Nitric oxide produces HLA-G nitration and induces metalloprotease-dependent shedding creating a tolerogenic milieu

Human leucocyte antigen G (HLA-G) is a tolerogenic molecule that protects the fetus from maternal immune attack, may favour tumoral immunoescape and is up-regulated in viral and inflammatory diseases. The aim of this work was to discover if nitric oxide (NO) could affect HLA-G expression or function because NO is an important modulator of innate and adaptive immunity. For this purpose HLA-G expression and function were analysed following treatment with a NO donor or a peroxynitrite donor in various cell lines expressing HLA-G either spontaneously or upon transfection. Results showed NO-dependent nitration of both cellular and soluble HLA-G protein, but not all HLA-G moieties underwent nitration. Endogenous biosynthesis of NO by both U-937-HLA-G1 and M8-HLA-G5 stable transfectants also caused HLA-G nitration. The NO decreased total HLA-G cellular protein content and expression on the cell surface, while increasing HLA-G shedding into the culture medium. This effect was post-transcriptional and the result of metalloprotease activity. By contrast, NO pretreatment did not affect HLA-G capability to suppress NK cytotoxicity and lymphocyte proliferation. Our studies show that NO regulates the availability of HLA-G molecules without modifying their biological activities

Linking Two Immuno-Suppressive Molecules: Indoleamine 2,3 Dioxygenase Can Modify HLA-G Cell-Surface Expression1

Nonclassical human leukocyte antigen (HLA) class I molecule HLA-G and indoleamine 2,3 dioxygenase (INDO) in humans and mice, respectively, have been shown to play crucial immunosuppressive roles in fetal-maternal tolerance. HLA-G inhibits natural killer and T cell function by high-affinity interaction with inhibitory receptors, and INDO acts by depleting the surrounding microenvironment of the essential amino acid tryptophan, thus inhibiting T cell proliferation. We investigated whether HLA-G expression and INDO function were linked. Working with antigen-presenting cell (APC) lines and monocytes, we found that functional inhibition of INDO by 1-methyl-tryptophan induced cell surface expression of HLA-G1 by HLA-G1- negative APCs that were originally cell-surface negative, and that in reverse, the functional boost of INDO by high concentrations of tryptophan induced a complete loss of HLA-G1 cell surface expression by APCs that were originally cell-surface HLA-G1-positive. This mechanism was shown to be posttranslational because HLA-G protein cell contents remained unaffected by the treatments used. Furthermore, HLA-G cell surface expression regulation by INDO seems to relate to INDO function, but not to tryptophan catabolism itself. Potentia

The Self Model and the Conception of Biological Identity in Immunology

The self/non-self model, first proposed by F.M. Burnet, has dominated immunology for sixty years now. According to this model, any foreign element will trigger an immune reaction in an organism, whereas endogenous elements will not, in normal circumstances, induce an immune reaction. In this paper we show that the self/non-self model is no longer an appropriate explanation of experimental data in immunology, and that this inadequacy may be rooted in an excessively strong metaphysical conception of biological identity. We suggest that another hypothesis, one based on the notion of continuity, gives a better account of immune phenomena. Finally, we underscore the mapping between this metaphysical deflation from self to continuity in immunology and the philosophical debate between substantialism and empiricism about identity

Multimeric structures of HLA-G isoforms function through differential binding to LILRB receptors

The non-classical Human leukocyte antigen G (HLA-G) differs from classical HLA class I molecules by its low genetic diversity, a tissue-restricted expression, the existence of seven isoforms, and immuno-inhibitory functions. Most of the known functions of HLA-G concern the membrane-bound HLA-G1 and soluble HLA-G5 isoforms, which present the typical structure of classical HLA class I molecule: a heavy chain of three globular domains α(1)-α(2)-α(3) non-covalently bound to β-2-microglobulin (B2M) and a peptide. Very little is known of the structural features and functions of other HLA-G isoforms or structural conformations other than B2M-associated HLA-G1 and HLA-G5. In the present work, we studied the capability of all isoforms to form homomultimers, and investigated whether they could bind to, and function through, the known HLA-G receptors LILRB1 and LILRB2. We report that all HLA-G isoforms may form homodimers, demonstrating for the first time the existence of HLA-G4 dimers. We also report that the HLA-G α(1)-α(3) structure, which constitutes the extracellular part of HLA-G2 and HLA-G6, binds the LILRB2 receptor but not LILRB1. This is the first report of a receptor for a truncated HLA-G isoform. Following up on this finding, we show that the α(1)-α(3)-Fc structure coated on agarose beads is tolerogenic and capable of prolonging the survival of skin allografts in B6-mice and in a LILRB2-transgenic mouse model. This study is the first proof of concept that truncated HLA-G isoforms could be used as therapeutic agents

Induction of CD4+ regulatory T cells by soluble HLA-G isoforms.

Purpose/Objective: 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. HLA-G inhibits a wide array of immune cells and has long-term immune- modulatory effects since it can induce the generation of suppressor/ regulatory cells. We recently demonstrated that tolerogenic DC, termed DC-10, promote the differentiation of Tr1 cells via the IL10-dependent membrane bound HLA-G1/ILT4 pathway. The role of membrane-bound HLA-G1 in promoting Tr1 cells via DC-10, raise the question whether soluble shed HLA-G1 or HLA-G5 can promote Tr1 cell differentiation. Materials and methods: Human naı¨ve CD4+ T cells were stimulated via anti-CD3 mAbs cross-linked on artificial APC consisting in murine L-cells co-transfected with human CD32, CD80, and CD58 in the presence of shed sHLA-G1 or HLA-G5 alone or in combination with IL-10. As control, we used Th0 and Tr1 cells differentiated with artificial APC alone or in the presence of IL-10 and IFN-a, respectively. Results: We showed that repetitive stimulation of human naı¨ve CD4+ T cells in the presence of shed HLA-G1 or HLA-G5, alone or in combination with IL-10, induced the differentiation of a population of CD4+ T cells that are phenotypically different from both Th0 and Tr1 cells. T cells differentiated with soluble HLA-Gs secrete lower levels of IFN-g and IL-2 as compared to Th0 cells. Interestingly, HLA-Ginduced T cells secrete low amounts of IL-10, which is slightly increased when IL-10 is present in culture, but never reach the levels produced by Tr1 cells. Despite the ability to proliferate upon polyclonal activation, HLA-G-induced T cells suppress the proliferation of autologous CD4+ T cells in vitro. Conclusions: We showed that activation of human CD4+ T cells in the presence of both soluble shed HLA-G1 or HLA-G5, alone or in combination with IL-10, promotes the induction of a population of suppressor CD4+ T cells, which are distinct from Tr1 cells

Role of HLA-G as a predictive marker of low risk of chronic rejection in lung transplant recipients: A clinical prospective study

Human leukocyte antigen G (HLA-G) expression is thought to be associated with a tolerance state following solid organ transplantation.In a lung transplant (LTx) recipient cohort, we assessed (1) the role of HLA-G expression as a predictor of graft acceptance, and (2) the relationship between (i) graft and peripheral HLA-G expression, (ii) HLA-G expression and humoral immunity and (iii) HLA-G expression and lung microenvironment.We prospectively enrolled 63 LTx recipients (median follow-up 3.26 years [min: 0.44-max: 5.03]).At 3 and 12 months post-LTx, we analyzed graft HLA-G expression by immunohistochemistry, plasma soluble HLA-G (sHLA-G) level by enzyme-linked immunosorbent assay, bronchoalveolar lavage fluid (BALF) levels of cytokines involved in chronic lung allograft dysfunction (CLAD) and anti-HLA antibodies (Abs) in serum.In a time-dependent Cox model, lung HLA-G expression had a protective effect on CLAD occurrence (hazard ratio: 0.13 [0.03-0.58]; p = 0.008).The same results were found when computing 3-month and 1-year conditional freedom from CLAD (p = 0.03 and 0.04, respectively [log-rank test]).Presence of anti-HLA Abs was inversely associated with graft HLA-G expression (p = 0.02).Increased BALF level of transforming growth factor-β was associated with high plasma sHLA-G level (p = 0.02).In conclusion, early graft HLA-G expression in LTx recipients with a stable condition was associated with graft acceptance in the long term

Nitric oxide produces HLA-G nitration and induces metalloprotease-dependent shedding creating a tolerogenic milieu

Human leucocyte antigen G (HLA-G) is a tolerogenic molecule that protects the fetus from maternal immune attack, may favour tumoral immunoescape and is up-regulated in viral and inflammatory diseases. The aim of this work was to discover if nitric oxide (NO) could affect HLA-G expression or function because NO is an important modulator of innate and adaptive immunity. For this purpose HLA-G expression and function were analysed following treatment with a NO donor or a peroxynitrite donor in various cell lines expressing HLA-G either spontaneously or upon transfection. Results showed NO-dependent nitration of both cellular and soluble HLA-G protein, but not all HLA-G moieties underwent nitration. Endogenous biosynthesis of NO by both U-937-HLA-G1 and M8-HLA-G5 stable transfectants also caused HLA-G nitration. The NO decreased total HLA-G cellular protein content and expression on the cell surface, while increasing HLA-G shedding into the culture medium. This effect was post-transcriptional and the result of metalloprotease activity. By contrast, NO pretreatment did not affect HLA-G capability to suppress NK cytotoxicity and lymphocyte proliferation. Our studies show that NO regulates the availability of HLA-G molecules without modifying their biological activities