HLA-G in the nervous system

CD4(+)HLA-G(+) regulatory T cells: Molecular signature and pathophysiological relevance

The regulation of potentially harmful immune responses by regulatory T (Treg) cells is essential for maintaining peripheral immune tolerance and homeostasis. Especially CD4(+) Treg cells have been regarded as pivotal regulators of autoreactive and inflammatory responses as well as inducers of immune tolerance by using a variety of immune suppressive mechanisms. Besides the well-known classical CD4(+)CD25(+)FoxP3(+) Treg cells, CD4(+) T cells expressing the immune tolerizing molecule human leukocyte antigen G (HLA-G) have been recently described as another potent thymus-derived Treg (tTreg) cell subset. Albeit both tTreg subsets share common molecular characteristics, the mechanisms of their immunosuppressive function differ fundamentally. Dysfunction and numerical abnormalities of classical CD4(+) tTreg cells have been implicated in the pathogenesis of several immune-mediated diseases such as multiple sclerosis (MS). Clearly, a deeper understanding of the various CD4(+) tTreg subsets and also the underlying mechanisms of impaired immune tolerance in these disorders are essential for the development of potential therapeutic strategies. This review focuses on the current knowledge on defining features and functioning of HLA-G(+)CD4(+) tTreg cells as well as their emerging role in various pathologies with special emphasis on the pathogenesis of MS. Furthermore, future research possibilities together with potential therapeutic applications are discussed.

Human Parthenogenetic Embryonic Stem Cell-Derived Neural Stem Cells Express HLA-G and Show Unique Resistance to NK Cell-Mediated Killing

Parent-of-origin imprints have been implicated in the regulation of neural differentiation and brain development. Previously we have shown that, despite the lack of a paternal genome, human parthenogenetic (PG) embryonic stem cells (hESCs) can form proliferating neural stem cells (NSCs) that are capable of differentiation into physiologically functional neurons while maintaining allele-specific expression of imprinted genes. Since biparental ("normal") hESC-derived NSCs (N NSCs) are targeted by immune cells, we characterized the immunogenicity of PG NSCs. Flow cytometry and immunocytochemistry revealed that both N NSCs and PG NSCs exhibited surface expression of human leukocyte antigen (HLA) class I but not HLA-DR molecules. Functional analyses using an in vitro mixed lymphocyte reaction assay resulted in less proliferation of peripheral blood mononuclear cells (PBMC) with PG compared with N NSCs. In addition, natural killer (NK) cells cytolyzed PG less than N NSCs. At a molecular level, expression analyses of immune regulatory factors revealed higher HLA-G levels in PG compared with N NSCs. In line with this finding, MIR152, which represses HLA-G expression, is less transcribed in PG compared with N cells. Blockage of HLA-G receptors ILT2 and KIR2DL4 on natural killer cell leukemia (NKL) cells increased cytolysis of PG NSCs. Together this indicates that PG NSCs have unique immunological properties due to elevated HLA-G expression.

Non-classical MHC-I human leukocyte antigen (HLA-G) in hepatotropic viral infections and in hepatocellular carcinoma

The human leukocyte antigen (HLA)-G is a "nonclassical" major histocompatibility complex (MHC) class Ib gene, located at chromosome 6, in the 6p21.3 region. The HLA-G presents immunomodulatory functions essential in pregnancy for the tolerance of the semi-allogenic fetus, but an abnormal expression of HLA-G has been observed in numerous pathological conditions, such as tumors, autoimmune diseases and viral infections. In recent years, numerous studies have assessed the clinical relevance of HLA-G expression in different types of cancer: in general, a higher HLA-G expression correlates with a lower survival rate or a shorter disease-free survival. Altered expression of HLA-G has been found in both HCV and HBV infection, and some genetic polymorphisms have been associated with altered susceptibility/disease development for these infections, however, whether the biologic role of HLA-G in HCV and HBV infection is beneficial or hazardous, it is not completely clear. In the context of hepatocellular carcinoma, HLA-G has shown a potential diagnostic role, moreover a prognostic value in HCC patients has been also attributed to HLA-G molecules. We revise here the role of HLA-G in hepatotropic HBV/HCV infections and in hepatocellular carcinoma (HCC).

Involvement of kynurenines in Huntington's disease and stroke-induced brain damage

Several components of the kynurenine pathway of tryptophan metabolism are now recognised to have actions of profound biological importance. These include the ability to modulate the activation of glutamate and nicotinic receptors, to modify the responsiveness of the immune system to inflammation and infection, and to modify the generation and removal of reactive oxygen species. As each of these factors is being recognised increasingly as contributing to major disorders of the central nervous system (CNS), so the potentially fundamental role of the kynurenine pathway in those disorders is presenting a valuable target both for understanding the progress of those disorders and for developing potential drug treatments. This review will summarise some of the evidence for an important contribution of the kynurenines to Huntington's disease and to stroke damage in the CNS. Together with preliminary evidence from a study of kynurenine metabolites after major surgery, an important conclusion is that kynurenine pathway activation closely reflects cognitive function, and may play a significant role in cognitive ability.

Janus head: the dual role of HLA-G in CNS immunity

The central nervous system (CNS) is considered an immune-privileged organ that maintains an adaptable immune surveillance system. Dysregulated immune function within the CNS contributes to the development of brain tumor growth, and robust immune activation results in excessive inflammation. Human lymphocyte antigen-G (HLA-G) proteins with tolerogenic immunoreactivity have been implicated in various pathophysiological processes including immune surveillance, governing homeostasis and immune regulation. In this review, we describe the wealth of evidence for the involvement of HLA-G in the CNS under physiological and pathological conditions. Further, we review regulatory functions that may be applicable as beneficial strategies in the therapeutic manipulation of immune-mediated CNS immune responses. Additionally, we try to understand how this molecule cooperates with other CNS-resident cells to maintain normal immune homeostasis, while still facilitating the development of the appropriate immune responses.

Emerging topics and new perspectives on HLA-G

Following the Fifth International Conference on non-classical HLA-G antigens (HLA-G), held in Paris in July 2009, we selected some topics which focus on emerging aspects in the setting of HLA-G functions. In particular, HLA-G molecules could play a role in: (1) various inflammatory disorders, such as multiple sclerosis, intracerebral hemorrhage, gastrointestinal, skin and rheumatic diseases, and asthma, where they may act as immunoregulatory factors; (2) the mechanisms to escape immune surveillance utilized by several viruses, such as human cytomegalovirus, herpes simplex virus type 1, rabies virus, hepatitis C virus, influenza virus type A and human immunodeficiency virus 1 (HIV-1); and (3) cytokine/chemokine network and stem cell transplantation, since they seem to modulate cell migration by the downregulation of chemokine receptor expression and mesenchymal stem cell activity blocking of effector cell functions and the generation of regulatory T cells. However, the immunomodulatory circuits mediated by HLA-G proteins still remain to be clarified.

Timing of serum soluble HLA-G levels in acute and subacute phases after spontaneous intracerebral hemorrhage

Serum levels of sHLA-G (sHLA-G1/HLA-G5) antigens and their soluble isoforms, sHLA-G1 and HLA-G5, were measured by ELISA in 22 patients with spontaneous intracerebral hemorrhage (SICH) at 24 h, 48 h and 7 days after bleeding. The perihematomal edema volume was calculated on non-enhanced computed tomography scans using the formula AxBxC/2 at the same time points. The mean serum concentrations of sHLA-G1/HLA-G5 and sHLA-G1 as well as the perihematomal edema volume changed significantly over time (p < 0.0001, p < 0.001 and p < 0.0001, respectively), whereas no statistical differences were found in serum HLA-G5 concentrations over the course of the experiment. In comparison to the values found at 24 h, sHLA-G1/HLA-G5 and sHLA-G1 increased at 48 h and then decreased at 7 days, whereas the perihematomal edema volume was more elevated at 48 h and, to a lesser extent, at 7 days. A positive correlation was detected between mean serum sHLA-G1/HLA-G5 and sHLA-G1 levels and perihematomal edema volume at 24 h (p < 0.02) and at 48 h (p < 0.01). Our results may indicate a role for sHLA-G in inflammatory mechanisms related to SICH, where these proteins probably act as anti-inflammatory molecules and are predominantly produced as the sHLA-G1 isoform.

Blood levels of kynurenines, interleukin-23 and soluble human leucocyte antigen-G at different stages of Huntington's disease

There is substantial evidence that abnormal concentrations of oxidised tryptophan metabolites, produced via the kynurenine pathway, contribute to progressive neurodegeneration in Huntington's disease. We have now examined the blood levels of these metabolites in patients at different stages of Huntington's disease, assessed both in terms of clinical disease severity and numbers of CAG repeats. Close relatives of the patients were included in the study as well as unrelated healthy controls. Levels of lipid peroxidation products, the pro-inflammatory cytokine interleukin (IL)-23 and the soluble human leucocyte antigen-G (sHLA-G) were also measured. There were lower levels of tryptophan and a higher kynurenine : tryptophan ratio, indicating activation of indoleamine-2,3-dioxygenase, in the most severely affected group of patients, with increased levels of IL-23 and sHLA-G. Marked correlations were noted between IL-23 and the patient severity group, anthranilic acid levels and the number of CAG repeats, and between anthranilic acid and IL-23, supporting our previous evidence of a relationship between anthranilic acid and inflammatory status. Tryptophan was negatively correlated with symptom severity and number of CAG repeats, and positively correlated with sHLA-G. The results support the proposal that tryptophan metabolism along the kynurenine pathway in Huntington's disease is related to the degree of genetic abnormality, to clinical disease severity and to aspects of immunopathogenesis.

T cell suppression by naturally occurring HLA-G-expressing regulatory CD4+ T cells is IL-10-dependent and reversible

CD4(+) T cells constitutively expressing the immune-tolerogenic HLA-G have been described recently as a new type of nT(reg) (HLA-G(pos) T(reg)) in humans. HLA-G(pos) T(reg) accumulate at sites of inflammation and are potent suppressors of T cell proliferation in vitro, suggesting their role in immune regulation. We here characterize the mechanism of how CD4(+) HLA-G(pos) T(reg) influence autologous HLA-G(neg) T(resp) function. Using a suppression system free of APC, we demonstrate a T-T cell interaction, resulting in suppression of HLA-G(neg) T(resp), which is facilitated by TCR engagement on HLA-G(pos) T(reg). Suppression is independent of cell-cell contact and is reversible, as the removal of HLA-G(pos) T(reg) from the established coculture restored the proliferative capability of responder cells. Further, HLA-G(pos) T(reg)-mediated suppression critically depends on the secretion of IL-10 but not TGF-beta.

Frequency of regulatory T cells is not affected by transient B cell depletion using anti-CD20 antibodies in rheumatoid arthritis

Objectives

Transient B cell depletion with the monoclonal anti-CD20 antibody rituximab has shown favourable clinical responses in patients with rheumatoid arthritis (RA). Recently a characteristic regeneration pattern of B cell subpopulations has been reported. However, little is known about the impact of B-cell depletion on peripheral T cells in particular regulatory T cells.

Materials and Methodology

17 patients with RA having failed anti-TNF were treated with rituximab. Four colour staining was performed using CD19, CD3, CD4, CD8, CD16, CD56, CD25, HLA-DR, HLA-G and intracellular Foxp3 at five time points spanning up to 12 months after rituximab. In addition, quantification of the soluble form of the HLA class I molecule HLA-G by ELISA has been performed.

Results

Peripheral B cell depletion lasted 6 to 9 months. The absolute number of CD3+, CD4+ and CD8+ lymphocytes showed no significant changes up to 1 year after B-cell depletion compared to before therapy. Only the relative frequency for CD3 and CD4 showed a significant increase (p < 0.05). In particular, CD4+CD25++ and Foxp3 positive regulatory T cells remained constant. The percentage of HLA-G positive cells in the CD4+ or CD8+ population did not change significantly either. The amount of sHLA-G remained without significant changes.

Conclusion

Absolute T cell counts showed no significant changes after rituximab compared to the time point before therapy.In particular, the frequency of regulatory T cells with a CD4+CD25++ phenotype as well as positive Foxp3 expression were numerically stable. Additionally, HLA-G positive regulatory T cells and soluble levels of HLA-G showed no significant changes.

The role of regulatory T cells in multiple sclerosis

The dysregulation of inflammatory responses and of immune self-tolerance is considered to be a key element in the autoreactive immune response in multiple sclerosis (MS). Regulatory T (T(REG)) cells have emerged as crucial players in the pathogenetic scenario of CNS autoimmune inflammation. Targeted deletion of T(REG) cells causes spontaneous autoimmune disease in mice, whereas augmentation of T(REG)-cell function can prevent the development of or alleviate variants of experimental autoimmune encephalomyelitis, the animal model of MS. Recent findings indicate that MS itself is also accompanied by dysfunction or impaired maturation of T(REG) cells. The development and function of T(REG) cells is closely linked to dendritic cells (DCs), which have a central role in the activation and reactivation of encephalitogenic cells in the CNS. DCs and T(REG) cells have an intimate bidirectional relationship, and, in combination with other factors and cell types, certain types of DCs are capable of inducing T(REG) cells. Consequently, T(REG) cells and DCs have been recognized as potential therapeutic targets in MS. This Review compiles the current knowledge on the role and function of various subsets of T(REG) cells in MS and experimental autoimmune encephalomyelitis. We also highlight the role of tolerogenic DCs and their bidirectional interaction with T(REG) cells during CNS autoimmunity.

CSF levels of soluble HLA-G and Fas molecules are inversely associated to MRI evidence of disease activity in patients with relapsing-remitting multiple sclerosis

Cerebrospinal fluid (CSF) concentrations of soluble human leukocyte antigen class I (HLA-I) (sHLA-I), HLA-G (sHLA-G) and anti-apoptotic Fas (sFas) molecules were measured by enzyme linked immunosorbent assay technique in 65 relapsing-remitting (RR) MS patients classified according to clinical and magnetic resonance imaging (MRI) evidence of disease activity. Sixty-four patients with other inflammatory neurological disorders (OIND) and 64 subjects with noninflammatory neurological disorders (NIND) served as controls. CSF concentrations were higher in RRMS and in OIND than in NIND patients for sHLA-I (P < 0.02), greater in RRMS than in OIND and in NIND for sHLA-G (P < 0.001 and P < 0.01, respectively) and lower in RRMS than in OIND and in NIND for sFas (P < 0.001 and P < 0.02, respectively). An increase in CSF levels was identified in MRI active RRMS for sHLA-I (P < 0.01) and in MRI stable RRMS for sHLA-G (P < 0.01), whereas CSF values of sFas were decreased in RRMS without Gd-enhancing lesions (P < 0.02). In MS patients with no evidence of MRI disease activity, a trend towards an inverse correlation was found between CSF concentrations of sHLA-G and sHLA-I and between CSF levels of sHLA-G and sFas. Our results indicate that enhanced CSF levels of sHLA-I antigens most likely represent an indirect manifestation of intrathecal immune activation taking place in neuroinflammation. Conversely, reciprocal fluctuations in CSF sHLA-G and sFas levels observed when MRI disease activity resolved suggest that sHLA-G could play an immunomodulatory role in MS through Fas/FasL-mediated mechanisms.

Soluble HLA-G molecules are released as HLA-G5 and not as soluble HLA-G1 isoforms in CSF of patients with relapsing-remitting multiple sclerosis

Cerebrospinal fluid (CSF) levels of sHLA-G (sHLA-G1/HLA-G5) molecules and their soluble isoforms HLA-G5 and sHLA-G1 were measured by ELISA procedures in 68 relapsing-remitting Multiple Sclerosis (RR MS) patients, in 67 patients with other inflammatory neurological disorders (OIND) and in 70 subjects with non-inflammatory neurological disorders (NIND). CSF concentrations of sHLA-G1/HLA-G5 and HLA-G5 were higher in RR MS than in OIND and NIND, and in Magnetic Resonance Imaging (MRI) inactive as compared to MRI active RR MS. Our results indicate that the potential implication of sHLA-G proteins in the resolution of MS intrathecal inflammatory response is probably due to HLA-G5 isoform.

The genetic influence of the nonclassical MHC molecule HLA-G on multiple sclerosis

Human leukocyte antigen (HLA)-G is a nonclassical major histocompatibility complex (MHC) molecule located at MHC complex at chromosome 6 and chiefly attributed immunoregulatory and tolerogenic functions. HLA-G is upregulated at sites of inflammation in multiple sclerosis (MS) and assumed to counterbalance immune responses. Different functionally relevant genetic variants of HLA-G have been described and shown to be statistically associated with human diseases such as fetal loss or sarcoidosis. We investigated the influence of three different variations in the HLA-G gene for disease susceptibility and course of MS (n = 698): (1) The -725 C/G exchange in the HLA-G promoter region, (2) HLA-G*0105N, a deletion that results in an irregular stopcodon in exon 3, and (3) a 14 bp insertion / deletion in the untranslated exon 8. None of these variations significantly influenced the susceptibility to multiple sclerosis. No association was seen with the age of onset of disease, disease severity or disease course. Although HLA-G is assumed to play an important role in the immunoregulatory processes of MS, our results do not support a role of genetic factors influencing disease susceptibility of the disease course.