Human TIM-1 associates with the TCR complex and up-regulates T cell activation signals

Sensing Dying Cells in Health and Disease: The Importance of Kidney Injury Molecule-1

Kidney injury molecule-1 (KIM-1), also known as T-cell Ig and mucin domain-1 (TIM-1), is a widely recognized biomarker for AKI, but its biological function is less appreciated. KIM-1/TIM-1 belongs to the T-cell Ig and mucin domain family of conserved transmembrane proteins, which bear the characteristic six-cysteine Ig-like variable domain. The latter enables binding of KIM-1/TIM-1 to its natural ligand, phosphatidylserine, expressed on the surface of apoptotic cells and necrotic cells. KIM-1/TIM-1 is expressed in a variety of tissues and plays fundamental roles in regulating sterile inflammation and adaptive immune responses. In the kidney, KIM-1 is upregulated on injured renal proximal tubule cells, which transforms them into phagocytes for clearance of dying cells and helps to dampen sterile inflammation. TIM-1, expressed in T cells, B cells, and natural killer T cells, is essential for cell activation and immune regulatory functions in the host. Functional polymorphisms in the gene for KIM-1/TIM-1, HAVCR1 , have been associated with susceptibility to immunoinflammatory conditions and hepatitis A virus-induced liver failure, which is thought to be due to a differential ability of KIM-1/TIM-1 variants to bind phosphatidylserine. This review will summarize the role of KIM-1/TIM-1 in health and disease and its potential clinical applications as a biomarker and therapeutic target in humans.

Kidney injury molecule 1 (KIM-1): a potential biomarker of acute kidney injury and tubulointerstitial injury in patients with ANCA-glomerulonephritis

Background

Kidney injury molecule 1 (KIM-1) is a transmembrane glycoprotein expressed by proximal tubular cells, recognized as an early, sensitive and specific urinary biomarker for kidney injury. Blood KIM-1 was recently associated with the severity of acute and chronic kidney damage but its value in antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis with glomerulonephritis (ANCA-GN) has not been studied. Thus, we analyzed its expression at ANCA-GN diagnosis and its relationship with clinical presentation, kidney histopathology and early outcomes.

Methods

We assessed KIM-1 levels and other pro-inflammatory molecules (C-reactive protein, interleukin-6, tumor necrosis factor α, monocyte chemoattractant protein-1 and pentraxin 3) at ANCA-GN diagnosis and after 6 months in patients included in the Maine-Anjou registry, which gathers data patients from four French Nephrology Centers diagnosed since January 2000.

Results

Blood KIM-1 levels were assessed in 54 patients. Levels were elevated at diagnosis and decreased after induction remission therapy. KIM-1 was associated with the severity of renal injury at diagnosis and the need for kidney replacement therapy. In opposition to other pro-inflammatory molecules, KIM-1 correlated with the amount of acute tubular necrosis and interstitial fibrosis/tubular atrophy (IF/TA) on kidney biopsy, but not with interstitial infiltrate or with glomerular involvement. In multivariable analysis, elevated KIM-1 predicted initial estimated glomerular filtration rate (β = -19, 95% CI -31, -7.6, P = .002).

Conclusion

KIM-1 appears as a potential biomarker for acute kidney injury and for tubulointerstitial injury in ANCA-GN. Whether KIM-1 is only a surrogate marker or is a key immune player in ANCA-GN pathogenesis remain to be determined.

Novel Roles of the Tim Family in Immune Regulation and Autoimmune Diseases

T cell Ig and mucin domain (Tim) protein family members were identified to be important regulators of the immune response. As their name indicates, Tim proteins were originally considered a T cell-specific markers, and they mainly regulate the responses of T helper cells. However, accumulating evidence indicates that Tims are also expressed on antigen-presenting cells (APCs), such as monocytes, macrophages, dendritic cells (DCs) and B cells, and even plays various roles in natural killer cells (NKs) and mast cells. In recent years, the expression and function of Tims on different cells and the identification of new ligands for the Tim family have suggested that the Tim family plays a crucial role in immune regulation. In addition, the relationship between Tim family gene polymorphisms and susceptibility to several autoimmune diseases has expanded our knowledge of the role of Tim proteins in immune regulation. In this review, we discuss how the Tim family affects immunomodulatory function and the potential role of the Tim family in typical autoimmune diseases, including multiple sclerosis (MS), rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and type 1 diabetes (T1D). A deeper understanding of the immunoregulatory mechanism of the Tim family might provide new insights into the clinical diagnosis and treatment of autoimmune diseases.

Kidney Injury Molecule 1 (KIM-1): a Multifunctional Glycoprotein and Biological Marker (Review)

KIM-1 (kidney injury molecule 1) is a transmembrane glycoprotein also known as HAVcr-1 and TIM-1 belongs to the T-cell immunoglobulin and mucin domain family (TIM) of proteins. TIM glycoproteins are presented on the immune cells and participate in the regulation of immune reactions. KIM-1 differs from other members of its family in that it is expressed not only by immunocompetent cells but epithelial cells as well. Cellular and humoral effects mediated by KIM-1 are involved in a variety of physiological and pathophysiological processes.

Current understanding of the mechanisms determining the participation of KIM-1 in viral invasion, the immune response regulation, adaptive reactions of the kidney epithelium to acute ischemic or toxic injury, in progression of chronic renal diseases, and kidney cancer development have been presented in this review. Data of clinical researches demonstrating the association of KIM-1 with viral diseases and immune disorders have also been analyzed. Potential application of KIM-1 as urinary or serological marker in renal and cardiovascular diseases has been considered.

EphA2 Interacts with Tim-4 through Association between Its FN3 Domain and the IgV Domain of Tim-4

Tim-4 promotes the engulfment of apoptotic cells or exogenous particles by securing them on phagocytes. It is unable to transduce signals by itself but helps other engulfment receptors sense and internalize them. However, the identity of the engulfment receptors collaborating with Tim-4 is still incompletely understood. In this study, we searched for a candidate transmembrane protein with a FN3 domain, important for interaction with Tim-4, in silico and investigated whether it indeed interacts with Tim-4 and is involved in Tim-4-mediated phagocytosis. We found that EphA2 containing a FN3 domain in the extracellular region interacted with Tim-4, which was mediated by the IgV domain of Tim-4 and the FN3 domain of EphA2. Nevertheless, we found that EphA2 expression failed to alter Tim-4-mediated phagocytosis of apoptotic cells or polystyrene beads. Taken together, our findings suggest that EphA2, a new Tim-4 interacting protein, may intervene in a Tim-4-mediated cellular event even if it is not phagocytosis of endogenous or exogenous particles and vice versa.

Rapid formation of human immunodeficiency virus-like particles

Understanding the molecular mechanisms involved in the assembly of viruses is essential for discerning how viruses transmit from cell to cell and host to host. Although molecular aspects of assembly have been studied for many viruses, we still have little information about these events in real time. Enveloped viruses such as HIV that assemble at, and bud from, the plasma membrane have been studied in some detail using live cell fluorescence imaging techniques; however, these approaches provide little information about the real-time morphological changes that take place as viral components come together to form individual virus particles. Here we used correlative scanning ion conductance microscopy and fluorescence confocal microscopy to measure the topological changes, together with the recruitment of fluorescently labeled viral proteins such as Gag and Vpr, during the assembly and release of individual HIV virus-like particles (VLPs) from the top, nonadherent surfaces of living cells. We show that 1) labeling of viral proteins with green fluorescent protein affects particle formation, 2) the kinetics of particle assembly on different plasma membrane domains can vary, possibly as a consequence of differences in membrane biophysical properties, and 3) VLPs budding from the top, unimpeded surface of cells can reach full size in 20 s and disappear from the budding site in 0.5 to 3 min from the moment curvature is initially detected, significantly faster than has been previously reported.

The Tim gene family in efferocytosis

One of the key features of the plasma membrane is the asymmetrical distribution of phospholipids across it. Especially, phosphatidylserine (PS) exclusively locates on its inner leaflet. Thus, the exposure of PS on the surface of cells could function as a signal initiating various cellular processes such as phagocytosis of apoptotic cells called efferocytosis, blood clotting, muscle formation, and viral entry. Indeed, PS on apoptotic cells stimulates phagocytes to engulf them and functions as an essential ligand for efferocytosis. Due to the importance of PS in efferocytosis, the existence of the PS receptor had been conceived. However, the PS receptor had not been revealed for a long time. Thus, the first identification of the PS receptor was significant excitement. Tim-4, a member of the T cell immunoglobulin and mucin domain containing family of genes, was one of PS receptors which first identified and received the greatest attention due to its expression in macrophages and relevance to autoimmune and allergic diseases. This review will serve to provide a comprehensive overview of Tim proteins as PS receptors.

Probiotics Can Boost the Antitumor Immunity of CD8+T Cells in BALB/c Mice and Patients with Colorectal Carcinoma

Background

The drug resistance and the immune suppression in the tumor microenvironment are important factors affecting tumor progression. Reversing drug resistance and changing tumor suppression microenvironment are ideal ways to inhibit tumor progression.

Objective

The aim of the study is to verify antitumor immune response of probiotics in patients with colorectal carcinoma and to explore its mechanism.

Methods

To detect the tumor samples of 122 patients with colorectal carcinoma after surgery, analyze the effect of probiotics on enhancing tumor-infiltrating CD8⁺T cells to inhibit colorectal carcinoma, and further verify the mechanism of probiotics on enhancing the antitumor immune response of CD8⁺T cells through animal experiments.

Results

The results of immunohistochemistry showed that the proportion of CD8⁺T cells in the patients treated with probiotics before surgery was increased significantly than that in other patients (P = 0.033). The results of flow cytometry also showed that the proportion of CD8⁺T cells in the probiotics group was higher than that in the nonprobiotics group (P = 0.029). Kaplan-Meier survival estimates also showed that the CD8⁺T cells, TNM stage, pathology grade, lymphatic metastasis, and probiotic treatment were significantly associated with the progression-free survival (PFS) (χ ² = 9.684, P = 0.002 for CD8⁺T cells; χ ² = 5.878, P = 0.015 for TNM stage; χ ² = 7.398, P = 0.004 for pathology grade; χ ² = 8.847, P = 0.003 for Lymphatic metastasis; and χ ² = 4.622, P = 0.032 for the group (group A was treated with probiotics before surgery; group B was not treated with probiotics)). The experimental results in mice showed that probiotics could inhibit tumor growth and increase the proportion of CD8⁺T cells in mice; the difference was statistically significant (P = 0.037). It was also found that probiotic feeding could upregulate the expression of T-cell immunoglobulin mucin receptor 1(TIM-1) in CD8⁺T cells of mice and also found that probiotic feeding could downregulate the expression of programmed cell death protein 1 (PD-1) in CD8⁺T cells of mice, compared with the nonfeeding group; the difference was statistically significant (P = 0.045 for TIM-1 and P = 0.02 for PD-1, respectively). In order to further understand the functional status of CD8⁺T cells, we analyzed interferon-gamma (IFN-γ)⁺ T cells and tumor necrosis factor-α (TNF-α)⁺CD8⁺T cells by flow cytometry. The results showed that the proportion of IFN-γ ⁺ T cells and TNF-α ⁺CD8⁺T cells significantly increased after probiotic treatment, compared with the nonprobiotic treatment group; the difference was statistically significant (P = 0.040 for IFN-γ ⁺ T cells and P = 0.014 for TNF-α ⁺CD8⁺T, respectively).

Conclusions

Probiotics can enhance the antitumor immune response of CD8⁺T cells. It can play a synergistic antitumor role. On the one hand, its mechanism is through regulating intestinal flora, and on the other hand, through regulating the antitumor immune function of CD8⁺T cells.

Costimulation Blockade in Transplantation

T cells play a pivotal role in orchestrating immune responses directed against a foreign (allogeneic) graft. For T cells to become fully activated, the T-cell receptor (TCR) must interact with the major histocompatibility complex (MHC) plus peptide complex on antigen-presenting cells (APCs), followed by a second "positive" costimulatory signal. In the absence of this second signal, T cells become anergic or undergo deletion. By blocking positive costimulatory signaling, T-cell allo-responses can be aborted, thus preventing graft rejection and promoting long-term allograft survival and possibly tolerance (Alegre ML, Najafian N, Curr Mol Med 6:843-857, 2006; Li XC, Rothstein DM, Sayegh MH, Immunol Rev 229:271-293, 2009). In addition, costimulatory molecules can provide negative "coinhibitory" signals that inhibit T-cell activation and terminate immune responses; strategies to promote these pathways can also lead to graft tolerance (Boenisch O, Sayegh MH, Najafian N, Curr Opin Organ Transplant 13:373-378, 2008). However, T-cell costimulation involves an incredibly complex array of interactions that may act simultaneously or at different times in the immune response and whose relative importance varies depending on the different T-cell subsets and activation status. In transplantation, the presence of foreign alloantigen incites not only destructive T effector cells but also protective regulatory T cells, the balance of which ultimately determines the fate of the allograft (Lechler RI, Garden OA, Turka LA, Nat Rev Immunol 3:147-158, 2003). Since the processes of alloantigen-specific rejection and regulation both require activation of T cells, costimulatory interactions may have opposing or synergistic roles depending on the cell being targeted. Such complexities present both challenges and opportunities in targeting T-cell costimulatory pathways for therapeutic purposes. In this chapter, we summarize our current knowledge of the various costimulatory pathways in transplantation and review the current state and challenges of harnessing these pathways to promote graft tolerance (summarized in Table 10.1).

Inhibition of T cell immunoglobulin and mucin-1 (TIM-1) protects against cerebral ischemia-reperfusion injury

Background

The T cell Ig domain and mucin domain (TIM)-1 protein expressed on the surface of Th2 cells regulates the immune response by modulating cytokine production. The present study aimed to investigate the role and possible mechanism of TIM-1 in cerebral ischemia-reperfusion injury.

Methods

Western blot was used to detect TIM-1 and apoptosis-related protein expression, whereas TIM-1 mRNA was examined using quantitative real-time reverse transcription PCR. Flow cytometry and a TdT-mediated biotin-16-dUTP nick-end labeling (TUNEL) assay were used to detect the percentage of apoptotic cells and a pathological examination was performed. The migration of neutrophils and macrophages was analyzed by immunohistochemistry.

Results

Our results suggest that TIM-1 expression was transiently increased 24 h or 48 h following middle cerebral artery occlusion (MCAO)/reperfusion. The infarct size was markedly increased in MCAO, whereas treatment with a TIM-1-blocking mAb could reduce the infarct size. TIM-1 blocking mAb effectively reduced the number of neutrophils, macrophage functionality, cytokine (i.e., IL-6, IL-1β, and TNF-α) and chemokine (i.e., CXCL-1 and CXCL-2) production in the brain tissue. The effect of in vitro T cell damage on neurons was significantly reduced following treatment with a TIM-1 blocking mAb or the knockdown of TIM-1 in co-cultured T cells and neurons.

Conclusion

Take together, these results indicated that TIM-1 blockade ameliorated cerebral ischemia-reperfusion injury. Thus, TIM-1 disruption may serve as a novel target for therapy following MCAO.

TIM-1 serves as a receptor for Ebola virus in vivo, enhancing viremia and pathogenesis

Background

T cell immunoglobulin mucin domain-1 (TIM-1) is a phosphatidylserine (PS) receptor, mediating filovirus entry into cells through interactions with PS on virions. TIM-1 expression has been implicated in Ebola virus (EBOV) pathogenesis; however, it remains unclear whether this is due to TIM-1 serving as a filovirus receptor in vivo or, as others have suggested, TIM-1 induces a cytokine storm elicited by T cell/virion interactions. Here, we use a BSL2 model virus that expresses EBOV glycoprotein to demonstrate the importance of TIM-1 as a virus receptor late during in vivo infection.

Methodology/Principal findings

Infectious, GFP-expressing recombinant vesicular stomatitis virus encoding either full length EBOV glycoprotein (EBOV GP/rVSV) or mucin domain deleted EBOV glycoprotein (EBOV GPΔO/rVSV) was used to assess the role of TIM-1 during in vivo infection. GFP-expressing rVSV encoding its native glycoprotein G (G/rVSV) served as a control. TIM-1-sufficient or TIM-1-deficient BALB/c interferon α/β receptor^-/- mice were challenged with these viruses. While G/rVSV caused profound morbidity and mortality in both mouse strains, TIM-1-deficient mice had significantly better survival than TIM-1-expressing mice following EBOV GP/rVSV or EBOV GPΔO/rVSV challenge. EBOV GP/rVSV or EBOV GPΔO/rVSV in spleen of infected animals was high and unaffected by expression of TIM-1. However, infectious virus in serum, liver, kidney and adrenal gland was reduced late in infection in the TIM-1-deficient mice, suggesting that virus entry via this receptor contributes to virus load. Consistent with higher virus loads, proinflammatory chemokines trended higher in organs from infected TIM-1-sufficient mice compared to the TIM-1-deficient mice, but proinflammatory cytokines were more modestly affected. To assess the role of T cells in EBOV GP/rVSV pathogenesis, T cells were depleted in TIM-1-sufficient and -deficient mice and the mice were challenged with virus. Depletion of T cells did not alter the pathogenic consequences of virus infection.

Conclusions

Our studies provide evidence that at late times during EBOV GP/rVSV infection, TIM-1 increased virus load and associated mortality, consistent with an important role of this receptor in virus entry. This work suggests that inhibitors which block TIM-1/virus interaction may serve as effective antivirals, reducing virus load at late times during EBOV infection.

T cell immunoglobulin mucin domain-1 (TIM-1) is one of a number of phosphatidylserine (PS) receptors that mediate clearance of apoptotic bodies by binding PS on the surface of dead or dying cells. Enveloped viruses mimic apoptotic bodies by exposing PS on the outer leaflet of the viral membrane. While TIM-1 has been shown to serve as an adherence factor/receptor for filoviruses in tissue culture, limited studies have investigated the role of TIM-1 as a receptor in vivo. Here, we sought to determine if TIM-1 was critical for Ebola virus glycoprotein-mediated infection using a BSL2 model virus. We demonstrate that loss of TIM-1 expression results in decreased virus load late during infection and significantly reduced virus-elicited mortality. These findings provide evidence that TIM-1 serves as an important receptor for Ebola virus in vivo. Blocking TIM-1/EBOV interactions may be effective antiviral strategy to reduce viral load and pathogenicity at late times of EBOV infection.

Urinary kidney injury molecule-1 in renal disease

Kidney injury molecule-1 (KIM-1), a type l transmembrane glycoprotein, is recognized as a potential biomarker for detection of tubular injury in the main renal diseases. Urinary KIM-1 increases rapidly upon the tubular injury, and its levels are associated with the degree of tubular injury, interstitial fibrosis, and inflammation in the injured kidney. Currently, the investigation of kidney diseases is usually performed through the assessment of serum creatinine and urinary albumin. However, these biomarkers are limited for the early detection of changes in renal function. Besides, the tubular injury appears to precede glomerular damage in the pathophysiology of renal diseases. For these reasons, the search for sensitive, specific and non-invasive biomarkers is of interest. Therefore, the purpose of this article is to review the physiological mechanisms of KIM-1, as well to present clinical evidence about the association between elevated urinary KIM-1 levels and the main renal diseases such as chronic kidney disease, diabetic kidney disease, acute kidney injury, and IgA nephropathy.

Tctex-1, a novel interaction partner of Kidney Injury Molecule-1, is required for efferocytosis

Kidney injury molecule-1 (KIM-1) is a phosphatidylserine receptor that is specifically upregulated on proximal tubular epithelial cells (PTECs) during acute kidney injury and mitigates tissue damage by mediating efferocytosis (the phagocytic clearance of apoptotic cells). The signaling molecules that regulate efferocytosis in TECs are not well understood. Using a yeast two-hybrid screen, we identified the dynein light chain protein, Tctex-1, as a novel KIM-1-interacting protein. Immunoprecipitation and confocal imaging studies suggested that Tctex-1 associates with KIM-1 in cells at baseline, but, dissociates from KIM-1 within 90 min of initiation of efferocytosis. Interfering with actin or microtubule polymerization interestingly prevented the dissociation of KIM-1 from Tctex-1. Moreover, the subcellular localization of Tctex-1 changed from being microtubule-associated to mainly cytosolic upon expression of KIM-1. Short hairpin RNA-mediated silencing of endogenous Tctex-1 in cells significantly inhibited efferocytosis to levels comparable to that of knock down of KIM-1 in the same cells. Importantly, Tctex-1 was not involved in the delivery of KIM-1 to the cell-surface. On the other hand, KIM-1 expression significantly inhibited the phosphorylation of Tctex-1 at threonine 94 (T94), a post-translational modification which is known to disrupt the binding of Tctex-1 to dynein on microtubules. In keeping with this, we found that KIM-1 bound less efficiently to the phosphomimic (T94E) mutant of Tctex-1 compared to wild type Tctex-1. Surprisingly, expression of Tctex-1 T94E did not influence KIM-1-mediated efferocytosis. Our studies uncover a previously unknown role for Tctex-1 in KIM-1-dependent efferocytosis in epithelial cells.

Determinants in the Ig Variable Domain of Human HAVCR1 (TIM-1) Are Required To Enhance Hepatitis C Virus Entry

Hepatitis C virus (HCV) is the leading cause of chronic hepatitis in humans. Several host molecules participate in HCV cell entry, but this process remains unclear. The complete unraveling of the HCV entry process is important to further understand viral pathogenesis and develop therapeutics. Human hepatitis A virus (HAV) cellular receptor 1 (HAVCR1), CD365, also known as TIM-1, functions as a phospholipid receptor involved in cell entry of several enveloped viruses. Here, we studied the role of HAVCR1 in HCV infection. HAVCR1 antibody inhibited entry in a dose-dependent manner. HAVCR1 soluble constructs neutralized HCV, which did not require the HAVCR1 mucinlike region and was abrogated by a mutation of N to A at position 94 (N94A) in the Ig variable (IgV) domain phospholipid-binding pocket, indicating a direct interaction of the HAVCR1 IgV domain with HCV virions. However, knockout of HAVCR1 in Huh7 cells reduced but did not prevent HCV growth. Interestingly, the mouse HAVCR1 ortholog, also a phospholipid receptor, did not enhance infection and a soluble form failed to neutralize HCV, although replacement of the mouse IgV domain with the human HAVCR1 IgV domain restored the enhancement of HCV infection. Mutations in the cytoplasmic tail revealed that direct HAVCR1 signaling is not required to enhance HCV infection. Our data show that the phospholipid-binding function and other determinant(s) in the IgV domain of human HAVCR1 enhance HCV infection. Although the exact mechanism is not known, it is possible that HAVCR1 facilitates entry by stabilizing or enhancing attachment, leading to direct interactions with specific receptors, such as CD81.IMPORTANCE Hepatitis C virus (HCV) enters cells through a multifaceted process. We identified the human hepatitis A virus cellular receptor 1 (HAVCR1), CD365, also known as TIM-1, as a facilitator of HCV entry. Antibody blocking and silencing or knockout of HAVCR1 in hepatoma cells reduced HCV entry. Our findings that the interaction of HAVCR1 with HCV early during infection enhances entry but is not required for infection support the hypothesis that HAVCR1 facilitates entry by stabilizing or enhancing virus binding to the cell surface membrane and allowing the correct virus-receptor positioning for interaction with the main HCV receptors. Furthermore, our data show that in addition to the phospholipid-binding function of HAVCR1, the enhancement of HCV infection involves other determinants in the IgV domain of HAVCR1. These findings expand the repertoire of molecules that HCV uses for cell entry, adding to the already complex mechanism of HCV infection and pathogenesis.

Acute stimulation generates Tim-3-expressing T helper type 1 CD4 T cells that persist in vivo and show enhanced effector function

T-cell immunoglobulin and mucin domain 3 (Tim-3) is a surface receptor expressed by T helper type 1 (Th1) effector CD4 T cells, which are critical for defence against intracellular pathogens and have been implicated in autoimmune disease. Previous studies showed that Tim-3 expression makes Th1 cells more susceptible to apoptosis and also marks functionally impaired T cells that arise due to chronic stimulation. However, other studies suggested that Tim-3-expressing Th1 cells do not always have these properties. To further define the relationship between Tim-3 and Th1 cell function, we analysed the characteristics of Th1 cells that expressed Tim-3 in response to brief stimulation in vitro or an acute viral infection in vivo. As expected, cultured CD4 T cells began expressing Tim-3 during Th1 differentiation and secondary stimulation generated Tim-3^- and Tim-3⁺ fractions that were separated and further analysed. When injected into naive mice, Tim-3⁺ cells down-regulated Tim-3 and survived equally well compared with Tim-3^- cells. Further, Tim-3^- and Tim-3⁺ Th1 cells had similar functional responses when transferred into naive mice that were subsequently infected with lymphocytic choriomeningitis virus (LCMV). Cultured Th1 cells that expressed Tim-3 following T-cell receptor stimulation had a greater capacity to express signature Th1 cytokines than their Tim-3^- counterparts and showed differential expression of genes that regulate CD4 T-cell function. Consistent with these findings, Tim-3⁺ Th1 cells generated in response to LCMV infection displayed augmented effector function relative to Tim-3^- cells. These results suggest that Tim-3 expression by Th1 cells responding to acute stimulation can mark cells that are functionally competent and have an augmented ability to produce cytokines.

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

TIM4-TIM1 interaction modulates Th2 pattern inflammation through enhancing SIRT1 expression

Skewed T helper 2 (Th2)-cell polarization plays a critical role in the pathogenesis of allergic inflammations; however, the underlying mechanisms require further elucidation. The aim of the present study was to investigate the mechanisms through which the interaction between T-cell immunoglobulin and mucin domain (TIM)4 and TIM1 regulates the expression of silent information regulator 1 (SIRT1) in Th2 cells, and the role of SIRT1 in Th2-cell polarization during nasal allergic inflammation. The results demonstrated that TIM4 expression by splenic dendritic cells was increased in mice with allergic rhinitis, and the TIM4̸TIM1 interaction promoted CD4⁺ T cells to express SIRT1 during allergic inflammation via enhancing phosphoinositide 3-kinase/Akt phosphorylation. SIRT1 then facilitated CD4⁺ T-cell proliferation through downregulating the expression of Fas ligand, caspase-3 and p53 in mice with nasal allergic inflammation. In conclusion, the interaction of TIM4̸TIM1 was found to promote Th2-cell proliferation through enhancing SIRT1 expression in mice with nasal allergic rhinitis.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Expression profile of novel cell surface molecules on different subsets of human peripheral blood antigen-presenting cells

Although major steps have been recently made in understanding the role of the distinct subsets of dendritic cells (DC)/antigen-presenting cells (APC), further studies are required to unravel their precise role, including in-depth immunophenotypic characterisation of these cells. Here, we used eight-colour flow cytometry to investigate the reactivity of a panel of 72 monoclonal antibodies (including those clustered in seven new Cluster of Differentiation, CD) on different subsets of APC in peripheral blood (PB) samples from five healthy adults. These experiments were performed in the context of the Tenth International Workshop on Human Leukocyte Differentiation Antigens (HLDA10). Plasmacytoid DC was the only cell population that expressed CD85g and CD195, whereas they lacked all of the other molecules investigated. In contrast, myeloid DC mostly expressed inhibitory C-type lectin receptors (CLRs) and other inhibitory-associated molecules, whereas monocytes expressed both inhibitory and activating CLRs, together with other phagocytosis-associated receptors. Within monocytes, progressively lower levels of expression were generally observed from classical monocytes (cMo) to SLAN^- and SLAN⁺ non-classical monocytes (ncMo) for most of the molecules expressed, except for the CD368 endocytic receptor. This molecule was found to be positive only in cMo, and the CD369 and CD371 modulating/signalling receptors. In addition, the CD101 inhibitory molecule was found to be expressed at higher levels in SLAN⁺ vs SLAN^- ncMo. In summary, the pattern of expression of the different signalling molecules and receptors analysed in this work varies among the distinct subsets of PB APCs, with similar profiles for molecules within each functional group. These findings suggest unique pattern-recognition and signalling capabilities for distinct subpopulations of APCs, and therefore, diverse functional roles.

Conjugated Bilirubin Differentially Regulates CD4+ T Effector Cells and T Regulatory Cell Function through Outside-In and Inside-Out Mechanisms: The Effects of HAV Cell Surface Receptor and Intracellular Signaling

We recently reported an immune-modulatory role of conjugated bilirubin (CB) in hepatitis A virus (HAV) infection. During this infection the immune response relies on CD4+ T lymphocytes (TLs) and it may be affected by the interaction of HAV with its cellular receptor (HAVCR1/TIM-1) on T cell surface. How CB might affect T cell function during HAV infection remains to be elucidated. Herein, in vitro stimulation of CD4+ TLs from healthy donors with CB resulted in a decrease in the degree of intracellular tyrosine phosphorylation and an increase in the activity of T regulatory cells (Tregs) expressing HAVCR1/TIM-1. A comparison between CD4+ TLs from healthy donors and HAV-infected patients revealed changes in the TCR signaling pathway relative to changes in CB levels. The proportion of CD4+CD25+ TLs increased in patients with low CB serum levels and an increase in the percentage of Tregs expressing HAVCR1/TIM-1 was found in HAV-infected patients relative to controls. A low frequency of 157insMTTTVP insertion in the viral receptor gene HAVCR1/TIM-1 was found in patients and controls. Our data revealed that, during HAV infection, CB differentially regulates CD4+ TLs and Tregs functions by modulating intracellular pathways and by inducing changes in the proportion of Tregs expressing HAVCR1/TIM-1.

Immune Regulation and Antitumor Effect of TIM-1

T cells play an important role in antitumor immunity, and the T cell immunoglobulin domain and the mucin domain protein-1 (TIM-1) on its surface, as a costimulatory molecule, has a strong regulatory effect on T cells. TIM-1 can regulate and enhance type 1 immune response of tumor association. Therefore, TIM-1 costimulatory pathways may be a promising therapeutic target in future tumor immunotherapy. This review describes the immune regulation and antitumor effect of TIM-1.

Characterization of Human and Murine T-Cell Immunoglobulin Mucin Domain 4 (TIM-4) IgV Domain Residues Critical for Ebola Virus Entry

Phosphatidylserine (PtdSer) receptors that are responsible for the clearance of dying cells have recently been found to mediate enveloped virus entry. Ebola virus (EBOV), a member of the Filoviridae family of viruses, utilizes PtdSer receptors for entry into target cells. The PtdSer receptors human and murine T-cell immunoglobulin mucin (TIM) domain proteins TIM-1 and TIM-4 mediate filovirus entry by binding to PtdSer on the virion surface via a conserved PtdSer binding pocket within the amino-terminal IgV domain. While the residues within the TIM-1 IgV domain that are important for EBOV entry are characterized, the molecular details of virion-TIM-4 interactions have yet to be investigated. As sequences and structural alignments of the TIM proteins suggest distinct differences in the TIM-1 and TIM-4 IgV domain structures, we sought to characterize TIM-4 IgV domain residues required for EBOV entry. Using vesicular stomatitis virus pseudovirions bearing EBOV glycoprotein (EBOV GP/VSVΔG), we evaluated virus binding and entry into cells expressing TIM-4 molecules mutated within the IgV domain, allowing us to identify residues important for entry. Similar to TIM-1, residues in the PtdSer binding pocket of murine and human TIM-4 (mTIM-4 and hTIM-4) were found to be important for EBOV entry. However, additional TIM-4-specific residues were also found to impact EBOV entry, with a total of 8 mTIM-4 and 14 hTIM-4 IgV domain residues being critical for virion binding and internalization. Together, these findings provide a greater understanding of the interaction of TIM-4 with EBOV virions.

IMPORTANCE

With more than 28,000 cases and over 11,000 deaths during the largest and most recent Ebola virus (EBOV) outbreak, there has been increased emphasis on the development of therapeutics against filoviruses. Many therapies under investigation target EBOV cell entry. T-cell immunoglobulin mucin (TIM) domain proteins are cell surface factors important for the entry of many enveloped viruses, including EBOV. TIM family member TIM-4 is expressed on macrophages and dendritic cells, which are early cellular targets during EBOV infection. Here, we performed a mutagenesis screening of the IgV domain of murine and human TIM-4 to identify residues that are critical for EBOV entry. Surprisingly, we identified more human than murine TIM-4 IgV domain residues that are required for EBOV entry. Defining the TIM IgV residues needed for EBOV entry clarifies the virus-receptor interactions and paves the way for the development of novel therapeutics targeting virus binding to this cell surface receptor.

Mast cell activation is enhanced by Tim1:Tim4 interaction but not by Tim-1 antibodies

Polymorphisms in the T cell (or transmembrane) immunoglobulin and mucin domain 1 ( TIM-1) gene, particularly in the mucin domain, have been associated with atopy and allergic diseases in mice and human. Genetic- and antibody-mediated studies revealed that Tim-1 functions as a positive regulator of Th2 responses, while certain antibodies to Tim-1 can exacerbate or reduce allergic lung inflammation. Tim-1 can also positively regulate the function of B cells, NKT cells, dendritic cells and mast cells. However, the precise molecular mechanisms by which Tim-1 modulates immune cell function are currently unknown. In this study, we have focused on defining Tim-1-mediated signaling pathways that enhance mast cell activation through the high affinity IgE receptor (FceRI). Using a Tim-1 mouse model lacking the mucin domain (Tim-1 ^Dmucin), we show for the first time that the polymorphic Tim-1 mucin region is dispensable for normal mast cell activation. We further show that Tim-4 cross-linking of Tim-1 enhances select signaling pathways downstream of FceRI in mast cells, including mTOR-dependent signaling, leading to increased cytokine production but without affecting degranulation.

G protein α12 (Gα12) is a negative regulator of kidney injury molecule-1-mediated efferocytosis

Kidney injury molecule-1 (KIM-1) is a receptor for the "eat me" signal, phosphatidylserine, on apoptotic cells. The specific upregulation of KIM-1 by injured tubular epithelial cells (TECs) enables them to clear apoptotic cells (also known as efferocytosis), thereby protecting from acute kidney injury. Recently, we uncovered that KIM-1 binds directly to the α-subunit of heterotrimeric G₁₂ protein (Gα₁₂) and inhibits its activation by reactive oxygen species during renal ischemia-reperfusion injury (Ismail OZ, Zhang X, Wei J, Haig A, Denker BM, Suri RS, Sener A, Gunaratnam L. Am J Pathol 185: 1207-1215, 2015). Here, we investigated the role that Gα₁₂ plays in KIM-1-mediated efferocytosis by TECs. We showed that KIM-1 remains bound to Gα₁₂ and suppresses its activity during phagocytosis. When we silenced Gα₁₂ expression using small interefering RNA, KIM-1-mediated engulfment of apoptotic cells was increased significantly; in contrast overexpression of constitutively active Gα₁₂ (QLGα₁₂) resulted in inhibition of efferocytosis. Inhibition of RhoA, a key effector of Gα₁₂, using a chemical inhibitor or expression of dominant-negative RhoA, had the same effect as inhibition of Gα₁₂ on efferocytosis. Consistent with this, silencing Gα₁₂ suppressed active RhoA in KIM-1-expressing cells. Finally, using primary TECs from Kim-1^+/+ and Kim-1^-/- mice, we confirmed that engulfment of apoptotic cells requires KIM-1 expression and that silencing Gα₁₂ enhanced efferocytosis by primary TECs. Our data reveal a previously unknown role for Gα₁₂ in regulating efferocytosis and that renal TECs require KIM-1 to mediate this process. These results may have therapeutic implications given the known harmful role of Gα₁₂ in acute kidney injury.

Distinct Trafficking of Cell Surface and Endosomal TIM-1 to the Immune Synapse

The T cell costimulatory molecule TIM-1 (T cell/transmembrane, mucin and immunoglobulin domain protein 1) sorts mainly to endosomes in lymphoid cells. At difference from the cell surface protein, endosomal TIM-1 translocates to the immune synapse (IS), where it can contribute to antigen-dependent T cell costimulation. TIM-1 ligands increase the amount of cell surface protein, preventing its traffic to the IS. The bipolar sorting of TIM-1 observed during IS formation is determined by differences in its subcellular location, and probably modulates antigen-driven immune responses.

Characterization of kidney injury molecule-1 in cats

BACKGROUND

Kidney disease (KD) is common in older cats and presumed to arise from subclinical kidney injuries throughout life. Sensitive markers for detecting kidney injury are lacking. Kidney injury molecule 1 (KIM-1) is a useful biomarker of kidney injury in humans and rodents.

HYPOTHESIS/OBJECTIVES

Feline KIM-1 is conserved across species, expressed in kidney, and shed into urine of cats with acute kidney injury (AKI). The objectives were to characterize the feline KIM-1 gene and protein, assess available immunoassays for detecting KIM-1 in urine of cats, and identify KIM-1 expression in kidney sections.

ANIMALS

Samples from 36 hospitalized and 7 clinically healthy cats were evaluated. Hospitalized cats were divided into 2 groups based on absence (n = 20) or presence (n = 16) of historical KD.

METHODS

Feline KIM-1 genomic and complementary DNA sequences were amplified, sequenced and analyzed to determine the presence of isoforms, exon-intron organization and similarity with orthologous sequences. Presence in urine was evaluated by immunoassay and expression in kidney by immunohistochemistry.

RESULTS

Three expressed feline KIM-1 transcript variants comprising 894, 810, and 705 bp were identified in renal tissue. KIM-1 immunoassays yielded positive results in urine of cats with conditions associated with AKI, but not chronic KD. Immunohistochemistry of kidney sections identified KIM-1 in proximal tubular cells of cats with positive urine immunoassay results.

CONCLUSIONS AND CLINICAL IMPORTANCE

Kidney injury molecule 1 was expressed in specific segments of the nephron and detected in urine of cats at risk of AKI. Urine KIM-1 immunoassay may be a useful indicator of tubular injury.

Regulation of T cell responses by the receptor molecule Tim-3

Tim-3 is a member of the T cell immunoglobulin and mucin domain (Tim) family of proteins, which are expressed by several cell types in the immune system, including CD4 and CD8 T cells activated under certain conditions. These molecules are generally thought to act as receptors for multiple ligands and thus to function by engaging intracellular signaling pathways in a ligand-dependent manner. In recent years, the function of the Tim-3 protein has been studied in some detail, particularly with respect to its role in the regulation of CD4 and CD8 T cell responses. Here, we review the structural features of Tim-3, known ligands for this molecule and the links established between Tim-3 and signal transduction pathways. In addition, we review the current literature regarding the role of Tim-3 in the regulation of effector responses by CD4 and CD8 T cells. Overall, findings published thus far strongly support the conclusion that Tim-3 functions to inhibit T cell responses, particularly under conditions involving chronic stimulation. Conversely, some reports have provided evidence that Tim-3 can stimulate T cells under conditions involving acute stimulation, suggesting that the role of Tim-3 may vary depending on context. Further study of Tim-3 is likely to advance our understanding of how CD4 and CD8 T cell responses are regulated and could uncover novel approaches for manipulating T cell function for therapeutic benefit.

Phosphatidylserine receptors: enhancers of enveloped virus entry and infection

A variety of both RNA and DNA viruses envelop their capsids in a lipid bilayer. One of the more recently appreciated benefits this envelope is incorporation of phosphatidylserine (PtdSer). Surface exposure of PtdSer disguises viruses as apoptotic bodies; tricking cells into engulfing virions. This mechanism is termed apoptotic mimicry. Several PtdSer receptors have been identified to enhance virus entry and we have termed this group of proteins PtdSer-mediated virus entry enhancing receptors or PVEERs. These receptors enhance entry of a range of enveloped viruses. Internalization of virions by PVEERs provides a broad mechanism of entry with little investment by the virus itself. PVEERs may allow some viruses to attach to cells, thereby making viral glycoprotein/cellular receptor interactions more probable. Alternatively, other viruses may rely entirely on PVEERs for internalization into endosomes. This review provides an overview of PtdSer receptors that serve as PVEERs and the biology behind virion/PVEER interaction.

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Phosphatidylserine (PtdSer) receptors can mediate entry of enveloped viruses.

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PtdSer is present on the outer leaflet of the virion envelope.

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PtdSer receptors are expressed on a variety of primary cells and cell lines.

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Characteristics of PtdSer receptors that mediate virus entry are defined.

TIM-family proteins inhibit HIV-1 release

Accumulating evidence indicates that T-cell immunoglobulin (Ig) and mucin domain (TIM) proteins play critical roles in viral infections. Herein, we report that the TIM-family proteins strongly inhibit HIV-1 release, resulting in diminished viral production and replication. Expression of TIM-1 causes HIV-1 Gag and mature viral particles to accumulate on the plasma membrane. Mutation of the phosphatidylserine (PS) binding sites of TIM-1 abolishes its ability to block HIV-1 release. TIM-1, but to a much lesser extent PS-binding deficient mutants, induces PS flipping onto the cell surface; TIM-1 is also found to be incorporated into HIV-1 virions. Importantly, TIM-1 inhibits HIV-1 replication in CD4-positive Jurkat cells, despite its capability of up-regulating CD4 and promoting HIV-1 entry. In addition to TIM-1, TIM-3 and TIM-4 also block the release of HIV-1, as well as that of murine leukemia virus (MLV) and Ebola virus (EBOV); knockdown of TIM-3 in differentiated monocyte-derived macrophages (MDMs) enhances HIV-1 production. The inhibitory effects of TIM-family proteins on virus release are extended to other PS receptors, such as Axl and RAGE. Overall, our study uncovers a novel ability of TIM-family proteins to block the release of HIV-1 and other viruses by interaction with virion- and cell-associated PS. Our work provides new insights into a virus-cell interaction that is mediated by TIMs and PS receptors.

Expression and polymorphisms of T cell immunoglobulin domain and mucin domain protein-1 in thymoma with or without myasthenia gravis

The present study aimed to investigate the expression and association of the single-nucleotide polymorphism (SNP) -1637A/G in the promoter region of the T cell immunoglobulin domain and mucin domain protein-1 (Tim-1) gene in patients diagnosed with thymoma with or without myasthenia gravis (MG). The expression of Tim-1 was detected using the streptavidin peroxidase immunohistochemical staining method on tissues obtained from thymoma patients with (n=58) and without (n=62) MG. The Tim-1 gene -1637A/G polymorphism was detected using the single allele-specific primer polymerase chain reaction. The positive rate of Tim-1 expression in thymoma patients with MG was 62.1% (32/58), which was significantly higher compared with that in thymoma patients without MG (33.9%, 21/62) (P=0.002). The genotype frequencies of GG, GA and AA in the -1637A/G polymorphism were 0.7931, 0.2069 and 0, respectively, in thymoma patients with MG, and 0.6129, 0.3871 and 0, respectively, in thymoma patients without MG. A significant difference in the genotypes between the thymoma patients with MG and those without MG was found (P=0.031). In addition, a significant difference in allele frequencies between thymoma patients with MG and those without MG (P=0.024) was observed. The high expression of Tim-1 in thymoma tissues may play an important role in the development of thymoma with MG. The -1637A/G polymorphism site of the promoter region in Tim-1 may be associated with thymoma with MG. These findings provide a basis for further genetic research of thymoma with MG.

Characterizing functional domains for TIM-mediated enveloped virus entry

T-cell immunoglobulin and mucin domain 1 (TIM-1) and other TIM family members were recently identified as phosphatidylserine (PtdSer)-mediated virus entry-enhancing receptors (PVEERs). These proteins enhance entry of Ebola virus (EBOV) and other viruses by binding PtdSer on the viral envelope, concentrating virus on the cell surface, and promoting subsequent internalization. The PtdSer-binding activity of the immunoglobulin-like variable (IgV) domain is essential for both virus binding and internalization by TIM-1. However, TIM-3, whose IgV domain also binds PtdSer, does not effectively enhance virus entry, indicating that other domains of TIM proteins are functionally important. Here, we investigate the domains supporting enhancement of enveloped virus entry, thereby defining the features necessary for a functional PVEER. Using a variety of chimeras and deletion mutants, we found that in addition to a functional PtdSer-binding domain PVEERs require a stalk domain of sufficient length, containing sequences that promote an extended structure. Neither the cytoplasmic nor the transmembrane domain of TIM-1 is essential for enhancing virus entry, provided the protein is still plasma membrane bound. Based on these defined characteristics, we generated a mimic lacking TIM sequences and composed of annexin V, the mucin-like domain of α-dystroglycan, and a glycophosphatidylinositol anchor that functioned as a PVEER to enhance transduction of virions displaying Ebola, Chikungunya, Ross River, or Sindbis virus glycoproteins. This identification of the key features necessary for PtdSer-mediated enhancement of virus entry provides a basis for more effective recognition of unknown PVEERs.

IMPORTANCE

T-cell immunoglobulin and mucin domain 1 (TIM-1) and other TIM family members are recently identified phosphatidylserine (PtdSer)-mediated virus entry-enhancing receptors (PVEERs). These proteins enhance virus entry by binding the phospholipid, PtdSer, present on the viral membrane. While it is known that the PtdSer binding is essential for the PVEER function of TIM-1, TIM-3 shares this binding activity but does not enhance virus entry. No comprehensive studies have been done to characterize the other domains of TIM-1. In this study, using a variety of chimeric proteins and deletion mutants, we define the features necessary for a functional PVEER. With these features in mind, we generated a TIM-1 mimic using functionally similar domains from other proteins. This mimic, like TIM-1, effectively enhanced transduction. These studies provide insight into the key features necessary for PVEERs and will allow for more effective identification of unknown PVEERs.

Role of the phosphatidylserine receptor TIM-1 in enveloped-virus entry

The cell surface receptor T cell immunoglobulin mucin domain 1 (TIM-1) dramatically enhances filovirus infection of epithelial cells. Here, we showed that key phosphatidylserine (PtdSer) binding residues of the TIM-1 IgV domain are critical for Ebola virus (EBOV) entry through direct interaction with PtdSer on the viral envelope. PtdSer liposomes but not phosphatidylcholine liposomes competed with TIM-1 for EBOV pseudovirion binding and transduction. Further, annexin V (AnxV) substituted for the TIM-1 IgV domain, supporting a PtdSer-dependent mechanism. Our findings suggest that TIM-1-dependent uptake of EBOV occurs by apoptotic mimicry. Additionally, TIM-1 enhanced infection of a wide range of enveloped viruses, including alphaviruses and a baculovirus. As further evidence of the critical role of enveloped-virion-associated PtdSer in TIM-1-mediated uptake, TIM-1 enhanced internalization of pseudovirions and virus-like proteins (VLPs) lacking a glycoprotein, providing evidence that TIM-1 and PtdSer-binding receptors can mediate virus uptake independent of a glycoprotein. These results provide evidence for a broad role of TIM-1 as a PtdSer-binding receptor that mediates enveloped-virus uptake. Utilization of PtdSer-binding receptors may explain the wide tropism of many of these viruses and provide new avenues for controlling their virulence.

Kidney injury molecule-1: more than just an injury marker of tubular epithelial cells?

Regardless of the original causes and etiology, the progression to renal function declines follows a final common pathway associated with tubulointerstitial injury, in which the proximal tubular epithelial cells (PTEC) are instrumental. Kidney injury molecule-1 (KIM-1) is an emerging biomarker, and its expression and release are induced in PTEC upon injury. KIM-1 plays the role as a double-edged sword and implicates in the process of kidney injury and healing. Expression of KIM-1 is also associated with tubulointerstitial inflammation and fibrosis. More importantly, KIM-1 expressing PTEC play the role as the residential phagocytes, contribute to the removal of apoptotic cells and facilitate the regeneration of injured tubules. The precise mechanism of KIM-1 and its sheded ectodomain on restoration of tubular integrity after injury is not fully understood. Other than PTEC, macrophages (Mø) also implicate in tubular repair. Understanding the crosstalk between Mø and the injured PTEC is essential for designing appropriate methods for controlling the sophisticated machinery in tubular regeneration and healing. This article will review the current findings of KIM-1, beginning with its basic structure, utility as a biomarker, and possible functions, with focus on the role of KIM-1 in regeneration and healing of injured PTEC.

TIM family proteins promote the lysosomal degradation of the nuclear receptor NUR77

T cell immunoglobulin and mucin domain (TIM) proteins are cell-surface signaling receptors in T cells and scavenger receptors in antigen-presenting cells and kidney tubular epithelia. Here, we demonstrated a function for TIM proteins in mediating the degradation of NUR77, a nuclear receptor implicated in apoptosis and cell survival. TIM proteins interacted with and mediated the lysosomal degradation of NUR77 in a phosphoinositide 3-kinase-dependent pathway. We also showed dynamic cycling of TIM-1 to and from the cell surface through clathrin-dependent constitutive endocytosis. Blocking this process or mutating the phosphatidylserine-binding pocket in TIM-1 abrogated TIM-1-mediated degradation of NUR77. In an in vitro model of kidney injury, silencing TIM-1 increased NUR77 abundance and decreased epithelial cell survival. These results show that TIM proteins may affect immune cell function and the response of the kidney to injury.

Murine Tim-1 is excluded from the immunological synapse

The interaction between T cells and APCs bearing cognate antigen results in the formation of an immunological synapse (IS). During this process, many receptors and signaling proteins segregate to regions proximal to the synapse. This protein movement is thought to influence T cell function. However, some proteins are transported away from the IS, which is controlled in part by ERM family proteins. Tim-1 is a transmembrane protein with co-stimulatory functions that is found on many immune cells, including T cells. However, the expression pattern of Tim-1 on T cells upon activation by APCs has not been explored. Interestingly, in this study we demonstrate that the majority of Tim-1 on activated T cells is excluded from the IS. Tim-1 predominantly resides outside of the IS, and structure/function studies indicate that the cytoplasmic tail influences Tim-1 polarization. Specifically, a putative ERM binding motif (KRK 244-246) in the Tim-1 cytoplasmic tail appears necessary for proper Tim-1 localization. Furthermore, mutation of the KRK motif results in enhanced early tyrosine phosphorylation downstream of TCR/CD28 stimulation upon ectopic expression of Tim-1. Paradoxically however, the KRK motif is necessary for Tim-1 co-stimulation of NFAT/AP-1 activation and co-stimulation of cytokine production. This work reveals unexpected complexity underlying Tim-1 localization and suggests potentially novel mechanisms by which Tim-1 modulates T cell activity.

Enhanced Tim3 activity improves survival after influenza infection

Influenza is a major cause of morbidity and mortality in the United States. Studies have shown that excessive T cell activity can mediate pneumonitis in the setting of influenza infection, and data from the 2009 H1N1 pandemic indicate that critical illness and respiratory failure postinfection were associated with greater infiltration of the lungs with CD8+ T cells. T cell Ig and mucin domain 3 (Tim3) is a negative regulator of Th1/Tc1-type immune responses. Activation of Tim3 on effector T cells has been shown to downregulate proliferation, cell-mediated cytotoxicity, and IFN-γ production, as well as induce apoptosis. In this article, we demonstrate that deletion of the terminal cytoplasmic domain of the Tim3 gene potentiates its ability to downregulate Tc1 inflammation, and that this enhanced Tim3 activity is associated with decreased phosphorylation of the TCR-CD3ζ-chain. We then show that mice with this Tim3 mutation infected with influenza are protected from morbidity and mortality without impairment in viral clearance or functional heterotypic immunity. This protection is associated with decreased CD8+ T cell proliferation and decreased production of inflammatory cytokines, including IFN-γ. Furthermore, the Tim3 mutation was protective against mortality in a CD8+ T cell-specific model of pneumonitis. These data suggest that Tim3 could be targeted to prevent immunopathology during influenza infection and demonstrate a potentially novel signaling mechanism used by Tim3 to downregulate the Tc1 response.

Binding of hepatitis A virus to its cellular receptor 1 inhibits T-regulatory cell functions in humans

BACKGROUND & AIMS

CD4+ T-regulatory (Treg) cells suppress immune responses and control self-tolerance and immunity to pathogens, cancer, and alloantigens. Most pathogens activate Treg cells to minimize immune-mediated tissue damage and prevent clearance, which promotes chronic infections. However, hepatitis A virus (HAV) temporarily inhibits Treg-cell functions. We investigated whether the interaction of HAV with its cellular receptor 1 (HAVCR1), a T-cell co-stimulatory molecule, inhibits the function of Treg cells to control HAV infection.

METHODS

We studied the effects of HAV interaction with HAVCR1 on human T cells using binding, signal transduction, apoptosis, activation, suppression, cytokine production, and confocal microscopy analyses. Cytokines were analyzed in sera from 14 patients with HAV infection using bead arrays.

RESULTS

Human Treg cells constitutively express HAVCR1. Binding of HAV to HAVCR1 blocked phosphorylation of Akt, prevented activation of the T-cell receptor, and inhibited function of Treg cells. At the peak viremia, patients with acute HAV infection had no Treg-cell suppression function, produced low levels of transforming growth factor-β , which limited leukocyte recruitment and survival, and produced high levels of interleukin-22, which prevented liver damage.

CONCLUSIONS

Interaction between HAV and its receptor HAVCR1 inhibits Treg-cell function, resulting in an immune imbalance that allows viral expansion with limited hepatocellular damage during early stages of infection-a characteristic of HAV pathogenesis. The mechanism by which HAV is cleared in the absence of Treg-cell function could be used as a model to develop anticancer therapies, modulate autoimmune and allergic responses, and prevent transplant rejection.

The emerging role of the TIM molecules in transplantation

Since their discovery in 2001, the T-cell immunoglobulin mucin (TIM) family members have been shown to play important roles in the regulation of immune responses. The TIM family comprises of eight genes in the mouse, three of which are conserved in humans (TIM-1, TIM-3 and TIM-4). Initially, TIM-1 and TIM-3 were thought to be expressed solely on T cells. However, emerging data suggest a much broader expression pattern where their presence on APCs confers differing functions, including the ability to mediate phagocytosis. In contrast, TIM-4 is exclusively expressed on APCs. Together, the TIM molecules provide a functional repertoire for determining the fate of T-cell activation and differentiation. To date, much of the knowledge about the TIM family members has been garnered from the models of asthma, allergy and autoimmunity. More recently, data from experimental models of transplantation demonstrate that TIM family members also have a key role in alloimmunity. This review will serve to highlight the emerging data regarding this unique family of molecules and to identify their potential in transplantation tolerance.

Phosphotyrosine-dependent coupling of Tim-3 to T-cell receptor signaling pathways

The transmembrane protein Tim-3 has been shown to negatively regulate T-cell-dependent immune responses and was recently demonstrated to be associated with the phenomenon of immune exhaustion, which can occur as a consequence of chronic viral infection. Unlike other negative regulators of T-cell function (e.g., PD-1), Tim-3 does not contain any obvious inhibitory signaling motifs. We have found that ectopic expression of Tim-3 in T cells leads to enhancement of T-cell receptor (TCR)-dependent signaling pathways, which was observed at the level of transcriptional reporters and endogenous cytokine production. We have exploited this observation to dissect what elements within the cytoplasmic tail of Tim-3 are required for coupling to downstream signaling pathways. Here we have demonstrated that two of the more membrane-proximal cytoplasmic tail tyrosines are required for Tim-3 signaling to T-cell activation pathways in a redundant fashion. Furthermore, we show that Tim-3 can directly bind to the Src family tyrosine kinase Fyn and the p85 phosphatidylinositol 3-kinase (PI3K) adaptor. Thus, at least under conditions of short-term stimulation, Tim-3 can augment T-cell activation, although this effect can be blocked by the inclusion of an agonistic antibody to Tim-3. These findings should help further the study of Tim-3 function in other physiological settings, such as those that lead to immune exhaustion.

Suppression of allograft rejection by Tim-1-Fc through cross-linking with a novel Tim-1 binding partner on T cells

Engagement of T-cell immunoglobulin mucin (Tim)-1 on T cells with its ligand, Tim-4, on antigen presenting cells delivers positive costimulatory signals to T cells. However, the molecular mechanisms for Tim-1-mediated regulation of T-cell activation and differentiation are relatively poorly understood. Here we investigated the role of Tim-1 in T-cell responses and allograft rejection using recombinant human Tim-1 extracellular domain and IgG1-Fc fusion proteins (Tim-1-Fc). In vitro assays confirmed that Tim-1-Fc selectively binds to CD4⁺ effector T cells, but not dendritic cells or natural regulatory T cells (nTregs). Tim-1-Fc was able to inhibit the responses of purified CD4⁺ T cells that do not express Tim-4 to stimulation by anti-CD3/CD28 mAbs, and this inhibition was associated with reduced AKT and ERK1/2 phosphorylation, but it had no influence on nTregs. Moreover, Tim-1-Fc inhibited the proliferation of CD4⁺ T cells stimulated by allogeneic dendritic cells. Treatment of recipient mice with Tim-1-Fc significantly prolonged cardiac allograft survival in a fully MHC-mismatched strain combination, which was associated with impaired Th1 response and preserved Th2 and nTregs function. Importantly, the frequency of Foxp3⁺ cells in splenic CD4⁺ T cells was increased, thus shifting the balance toward regulators, even though Tim-1-Fc did not induce Foxp3 expression in CD4⁺CD25⁻ T cells directly. These results indicate that Tim-1-Fc can inhibit T-cell responses through an unknown Tim-1 binding partner on T cells, and it is a promising immunosuppressive agent for preventing allograft rejection.

Tim-1 stimulation of dendritic cells regulates the balance between effector and regulatory T cells

We show that the T-cell immunoglobalin mucin, Tim-1, initially reported to be expressed on CD4(+) T cells, is constitutively expressed on dendritic cells (DCs) and that its expression further increases after DC maturation. Tim-1 signaling into DCs upregulates costimulatory molecule expression and proinflammatory cytokine production, thereby promoting effector T-cell responses, while inhibiting Foxp3(+) Treg responses. By contrast, Tim-1 signaling in T cells only regulates Th2 responses. Using a high-avidity/agonistic anti-Tim-1 antibody as a co-adjuvant enhances the immunogenic function of DCs, decreases the suppressive function of Tregs, and substantially increases proinflammatory Th17 responses in vivo. The treatment with high- but not low-avidity anti-Tim-1 not only worsens experimental autoimmune encephalomyelitis (EAE) in susceptible mice but also breaks tolerance and induces EAE in a genetically resistant strain of mice. These findings indicate that Tim-1 has an important role in regulating DC function and thus shifts the balance between effector and regulatory T cells towards an enhanced immune response. By understanding the mechanisms by which Tim-1 regulates DC and T-cell responses, we may clarify the potential utility of Tim-1 as a target of therapy against autoimmunity, cancer, and infectious diseases.

T-cell immunoglobulin and mucin domain 1 (TIM-1) is a receptor for Zaire Ebolavirus and Lake Victoria Marburgvirus

The glycoproteins (GP) of enveloped viruses facilitate entry into the host cell by interacting with specific cellular receptors. Despite extensive study, a cellular receptor for the deadly filoviruses Ebolavirus and Marburgvirus has yet to be identified and characterized. Here, we show that T-cell Ig and mucin domain 1 (TIM-1) binds to the receptor binding domain of the Zaire Ebola virus (EBOV) glycoprotein, and ectopic TIM-1 expression in poorly permissive cells enhances EBOV infection by 10- to 30-fold. Conversely, reduction of cell-surface expression of TIM-1 by RNAi decreased infection of highly permissive Vero cells. TIM-1 expression within the human body is broader than previously appreciated, with expression on mucosal epithelia from the trachea, cornea, and conjunctiva--tissues believed to be important during in vivo transmission of filoviruses. Recognition that TIM-1 serves as a receptor for filoviruses on these mucosal epithelial surfaces provides a mechanistic understanding of routes of entry into the human body via inhalation of aerosol particles or hand-to-eye contact. ARD5, a monoclonal antibody against the IgV domain of TIM-1, blocked EBOV binding and infection, suggesting that antibodies or small molecules directed against this cellular receptor may provide effective filovirus antivirals.

Fyn binds to and phosphorylates T cell immunoglobulin and mucin domain-1 (Tim-1)

The gene encoding T cell immunoglobulin and mucin domain-1 (Tim-1) is linked to atopy and asthma susceptibility in mice and humans. Tim-1 is a transmembrane protein expressed on activated lymphocytes and appears to have a role as a co-stimulatory receptor in T cells. The protein has not been shown to have enzymatic activity but contains a site within its cytoplasmic tail predicted to be a target for tyrosine kinases. Here, we show that Tim-1 can associate with the kinase Fyn, a member of the Src family of tyrosine kinases. This association does not require Fyn's kinase activity and is independent of the phosphorylation of a conserved tyrosine present within the cytoplasmic tail of Tim-1. Fyn is necessary for phosphorylation of this tyrosine in Tim-1 and the phosphorylation of Tim-1 varies with the levels of Fyn present in cells. These data suggest a role for Fyn in the signaling downstream of Tim-1.

TIM-4, a receptor for phosphatidylserine, controls adaptive immunity by regulating the removal of antigen-specific T cells

Adaptive immunity is characterized by the expansion of an Ag-specific T cell population following Ag exposure. The precise mechanisms, however, that control the expansion and subsequent contraction in the number of Ag-specific T cells are not fully understood. We show that T cell/transmembrane, Ig, and mucin (TIM)-4, a receptor for phosphatidylserine, a marker of apoptotic cells, regulates adaptive immunity in part by mediating the removal of Ag-specific T cells during the contraction phase of the response. During Ag immunization or during infection with influenza A virus, blockade of TIM-4 on APCs increased the expansion of Ag-specific T cells, resulting in an increase in secondary immune responses. Conversely, overexpression of TIM-4 on APCs in transgenic mice reduced the number of Ag-specific T cells that remained after immunization, resulting in reduced secondary T cell responses. There was no change in the total number of cell divisions that T cells completed, no change in the per cell proliferative capacity of the remaining Ag-specific T cells, and no increase in the development of Ag-specific regulatory T cells in TIM-4 transgenic mice. Thus, TIM-4-expressing cells regulate adaptive immunity by mediating the removal of phosphatidylserine-expressing apoptotic, Ag-specific T cells, thereby controlling the number of Ag-specific T cells that remain after the clearance of Ag or infection.

Apoptotic cells activate NKT cells through T cell Ig-like mucin-like-1 resulting in airway hyperreactivity

T cell Ig-like mucin-like-1 (TIM-1) is an important asthma susceptibility gene, but the immunological mechanisms by which TIM-1 functions remain uncertain. TIM-1 is also a receptor for phosphatidylserine (PtdSer), an important marker of cells undergoing programmed cell death, or apoptosis. We now demonstrate that NKT cells constitutively express TIM-1 and become activated by apoptotic cells expressing PtdSer. TIM-1 recognition of PtdSer induced NKT cell activation, proliferation, and cytokine production. Moreover, the induction of apoptosis in airway epithelial cells activated pulmonary NKT cells and unexpectedly resulted in airway hyperreactivity, a cardinal feature of asthma, in an NKT cell-dependent and TIM-1-dependent fashion. These results suggest that TIM-1 serves as a pattern recognition receptor on NKT cells that senses PtdSer on apoptotic cells as a damage-associated molecular pattern. Furthermore, these results provide evidence for a novel innate pathway that results in airway hyperreactivity and may help to explain how TIM-1 and NKT cells regulate asthma.

TIM genes: a family of cell surface phosphatidylserine receptors that regulate innate and adaptive immunity

The TIM (T cell/transmembrane, immunoglobulin, and mucin) gene family plays a critical role in regulating immune responses, including allergy, asthma, transplant tolerance, autoimmunity, and the response to viral infections. The unique structure of TIM immunoglobulin variable region domains allows highly specific recognition of phosphatidylserine (PtdSer), exposed on the surface of apoptotic cells. TIM-1, TIM-3, and TIM-4 all recognize PtdSer but differ in expression, suggesting that they have distinct functions in regulating immune responses. TIM-1, an important susceptibility gene for asthma and allergy, is preferentially expressed on T-helper 2 (Th2) cells and functions as a potent costimulatory molecule for T-cell activation. TIM-3 is preferentially expressed on Th1 and Tc1 cells, and generates an inhibitory signal resulting in apoptosis of Th1 and Tc1 cells. TIM-3 is also expressed on some dendritic cells and can mediate phagocytosis of apoptotic cells and cross-presentation of antigen. In contrast, TIM-4 is exclusively expressed on antigen-presenting cells, where it mediates phagocytosis of apoptotic cells and plays an important role in maintaining tolerance. TIM molecules thus provide a functional repertoire for recognition of apoptotic cells, which determines whether apoptotic cell recognition leads to immune activation or tolerance, depending on the TIM molecule engaged and the cell type on which it is expressed.

TIM genes: a family of cell surface phosphatidylserine receptors that regulate innate and adaptive immunity

The TIM (T cell/transmembrane, immunoglobulin, and mucin) gene family plays a critical role in regulating immune responses, including allergy, asthma, transplant tolerance, autoimmunity, and the response to viral infections. The unique structure of TIM immunoglobulin variable region domains allows highly specific recognition of phosphatidylserine (PtdSer), exposed on the surface of apoptotic cells. TIM-1, TIM-3, and TIM-4 all recognize PtdSer but differ in expression, suggesting that they have distinct functions in regulating immune responses. TIM-1, an important susceptibility gene for asthma and allergy, is preferentially expressed on T-helper 2 (Th2) cells and functions as a potent costimulatory molecule for T-cell activation. TIM-3 is preferentially expressed on Th1 and Tc1 cells, and generates an inhibitory signal resulting in apoptosis of Th1 and Tc1 cells. TIM-3 is also expressed on some dendritic cells and can mediate phagocytosis of apoptotic cells and cross-presentation of antigen. In contrast, TIM-4 is exclusively expressed on antigen-presenting cells, where it mediates phagocytosis of apoptotic cells and plays an important role in maintaining tolerance. TIM molecules thus provide a functional repertoire for recognition of apoptotic cells, which determines whether apoptotic cell recognition leads to immune activation or tolerance, depending on the TIM molecule engaged and the cell type on which it is expressed.