In vivo identification of an HLA-G complex as ubiquitinated protein circulating in exosomes

The nonclassical human leukocyte antigen-G (HLA-G) is a tolerogenic molecule that can be released to the circulation by expressing cells. This molecule can form dimers but some other complexed HLA-G forms have been proposed to be present in vivo. Here, we further characterized these other complexed HLA-G forms in vivo. Ascitic and pleural exudates from patients were selected based on positivity for HLA-G by ELISA. Complexed HLA-G was detected in exosomes, which indicates an intracellular origin of these forms. 2D-PAGE analysis of exudates and isolated exosomes showed that these high molecular weight complexes were more heterogeneous than the HLA-G1 expressed by cell cultures. Treatment with deglycosylating enzymes did not change the molecular weight of HLA-G complexes. Immunoblot analysis of exudates and exosomes with an anti-ubiquitin antibody showed that at least some of these structures correspond to ubiquitinated HLA-G. HLA-G ubiquitination could be reproduced in vitro in HLA-G1-transfected cell lines, although with a lower modified/nonmodified protein proportion than in exudates. In summary, we demonstrate new circulating HLA-G forms in vivo that open a new perspective in the study of HLA-G function and analysis.

HLA-G inhibition of NK-cell cytolytic function is uncoupled from tumor cell lipid raft reorganization

HLA-G is a non-classical HLA class I molecule with tolerogenic properties and restricted tissue distribution. The expression of HLA-G can be induced by tumors thus providing an efficient way to escape the anti-tumoral immune response. Although lipid rafts regulate diverse immunological mechanisms their relationship with HLA-G remains controversial. Our results show that HLA-G-mediated inhibition of both the interaction between NK and tumor cells, and of intracellular calcium flux in NK cells conjugated to their target cells were independent of lipid raft integrity. In addition, cytotoxicity assays indicated that HLA-G continued to efficiently inhibit NK-cell cytolytic function in several different tumor cells independently of lipid raft integrity. Confocal microscopy with 3D reconstruction combined with biochemical analysis showed that HLA-G was mainly localized outside the lipid rafts of tumor cells after cross-linking with specific antibody and remained excluded from lipid rafts during interaction with the ILT2 inhibitory receptor of NK cells. This study indicates that the inhibitory function of HLA-G is uncoupled from lipid raft organization, further distinguishing HLA-G from classical HLA molecules and providing novel information in the understanding of tumor immune escape mechanism mediated through HLA-G.

ILT2/HLA-G interaction impairs NK-cell functions through the inhibition of the late but not the early events of the NK-cell activating synapse

Natural killer (NK) cells play a crucial role in the antitumoral responses through cytolytic function and cytokine production. Expression of HLA-G at the surface of tumoral cells confers a protection against NK-cell cytolysis through its interaction with the ILT2 inhibitory receptor. Even though the role of this interaction on the inhibition of NK-cell cytotoxicity is well established, its effect on the molecular events occurring at the NK/target-cell synapse is not well characterized. We found that the interaction of the inhibitory receptor ILT2 with HLA-G inhibited the polarization of NK-cell lytic granules and the microtubule organizing center (MTOC) as well as the accumulation of filamentous actin (F-actin) at the area of contact. However, it did not affect the recruitment of the activatory receptor CD2 at the NK/target-cell interface. Even though CD2 was accumulated to the NK-cell synapse, the interaction of ILT2 with HLA-G efficiently inhibited intracellular calcium mobilization and IFN-gamma polarized production of NK cells. These results indicate that while the ILT2/HLA-G interaction leads to the inhibition of NK-cell functions, it displays differential effects on cytoskeleton reorganization and CD2 localization at the NK-cell synapse.-Favier, B., LeMaoult, J., Lesport, E., Carosella, E. D. ILT2/HLA-G interaction impairs NK-cell functions through the inhibition of the late but not the early events of the NK-cell-activating synapse.

Maternal antigen presenting cells are a source of plasmatic HLA-G during pregnancy: longitudinal study during pregnancy

The aim of this work was to investigate the longitudinal evolution of plasmatic soluble HLA-G (sHLA-G: shed HLA-G1 plus HLA-G5) during pregnancy, and if peripheral maternal antigen presenting cells (APC) can be a source of sHLA-G. Blood samples were obtained from 45 volunteers during normal pregnancy, 8 of them monthly; from 8 pregnant volunteers in the first weeks of pregnancy who had later a miscarriage, and from 14 healthy nonpregnant control women. Monocytes obtained during pregnancy showed a moderately HLA-G cell surface expression and stimulation with interferon (IFN)-gamma increased this expression. Monocytes-derived dendritic cells obtained from pregnant women during the first and third trimester of pregnancy secreted more sHLA-G than those obtained from nonpregnant women. Plasmatic sHLA-G concentration in pregnant women was significatively higher than in nonpregnant women, with a peak in the third month. We can conclude that maternal APC are a source of sHLA-G. Women who experienced miscarriage had previously very low or undetectable plasmatic sHLA-G levels in the second month of pregnancy. Data suggest that undetectable sHLA-G could be a risk of complications.

Intercellular exchanges of membrane patches (trogocytosis) highlight the next level of immune plasticity

Although they have been evidenced a long time ago, exchanges of antigens between cells have remained poorly understood and their significance is still unclear. Yet, intercellular antigen exchanges, and most particularly intercellular exchanges of intact membrane patches, also called trogocytosis, have recently been the subject of renewed interest. We here briefly outline the key findings concerning trogocytosis, and discuss their implications.