Tim4 enables large peritoneal macrophages to cross-present tumor antigens at early stages of tumorigenesis

Receptors controlling the cross-presentation of tumor antigens by macrophage subsets in cancer tissues are poorly explored. Here, we show that TIM4+ large peritoneal macrophages efficiently capture and cross-present tumor-associated antigens at early stages of peritoneal infiltration by ovarian cancer cells. The phosphatidylserine (PS) receptor TIM4 promotes maximal uptake of dead cells or PS-coated artificial targets and triggers inflammatory and metabolic gene programs in combination with cytoskeletal remodeling and upregulation of transcriptional signatures related to antigen processing. At the cellular level, TIM4-mediated engulfment induces nucleation of F-actin around nascent phagosomes, delaying the recruitment of vacuolar ATPase, acidification, and cargo degradation. In vivo, TIM4 deletion blunts induction of early anti-tumoral effector CD8 T cells and accelerates the progression of ovarian tumors. We conclude that TIM4-mediated uptake drives the formation of specialized phagosomes that prolong the integrity of ingested antigens and facilitate cross-presentation, contributing to immune surveillance of the peritoneum.

Platelet-derived extracellular vesicles are increased in sera of Alzheimer's disease patients, as revealed by Tim4-based assays

Alzheimer's disease (AD) is the most common form of dementia, characterized by the accumulation of β‐amyloid plaques and the formation of neurofibrillary tangles. Extracellular vesicles (EVs) are small vesicles surrounded by a lipid bilayer membrane, which may be involved in the progression of AD. Glycans are essential building blocks of EVs, and we hypothesized that EV glycans may reflect pathological conditions of various diseases. Here, we performed glycan profiling of EVs prepared from sera of three AD patients (APs) compared to three healthy donors (HDs) using lectin microarray. Distinct glycan profiles were observed. Mannose‐binding lectins exhibited significantly higher signals for AP‐derived EVs than HD‐derived EVs. Lectin blotting using mannose‐binding lectin (rPALa) showed a single protein band at ~ 80 kDa exclusively in AP‐derived EVs. LC‐MS/MS analysis identified a protein band precipitated by rPALa as CD61, a marker of platelet‐derived exosomes (P‐Exo). Sandwich assays using Tim4 with specificity for phosphatidylserine on EVs and antibodies against P‐Exo markers (CD61, CD41, CD63, and CD9) revealed that P‐Exo is significantly elevated in sera of APs (n = 16) relative to age‐ and sex‐matched HDs (n = 16). Tim4‐αCD63 showed the highest value for the area under the curve (0.957) for discriminating APs from HDs, which should lead to a better understanding of AD pathology and may facilitate the development of a novel diagnostic method for AD.

Urinary Extracellular Vesicles: Ultracentrifugation Method

Recently, urinary extracellular vesicles (EVs) have garnered interest as a potential source of noninvasive biomarkers of diseases related to urinary organs (kidney, bladder, urethra, and prostate).Ultracentrifugation is considered the gold standard method for isolation of EVs. However, the precipitates after ultracentrifugation steps are usually contaminated with soluble proteins, such as the Tamm-Horsfall protein (uromodulin).Therefore, ultracentrifugation on a sucrose-deuterium oxide (D₂O) cushion for purer EV isolation is performed to remove these proteins. In addition, as a nonultracentrifugation method for EV isolation, we have also adopted the phosphatidylserine (PS) affinity method, which is a novel method for EV purification using the T-cell immunoglobulin domain and the mucin domain-containing protein 4 (Tim4).Here, we describe an ultracentrifugation protocol based on a sucrose-D₂O cushion and the PS affinity method protocol for the isolation of urinary EVs.

Tim4 regulates NALP3 inflammasome expression and activity during monocyte/macrophage dysfunction in septic shock patients

Sepsis is the leading cause of death in burn patients. Monocytes/macrophages rapidly exhibit impaired production of proinflammatory cytokines and an elevated generation of anti-inflammatory cytokines in septic patients with immunosuppression. However, the expression patterns of Tim4 and Nod-like receptor protein 3 (NALP3) inflammasome and their roles during immunosuppression in septic shock patients are not well understood. Tim4 and NALP3 inflammasome expression in monocytes were downregulated in immunosuppressive patients with sepsis compared with healthy volunteers. Meanwhile, NALP3 inflammasome expression was upregulated by Tim4 overexpression in murine bone marrow-derived macrophages (BMDMs) and J774A.1 macrophages. Tim4 overexpression improved the ability of BMDMs and J774A.1 macrophages to produce proinflammatory cytokines and increased the expression of cleaved-caspase-1 (p10) after LPS/ATP stimulation. In addition, overexpression of Tim4 enhanced phagocytosis of apoptotic polymorphonuclear neutrophils (PMNs) by BMDMs and J774A.1 macrophages, while depletion of NALP3 in Tim4 overexpressing BMDMs and J774A.1 macrophages decreased phagocytosis of apoptotic PMNs. In summary, the expression of Tim4 and NALP3 inflammasome in monocytes/macrophages was downregulated in septic shock patients, and diminished expression of Tim4 and NALP3 inflammasome in monocytes/macrophages might play a critical role in sepsis-elicited immunosuppression.

Incomplete clearance of apoptotic cells by core 1-derived O-glycan-deficient resident peritoneal macrophages

The core 1 β1,3-galactosyltransferase-specific molecular chaperon (Cosmc) is essential for the synthesis of the core 1 structure of mucin-type O-glycans. To clarify the physiological role of core 1-derived O-glycans in macrophages, we exploited the LysM-Cre transgene to generate a conditional Cosmc mutant allele (conditional Cosmc knockout; cKO) in myeloid cells. cKO mice developed normally with no gross phenotypic abnormalities or abnormal peripheral blood counts. Resident peritoneal macrophages (rpMacs) of cKO mice exhibited impaired engulfment of apoptotic cells but showed normal macrophage differentiation and counts. T-cell immunoglobulin and mucin domain-containing molecule 4 (Tim4) is a phosphatidylserine (PS) receptor expressed on rpMacs and possesses a heavily O-glycosylated domain. Tim4 tethers apoptotic cells through PS binding. Expression of the Tim4 transcript was unchanged in cKO rpMacs, whereas flow cytometric, Western and dot blot analyses revealed that Tim4 protein expression in cKO rpMacs was significantly lower than that in wild-type (WT) rpMacs. Moreover, the expression levels of other efferocytosis-related molecules, Mertk, Itgav and Itgb3, were normal in rpMacs. In addition, hypoglycosylated Tim4-FLAG fusion protein sufficiently recognized PS. These results demonstrated that core 1-derived O-glycan is required for Tim4-dependent normal efferocytosis and may contribute to the stable expression of the Tim4 glycoprotein.

Mouse macrophages show different requirements for phosphatidylserine receptor Tim4 in efferocytosis

Protein S (ProS) and growth arrest-specific 6 (Gas6) bind to phosphatidylserine (PtdSer) and induce efferocytosis upon binding TAM-family receptors (Tyro3, Axl, and Mer). Here, we produced mouse ProS, Gas6, and TAM-receptor extracellular region fused to IgG fragment crystallizable region in HEK293T cells. ProS and Gas6 bound Ca²⁺ dependently to PtdSer (K_d 20-40 nM), Mer, and Tyro3 (K_d 15-50 nM). Gas6 bound Axl strongly (K_d < 1.0 nM), but ProS did not bind Axl. Using NIH 3T3-based cell lines expressing a single TAM receptor, we showed that TAM-mediated efferocytosis was determined by the receptor-binding ability of ProS and Gas6. Tim4 is a membrane protein that strongly binds PtdSer. Tim4 alone did not support efferocytosis, but enhanced TAM-dependent efferocytosis. Resident peritoneal macrophages, Kupffer cells, and CD169⁺ skin macrophages required Tim4 for TAM-stimulated efferocytosis, whereas efferocytosis by thioglycollate-elicited peritoneal macrophages or primary cultured microglia was TAM dependent, but not Tim4 dependent. These results indicate that TAM and Tim4 collaborate for efficient efferocytosis in certain macrophage populations.

Macrophages engulfing apoptotic thymocytes produce retinoids to promote selection, differentiation, removal and replacement of double positive thymocytes

The thymus provides the microenvironment in which thymocytes develop into mature T-cells, and interactions with thymic stromal cells are thought to provide the necessary signals for thymocyte maturation. Recognition of self-MHC by T-cells is a basic requirement for mature T-cell functions, and those thymocytes that do not recognize or respond too strongly to the peptide-loaded self-MHC molecules found in the thymus undergo apoptosis. As a result, 95% of the thymocytes produced will die and be subsequently cleared by macrophages. This review describes a complex crosstalk between developing thymocytes and engulfing macrophages which is mediated by retinoids produced by engulfing macrophages. The interaction results in the harmonization of the rate of cell death of dying double positive cells with their clearance and replacement, and in promotion of the differentiation of the selected cells in the thymus.