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[This corrects the article DOI: 10.3389/fimmu.2022.903796.].

Tetherin is an IFN-inducible transmembrane protein that inhibits the detachment of enveloped viruses from infected cells. HIV-1 overcomes this restriction factor by expressing HIV-1 viral protein U (Vpu), which down-regulates and degrades tetherin. We report that mutations in Vpu that impair tetherin antagonism increase the susceptibility of HIV-infected cells to antibody-dependent cell-mediated cytotoxicity (ADCC), and conversely that RNAi knockdown of tetherin, but not other cellular proteins down-modulated by Vpu, decreases the susceptibility of HIV-infected cells to ADCC. These results reveal that Vpu protects HIV-infected cells from ADCC as a function of its ability to counteract tetherin. By serving as link between innate and adaptive immunity, the antiviral activity of tetherin may be augmented by virus-specific antibodies, and hence much greater than previously appreciated.

Natalizumab, a humanized monoclonal antibody (mAb) against α4-integrin, reduces the number of dendritic cells (DC) in cerebral perivascular spaces in multiple sclerosis (MS). Selective deletion of α4-integrin in CD11c⁺ cells should curtail their migration to the central nervous system (CNS) and ameliorate experimental autoimmune encephalomyelitis (EAE). We generated CD11c.Cre+/− ITGA4 fl/fl C57BL/6 mice to selectively delete α4-integrin in CD11c⁺ cells. Active immunization and adoptive transfer EAE models were employed and compared with WT controls. Multiparameter flow cytometry was utilized to immunophenotype leukocyte subsets. Single-cell RNA sequencing was used to profile individual cells. α4-Integrin expression by CD11c⁺ cells was significantly reduced in primary and secondary lymphoid organs in CD11c.Cre+/− ITGA4 fl/fl mice. In active EAE, a delayed disease onset was observed in CD11c.Cre+/− ITGA4 fl/fl mice, during which CD11c⁺CD88⁺ cells were sequestered in the blood. Upon clinical EAE onset, CD11c⁺CD88⁺ cells appeared in the CNS and expressed CD317⁺. In adoptive transfer experiments, CD11c.Cre+/− ITGA4 fl/fl mice had ameliorated clinical disease phenotype associated with significantly diminished numbers of CNS CD11c⁺CD88⁺CD317⁺ cells. In human cerebrospinal fluid from subjects with neuroinflammation, microglia-like cells display coincident expression of ITGAX (CD11c), C5AR1 (CD88), and BST2 (CD317). In mice, we show that only activated, but not naïve microglia expressed CD11c, CD88, and CD317. Finally, anti-CD317 treatment prior to clinical EAE substantially enhanced recovery in mice.

Mammalian cells employ numerous innate cellular mechanisms to inhibit viral replication and spread. Tetherin, also known as Bst-2 or CD317, is a recently identified, IFN-induced, cellular response factor that blocks release of HIV-1 and other retroviruses from infected cells. The means by which tetherin retains retroviruses on the cell surface, as well as the mechanism used by the HIV-1 accessory protein Vpu to antagonize tetherin function and promote HIV-1 release, are unknown. Here, we document that tetherin functions as a broadly acting antiviral factor by demonstrating that both human and murine tetherin potently inhibit the release of the filovirus, Ebola, from the surface of cells. Expression of the Ebola glycoprotein (GP) antagonized the antiviral effect of human and murine tetherin and facilitated budding of Ebola particles, as did the HIV-1 Vpu protein. Conversely, Ebola GP could substitute for Vpu to promote HIV-1 virion release from tetherin-expressing cells, demonstrating a common cellular target for these divergent viral proteins. Ebola GP efficiently coimmunoprecipitated with tetherin, suggesting that the viral glycoprotein directly interferes with this host antiviral factor. These results demonstrate that tetherin is a cellular antiviral factor that restricts budding of structurally diverse enveloped viruses. Additionally, Ebola has evolved a highly effective strategy to combat this antiviral response elicited in the host during infection.

Heterogeneity of bone marrow mesenchymal stromal cells (MSCs, frequently referred to as “mesenchymal stem cells”) clouds biological understanding and hampers their clinical development. In MSC cultures most commonly used in research and therapy, we have identified an MSC subtype characterized by CD317 expression (CD317 pos (29.77 ± 3.00% of the total MSC population), comprising CD317 dim (28.10 ± 4.60%) and CD317 bright (1.67 ± 0.58%) MSCs) and a constitutive interferon signature linked to human disease. We demonstrate that CD317 pos MSCs induced cutaneous tissue damage when applied a skin explant model of inflammation, whereas CD317 neg MSCs had no effect. Only CD317 neg MSCs were able to suppress proliferative cycles of activated human T cells in vitro , whilst CD317 pos MSCs increased polarization towards pro-inflammatory Th1 cells and CD317 neg cell lines did not. Using an in vivo peritonitis model, we found that CD317 neg and CD317 pos MSCs suppressed leukocyte recruitment but only CD317 neg MSCs suppressed macrophage numbers. Using MSC-loaded scaffolds implanted subcutaneously in immunocompromised mice we were able to observe tissue generation and blood vessel formation with CD317 neg MSC lines, but not CD317 pos MSC lines. Our evidence is consistent with the identification of an immune stromal cell, which is likely to contribute to specific physiological and pathological functions and influence clinical outcome of therapeutic MSCs.

HIV-1 Vpu enhances the release of virions from infected cells. Recent work identified Bst-2/CD317/tetherin as a host factor whose inhibitory activity on viral release is counteracted by Vpu. A current working model proposes that Bst-2 inhibits virus release by tethering viral particles to the cell surface. Here, we analyzed endogenous Bst-2 with respect to its effect on virus release from HeLa cells, T cells, and macrophages. We noted significant cell type-dependent variation in Bst-2 expression. Vpu caused a reduction in Bst-2 expression in transfected HeLa cells and long-term infected macrophages. However, Vpu expression did not result in cell surface down-modulation of Bst-2 or a reduction in intracellular Bst-2 expression in CEMx174 or H9 cells, yet virus replication in these cells was Vpu-responsive. Surprisingly, Bst-2 was undetectable in cell-free virions that were recovered from the surface of HeLa cells by physical shearing, suggesting that a tethering model may not explain all of the functional properties of Bst-2. Taken together we conclude that enhancement of virus release by Vpu does not, at least in CEMx174 and H9 cells, require cell surface down-modulation or intracellular depletion of Bst-2, nor does it entail exclusion of Bst-2 from viral particles.

Background Previously, we have demonstrated that the batch variations of human platelet lysate (conventional MSC expansion medium) induce MSC heterogeneity and therapeutic inconsistency. On the other hand, the MSCs expanded with chemical defined medium have improved therapeutic consistency. Methods In the current study, we studied the MSC subpopulation composition and variation in different types and batches of MSC expansion medium with scRNA-seq analysis. Results MSCs expanded with different batches of media have higher levels of heterogeneity from the perspective of cell subpopulation composition at transcriptome levels and therapeutic inconsistency. The CD317 + subpopulation has enhanced immune suppression activities. And the percentage of CD317 + MSCs within MSCs is tightly correlated with its immune suppression activities, and also contributes to the heterogeneity and therapeutic inconsistency of MSCs. the CD317 + MSCs have increased expression levels of PTX3, which might stabilize the TSG6 protein and improve the therapeutic effects Conclusions Thus, purifying CD317 + MSCs is one efficient strategy to reduce MSC heterogeneity and increase the therapeutic consistency of MSCs.

Background Although both preclinical and clinical studies have shown the great application potential of MSCs (mesenchymal stem/stromal cells) in treating many kinds of diseases, therapeutic inconsistency resulting from cell heterogeneity is the major stumbling block to their clinical applications. Cell population diversity and batch variation in the cell expansion medium are two major inducers of MSC heterogeneity. Methods Cell population diversity was investigated through single-cell RNA sequencing analysis of human MSCs derived from the umbilical cord and expanded with fully chemically defined medium in the current study. Then, the MSC subpopulation with enhanced anti-inflammatory effects was studied in vitro and in vivo. Results Our data showed that MSCs contain different populations with different functions, including subpopulations with enhanced functions of exosome secretion, extracellular matrix modification and responses to stimuli (regeneration and immune response). Among them, CD317 + MSCs have improved differentiation capabilities and enhanced immune suppression activities. Underlying mechanism studies showed that higher levels of TSG6 confer enhanced anti-inflammatory functions of CD317 + MSCs. Conclusions Thus, CD317 + MSCs might be a promising candidate for treating immunological disorder-related diseases.

Background Although both pre-clinical and clinical studies show promising outcomes, resulting in rapid growth of clinical trials of MSC-based therapies in recent years, the heterogeneity and therapeutic inconsistency of MSCs have severely hampered their clinical applications. Purifying homogenous MSC populations with enhanced specific functions represents one promising approach. We have demonstrated recently that the CD317 + MSCs have enhanced anti-inflammatory functions and improved therapeutic efficacy and consistency. Methods In the current study, we performed both in vitro and in vivo investigations to delineate whether and how CD317 regulates the immune modulation function of MSCs. Results Our data here indicate that the CD317 directly contributes to the immune suppression function of MSCs stimulated by TNF-α through up-regulating TSG6 via CD317/lipid-raft/TNFR1 complex. The CD317 stabilizes the TNFR1 complex, resulting in hyper-activation of the NF-κB pathway and up-regulation of TSG6, which confers the therapeutic effects of MSCs on the mouse model of ALI (acute lung injury) and IBD (inflammatory bowel disease). Conclusions Thus, the CD317 stabilizes TNFR1 and confers the anti-inflammatory functions of MSCs via NF-κB/TSG6 Pathway.

Adult hematopoietic stem cells (HSCs) in the bone marrow (BM) are quiescent. Following perturbations, such as blood loss or infection, HSCs may undergo activation. Surprisingly, little is known about the earliest stages of HSCs activation. We utilize surface markers of HSCs activation, CD69 and CD317, revealing a response as early as 2 h after stimulation. The dynamic expression of HSCs activation markers varies between viral-like (poly-Inosinic-poly-Cytidylic) or bacterial-like (Lipopolysaccharide) immune stimuli. We further quantify dose response, revealing a low threshold, and similar sensitivity of HSCs and progenitors in the BM. Finally, we find a positive correlation between the expression of surface activation markers and early exit from quiescence. Our data show that the response of adult stem cells to immune stimulation is rapid and sensitive, rapidly leading HSCs out of quiescence.