Fibroblastic reticular cells enhance T cell metabolism and survival via epigenetic remodeling.

Lymph node fibroblastic reticular cells (FRCs) respond to signals from activated T cells by releasing nitric oxide, which inhibits T cell proliferation and restricts the size of the expanding T cell pool. Whether interactions with FRCs also support the function or differentiation of activated CD8+ T cells is not known. Here we report that encounters with FRCs enhanced cytokine production and remodeled chromatin accessibility in newly activated CD8+ T cells via interleukin-6. These epigenetic changes facilitated metabolic reprogramming and amplified the activity of pro-survival pathways through differential transcription factor activity. Accordingly, FRC conditioning significantly enhanced the persistence of virus-specific CD8+ T cells in vivo and augmented their differentiation into tissue-resident memory T cells. Our study demonstrates that FRCs play a role beyond restricting T cell expansion-they can also shape the fate and function of CD8+ T cells.

B cell homeostasis and follicle confines are governed by fibroblastic reticular cells.

Fibroblastic reticular cells (FRCs) are known to inhabit T cell-rich areas of lymphoid organs, where they function to facilitate interactions between T cells and dendritic cells. However, in vivo manipulation of FRCs has been limited by a dearth of genetic tools that target this lineage. Here, using a mouse model to conditionally ablate FRCs, we demonstrated their indispensable role in antiviral T cell responses. Unexpectedly, loss of FRCs also attenuated humoral immunity due to impaired B cell viability and follicular organization. Follicle-resident FRCs established a favorable niche for B lymphocytes via production of the cytokine BAFF. Thus, our study indicates that adaptive immunity requires an intact FRC network and identifies a subset of FRCs that control B cell homeostasis and follicle identity.

High-density lipoprotein mediates anti-inflammatory reprogramming of macrophages via the transcriptional regulator ATF3.

High-density lipoprotein (HDL) mediates reverse cholesterol transport and is known to be protective against atherosclerosis. In addition, HDL has potent anti-inflammatory properties that may be critical for protection against other inflammatory diseases. The molecular mechanisms of how HDL can modulate inflammation, particularly in immune cells such as macrophages, remain poorly understood. Here we identify the transcriptional regulator ATF3, as an HDL-inducible target gene in macrophages that downregulates the expression of Toll-like receptor (TLR)-induced proinflammatory cytokines. The protective effects of HDL against TLR-induced inflammation were fully dependent on ATF3 in vitro and in vivo. Our findings may explain the broad anti-inflammatory and metabolic actions of HDL and provide the basis for predicting the success of new HDL-based therapies.

Intrahepatic myeloid-cell aggregates enable local proliferation of CD8(+) T cells and successful immunotherapy against chronic viral liver infection.

Chronic infection is difficult to overcome because of exhaustion or depletion of cytotoxic effector CD8(+) T cells (cytotoxic T lymphoytes (CTLs)). Here we report that signaling via Toll-like receptors (TLRs) induced intrahepatic aggregates of myeloid cells that enabled the population expansion of CTLs (iMATEs: 'intrahepatic myeloid-cell aggregates for T cell population expansion') without causing immunopathology. In the liver, CTL proliferation was restricted to iMATEs that were composed of inflammatory monocyte-derived CD11b(+) cells. Signaling via tumor-necrosis factor (TNF) caused iMATE formation that facilitated costimulation dependent on the receptor OX40 for expansion of the CTL population. The iMATEs arose during acute viral infection but were absent during chronic viral infection, yet they were still induced by TLR signaling. Such hepatic expansion of the CTL population controlled chronic viral infection of the liver after vaccination with DNA. Thus, iMATEs are dynamic structures that overcome regulatory cues that limit the population expansion of CTLs during chronic infection and can be used in new therapeutic vaccination strategies.

Coinhibitory Pathways in the B7-CD28 Ligand-Receptor Family.

Immune responses need to be controlled for optimal protective immunity and tolerance. Coinhibitory pathways in the B7-CD28 family provide critical inhibitory signals that regulate immune homeostasis and defense and protect tissue integrity. These coinhibitory signals limit the strength and duration of immune responses, thereby curbing immune-mediated tissue damage, regulating resolution of inflammation, and maintaining tolerance to prevent autoimmunity. Tumors and microbes that cause chronic infections can exploit these coinhibitory pathways to establish an immunosuppressive microenvironment, hindering their eradication. Advances in understanding T cell coinhibitory pathways have stimulated a new era of immunotherapy with effective drugs to treat cancer, autoimmune and infectious diseases, and transplant rejection. In this review we discuss the current knowledge of the mechanisms underlying the coinhibitory functions of pathways in the B7-CD28 family, the diverse functional consequences of these inhibitory signals on immune responses, and the overlapping and unique functions of these key immunoregulatory pathways.

Biologic principles of minced cartilage implantation: a narrative review.

Cartilage tissue has a very limited ability to regenerate. Symptomatic cartilage lesions are currently treated by various cartilage repair techniques. Multiple treatment techniques have been proposed in the last 30 years. Nevertheless, no single technique is accepted as a gold standard. Minced cartilage implantation is a newer technique that has garnered increasing attention. This procedure is attractive because it is autologous, can be performed in a single surgery, and is therefore given it is cost-effective. This narrative review provides an overview of the biological potential of current cartilage regenerative repair techniques with a focus on the translational evidence of minced cartilage implantation.

Liver stromal cells restrict macrophage maturation and stromal IL-6 limits the differentiation of cirrhosis-linked macrophages.

BACKGROUND & AIMS: Myeloid cells are key regulators of cirrhosis, a major cause of mortality worldwide. Because stromal cells can modulate the functionality of myeloid cells in vitro, targeting stromal-myeloid interactions has become an attractive potential therapeutic strategy. We aimed to investigate how human liver stromal cells impact myeloid cell properties and to understand the utility of a stromal-myeloid coculture system to study these interactions in the context of cirrhosis. METHODS: Single-cell RNA-sequencing analyses of non-cirrhotic (n = 7) and cirrhotic (n = 5) human liver tissue were correlated to the bulk RNA-sequencing results of in vitro cocultured human CD14+ and primary liver stromal cells. Complimentary mechanistic experiments and flow cytometric analysis were performed on human liver stromal-myeloid coculture systems. RESULTS: We found that stromal-myeloid coculture reduces the frequency CD14+ cell subsets transcriptionally similar to liver macrophages, showing that stromal cells inhibit the maturation of monocytes into macrophages. Stromal cells also influenced in vitro macrophage differentiation by skewing away from cirrhosis-linked CD9+ scar-associated macrophage-like cells and towards CD163+ Kupffer cell-like macrophages. We identify IL-6 production as a mechanism by which stromal cells limit CD9+ macrophage differentiation and find that local IL-6 levels are decreased in early-stage human liver disease compared to healthy liver tissue, suggesting a protective role for local IL-6 in the healthy liver. CONCLUSIONS: Our work reveals an unanticipated role for liver stromal cells in impeding the maturation and altering the differentiation of macrophages and should prompt investigations into the role of local IL-6 production in the pathogenesis of liver disease. These studies provide a framework for investigating macrophage-stromal interactions during cirrhosis. LAY SUMMARY: The impact of human liver stromal cells on myeloid cell maturation and differentiation in liver disease is incompletely understood. In this study, we present a mechanistic analysis using a primary in vitro human liver stromal-myeloid coculture system that is translated to liver disease using single-cell RNA sequencing analysis of cirrhotic and non-cirrhotic human liver tissue. Our work supports a role for stromal cell contact in restricting macrophage maturation and for stromal-derived IL-6 in limiting the differentiation of a cirrhotic macrophage subset.

Dendritic Cell PD-L1 Limits Autoimmunity and Follicular T Cell Differentiation and Function.

The programmed death (PD)-1 coinhibitory receptor regulates the balance between T cell activation and tolerance. Although the PD-1 ligands, PD-L1 and PD-L2, are expressed on a variety of cell types, the cell type-specific functions of PD-1 ligands in inducing signals through PD-1 are unknown. In this study, we use PD-L1 conditional knockout mice to investigate the cell type-specific functions of PD-L1. We demonstrate that PD-L1 expressed on dendritic cells (DCs), and to a lesser extent on B cells, attenuates the progression of experimental autoimmune encephalomyelitis and inhibits naive and effector T cells. PD-1 is highly expressed on effector populations, including T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells, which reside in germinal centers. We also show that DC PD-L1 is essential for limiting Tfh and Tfr cell differentiation. In addition, we find that PD-1 suppresses Tfh cell differentiation and help for Ig class switching, even in the presence of wild-type Tfr cells. Our work points to critical roles for PD-L1 expressed on DCs in mediating PD-1 functions.

Investigation of Cytotoxicity, Oxidative Stress, and Inflammatory Responses of Tantalum Nanoparticles in THP-1-Derived Macrophages.

Tantalum (Ta) is gaining attention as a biomaterial in bone tissue engineering. Although the clinical advantage of Ta-based implants for primary and revision total joint replacement (TJA) has been well documented, few studies investigated the effect of wear products of Ta implants on peri-implant cells, and their potential contribution to aseptic implant loosening. This study is aimed at examining the cytotoxicity, oxidative stress, and proinflammatory potential of Ta and TiO2 nanoparticles (NPs) on macrophages in vitro. NPs were characterized using scanning electron microscopy, dynamic light scattering, and energy-dispersive X-ray. To test the NP-mediated cellular response in macrophages, THP-1-derived macrophages were challenged with both NPs, and cytotoxicity was analyzed using CCK-8 and LDH assays. Flow cytometry was used to investigate particle uptake and their internalization routes. NP-mediated oxidative stress was investigated by measuring the production of reactive oxygen species, and their proinflammatory potential was determined by quantifying the production of TNFα and IL-1ß in cell culture supernatants using ELISA. We found that both Ta and TiO2 NPs were taken up through actin-dependent phagocytosis, although TiO2 NPs did also show some involvement of macropinocytosis and clathrin-mediated endocytosis. Ta NPs caused no apparent toxicity, while TiO2 NPs demonstrated significant cytotoxicity at a concentration of over 100µg/mL at 24 h. Ta NPs induced negligible ROS generation and proinflammatory cytokines (TNFα, IL-1ß) in macrophages. In contrast, TiO2 NPs markedly induced these effects in a dose-dependent manner. Our findings indicate that Ta NPs are inert, nontoxic, and noninflammatory. Therefore, Ta could be considered an excellent biomaterial in primary and revision joint arthroplasty implants.

Vertebral Bone Marrow-Derived Mesenchymal Stromal Cells from Osteoporotic and Healthy Patients Possess Similar Differentiation Properties In Vitro.

Osteoporosis is a disease characterized by low bone mass and an increased risk of fractures. Although several cellular players leading to osteoporosis have been identified, the role of mesenchymal stromal cells (MSC) is still not fully elaborated. The aim of this study was, therefore, to isolate and characterize MSCs from vertebral body of healthy non-osteoporotic and osteoporotic patients, with a particular focus on their osteogenic differentiation potential. Isolated MSCs were characterized by their osteogenic, adipogenic, and chondrogenic differentiation, as well as surface marker expression, proliferation behavior, and immunomodulatory capacity. The mineralization process was confirmed using Alizarin Red S and alkaline phosphatase (ALP) stains and further evaluated by determining ALP activity, mineral deposition, and free phosphate ion release. MSCs from both healthy and osteoporotic patients showed common fibroblast-like morphology and similar proliferation behavior. They expressed the typical MSC surface markers and possessed immunomodulatory capacity. Both groups demonstrated solid trilineage differentiation potential; osteogenic differentiation was further confirmed by increased ALP activity, deposition of inorganic crystals, phosphate ion release, and expression of osteoblast marker genes. Overall, MSCs from osteoporotic and non-osteoporotic patients showed neither a difference in general MSC features nor in the detailed analysis regarding osteogenic differentiation. These data suggest that vertebral body MSCs from osteoporotic patients were not impaired; rather, they possessed full osteogenic potential compared to MSCs from non-osteoporotic patients.

Characterization and Comparison of Human and Ovine Mesenchymal Stromal Cells from Three Corresponding Sources.

Currently, there is an increasing focus on mesenchymal stromal cells (MSC) as therapeutic option in bone pathologies as well as in general regenerative medicine. Although human MSCs have been extensively characterized and standardized, ovine MSCs are poorly understood. This limitation hampers clinical progress, as sheep are an excellent large animal model for orthopedic studies. Our report describes a direct comparison of human and ovine MSCs from three corresponding sources under the same conditions. All MSCs presented solid growth behavior and potent immunomodulatory capacities. Additionally, we were able to identify common positive (CD29, CD44, CD73, CD90, CD105, CD166) and negative (CD14, CD34, CD45, HLA-DR) surface markers. Although both human and ovine MSCs showed strong osteogenic potential, direct comparison revealed a slower mineralization process in ovine MSCs. Regarding gene expression level, both human and ovine MSCs presented a comparable up-regulation of Runx2 and a trend toward down-regulation of Col1A during osteogenic differentiation. In summary, this side by side comparison defined phenotypic similarities and differences of human and ovine MSCs from three different sources, thereby contributing to a better characterization and standardization of ovine MSCs. The key findings shown in this report demonstrate the utility of ovine MSCs in preclinical studies for MSC-based therapies.

Molecular and Functional Phenotypes of Human Bone Marrow-Derived Mesenchymal Stromal Cells Depend on Harvesting Techniques.

Mesenchymal stromal cells (MSC) harvested in different tissues from the same donor exhibit different phenotypes. Each phenotype is not only characterized by a certain pattern of cell surface markers, but also different cellular functionalities. Only recently were different harvesting and processing techniques found to contribute to this phenomenon as well. This study was therefore set up to investigate proteomic and functional properties of human bone marrow-derived MSCs (hBM-MSC). These were taken from the same tissue and donor site but harvested either as aspirate or bone chip cultures. Both MSC populations were profiled for MSC markers defined by the International Society for Cellular Therapy (ISCT), MSC markers currently under discussion and markers of particular interest. While classic ISCT MSC markers did not show any significant difference between aspirate and outgrowth hBM-MSCs, our additional characterization panel revealed distinct patterns of differentially expressed markers. Furthermore, hBM-MSCs from aspirate cultures demonstrated a significantly higher osteogenic differentiation potential than outgrowth MSCs, which could be confirmed using a transcriptional approach. Our comparison of MSC phenotypes obtained by different harvesting techniques suggests the need of future standardized harvesting, processing and phenotyping procedures in order to gain better comparability in the MSC field.

Pediatric Scoliosis Surgery-A Comprehensive Analysis of Treatment-Specific Variables and Trends in Latvia.

Background and Objectives: There are currently no data available regarding pediatric scoliosis surgery in Latvia. The aim of this article is to present treatment specific variables, investigate their interrelation, and identify predictors for the length of stay after surgical pediatric scoliosis correction. Materials and Methods: This retrospective study included all surgical pediatric scoliosis corrections in Latvia for the years 2012 to 2016. Analyzed parameters were chosen to portray the patients' demographics, pathology, as well as treatment specific variables. Descriptive, inferential, and linear regression statistics were calculated. Results: A total of 69 cases, 74% female and 26% male, were identified. The diagnostic subgroups consisted of 62% idiopathic (IDI) and 38% non-idiopathic (non-IDI) scoliosis cases. Non-IDI cases had significantly increased operation time, hospital stay, Cobb angle before surgery, and instrumented levels, while IDI cases showed significantly higher Cobb angle percentage correction. For all operated cases, the operation time and the hospital stay decreased significantly over the investigated time period. Early post-operative complications (PCs) occurred in 15.9% of the cases and were associated with increased hospital stay, instrumented levels, and Cobb angle before surgery. The linear regression analysis revealed that operation time and the presence of PCs were significant predictors for the length of the hospital stay. Conclusions: This is the first study to provide comprehensive insight into pediatric scoliosis surgery since its establishment in Latvia. Our regression model offers clinically applicable predictors and further underlines the significance of the operation length on the hospital stay. These results build the foundation for international comparison and facilitate improvement in the field.

Laser-based Techniques for Microcirculatory Assessment in Orthopedics and Trauma Surgery: Past, Present, and Future.

: Microcirculatory integrity and proper function are the cornerstones to tissue nourishment and viability. In the clinical environment extended immobility, injuries, and inflammatory reactions demand local microcirculatory adaption to provide adequate supply. Assessment of endothelial adjustment capability and microcirculatory perfusion status, as direct or surrogate markers of disease, are therefore of uttermost interest to the treating physician. Given the simple, noninvasive, nonradiating nature of laser-based techniques for bedside or intraoperative microcirculatory perfusion assessment, this article's objective is to present a comprehensive overview of available techniques, their technological aspects, and current application. Advantages of individual methods are pointed out and compared with each other. The areas of medical utilization relevant to orthopedics and trauma surgery are exemplified and their available evidence elaborated. A particular focus is put on laser speckle contrast imaging, with its current and future influence on medical practice.

Predictors for secondary patellar resurfacing after primary total knee arthroplasty using a "patella-friendly" total knee arthroplasty system.

PURPOSE: Patellar resurfacing (PR) in total knee arthroplasty (TKA) is still one of the major controversies in orthopaedic surgery today. The aim of the present retrospective case-control study was to identify predictors for secondary patellar resurfacing (SPR) after initial TKA to create a rationale for surgeons to decide which patients to resurface primarily. It was hypothesized that proper TKA implantation and component positioning as well as a maintained physiological patellar geometry will lead to a reduced risk of SPR. Overmore, it was hypothesized that intrinsic factors like overweight might also have an influence on the need for SPR. METHODS: After identification of suitable patients and age/sex matching in a 1:2 fashion, 29 cases (TKA/SPR) and 58 controls (TKA) were included and screened for available clinical and epidemiological data as well as for radiographic data after primary TKA. Pearson's correlation analysis as well as logistic regression modeling was performed to identify possible predictors for SPR following TKA. RESULTS: Binary logistic regression was able to correctly classify 88.5% of patients into case or control groups. It indicated that patella tilt, patella height, and thickness as well as the delta angle were significant predictors of a need for SPR following primary TKA. An increase in patellar width by 1 mm will increase the risk of SPR, while an increase in patellar thickness by 1 mm will reduce it. An increase in patellar tilt by 1° will also increase the risk of SPR. Finally, an increase in delta angle by 1° will again reduce the risk of SPR. CONCLUSIONS: Easy and accessible radiographic measurements have been identified as possible predictors of SPR following primary TKA. Although indication for primary PR may still remain a controversial topic, a rationale has been proposed in this study to support surgeons in objectively estimating an individual patient's risk for SPR prior to primary TKA measuring the patella tilt, width, and thickness. Overmore, regarding surgical aspects of TKA, tibial component positioning has also been shown to be of importance to reduce the risk of SPR.

Articular cartilage regeneration and tissue engineering models: a systematic review.

INTRODUCTION: Cartilage regeneration and restoration is a major topic in orthopedic research as cartilaginous degeneration and damage is associated with osteoarthritis and joint destruction. This systematic review aims to summarize current research strategies in cartilage regeneration research. MATERIALS AND METHODS: A Pubmed search for models investigating single-site cartilage defects as well as chondrogenesis was conducted and articles were evaluated for content by title and abstract. Finally, only manuscripts were included, which report new models or approaches of cartilage regeneration. RESULTS: The search resulted in 2217 studies, 200 of which were eligible for inclusion in this review. The identified manuscripts consisted of a large spectrum of research approaches spanning from cell culture to tissue engineering and transplantation as well as sophisticated computational modeling. CONCLUSIONS: In the past three decades, knowledge about articular cartilage and its defects has multiplied in clinical and experimental settings and the respective body of research literature has grown significantly. However, current strategies for articular cartilage repair have not yet succeeded to replicate the structure and function of innate articular cartilage, which makes it even more important to understand the current strategies and their impact. Therefore, the purpose of this review was to globally summarize experimental strategies investigating cartilage regeneration in vitro as well as in vivo. This will allow for better referencing when designing new models or strategies and potentially improve research translation from bench to bedside.

Follicular Dendritic Cells Retain Infectious HIV in Cycling Endosomes.

Despite the success of antiretroviral therapy (ART), it does not cure Human Immunodeficiency Virus (HIV) and discontinuation results in viral rebound. Follicular dendritic cells (FDC) are in direct contact with CD4+ T cells and they retain intact antigen for prolonged periods. We found that human FDC isolated from patients on ART retain infectious HIV within a non-degradative cycling compartment and transmit infectious virus to uninfected CD4 T cells in vitro. Importantly, treatment of the HIV+ FDC with a soluble complement receptor 2 purges the FDC of HIV virions and prevents viral transmission in vitro. Our results provide an explanation for how FDC can retain infectious HIV for extended periods and suggest a therapeutic strategy to achieve cure in HIV-infected humans.

Diagnostic relevance of a novel multiplex immunoassay panel in breast cancer.

Multiple factors contribute to the development and progression of breast cancer. Markers of tumor growth and invasion, cell death, immune activation, and angiogenesis can be assessed in parallel by a novel multiplex immunoassay panel. The diagnostic performance of a multiplex cancer biomarker magnetic bead panel comprising 24 tumor associated parameters was evaluated in sera of 154 women including 77 patients with breast cancer, 10 with precancerous lesions, 31 with benign breast diseases, and 36 healthy controls. Marker levels were log-transformed for variance stabilization. Significance testing was done using t-test or Wilcoxon rank-sum test with correction of p values for multiple testing. Furthermore, receiver operating characteristic analyses were performed. Serum levels of several biomarkers were significantly (p ≤ 0.001) higher in cancer patients than in healthy controls, particularly alpha-fetoprotein, cancer antigen 15-3, cancer antigen 19-9, migration inhibitory factor, carcinoembryonic antigen, cancer antigen 125, hepatocyte growth factor, soluble Fas, tumor necrosis factor-α, stem cell factor, and osteopontin. As most markers were also elevated in benign breast diseases, only cancer antigen 15-3 showed significant differences to cancer patients (p ≤ 0.001). The resulting areas under the curve in receiver operating characteristic curves for discrimination between benign and malignant breast diseases achieved 0.71 with a sensitivity of 33.8% at 95% specificity. Multiplexing enables parallel analysis of different biomarker classes for cancer detection. Established cancer antigen 15-3 proved to be most relevant for differential diagnosis.

Angiotensin-II type 1 receptor-mediated Janus kinase 2 activation induces liver fibrosis.

UNLABELLED: Activation of the renin angiotensin system resulting in stimulation of angiotensin-II (AngII) type I receptor (AT1R) is an important factor in the development of liver fibrosis. Here, we investigated the role of Janus kinase 2 (JAK2) as a newly described intracellular effector of AT1R in mediating liver fibrosis. Fibrotic liver samples from rodents and humans were compared to respective controls. Transcription, protein expression, activation, and localization of JAK2 and downstream effectors were analyzed by real-time polymerase chain reaction, western blotting, immunohistochemistry, and confocal microscopy. Experimental fibrosis was induced by bile duct ligation (BDL), CCl4 intoxication, thioacetamide intoxication or continuous AngII infusion. JAK2 was inhibited by AG490. In vitro experiments were performed with primary rodent hepatic stellate cells (HSCs), Kupffer cells (KCs), and hepatocytes as well as primary human and human-derived LX2 cells. JAK2 expression and activity were increased in experimental rodent and human liver fibrosis, specifically in myofibroblastic HSCs. AT1R stimulation in wild-type animals led to activation of HSCs and fibrosis in vivo through phosphorylation of JAK2 and subsequent RhoA/Rho-kinase activation. These effects were prevented in AT1R(-/-) mice. Pharmacological inhibition of JAK2 attenuated liver fibrosis in rodent fibrosis models. In vitro, JAK2 and downstream effectors showed increased expression and activation in activated HSCs, when compared to quiescent HSCs, KCs, and hepatocytes isolated from rodents. In primary human and LX2 cells, AG490 blocked AngII-induced profibrotic gene expression. Overexpression of JAK2 led to increased profibrotic gene expression in LX2 cells, which was blocked by AG490. CONCLUSION: Our study substantiates the important cell-intrinsic role of JAK2 in HSCs for development of liver fibrosis. Inhibition of JAK2 might therefore offer a promising therapy for liver fibrosis.

Transfer of MHC-class-I molecules among liver sinusoidal cells facilitates hepatic immune surveillance.

BACKGROUND & AIMS: In the liver, antigen-presenting cell populations such as Kupffer cells, liver dendritic cells, and liver sinusoidal endothelial cells (LSECs) participate through cross-presentation to CD8 T cells (CTLs) in hepatic immune-regulation and immune-surveillance. The participation of hepatic stellate cells (HSCs) in immune regulation is controversial. Here we studied HSC's contribution to antiviral CTL immunity. METHODS: Flow cytometric analysis of MHC-I molecules at the cell surface of liver cells from mice with cell-type restricted MHC-I expression. Mice with HSC-restricted MHC-I expression were infected with a hepatotropic virus and analyzed for development of viral hepatitis after CTL transfer. RESULTS: HSCs transferred MHC-I molecules to LSECs and these molecules were employed for LSEC cross-presentation to CTLs. Such transfer of MHC-I molecules was sufficient to support in vivo LSEC cross-presentation of soluble antigens to CTLs. Importantly, this transfer of MHC-I molecules contributed to anti-viral CTL immunity leading to development of immune-mediated hepatitis. CONCLUSIONS: Our findings demonstrate transfer of MHC-I molecules among sinusoidal liver cell populations as a potent mechanism to increase anti-viral CTL effector function. The transfer of MHC-I molecules from HSCs supplies LSECs with additional MHC-I molecules for their own cell-intrinsic cross-presentation. Such cross-allocation of MHC-I molecules in liver cell populations is distinct from cross-dressing that occurs among immune cell populations in lymphoid tissues where peptide-loaded MHC-I molecules are transferred. Our findings thus reveal a novel mechanism that increases local cross-presentation and CTL effector function in the liver, which may be instrumental for immune-surveillance during viral infection of antigen-presenting liver cells.