of antigen-loaded major histocompatibility complex (MHC) class II molecules from antigen-presenting cells to CD4 T cells. This transfer results in the formation of antigen-loaded MHC class II molecule-dressed CD4 T cells. These "dressed" CD4 T cells subsequently participate in antigen presentation to other CD4 T cells. Additionally, trogocytosis enables the acquisition of immune-regulatory molecules, such as CTLA-4 and Tim3, in recipient cells, thereby modulating their anti-tumor immunity. Concurrently, donor cells undergo plasma membrane loss, and substantial loss can trigger trogocytosis-mediated cell death, termed trogoptosis. This review aims to explore the trogocytosis-mediated transfer of immune regulatory molecules and their implications within the tumor microenvironment to elucidate the underlying mechanisms of immune evasion in cancers.

HLA-G can be transfered via trogocytosis from leukemic cells to T cells in chronic lymphocytic leukemia

In chronic lymphocytic leukemia (CLL) immune escape mechanism allows leukemia cells to proliferate and expand and it might also be responsible for disease progression. Some molecules involved in the regulation of an immune system might represent prognostic value for CLL patients. Among numerous immune escape mechanisms it was shown that the expression of human leukocyte antigen G (HLA-G) might represent one of the agents damaging cellular immune response. In the present study, the expression of the HLA-G molecule and ILT-2 receptor on the surface of leukemic cells, as well as a plasma concentration of soluble HLA-G (sHLA-G) was evaluated. Also, we investigated whether HLA-G could be transferred from leukemic cells to T cells by the mechanism of trogocytosis. We showed higher proportion of leukemic cells expressing HLA-G and increased levels of sHLA-G in CLL patients compared to that of B-cells in healthy volunteers (HVs). Results of our work showed a time-dependent increase in HLA-G expression on CD4+ T cells co-incubated with HLA-G-positive CD19+ cells. Longer coincubation times did significantly increase these proportions (p < 0.001). We have shown that a higher proportion of HLA-G-expressing CD4+ T cells correlated with the clinical stage of the disease according to the Rai classification. Interestingly, we found a higher CD4+HLA-G+ percentage in the group with unmutated immunoglobulin heavy chain variable region (IGHV) genes compared to the group with mutated IGHV gene after 48 h co-culture. In summary, increasing evidence has revealed that, in addition to HLA-G expressed on tumor cells, intercellular transfer of HLA-G among cancer cells and immune cells through trogocytosis plays important roles in mechanism of immune escape, disease progression and poor clinical outcome.

Intercellular transfer of MHC molecules in T cell alloimmunity and allotransplantation

After transplantation of allogeneic tissues and organs, recognition by recipient T cells of donor MHC molecules initiates the pro-inflammatory adaptive immune response leading to allograft rejection. T cell allorecognition has long been known to be mediated via two distinct pathways: the direct pathway in which T cells recognize intact allogeneic MHC molecules displayed on donor cells and the indirect pathway whereby T cells recognize donor MHC peptides processed and presented by recipient antigen-presenting cells (APCs). It is believed that direct allorecognition is the driving force behind early acute allograft rejection while indirect allorecognition is involved in chronic allograft rejection, a progressive condition characterized by graft vasculopathy and tissue fibrosis. Recently, we and others have reported that after transplantation of allogeneic skin and organs, donor MHC molecules are transferred from donor cells to the host's APCs via trogocytosis or extracellular vesicles. Recipient APCs having captured donor MHC molecules can either present them to T cells in their intact form on their surface (semi-direct pathway) or the form of peptides bound to self-MHC molecules (indirect pathway). The present article provides an overview of recent studies evaluating the role of intercellular exchange of MHC molecules in T cell alloimmunity and its contribution to allograft rejection and tolerance.

Chimeric Cell Therapies as a Novel Approach for Duchenne Muscular Dystrophy (DMD) and Muscle Regeneration

Chimerism-based strategies represent a pioneering concept which has led to groundbreaking advancements in regenerative medicine and transplantation. This new approach offers therapeutic potential for the treatment of various diseases, including inherited disorders. The ongoing studies on chimeric cells prompted the development of Dystrophin-Expressing Chimeric (DEC) cells which were introduced as a potential therapy for Duchenne Muscular Dystrophy (DMD). DMD is a genetic condition that leads to premature death in adolescent boys and remains incurable with current methods. DEC therapy, created via the fusion of human myoblasts derived from normal and DMD-affected donors, has proven to be safe and efficacious when tested in experimental models of DMD after systemic-intraosseous administration. These studies confirmed increased dystrophin expression, which correlated with functional and morphological improvements in DMD-affected muscles, including cardiac, respiratory, and skeletal muscles. Furthermore, the application of DEC therapy in a clinical study confirmed its long-term safety and efficacy in DMD patients. This review summarizes the development of chimeric cell technology tested in preclinical models and clinical studies, highlighting the potential of DEC therapy in muscle regeneration and repair, and introduces chimeric cell-based therapies as a promising, novel approach for muscle regeneration and the treatment of DMD and other neuromuscular disorders.

Harnessing the potential of HLA-G in cancer therapy: advances, challenges, and prospects

Immune checkpoint blockades have been prized in circumventing and ablating the impediments posed by immunosuppressive receptors, reaching an exciting juncture to be an innovator in anticancer therapy beyond traditional therapeutics. Thus far, approved immune checkpoint blockades have principally targeted PD-1/PD-L1 and CTLA-4 with exciting success in a plethora of tumors and yet are still trapped in dilemmas of limited response rates and adverse effects. Hence, unveiling new immunotherapeutic targets has aroused immense scientific interest in the hope of expanding the clinical application of immune checkpoint blockades to scale new heights. Human leukocyte antigen-G (HLA-G), a non-classical major histocompatibility complex (MHC) class I molecule, is enriched on various malignant cells and is involved in the hindrance of immune effector cells and the facilitation of immunosuppressive cells. HLA-G stands out as a crucial next-generation immune checkpoint showing great promise for the benefit of cancer patients. Here, we provide an overview of the current understanding of the expression pattern and immunological functions of HLA-G, as well as its interaction with well-characterized immune checkpoints. Since HLA-G can be shed from the cell surface or released by various cells as free soluble HLA-G (sHLA-G) or as part of extracellular vesicles (EVs), namely HLA-G-bearing EVs (HLA-GEV), we discuss the potential of sHLA-G and HLA-GEV as predictive biomarkers. This review also addresses the advancement of HLA-G-based therapies in preclinical and clinical settings, with a focus on their clinical application in cancer.

The Biological Significance of Trogocytosis

Trogocytosis is the intercellular transfer of membrane and membrane-associated proteins between cells. Trogocytosis is an underappreciated phenomenon that has historically routinely been dismissed as an artefact. With a greater understanding of the process and the implications it has on biological systems, trogocytosis has the potential to become a paradigm changer. The presence on a cell of molecules they don't endogenously express can alter the biological activity of the cell and could also lead to the acquisition of new functions. To better appreciate this phenomenon, it is important to understand how these intercellular membrane exchanges influence the function and activity of the donor and the recipient cells. In this chapter, we will examine how the molecules acquired by trogocytosis influence the biology of a variety of systems including mammalian fertilization, treatment of hemolytic disease of the newborn, viral and parasitic infections, cancer immunotherapy, and immune modulation.

Extracellular Vesicles in Liver Transplantation: Current Evidence and Future Challenges

Extracellular vesicles (EVs) are emerging as a promising field of research in liver disease. EVs are small, membrane-bound vesicles that contain various bioactive molecules, such as proteins, lipids, and nucleic acids and are involved in intercellular communication. They have been implicated in numerous physiological and pathological processes, including immune modulation and tissue repair, which make their use appealing in liver transplantation (LT). This review summarizes the current state of knowledge regarding the role of EVs in LT, including their potential use as biomarkers and therapeutic agents and their role in graft rejection. By providing a comprehensive insight into this emerging topic, this research lays the groundwork for the potential application of EVs in LT.

The unique expression pattern of human leukocyte antigen in trophoblasts potentially explains the key mechanism of maternal-fetal tolerance and successful pregnancy

The success of pregnancy mainly depends on immune tolerance of the mother for the semi-allogeneic fetus. The placenta carrying paternal antigens develops in the maternal uterus without suffering immune attack, making the underlying mechanism of maternal tolerance an enduring mystery. As we all know, human leukocyte antigen (HLA) plays an important role in antigen processing and presentation, thus inducing specific immune responses. Therefore, it is reasonable to speculate that the absence of classical HLA class-I（HLA-I) and HLA class-II (HLA-II) molecules in trophoblasts may account for the maternal-fetal tolerance. Here, we review the HLA-involved interactions between trophoblast cells and decidual immune cells, which contribute to the immunotolerance in the development of normal pregnancy. We also compare the similarity between the maternal-fetal interface and tumor-immune microenvironment because the important role of HLA molecules in tumor immune invasion can provide some references to studies of maternal-fetal immune tolerance. Besides, the abnormal HLA expression is likely to be associated with unexplained miscarriage, making HLA molecules potential therapeutic targets. The advances reported by these studies may exert profound influences on other research areas, including tumor immunity, organ transplantation and autoimmune disease in the future.

HLA-G 5'URR regulatory polymorphisms are associated with the risk of developing gliomas

BACKGROUND

Human leukocyte antigen G (HLA-G) belongs to non-classical MHC class I molecules that is involved in the suppression of immune response. As HLA-G plays important role in the maintenance of fetal tolerance, its overexpression has been associated with tumor progression. For the regulation of HLA-G levels, genetic variants within the 5' upstream regulatory region (5'URR) are of crucial importance. Our study aimed to analyze the association between 16 HLA-G 5'URR variants, sHLA-G level and clinical variables in glioma patients.

METHODS

We investigated 59 patients with gliomas (mean age 54.70 ± 15.10 years) and 131 healthy controls (mean age 41.45 ± 9.75 years). Patient's blood was obtained on the day of surgical treatment. The HLA-G 5'URR polymorphisms were typed by direct sequencing and the plasma level of sHLA-G assessed by ELISA.

RESULTS

Haploblock within HLA-G 5'URR consisting of -762T, -716G, -689G, -666T, -633A, followed by -486C and -201A alleles were significantly more frequent in patients with gliomas than in the controls (p < 0.05). No correlation of HLA-G 5'URR variants with sHLA-G plasma level was found. Analysis of HLA-G 5'URR variants with main clinical variables in patients with grade IV gliomas revealed that haploblock carriers of -762CT, -716TG, -689AG, -666GT, -633GA, -486AC, -477GC, -201GA followed by -369AC carriers tend to have lower age at onset as compared to other genotype carriers (p = 0.04).

CONCLUSION

Our results suggest genetic association of HLA-G 5'URR variants with risk of developing gliomas and possible contribution of HLA-G to disease pathology.

Leukocyte immunoglobulin-like receptor subfamily B: A novel immune checkpoint molecule at the maternal-fetal interface

Due to their crucial roles in embryo implantation, maternal-fetal tolerance induction, and pregnancy progression, immune checkpoint molecules (ICMs), such as programmed cell death-1, cytotoxic T-lymphocyte antigen 4, and T cell immunoglobulin mucin 3, are considered potential targets for clinical intervention in pregnancy complications. Despite the considerable progress on these molecules, our understanding of ICMs at the maternal-fetal interface is still limited. Identification of alternative and novel ICMs and the combination of multiple ICMs is urgently needed for deeply understanding the mechanism of maternal-fetal tolerance and to discover the causes of pregnancy complications. Leukocyte immunoglobulin-like receptor subfamily B (LILRB) is a novel class of ICMs with strong negative regulatory effects on the immune response. Recent studies have revealed that LILRB is enriched in decidual immune cells and stromal cells at the maternal-fetal interface, which can modulate the biological behavior of immune cells and promote immune tolerance. In this review, we introduce the structural features, expression profiles, ligands, and orthologs of LILRB. In addition, the potential mechanisms and functions mediated by LILRB for sustaining the maternal-fetal tolerance microenvironment, remodeling the uterine spiral artery, and induction of pregnancy immune memory are summarized. We have also provided new suggestions for further understanding the roles of LILRB and potential therapeutic strategies for pregnancy-related diseases.

Activation and regulation of alloreactive T cell immunity in solid organ transplantation

Transplantation is the only curative treatment for patients with kidney failure but it poses unique immunological challenges that must be overcome to prevent allograft rejection and ensure long-term graft survival. Alloreactive T cells are important contributors to graft rejection, and a clearer understanding of the mechanisms by which these cells recognize donor antigens - through direct, indirect or semi-direct pathways - will facilitate their therapeutic targeting. Post-T cell priming rejection responses can also be modified by targeting pathways that regulate T cell trafficking, survival cytokines or innate immune activation. Moreover, the quantity and quality of donor-reactive memory T cells crucially shape alloimmune responses. Of note, many fundamental concepts in transplant immunology have been derived from models of infection. However, the programmed differentiation of allograft-specific T cell responses is probably distinct from that of pathogen-elicited responses, owing to the dearth of pathogen-derived innate immune activation in the transplantation setting. Understanding the fundamental (and potentially unique) immunological pathways that lead to allograft rejection is therefore a prerequisite for the rational development of therapeutics that promote transplantation tolerance.

Advances in the study of HLA class Ib in maternal-fetal immune tolerance

The HLA class Ib molecule is an alloantigen that causes transplant rejection on behalf of individual human and plays an important role in maternal-fetal immune tolerance. Early studies on HLA class Ib focused on the mechanism of HLA-G-induced immune escape, but in recent years, studies on the mechanism of HLA-G have deepened and gradually explored the mechanism of HLA-E and HLA-F, which are also HLA class Ib molecules. In the maternal-fetal interface, trophoblast cells express HLA class Ib molecules to protect the fetus from maternal immune cells by binding to inhibitory receptors of decidual immune cells (DICs) and shifting Th1/Th2 balance toward Th2 bias. Further studies on the molecular mechanism of HLA class Ib molecules provide a reference for its application in the field of clinical assisted reproduction.

Sequence Variations Within HLA-G and HLA-F Genomic Segments at the Human Leukocyte Antigen Telomeric End Associated With Acute Graft-Versus-Host Disease in Unrelated Bone Marrow Transplantation

Acute graft-versus-host disease (aGVHD) is defined as a syndrome of an immunological response of graft to the host that occurs early after allogeneic hematopoietic stem cell transplantation (HCT). This disease is frequently observed even in HCT matched for human leukocyte antigen (HLA) alleles at multiple gene loci. Although the HLA region represents complex and diverse genomic characteristics, detailed association analysis is required for the identification of uncharacterized variants that are strongly associated with aGVHD. We genotyped three loci, OR2H2, HLA-F-AS1, and HLA-G, that are located in the 460 kb of HLA telomeric region and statistically analyzed the genotypes including HLA-DPB1 with clinical and transplantation outcomes using 338 unrelated bone marrow transplantation (UR-BMT) patient–donor pairs who were matched for HLA-A, HLA-B, HLA-C, HLA-DRB1, and HLA-DQB1 (HLA-10/10). Multivariate analyses demonstrated that HLA-F-AS1 and HLA-DPB1 mismatches were associated with grade II–IV aGVHD (hazard ratio (HR), 1.76; 95% CI, 1.07–2.88; p = 0.026; and HR, 1.59; CI, 1.02–2.49; p = 0.042, respectively). There was no confounding between HLA-F-AS1 and HLA-DPB1 (p = 0.512), suggesting that the HLA-F-AS1 mismatch has a strong effect on aGVHD independently of HLA-DPB1. Moreover, a stratified analysis suggested possible associations of HLA-F-AS1, HLA-DPB1, and/or HLA-G mismatches with grade II–IV aGVHD and the more severe grade III–IV aGVHD. These findings provide new insights into understanding the molecular mechanism of aGVHD caused by HLA-matched UR-BMT.

Investigating Serum sHLA-G Cooperation With MRI Activity and Disease-Modifying Treatment Outcome in Relapsing-Remitting Multiple Sclerosis

Relapsing-remitting multiple sclerosis (RRMS) is a demyelinating disease in which pathogenesis T cells have a major role. Despite the unknown etiology, several risk factors have been described, including a strong association with human leukocyte antigen (HLA) genes. Recent findings showed that HLA class I-G (HLA-G) may be tolerogenic in MS, but further insights are required. To deepen the HLA-G role in MS inflammation, we measured soluble HLA-G (sHLA-G) and cytokines serum level in 27 patients with RRMS at baseline and after 12 and 24 months of natalizumab (NTZ) treatment. Patients were divided into high (sHLA-G&gt;20 ng/ml), medium (sHLA-G between 10 and 20 ng/ml), and low (sHLA-G &lt;10 ng/ml) producers. Results showed a heterogeneous distribution of genotypes among producers, with no significant differences between groups. A significant decrease of sHLA-G was found after 24 months of NTZ in low producers carrying the +3142 C/G genotype. Finally, 83.3% of high and 100% of medium producers were MRI-activity free after 24 months of treatment, compared to 63.5% of low producers. Of note, we did not find any correlation of sHLA-G with peripheral cell counts or cytokines level. These findings suggest that serum sHLA-G level may partly depend on genotype rather than peripheral inflammation, and that may have impacted on MRI activity of patients over treatment.

When killers become thieves: Trogocytosed PD-1 inhibits NK cells in cancer

Trogocytosis modulates immune responses, with still unclear underlying molecular mechanisms. Using leukemia mouse models, we found that lymphocytes perform trogocytosis at high rates with tumor cells. While performing trogocytosis, both Natural Killer (NK) and CD8+ T cells acquire the checkpoint receptor PD-1 from leukemia cells. In vitro and in vivo investigation revealed that PD-1 on the surface of NK cells, rather than being endogenously expressed, was derived entirely from leukemia cells in a SLAM receptor-dependent fashion. PD-1 acquired via trogocytosis actively suppressed NK cell antitumor immunity. PD-1 trogocytosis was corroborated in patients with clonal plasma cell disorders, where NK cells that stained for PD-1 also stained for tumor cell markers. Our results, in addition to shedding light on a previously unappreciated mechanism underlying the presence of PD-1 on NK and cytotoxic T cells, reveal the immunoregulatory effect of membrane transfer occurring when immune cells contact tumor cells.

The Human Leukocyte Antigen G as an Immune Escape Mechanism and Novel Therapeutic Target in Urological Tumors

The non-classical human leukocyte antigen G (HLA-G) is a potent regulatory protein involved in the induction of immunological tolerance. This is based on the binding of membrane-bound as well as soluble HLA-G to inhibitory receptors expressed on various immune effector cells, in particular NK cells and T cells, leading to their attenuated functions. Despite its restricted expression on immune-privileged tissues under physiological conditions, HLA-G expression has been frequently detected in solid and hematopoietic malignancies including urological cancers, such as renal cell and urothelial bladder carcinoma and has been associated with progression of urological cancers and poor outcome of patients: HLA-G expression protects tumor cells from anti-tumor immunity upon interaction with its inhibitory receptors by modulating both the phenotype and function of immune cells leading to immune evasion. This review will discuss the expression, regulation, functional and clinical relevance of HLA-G expression in urological tumors as well as its use as a putative biomarker and/or potential therapeutic target for the treatment of renal cell carcinoma as well as urothelial bladder cancer.

HLA-G and Other Immune Checkpoint Molecules as Targets for Novel Combined Immunotherapies

HLA-G is an HLA-class Ib molecule that is involved in the establishment of tolerance at the maternal/fetal interface during pregnancy. The expression of HLA-G is highly restricted in adults, but the de novo expression of this molecule may be observed in different hematological and solid tumors and is related to cancer progression. Indeed, tumor cells expressing high levels of HLA-G are able to suppress anti-tumor responses, thus escaping from the control of the immune system. HLA-G has been proposed as an immune checkpoint (IC) molecule due to its crucial role in tumor progression, immune escape, and metastatic spread. We here review data available in the literature in which the interaction between HLA-G and other IC molecules is reported, in particular PD-1, CTLA-4, and TIM-3, but also IDO and TIGIT. Clinical trials using monoclonal antibodies against HLA-G and other IC are currently ongoing with cancer patients where antibodies and inhibitors of PD-1 and CTLA-4 showed encouraging results. With this background, we may envisage that combined therapies using antibodies targeting HLA-G and another IC may be successful for clinical purposes. Indeed, such immunotherapeutic protocols may achieve a better rescue of effective anti-tumor immune response, thus improving the clinical outcome of patients.

HLA-G: Too Much or Too Little? Role in Cancer and Autoimmune Disease

HLA-G is a non-classical HLA class I molecule with immunomodulatory properties. It was initially described at the maternal-fetal interface, and it was later found that this molecule was constitutively expressed on certain immuneprivileged tissues, such as cornea, endothelial and erythroid precursors, and thymus. The immunosuppressive effect of HLA-G is exerted through the interaction with its cognate receptors, expressed on immunocompetent cells, like ILT2, expressed on NK, B, T cells and APCs; ILT4, on APCs; KIR, found on the surface of NK cells; and finally, the co-receptor CD8. Because of these immunomodulatory functions, HLA-G has been involved in several processes, amongst which organ transplantation, viral infections, cancer progression, and autoimmunity. HLA-G neo-expression on tumors has been recently described in several types of malignancies. In fact, tumor progression is tightly linked to the presence of the molecule, as it exerts its tolerogenic function, inhibiting the cells of the immune system and favoring tumor escape. Several polymorphisms in the 3’UTR region condition changes in HLA-G expression (14bp and +3142C/G, among others), which have been associated with both the development and outcome of patients with different tumor types. Also, in recent years, several studies have shown that HLA-G plays an important role in the control of autoimmune diseases. The ability of HLA-G to limit the progression of these diseases has been confirmed and, in fact, levels of the molecule and several of its polymorphisms have been associated with increased susceptibility to the development of autoimmune diseases, as well as increased disease severity. Thus, modulating HLA-G expression in target tissues of oncology patients or patients with autoimmune diseases may be potential therapeutic approaches to treat these pathological conditions.

HLA-G in Allergy: Does It Play an Immunoregulatory Role?

Allergy is an inflammatory process determined by a cascade of immune events characterized by T-helper 2 lymphocytes polarization leading to interleukin-4 upregulation, IgE secretion, and mast cell and eosinophil activation. HLA-G molecules, both in membrane-bound and in soluble forms, are known to play a key immunoregulatory role and their involvement in allergic diseases is supported by increasing literature data. HLA-G expression and secretion is specifically induced in peripheral blood mononuclear cells of allergic patients after in vitro incubation with the causal allergen. Elevated levels of soluble HLA-G molecules are detected in serum of patients with allergic rhinitis correlating with allergen-specific IgE levels, clinical severity, drug consumption and response to allergen-specific immunotherapy. HLA-G genetic polymorphisms confer susceptibility to allergic asthma development and high levels of soluble HLA-G molecules are found in plasma and bronchoalveolar lavage fluid of patients with allergic asthma correlating with allergen-specific IgE levels. Interestingly, allergic pregnant women have lower plasma sHLA-G levels than non-allergic women during the 3^rd trimester of pregnancy and at delivery. Finally, in allergic patients with atopic dermatitis HLA-G molecules are expressed by T cells, monocytes-macrophages and Langerhans cells infiltrating the dermis. Although at present is difficult to completely define the role of HLA-G molecules in allergic diseases, it may be suggested that they are specifically expressed and secreted by immune cells during the allergic reaction in an attempt to suppress allergic inflammation.

The HLA-G Immune Checkpoint Plays a Pivotal Role in the Regulation of Immune Response in Autoimmune Diseases

The human G-leukocyte antigen (HLA-G) molecule is a non-classical major histocompatibility complex (MHC) class I molecule. The pertinence of HLA-G has been investigated in numerous studies which have sought to elucidate the relevance of HLA-G in pathologic conditions, such as autoimmune diseases, cancers, and hematologic malignancies. One of the main goals of the current research on HLA-G is to use this molecule in clinical practice, either in diagnostics or as a therapeutic target. Since HLA-G antigens are currently considered as immunomodulatory molecules that are involved in reducing inflammatory and immune responses, in this review, we decided to focus on this group of antigens as potential determinants of progression in autoimmune diseases. This article highlights what we consider as recent pivotal findings on the immunomodulatory function of HLA-G, not only to establish the role of HLA-G in the human body, but also to explain how these proteins mediate the immune response.

Targeting monocytes/macrophages in fibrosis and cancer diseases: Therapeutic approaches

Over almost 140 years since their identification, the knowledge about macrophages has unbelievably evolved. The 'big eaters' from being thought of as simple phagocytic cells have been recognized as master regulators in immunity, homeostasis, healing/repair and organ development. Long considered to originate exclusively from bone marrow-derived circulating monocytes, macrophages have been also demonstrated to be the first immune cells colonizing tissues in the developing embryo and persisting in adult life by self-renewal, as long-lived tissue resident macrophages. Therefore, heterogeneous populations of macrophages with different ontogeny and functions co-exist in tissues. Macrophages act as sentinels of homeostasis and are intrinsically programmed to lead the wound healing and repair processes that occur after injury. However, in certain pathological circumstances macrophages get dysfunctional, and impaired or aberrant macrophage activities become key features of diseases. For instance, in both fibrosis and cancer, that have been defined 'wounds that do not heal', dysfunctional monocyte-derived macrophages overall play a key detrimental role. On the other hand, due to their plasticity these cells can be 're-educated' and exert anti-fibrotic and anti-cancer functions. Therefore macrophages represent an important therapeutic target in both fibrosis and cancer diseases. The current review will illustrate new insights into the role of monocytes/macrophages in these devastating diseases and summarize new therapeutic strategies and applications of macrophage-targeted drug development in their clinical setting.

High expression of CD38 and MHC class II on CD8+ T cells during severe influenza disease reflects bystander activation and trogocytosis

Objectives

Although co‐expression of CD38 and HLA‐DR reflects T‐cell activation during viral infections, high and prolonged CD38⁺HLA‐DR⁺ expression is associated with severe disease. To date, the mechanism underpinning expression of CD38⁺HLA‐DR⁺ is poorly understood.

Methods

We used mouse models of influenza A/H9N2, A/H7N9 and A/H3N2 infection to investigate mechanisms underpinning CD38⁺MHC‐II⁺ phenotype on CD8⁺ T cells. To further understand MHC‐II trogocytosis on murine CD8⁺ T cells as well as the significance behind the scenario, we used adoptively transferred transgenic OT‐I CD8⁺ T cells and A/H3N2‐SIINKEKL infection.

Results

Analysis of influenza‐specific immunodominant D^bNP₃₆₆⁺CD8⁺ T‐cell responses showed that CD38⁺MHC‐II⁺ co‐expression was detected on both virus‐specific and bystander CD8⁺ T cells, with increased numbers of both CD38⁺MHC‐II⁺CD8⁺ T‐cell populations observed in immune organs including the site of infection during severe viral challenge. OT‐I cells adoptively transferred into MHC‐II^−/− mice had no MHC‐II after infection, suggesting that MHC‐II was acquired via trogocytosis. The detection of CD19 on CD38⁺MHC‐II⁺ OT‐I cells supports the proposition that MHC‐II was acquired by trogocytosis sourced from B cells. Co‐expression of CD38⁺MHC‐II⁺ on CD8⁺ T cells was needed for optimal recall following secondary infection.

Conclusions

Overall, our study demonstrates that both virus‐specific and bystander CD38⁺MHC‐II⁺ CD8⁺ T cells are recruited to the site of infection during severe disease, and that MHC‐II presence occurs via trogocytosis from antigen‐presenting cells. Our findings highlight the importance of the CD38⁺MHC‐II⁺ phenotype for CD8⁺ T‐cell recall.

Lymphocytes and Trogocytosis-Mediated Signaling

Trogocytosis is the intercellular transfer of membrane and membrane-associated molecules. This underappreciated process has been described in a variety of biological settings including neuronal remodeling, fertilization, viral and bacterial spread, and cancer, but has been most widely studied in cells of the immune system. Trogocytosis is performed by multiple immune cell types, including basophils, macrophages, dendritic cells, neutrophils, natural killer cells, B cells, γδ T cells, and CD4+ and CD8+ αβ T cells. Although not expressed endogenously, the presence of trogocytosed molecules on cells has the potential to significantly impact an immune response and the biology of the individual trogocytosis-positive cell. Many studies have focused on the ability of the trogocytosis-positive cells to interact with other immune cells and modulate the function of responders. Less understood and arguably equally important is the impact of these molecules on the individual trogocytosis-positive cell. Molecules that have been reported to be trogocytosed by cells include cognate ligands for receptors on the individual cell, such as activating NK cell ligands and MHC:peptide. These trogocytosed molecules have been shown to interact with receptors on the trogocytosis-positive cell and mediate intracellular signaling. In this review, we discuss the impact of this trogocytosis-mediated signaling on the biology of the individual trogocytosis-positive cell by focusing on natural killer cells and CD4+ T lymphocytes.

HLA-G and single nucleotide polymorphism (SNP) associations with cancer in African populations: Implications in personal medicine

The immune system plays an important role in protecting the body against malignancy. During cancer immunoediting, the immune system can recognize and keep checking the tumor cells by down-expression of some self-molecules or by increasing expression of some novel molecules. However, the microenvironment created in the course of cancer development hampers the immune ability to recognize and destroy the transforming cells. Human Leukocyte Antigen G (HLA-G) is emerging as immune checkpoint molecule produced more by cancer cells to weaken the immune response against them. HLA-G is a non-classical HLA class I molecule which is normally expressed in immune privileged tissues as a soluble or membrane-bound protein. HLA-G locus is highly polymorphic in the non-coding 3′ untranslated region (UTR) and in the 5′ upstream regulatory region (5′ URR). HLA-G expression is controlled by polymorphisms located in these regions, and several association studies between these polymorphic sites and disease predisposition, response to therapy, and/or HLA-G protein expression have been reported. Various polymorphisms are demonstrated to modulate its expression and this is increasingly finding more significance in cancer biology. This review focuses on the relevance of the HLA-G gene and its polymorphisms in cancer development. We highlight population genetics of HLA-G as evidence to espouse the need and importance of exploring potential utility of HLA-G in cancer diagnosis, prognosis and immunotherapy in the currently understudied African population.

Role of CD8+ T lymphocyte cells: Interplay with stromal cells in tumor microenvironment

CD8+ T lymphocytes are pivotal cells in the host response to antitumor immunity. Tumor-driven microenvironments provide the conditions necessary for regulating infiltrating CD8+ T cells in favor of tumor survival, including weakening CD8+ T cell activation, driving tumor cells to impair immune attack, and recruiting other cells to reprogram the immune milieu. Also in tumor microenvironment, stromal cells exert immunosuppressive skills to avoid CD8+ T cell cytotoxicity. In this review, we explore the universal function and fate decision of infiltrated CD8+ T cells and highlight their antitumor response within various stromal architectures in the process of confronting neoantigen-specific tumor cells. Thus, this review provides a foundation for the development of antitumor therapy based on CD8+ T lymphocyte manipulation.

The Role of Trogocytosis in the Modulation of Immune Cell Functions

Trogocytosis is an active process, in which one cell extracts the cell fragment from another cell, leading to the transfer of cell surface molecules, together with membrane fragments. Recent reports have revealed that trogocytosis can modulate various biological responses, including adaptive and innate immune responses and homeostatic responses. Trogocytosis is evolutionally conserved from protozoan parasites to eukaryotic cells. In some cases, trogocytosis results in cell death, which is utilized as a mechanism for antibody-dependent cytotoxicity (ADCC). In other cases, trogocytosis-mediated intercellular protein transfer leads to both the acquisition of novel functions in recipient cells and the loss of cellular functions in donor cells. Trogocytosis in immune cells is typically mediated by receptor–ligand interactions, including TCR–MHC interactions and Fcγ receptor-antibody-bound molecule interactions. Additionally, trogocytosis mediates the transfer of MHC molecules to various immune and non-immune cells, which confers antigen-presenting activity on non-professional antigen-presenting cells. In this review, we summarize the recent advances in our understanding of the role of trogocytosis in immune modulation.

Development of Donor Recipient Chimeric Cells of bone marrow origin as a novel approach for tolerance induction in transplantation

Background

Cell therapies and chimerism-based strategies are currently the most successful approach for tolerance induction in transplantation. This study aimed to establish and characterize novel Donor Recipient Chimeric Ccell (DRCC) therapy of bone marrow (BM) origin presenting donor-recipient phenotype to support tolerance induction.

Methods

Ex vivo fusions of fully MHC-mismatched BM cells from ACI (RT1^a) and Lewis (RT1^l) rats were performed using polyethylene-glycol (PEG). The creation of rat DRCC was tested by flow cytometry (FC), confocal microscopy and PCR. FC characterized DRCC's phenotype (CD3, CD4, CD8, CD45, CD90, CD11b/c, CD45RA, OX-82, or CD4/CD25) and apoptosis, while mixed lymphocyte reaction assessed DRCC's immunogenicity and colony forming unit assay tested DRCC's differentiation and proliferation. DRCC's polyploidy was evaluated using Hoechst33342 staining and COMET assay tested genotoxicity of fusion procedure. ELISA analyzed the secretion of IL-2, IL-4, IL-10, TGFß1, IFNγ and TNFα by DRCC at day 1, 5 and 14 post-fusion. The DRCC's phenotype after long-term culturing was assessed by reverse-transcription PCR.

Results

The chimeric state of DRCC was confirmed. Fusion did not change the expression of hematopoietic markers compared to BM controls. Although an increased number of early and late apoptotic (Annexin V⁺/Sytox blue^- and Annexin V⁺/Sytox blue⁺, respectively) DRCC was detected at 24h post-fusion, the number significantly decreased at day 5 (38.4%±3.1% and 22.6%±2.5%, vs. 28.3%±2.5% and 13.9%±2.6%, respectively, P<0.05). DRCC presented decreased immunogenicity, increased expression of IL-10 and TGFβ1 and proliferative potential comparable to BM controls. The average percentage of tetraploid DRCC was 3.1%±0.2% compared to 0.96%±0.1% in BM controls. The lack of damage to the DRCC's DNA content supported the DRCC's safety. In culture, DRCC maintained proliferation for up to 28 days while preserving hematopoietic profile.

Conclusions

This study confirmed feasibility of DRCC creation via ex vivo PEG mediated fusion. The created DRCC revealed pro-tolerogenic properties indicating potential immunomodulatory effect of DRCC therapy when applied in vivo to support tolerance induction in solid organ and vascularized composite allograft transplantation.

Role of the HLA-G immune checkpoint molecule in pregnancy

The non-classical HLA class I molecule HLA-G is expressed in trophoblasts where it contributes to maternal-fetal tolerance. HLA-G has been implicated in the control of trophoblast invasion, uterine vascular remodeling, and maintenance of a local immunosuppressive state. Understanding HLA-G biology at the maternal-fetal interface is therefore a critical issue in reproduction. In this regard, we review here: (i) the effects of HLA-G on decidual leucocytes and stromal cells, (ii) the contribution of trogocytosis in HLA-G expression on decidual cells, (iii) its interaction with the ILT2, ILT4 and KIR2DL4 receptors, (iv) the link between HLA-G polymorphism and pregnancy disorders, and (v) the expression of newly-described HLA-G isoforms at the maternal-fetal interface.

First immunotherapeutic CAR-T cells against the immune checkpoint protein HLA-G

BACKGROUND

CAR-T cells immunotherapy is a breakthrough in the treatment of hematological malignancies such as acute lymphoblastic leukemia (ALL) and B-cell malignancies. However, CAR-T therapies face major hurdles such as the lack of tumor-specific antigen (TSA), and immunosuppressive tumor microenvironment sometimes caused by the tumorous expression of immune checkpoints (ICPs) such as HLA-G. Indeed, HLA-G is remarkable because it is both a potent ICP and a TSA. HLA-G tumor expression causes immune escape by impairing innate and adaptive immune responses and by inducing a suppressive microenvironment. Yet, to date, no immunotherapy targets it.

METHODS

We have developed two anti-HLA-G third-generation CARs based on new anti-HLA-G monoclonal antibodies.

RESULTS

Anti-HLA-G CAR-T cells were specific for immunosuppressive HLA-G isoforms. HLA-G-activated CAR-T cells polarized toward T helper 1, and became cytotoxic against HLA-G+ tumor cells. In vivo, anti-HLA-G CAR-T cells were able to control and eliminate HLA-G+ tumor cells. The interaction of tumor-HLA-G with interleukin (IL)T2-expressing T cells is known to result in effector T cell functional inhibition, but anti-HLA-G CAR-T cells were insensitive to this inhibition and still exerted their function even when expressing ILT2. Lastly, we show that anti-HLA-G CAR-T cells differentiated into long-term memory effector cells, and seemed not to lose function even after repeated stimulation by HLA-G-expressing tumor cells.

CONCLUSION

We report for the first time that HLA-G, which is both a TSA and an ICP, constitutes a valid target for CAR-T cell therapy to specifically target and eliminate both tumor cells and HLA-G+ suppressive cells.

Trogocytosis between Non-Immune Cells for Cell Clearance, and among Immune-Related Cells for Modulating Immune Responses and Autoimmunity

The term trogocytosis refers to a rapid bidirectional and active transfer of surface membrane fragment and associated proteins between cells. The trogocytosis requires cell-cell contact, and exhibits fast kinetics and the limited lifetime of the transferred molecules on the surface of the acceptor cells. The biological actions of trogocytosis include information exchange, cell clearance of unwanted tissues in embryonic development, immunoregulation, cancer surveillance/evasion, allogeneic cell survival and infectious pathogen killing or intercellular transmission. In the present review, we will extensively review all these aspects. In addition to its biological significance, aberrant trogocytosis in the immune system leading to autoimmunity and immune-mediated inflammatory diseases will also be discussed. Finally, the prospective investigations for further understanding the molecular basis of trogocytosis and its clinical applications will also be proposed.

Trogocytosis generates acquired regulatory T cells adding further complexity to the dysfunctional immune response in multiple myeloma

Trogocytosis, which results in the acquisition of myeloma cell-derived membrane proteins by T cells, and hence generates novel regulatory T cells, adds to the growing list of immune defects of multiple myeloma patients. The increasing complexity of the cancer-associated immune defects must be attentively considered for attempting to improve the so-far unsatisfactory rates of clinical responses to immunotherapy in patients affected by multiple myeloma and other malignancies.

Roles of HLA-G in the Maternal-Fetal Immune Microenvironment

During pregnancy, the maternal uterus and fetus form a special microenvironment at the maternal-fetal interface to support fetal development. Extravillous trophoblasts (EVTs), differentiated from the fetus, invade into the decidua and interact with maternal cells. Human leukocyte antigen (HLA)-G is a non-classical MHC-I molecule that is expressed abundantly and specifically on EVTs in physiological conditions. Soluble HLA-G (sHLA-G) is also found in maternal blood, amniotic fluid, and cord blood. The abnormal expression and polymorphisms of HLA-G are related to adverse pregnancy outcomes such as preeclampsia (PE) and recurrent spontaneous abortion (RSA). Here we summarize current findings about three main roles of HLA-G during pregnancy, namely its promotion of spiral artery remodeling, immune tolerance, and fetal growth, all resulting from its interaction with immune cells. These findings are not only of great significance for the treatment of pregnancy-related diseases but also provide clues to tumor immunology research since HLA-G functions as a checkpoint in tumors.

Understanding HLA-G driven journey from HPV infection to cancer cervix: Adding missing pieces to the jigsaw puzzle

Human Papillomavirus (HPV) is a vital risk-factor for cancer cervix. However, persistent HPV infection results in cervical cancer in only a minority. Probably, HPV subdues the host immune response for persistence, which includes augmentation of HLA-G and plausibly aids in progression to cervical cancer. HLA-G, which comprises of membrane and soluble form, downregulates the host's immune response and generate tolerance. The current study aimed to analyze both forms of HLA-G in fresh tissue and plasma of women with HPV-infected and uninfected cervix and cancer cervix using Western blot and ELISA. The study cohort included 30 women with cervical carcinoma and equal number with normal cervix and 6 with HPV infected cervix. We observed a significant upregulation of membranous HLA-G expression in HPV infected cervix and cervical carcinoma (P < 0.001). Interestingly, the pairwise comparison of HLA-G tissue protein expression of the normal cervix and cervical carcinoma, as well as the normal cervix with HPV infected cervix, was significant (P < 0.001). Levels of soluble HLA-G were significantly raised in carcinoma cervix. We observed a progressive increase in HLA-G protein expression in HPV infected cervix and cervical carcinoma. These findings compel us to hypothesize that the upregulation of HLA-G expression favors the persistence of HPV in a microenvironment of a submissive host response. This progressive upregulation further leads to cervical cancer. Thus elimination of HPV infection seems to be a desirable proposition to prevent cervical cancer. In the absence of antiviral therapy for HPV, exploration of HLA-G antibody-based therapeutic strategies appear promising.

HLA-G and HLA-F protein isoform expression in breast cancer patients receiving neoadjuvant treatment

The immunosuppressive human leukocyte antigens HLA-G and HLA-F are expressed on trophoblast and malignant cells. Four membrane-bound and three soluble HLA-G protein isoforms have been described, which have different immunosuppressive potentials. HLA-F has three transcript variants, resulting in three different protein isoforms. The aim of this study was to evaluate the prognostic and predictive value of HLA-G and HLA-F protein isoform expression patterns in patients with breast cancer. Core biopsies were taken at diagnosis in patients with HER2+ (n = 28), luminal B-like (n = 49) and triple-negative (n = 38) breast cancers who received neoadjuvant chemotherapy. Expression levels of HLA-F and -G were correlated with the pathological complete response (pCR). Protein expression was determined by Western blot analysis, using two antibodies for each HLA, specific for different isoforms. The protein expression of HLA isoforms did not significantly differ between breast cancer subtypes. However, some initial indications were found for an association between the soluble HLA-G6 protein isoform and pCR in HER2+ breast cancer. The study provides preliminary evidence for the evaluation of HLA-G isoform expression, in particular HLA-G6, as a possible new marker for pCR in HER2+ breast cancer.

Generation of hypoimmunogenic human pluripotent stem cells via expression of membrane-bound and secreted β2m-HLA-G fusion proteins

Allogeneic immune rejection is a major barrier for the application of human pluripotent stem cells (hPSCs) in regenerative medicine. A broad spectrum of immune cells, including T cells, natural killer (NK) cells, and antigen-presenting cells, which either cause direct cell killing or constitute an immunogenic environment, are involved in allograft immune rejection. A strategy to protect donor cells from cytotoxicity while decreasing the secretion of inflammatory cytokines of lymphocytes is still lacking. Here, we engineered hPSCs with no surface expression of classical human leukocyte antigen (HLA) class I proteins via beta-2 microglobulin (B2M) knockout or biallelic knockin of HLA-G1 within the frame of endogenous B2M loci. Elimination of the surface expression of HLA class I proteins protected the engineered hPSCs from cytotoxicity mediated by T cells. However, this lack of surface expression also resulted in missing-self response and NK cell activation, which were largely compromised by expression of β2m-HLA-G1 fusion proteins. We also proved that the engineered β2m-HLA-G5 fusion proteins were soluble, secretable, and capable of safeguarding low immunogenic environments by lowering inflammatory cytokines secretion in allografts. Our current study reveals a novel strategy that may offer unique advantages to construct hypoimmunogenic hPSCs via the expression of membrane-bound and secreted β2m-HLA-G fusion proteins. These engineered hPSCs are expected to serve as an unlimited cell source for generating universally compatible "off-the-shelf" cell grafts in the future.

HLA-G Expressing Immune Cells in Immune Mediated Diseases

HLA-G is a HLA class Ib antigen that possesses immunomodulatory properties. HLA-G-expressing CD4+ and CD8+ T lymphocytes, NK cells, monocytes, and dendritic cells with immunoregulatory functions are present in small percentages of patients with physiologic conditions. Quantitative and qualitative derangements of HLA-G+ immune cells have been detected in several conditions in which the immune system plays an important role, such as infectious, neoplastic, and autoimmune diseases as well as in complications from transplants and pregnancy. These observations strongly support the hypothesis that HLA-G+ immune cells may be implicated in the complex mechanisms underlying the pathogenesis of these disorders.

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.

Regulatory T Cells in Pregnancy: It Is Not All About FoxP3

In pregnancy, the semi-allogeneic fetus needs to be tolerated by the mother's immune system. Regulatory T cells (Tregs) play a prominent role in this process. Novel technologies allow for in-depth phenotyping of previously unidentified immune cell subsets, which has resulted in the appreciation of a vast heterogeneity of Treg subsets. Similar to other immunological events, there appears to be great diversity within the Treg population during pregnancy, both at the maternal-fetal interface as in the peripheral blood. Different Treg subsets have distinct phenotypes and various ways of functioning. Furthermore, the frequency of individual Treg subsets varies throughout gestation and is altered in aberrant pregnancies. This suggests that distinct Treg subsets play a role at different time points of gestation and that their role in maintaining healthy pregnancy is crucial, as reflected for instance by their reduced frequency in women with recurrent pregnancy loss. Since pregnancy is essential for the existence of mankind, multiple immune regulatory mechanisms and cell types are likely at play to assure successful pregnancy. Therefore, it is important to understand the complete microenvironment of the decidua, preferably in the context of the whole immune cell repertoire of the pregnant woman. So far, most studies have focused on a single mechanism or cell type, which often is the FoxP3 positive regulatory T cell when studying immune regulation. In this review, we instead focus on the contribution of FoxP3 negative Treg subsets to the decidual microenvironment and their possible role in pregnancy complications. Their phenotype, function, and effect in pregnancy are discussed.

Maternal and fetal T cells in term pregnancy and preterm labor

Pregnancy is a state of immunological balance during which the mother and the developing fetus must tolerate each other while maintaining sufficient immunocompetence to ward off potential threats. The site of closest contact between the mother and fetus is the decidua, which represents the maternal-fetal interface. Many of the immune cell subsets present at the maternal-fetal interface have been well described; however, the importance of the maternal T cells in this compartment during late gestation and its complications, such as preterm labor and birth, has only recently been established. Moreover, pioneer and recent studies have indicated that fetal T cells are activated in different subsets of preterm labor and may elicit distinct inflammatory responses in the amniotic cavity, leading to preterm birth. In this review, we describe the established and proposed roles for maternal T cells at the maternal-fetal interface in normal term parturition, as well as the demonstrated contributions of such cells to the pathological process of preterm labor and birth. We also summarize the current knowledge of and proposed roles for fetal T cells in the pathophysiology of the preterm labor syndrome. It is our hope that this review provides a solid conceptual framework highlighting the importance of maternal and fetal T cells in late gestation and catalyzes new research questions that can further scientific understanding of these cells and their role in preterm labor and birth, the leading cause of neonatal mortality and morbidity worldwide.

Assessing in vitro and in vivo Trogocytosis By Murine CD4+ T cells

Recognition of antigens by lymphocytes (B, T, and NK) on the surface of an antigen-presenting cell (APC) leads to lymphocyte activation and the formation of an immunological synapse between the lymphocyte and the APC. At the immunological synapse APC membrane and associated membrane proteins can be transferred to the lymphocyte in a process called trogocytosis. The detection of trogocytosed molecules provides insights to the activation state, antigen specificity, and effector functions and differentiation of the lymphocytes. Here we outline our protocol for identifying trogocytosis-positive CD4+ T cells in vitro and in vivo. In vitro, antigen presenting cells are surface biotinylated and pre-loaded with magnetic polystyrene beads before incubating for a short time with in vitro activated CD4+ T cell blasts (90 min) or naïve T cells (3-24 h). After T cell recovery and APC depletion by magnetic separation trogocytosis positive (trog+) cells are identified by streptavidin staining of trogocytosed, biotinylated APC membrane proteins. Their activation phenotype, effector function, and effector differentiation are subsequently analyzed by flow cytometry immediately or after subsequent incubation. Similarly, trogocytosis-positive cells can be identified and similarly analyzed by flow cytometry. Previous studies have described methods for analyzing T cell trogocytosis to identify antigen-specific cells or the antigenic epitopes recognized by the cells. With the current protocol, the effects of trogocytosis on the individual T cell or the ability of trog+ T cells to modulate the activation and function of other immune cells can be assessed over an extended period of time.

Modes of Communication between T Cells and Relevance for Immune Responses

T cells are essential mediators of the adaptive immune system, which constantly patrol the body in search for invading pathogens. During an infection, T cells that recognise the pathogen are recruited, expand and differentiate into subtypes tailored to the infection. In addition, they differentiate into subsets required for short and long-term control of the pathogen, i.e., effector or memory. T cells have a remarkable degree of plasticity and heterogeneity in their response, however, their overall response to a given infection is consistent and robust. Much research has focused on how individual T cells are activated and programmed. However, in order to achieve a critical level of population-wide reproducibility and robustness, neighbouring cells and surrounding tissues have to provide or amplify relevant signals to tune the overall response accordingly. The characteristics of the immune response-stochastic on the individual cell level, robust on the global level-necessitate coordinated responses on a system-wide level, which facilitates the control of pathogens, while maintaining self-tolerance. This global coordination can only be achieved by constant cellular communication between responding cells, and faults in this intercellular crosstalk can potentially lead to immunopathology or autoimmunity. In this review, we will discuss how T cells mount a global, collective response, by describing the modes of T cell-T cell (T-T) communication they use and highlighting their physiological relevance in programming and controlling the T cell response.

Differentiation of cancer cells upregulates HLA‑G and PD‑L1

A tumor contains special types of cells that have characteristics similar to stem cells that aid in tumor initiation, evasion and proliferation and are often resistant to chemotherapy. These cancer stem cells can be differentiated to eradicate their stemness and proliferative capacity by differentiating agents. This study investigated the effect of differentiation on the expression of two immune checkpoint inhibitors, human leukocyte antigen-G (HLA-G) and programmed death ligand-1 (PD-L1). Two cancer cell lines (OVCAR-3-NIH and KATO-III) were treated with adipocyte and neurocyte differentiation media for 14 days. Bone-marrow derived mesenchymal stem cells (BM-MSCs) were used as control healthy stem cells. We found that the cancer cell lines (OVCAR-3-NIH and KATO-III) when subjected to differentiation lost their proliferation ability. BM-MSC proliferation was not halted but was decreased in the adipocyte differentiation media. There was no decrease in the CD90 stem cell marker in the BM-MSCs; however, both cancer cell lines showed decreased CD90 stem cell marker. A significant increase in HLA-G was noted for both the cancer cell lines following adipocyte differentiation. No effect was found for BM-MSCs. Moreover, an increase in PD-L1 in cancer cell lines was found following neurocyte differentiation. Moreover, we found that differentiation resulted in decreased PD-L1 expression in BM-MSCs. Differentiation therapy of cancer stem cells may result in increased immunosuppression ability, hence causing hindrance in the removal of cancer cells. Moreover, the differentiation of healthy stem cells can result in increased immunogenic reactivity owing to a decrease in PD-L1 expression.

Extracellular vesicles in allograft rejection and tolerance

Extracellular vesicles (EVs), including exosomes, ectosomes and apoptotic vesicles, play an essential role in communication between cells of the innate and adaptive immune systems. Recent studies showed that EVs released after transplantation of allogeneic tissues and organs are involved in the immune recognition and response leading to rejection or tolerance in mice. After skin, pancreatic islet, and solid organ transplantation, donor-derived EVs were shown to initiate direct inflammatory alloresponses by T cells leading to acute rejection. This occurred through presentation of intact allogeneic MHC molecules on recipient antigen presenting cells (MHC cross-dressing) and subsequent activation of T cells via semi-direct allorecognition. On the other hand, some studies have documented the role of EVs in maternal tolerance of fetal alloantigens during pregnancy and immune privilege associated with spontaneous tolerance of liver allografts in laboratory rodents. The precise nature of the EVs, which are involved in rejection or tolerance, and the cells which produce them, is still unclear. Nevertheless, several reports showed that EVs released in the blood and urine by allografts can be used as biomarkers of rejection. This article reviews current knowledge on the contribution of EVs in allorecognition by T cells and discusses some mechanisms underlying their influence on T cell alloimmunity in allograft rejection or tolerance.

Expression of membrane-bound human leucocyte antigen-G in systemic sclerosis and systemic lupus erythematosus

Human leucocyte antigen-G (HLA-G) is a nonclassical class I major histocompatibility complex (MHC) molecule characterized by complex immunoregulatory and tolerogenic functions. Membrane-bound HLA-G is expressed on the surface of different cell populations in both physiological and pathological conditions. Systemic sclerosis (SSc) is a multisystem autoimmune disease characterized by widespread tissue fibrosis, vascular lesions and immunological alterations. Systemic lupus erythematosus is the prototypic systemic autoimmune disease affecting virtually any organ system, such as skin, joints, central nervous system, or kidneys. In SSc and SLE patients, the membrane expression of HLA-G on monocytes (0.88 ± 1.54 and 0.43 ± 0.75, respectively), CD4+ (0.42 ± 0.78 and 0.63 ± 0.48, respectively), CD8+ (2.65 ± 3.47 and 1.29 ± 1.34, respectively) and CD4+ CD8+ double-positive cells (13.87 ± 15.97 and 3.79 ± 3.11, respectively) was significantly higher than in healthy controls (0.12 ± 0.07; 0.01 ± 0.01; 0.14 ± 0.20 and 0.32 ± 0.38, respectively) (p < 0.0001). Our results show that in SSc and SLE the membrane expression of HLA-G by different subpopulations of peripheral blood mononuclear cells (PBMC) is increased, suggesting a potential role of HLA-G molecules in the complex immunological pathogenesis of these two autoimmune disorders.

Intercellular transfer of HLA-G: its potential in cancer immunology

Intercellular protein transfer between cancer cells and immune cells is a very common phenomenon that can affect different stages of host antitumor immune responses. HLA-G, a non-classical HLA class I antigen, has been observed to be widely expressed in various malignancies, and its immune-suppressive functions have been well recognised. HLA-G expression in cancer cells can directly mediate immune tolerance by interacting with inhibitory receptors such as ILT2 and ILT4 expressed on immune cells. Moreover, a network of multiple directional intercellular transfers of HLA-G among cancer cells and immune cells through trogocytosis, exosomes and tunnelling nanotubes provides malignant cells with an alternative ploy for antigen sharing and induces more complex heterogeneity, to modulate immune responses, ultimately leading to immune evasion, therapy resistance, disease progression and poor clinical outcome. Herein, we discuss the relative aspects of the intercellular transfer of HLA-G between tumor cells and immune cells and its potential use in tumor immunology research and translational cancer therapy.

Adaptive innate immunity or innate adaptive immunity?

The innate immunity is frequently accepted as a first line of relatively primitive defense interfering with the pathogen invasion until the mechanisms of ‘privileged’ adaptive immunity with the production of antibodies and activation of cytotoxic lymphocytes ‘steal the show’. Recent advancements on the molecular and cellular levels have shaken the traditional view of adaptive and innate immunity. The innate immune memory or ‘trained immunity’ based on metabolic changes and epigenetic reprogramming is a complementary process insuring adaptation of host defense to previous infections.

Innate immune cells are able to recognize large number of pathogen- or danger- associated molecular patterns (PAMPs and DAMPs) to behave in a highly specific manner and regulate adaptive immune responses. Innate lymphoid cells (ILC1, ILC2, ILC3) and NK cells express transcription factors and cytokines related to subsets of T helper cells (Th1, Th2, Th17). On the other hand, T and B lymphocytes exhibit functional properties traditionally attributed to innate immunity such as phagocytosis or production of tissue remodeling growth factors. They are also able to benefit from the information provided by pattern recognition receptors (PRRs), e.g. γδT lymphocytes use T-cell receptor (TCR) in a manner close to PRR recognition. Innate B cells represent another example of limited combinational diversity usage participating in various innate responses. In the view of current knowledge, the traditional black and white classification of immune mechanisms as either innate or an adaptive needs to be adjusted and many shades of gray need to be included.

European Patent in Immunoncology: From Immunological Principles of Implantation to Cancer Treatment

The granted European patent EP 2 561 890 describes a procedure for an immunological treatment of cancer. It is based on the principles of the HLA-supported communication of implantation and pregnancy. These principles ensure that the embryo is not rejected by the mother. In pregnancy, the placenta, more specifically the trophoblast, creates an “interface” between the embryo/fetus and the maternal immune system. Trophoblasts do not express the “original” HLA identification of the embryo/fetus (HLA-A to -DQ), but instead show the non-classical HLA groups E, F, and G. During interaction with specific receptors of NK cells (e.g., killer-immunoglobulin-like receptors (KIR)) and lymphocytes (lymphocyte-immunoglobulin-like receptors (LIL-R)), the non-classical HLA groups inhibit these immunocompetent cells outside pregnancy. However, tumors are known to be able to express these non-classical HLA groups and thus make use of an immuno-communication as in pregnancies. If this occurs, the prognosis usually worsens. This patent describes, in a first step, the profiling of the non-classical HLA groups in primary tumor tissue as well as metastases and recurrent tumors. The second step comprises tailored antibody therapies, which is the subject of this patent. In this review, we analyze the underlying mechanisms and describe the currently known differences between HLA-supported communication of implantation and that of tumors.

Trogocytosis-Mediated Intracellular Signaling in CD4+ T Cells Drives TH2-Associated Effector Cytokine Production and Differentiation

CD4⁺ T cells have been observed to acquire APC-derived membrane and membrane-associated molecules through trogocytosis in diverse immune settings. Despite this, the consequences of trogocytosis on the recipient T cell remain largely unknown. We previously reported that trogocytosed molecules on CD4⁺ T cells engage their respective surface receptors, leading to sustained TCR signaling and survival after APC removal. Using peptide-pulsed bone marrow-derived dendritic cells and transfected murine fibroblasts expressing antigenic MHC:peptide complexes as APC, we show that trogocytosis-positive CD4⁺ T cells display effector cytokines and transcription factor expression consistent with a T_H2 phenotype. In vitro-polarized T_H2 cells were found to be more efficient at performing trogocytosis than T_H1 or nonpolarized CD4⁺ cells, whereas subsequent trogocytosis-mediated signaling induced T_H2 differentiation in polarized T_H1 and nonpolarized cells. Trogocytosis-positive CD4⁺ T cells generated in vivo also display a T_H2 phenotype in both TCR-transgenic and wild-type models. These findings suggest that trogocytosis-mediated signaling impacts CD4⁺ T cell differentiation and effector cytokine production and may play a role in augmenting or shaping a T_H2-dominant immune response.

The anti-fibrotic role of mast cells in the liver is mediated by HLA-G and interaction with hepatic stellate cells

BACKGROUND & AIMS

We have reported a significant association between HLA-G expression or the number of hepatic mast cells and liver fibrosis. Here, we investigated the role of HLA-G and mast cells in liver fibrosis, focusing, in particular, on interactions between human mast and stellate cells.

METHODS

Human mast cells (HMC cell line, CD34-derived mast cells, or tissue-derived mast cells) were co-cultured with purified human hepatic stellate cells (HSCs), and collagen I production by HSCs was evaluated. Mast cells and HSCs were characterized by immunocytochemistry. Various conditions were tested: different times in direct or indirect contact, presence or absence of cytokines, addition or not of HLA-G, and presence or absence of specific protease inhibitors.

RESULTS

The reciprocal interaction between HSCs and mast cells led to the attraction of mast cells to HSCs in vivo and in vitro, and to a significant decrease in collagen production, at all times of co-culture, following the direct or indirect contact of mast cells with HSCs alone or in the presence of TGF-β, IFN-α or IL-10. We identified the diffusible factors involved in collagen I degradation as mast cell proteases. Moreover, HLA-G expression increased during the co-culture of HSCs and mast cells, with HLA-G acting on both mast cells and HSCs, to enhance collagen I degradation.

CONCLUSIONS

Mast cells play a beneficial, anti-fibrotic role in liver fibrosis, via the HLA-G-mediated decrease of collagen I. These findings are consistent with high levels of cross-communication between mast cells and hepatic stellate cells and the role of HLA-G.