Milky Spots Research Articles

Eosinophil and B cell dynamics in the milky spots from Schistosoma mansoni-infected mice - Comparison with spleen and bone marrow, and extramedullary eosinopoiesis.

The milky spots are structures found in the omentum of humans and other vertebrates, representing a fraction of the lymphomyeloid tissue associated with the celom. They majorly consist of B lymphocytes, T lymphocytes, and macrophages. Also found in smaller quantities are mesothelial, stromal, dendritic, and rare mast cells. In an experimental model of Schistosoma mansoni infection, there is significant activation of the omentum and milky spots, which exhibit numerous eosinophils. Despite being described for many years, the complete profile of cells found in milky spots and their functions remains largely unexplored. Here, we evaluate the leukocyte populations of the milky spots in homeostasis and a murine model of Schistosoma mansoni infection. The histopathological characterizations, phenotypic profile analysis, and characterization of the eosinophilic potential of progenitors and precursors comparing the milky spots with the spleen and bone marrow showed significant activation of milky spots in infected mice, with changes in the profile over the analyzed times, showing signs of migration and activation of eosinophils, with local eosinopoiesis and maintenance of the eosinophilic population. In naive mice, B1a and B1b cells comprise only a small fraction of B lymphocytes. However, B1b cells expand significantly during infection, peaking at 60 DPI before stabilizing by 90 DPI. B1a cells also increase initially but decrease over time. The behavior of milky spots differs from other primary and secondary lymphoid organs, acting as a central lymphoid organ in cavity immunity.

Morphological differences between the lesser and the greater omenta in albino rats

BackgroundPublications report that all mammals have two omenta, namely, lesser omentum and greater omentum. Basically, these organs, which share the same name except for the adjective “lesser” or “greater,” should not differ from each other. However, no clear description of the structure of the lesser omentum, as well as comparative morphological analysis between the lesser and greater omenta have been found in the literature, which necessitates a thorough investigation. Therefore, the aim of our study was to analyze the morphofunctional differences between the greater and lesser omenta in albino rats. MethodThe experiment involved 20 mature male albino rats, weighing 298,28±7,36 grams. The material for our study were preparations of lesser and greater omenta, fixed in 10 % of neutral buffered formalin. Paraffin sections were stained with hematoxylin-eosin and Van Gieson stain. ResultsThe findings of the study showed that the greater omentum in albino rats, unlike other derivatives of the omentum (ligaments and mesenteries), represents a free extension (mostly from the greater curvature of the stomach), in the form of an “apron,” into a specific depth of the peritoneal cavity, duplicating the serous membrane. This duplication is characterized by the composition of two structurally interdependent formations. These include vascular-fatty arcades, associated with lymphoid nodules known as milky spots, and binding serous-reticular membranes. The findings of the study of the lesser omentum have established that in all cases it is located beneath the liver and becomes visualized only after hepatolifting. It is presented in the form of two ligaments: hepatoduodenal and hepatogastric, which contain two main structured formations, which we called vascular-fatty spurs, between these spurs, serous-reticular membranes are located. Conclusiondespite having similar names, the lesser omentum, a derivative of the peritoneum, is fundamentally different. As it is well known, the lesser omentum is represented by ligaments that extend from the liver hilus to the lesser curvature of the stomach and the duodenum. Due to this arrangement, the lesser omentum lacks the mobile activity characteristic of the greater omentum, which plays a crucial role in rapid response to damage in the gastrointestinal tract. Despite sharing the same names, both formations differ in shape, morphological structure, development and function

Lesser omentum of white rats: structural and functional characteristics

Background. According to the literature, all mammals have two omenta, one of which is the greater omentum and the other one is the lesser omentum, but white rats have two additional derivatives of the visceral peritoneum, homologous in structure to the greater omentum - called the epididymal omentum. Today, the greater omentum is comprehensively studied and arouses great interest among researchers, in turn, the lesser omentum is not popular among scientists, as a result of which many details of its anatomical and histological structure remain undisclosed. Objective. To study the morpho-functional features of the structure of the lesser omentum of white rats through a bibliographic analysis of the literature. Methods. This bibliographic analysis is based on published articles, books, textbooks, and monographs. The search for which was carried out on the worldwide network "Internet" in the scientific and metric databases of Google Scholar, Web of Science and PubMed, foreign sources of literature and the National Library of Medicine. Results. The lesser omentum is formed due to the successive transition of the duplication of the peritoneum, from the lower surface of the liver to the lesser curvature of the stomach and duodenum. At the small curvature of the stomach, the leaves that form the lesser omentum diverge and cover the front-upper and back-lower walls of the stomach. The lesser omentum consists of the hepatogastric ligament, which is an extremely thin and transparent membrane, and a thicker, transparent membrane - the hepatoduodenal ligament, which is attached to the duodenum. Microscopically, it consists of two main structural elements: thin fenestrated translucent membranes and adipose tissue-rich areas containing milky spots that may play a role in the immune defense of the peritoneal cavity. Conclusion. So, according to the analysis, the lesser omentum mainly performs a connecting role and provides support and stabilization of the organs of the abdominal cavity between which it is located.

Development and organization of omental milky spots.

The milky spots in omentum are atypical lymphoid tissues that play a pivotal role in regulating immune responses in the peritoneal cavity. The milky spots act as central hubs for collecting antigens and particles from the peritoneal cavity, regulating lymphocyte trafficking, promoting the differentiation and self-renewal of immune cells, and supporting the local germinal centre response. In addition, the milky spots exhibit unique developmental characteristics that combine the features of secondary and tertiary lymphoid tissues. These structures are innately programmed to form during foetal development; however, they can also be formed postnatally in response to peritoneal irritation such as inflammation, infection, obesity, or tumour metastasis. In this review, I discuss emerging perspectives on homeostatic development and organization of the milky spots.

Vagus nerve signal has an inhibitory influence on the development of peritoneal metastasis in murine gastric cancer

The vagus nerve is the only pathway for transmitting parasympathetic signals between the brain and thoracoabdominal organs, thereby exhibiting anti-inflammatory functions through the cholinergic anti-inflammatory pathway. Despite often being resected during lymph node dissection in upper gastrointestinal cancer surgery, the impact of vagotomy on postoperative outcomes in gastric cancer patients remains unclear. Sub-diaphragmatic vagotomy was performed on C57BL/6 mice. Three weeks later, syngeneic murine gastric cancer cell line YTN16P was injected into the peritoneal cavity, and the number of peritoneal metastases (PM) on the mesentery and omentum compared with control mice. The phenotypes of immune cells in peritoneal lavage and omental milky spots one day after tumor inoculation were analyzed using flow cytometry and immunohistochemistry. Intraperitoneal transfer of 3 × 105 YTN16P significantly increased the number of metastatic nodules on the mesentery in the vagotomy group compared to the control group. The omental metastasis grade was also significantly higher in the vagotomy group. Phenotypic analysis of immune cells in peritoneal lavage did not reveal significant differences after vagotomy. However, vagotomized mice exhibited a notable increase in milky spot area, with a higher presence of cytokeratin(+) tumor cells, F4/80(+) macrophages, and CD3(+) T cells. Vagus nerve signaling appears to regulate the immune response dynamics within milky spots against disseminated tumor cells and inhibits the development of PM. Preserving the vagus nerve may offer advantages in advanced gastric cancer surgery to reduce peritoneal recurrence.

Abstract 6810: Macrophage secreted CCL9/CCL5 induces CCR1-mediated ovarian cancer metastasis to the omentum in the absence of CCL6

Abstract The tumor-microenvironment (TME) of the omentum plays an important role in providing a pre-metastatic niche for high-grade serous ovarian cancer (HGSC-OvCa) progression. Omentum, a layer of fatty adipose tissue within the peritoneal cavity, is the preferred primary metastatic site for OvCa. Omentum is a well-vascularized fatty tissue containing wide-spread immune clusters (milky spots). Milky spots in the omentum are primarily composed of B cells, T cells, macrophages, NK cells, and dendritic cells. Previous studies from our laboratory have highlighted the importance of CCL6 secreted by omental macrophages as a key chemokine in providing a pre-metastatic niche by activating the CCR1 axis on the cancer cells required for OvCa invasion. We engineered and characterized a CCL6-depleted in vivo mouse model using CRISPR/Cas9 to study the effect on OvCa progression and macrophage biology. We demonstrate that deletion of CCL6 in mice alters characteristics of omental tissue-resident (OM-Macφ) and bone marrow-derived macrophages (BMDM) derived from circulating monocytes. RNA sequencing of OM-Macφ and microarray study of BMDM revealed that CCL6 deletion produces enhanced TGFβ1+CSF-1+CCL5+ expression in OM-Macφ and an increase in TGFβ1+CSF-1+CCL9+ in BMDM. This acts as a rescue mechanism to maintain activation of their shared CCR1 receptor with a pro-M2 stimulation profile. Deep multiplexed imaging of the omentum tissue by CODEX showed that CCL6 deletion alters the architecture of milky spots, with enriched macrophage content and reduced B-cell and T-cell-rich neighborhoods. While our previous studies indicated that CCL6 deletion could potentially have an anti-tumoral effect, we found that it does not significantly affect murine ID8 ovarian cancer tumor burden in both short-term (7 days) and long-term in vivo studies and provides no added survival advantage. We report that CCL9+F4/80+ macrophages accumulate in the omentum post-tumor infiltration in the CCL6-KO model, and neutralizing CCL9 in the CCL6-KO BMDM macrophage-derived conditioned media significantly reduces migration of ID8 cancer cells in vitro. Furthermore, we demonstrate that murine CCL9 and its analog, human CCL15, promote the epithelial-mesenchymal transition (EMT) of mouse ID8 and human A2780 OvCa cells. Moreover, our findings indicate that human CCL15 is a prognostic biomarker for reduced survival in OvCa patient cohorts (Stages I&II and Stage III&IV). Further classification of HGSC-OV patients (n=375) from TCGA-OV based on CCL15 and CCR1-high expression correlated with a higher expression of TGFβ1, CCL5, and a T-cell exhaustion signature in the TME. Taken together, given the apparent redundancy of the chemokine network controlling CCR1 signaling, our findings support the strategy of therapeutically blocking the CCR1 receptor to potentially produce a favorable anti-tumor response in HGSC-OvCa patients. Citation Format: Supreeti Tallapragada, Justine Chan, Venkatesh Krishnan, Oliver Dorigo. Macrophage secreted CCL9/CCL5 induces CCR1-mediated ovarian cancer metastasis to the omentum in the absence of CCL6 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6810.

Abstract B091: Mechanisms underlying the omental support of ovarian cancer peritoneal metastasis

Abstract More than 80% of ovarian cancer (OC) cases have metastasized to the peritoneal cavity at diagnosis. The five-year survival rate for these patients is 25%. Within the peritoneal cavity, the most common and early site of metastasis is the omentum, a well-vascularized, specialized adipose tissue that arises off the greater curvature of the stomach. Because OC homes to the omentum, it is removed as a standard of care for patients with peritoneal metastases. In mouse models, the omentum is also an early metastatic site, and omentectomy reduces tumor expansion. The mechanism by which the omentum promotes OC growth is unknown, perhaps due to the complex omental microenvironment. The omentum contains many features that could impact OC seeding. It is covered by a mesothelial lining that extends in a sheet anchoring the omentum to surrounding tissues and is highly perfused by blood vessels. It contains lymphoid aggregates or milky spots that may contribute to pro- and anti-tumor immunity. This study explores the association of OC metastases with specific omentum features. We intraperitoneally injected luciferase-expressing ID8 OC cells to mimic high-grade serous OC metastasis. Using high-resolution, spatial ex vivo bioluminescence imaging (BLI), we found that tumor nodules preferentially formed in adipocyte-rich regions of the omentum and the mesentery, another peritoneal tissue favorable for OC seeding. To determine whether adipocytes, the most abundant omental cell type, are the primary support of OC, we crossed Adiponectin Cre+ and DTA mice to generate mice lacking white and brown adipocytes from birth. To overcome the metabolic syndrome the fatless mice develop, they receive subcutaneous fat transplants dorsally, referred to as distal adipocyte rescue of lipodystrophy (DARL) mice. Littermate controls receive sham surgeries. The blood triglyceride, glucose, and insulin levels are normalized post-transplant. Thus, the peritoneal cavity lacks mature adipocytes, but the mice have typical metabolic profiles, allowing us to study the role of omental adipocytes in peritoneal metastasis. Once the mice recovered from surgery, they were intraperitoneally injected with luciferase-expressing ID8s. While it was hypothesized that OC metastatic seeding within the adipocyte-free omentum and associated peritoneum would be impaired without local mature adipocytes, BLI revealed that the tumor burden was not decreased in DARL mice. Notably, specific regions of OC seeding, where mature adipocytes would have localized if present, such as the omentum and mesentery, retained tumor nodules in DARL mice. This conserved OC localization suggests that other cells in this specific microenvironment might be responsible for metastatic spread. To identify these cells, studies are ongoing to profile these regions in the DARL and control mice via flow cytometry and single-cell RNA sequencing. By unraveling the support for OC cells in the omentum, therapeutic strategies can be identified to repolarize the metastatic milieu and halt further spread to improve patient outcomes. Citation Format: Rachel Mintz, Alexandre Gallerand, Jichang Han, Sarah Ning, Wei Zou, Mary Wohltmann, Bernd Zinselmeyer, Gwendalyn Randolph. Mechanisms underlying the omental support of ovarian cancer peritoneal metastasis [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr B091.

Greater Omentum: Multifaceted Interactions in Neurological Recovery and Disease Progression.

The greater omentum, a unique anatomical structure composed of adipocytes, loose connective tissue, and a dense vascular network. Plays a pivotal role beyond its traditional understanding. It houses specialized immunological units known as 'Milky spots,' making it a key player in immune response. Moreover, the omentum's capacity to enhance tissue perfusion, absorb edema fluid, boost acetylcholine synthesis, and foster neuron repair have rendered it a topic of interest in the context of various diseases, especially neurological disorders. This review provides a comprehensive overview of the intricate anatomy and histology of the greater omentum, casting light on its multifaceted functions and its associations with a spectrum of diseases. With a specific focus on neurological ailments, we delineate the intricate relationship that the omentum shares with other pathologies like stroke and we underly its contribution to serving as a therapeutic agent in neurological disorders. By deciphering the underlying mechanisms and emphasizing areas that demand further investigation. This review aims to spark renewed interest and pave the way for comprehensive studies exploring the greater omentum's potential in neurology and broader medicine overall. Given these diverse interactions that yet remain elusive, we must investigate and understand the nuanced relationship between the greater omentum and pathologies, especially its role in stroke's pathophysiology and therapeutic interventions so as to enhance patient care.

Exosomes secreted by ST3GAL5high cancer cells promote peritoneal dissemination by establishing a premetastatic microenvironment.

Peritoneal dissemination of cancer affects patient survival. The behavior of peritoneal mesothelial cells (PMCs) and immune cells influences the establishment of a microenvironment that promotes cancer cell metastasis in the peritoneum. Here, we investigated the roles of lactosylceramide alpha-2,3-sialyltransferase (ST3G5; also known as ST3GAL5 and GM3 synthase) in the exosome-mediated premetastatic niche in peritoneal milky spots (MSs). Exosomes secreted from ST3G5high cancer cells (ST3G5high -cExos) were found to contain high levels of hypoxia-inducible factor 1-alpha (HIF1α) and accumulated in MSs via uptake in macrophages (MΦs) owing to increased expression of sialic acid-binding Ig-like lectin 1 (CD169; also known as SIGLEC1). ST3G5high -cExos induced pro-inflammatory cytokines and glucose metabolic changes in MΦs, and the interaction of these MΦs with PMCs promoted mesothelial-mesenchymal transition (MMT) in PMCs, thereby generating αSMA+ myofibroblasts. ST3G5high -cExos also increased the expression of immune checkpoint molecules and T-cell exhaustion in MSs, which accelerated metastasis to the omentum. These events were prevented following ST3G5 depletion in cancer cells. Mechanistically, ST3G5high -cExos upregulated chemokines, including CC-chemokine ligand 5 (CCL5), in recipient MΦs and dendritic cells (DCs), which induced MMT and immunosuppression via activation of signal transducer and activator of transcription 3 (STAT3). Maraviroc, a C-C chemokine receptor type 5 (CCR5) antagonist, prevented ST3G5high -cExo-mediated MMT, T-cell suppression, and metastasis in MSs. Our results suggest ST3G5 as a suitable therapeutic target for preventing cExo-mediated peritoneal dissemination.

Normal saline remodels the omentum and stimulates its receptivity for transcoelomic metastasis.

The omentum contains immune cell structures called milky spots that are niches for transcoelomic metastasis. It is difficult to remove the omentum completely, and there are no effective strategies to minimize the risk of colonization of preserved omental tissues by cancer cells that circulate in the peritoneal fluid. Normal saline is commonly administered into the peritoneal cavity for diagnostic and intraoperative lavage. Here we show that normal saline, when administered into the peritoneal cavity of mice, is prominently absorbed by the omentum, exfoliates its mesothelium, and induces expression of CX3CL1, the ligand for CX3CR1, within and surrounding the omental vasculature. Studies using CX3CR1-competent and CX3CR1-deficient mice showed that the predominant response in the omentum following saline administration is an accumulation of CX3CR1+ monocytes/macrophages that expand milky spots and promote neoangiogenesis within these niches. Moreover, saline administration promoted the implantation of cancer cells of ovarian and colorectal origin onto the omentum. By contrast, these deleterious effects were not observed following i.p. administration of lactated Ringer's solution. Our findings suggest that normal saline stimulates the receptivity of the omentum for cancer cells and that the risk of colonization can be minimized by using a biocompatible crystalloid for lavage procedures.

MILKY SPOTS IN THE GREATER OMENTUM

The article presents a review of data from foreign and domestic literature focusing on various aspects of the morphology and functions of the milky spots of the greater omentum in the health and in the pathological process in the abdominal cavity. The milk spots of the greater omentum do not have a capsule; the cellular composition is normally represented in the majority by macrophages and lymphocytes. However, in pathological conditions within the abdominal cavity, additional cells such as fibroblasts, plasmocytes, dendritic (antigen-presenting) cells, and mast cells are detected in the milky spot.
 Normally, in the structure of the milky spot there are fat cells (adipocytes), undifferentiated mesenchymal cells, elastic, reticular and nerve fibres. A thorough analysis of existing literature has revealed the dual role of milky spots, encompassing both protective and immune functions.
 The protective role involves the mobilization of resident macrophages from the milky spot into the abdominal cavity in response to the presence of pathogens. During the initial stages of the inflammatory response, mesothelial cells, when activated by pathogens, release chemokines that intensify inflammation. This process creates a gradient of chemotactic cytokines, enabling the migration of leukocytes from fenestrated postcapillary venules through the stigmata and into the abdominal cavity. The immune role is characterized by the secretion of cytokines by mature dendritic (antigen-presenting) cells and their interaction with milky spot T cells to achieve an immune response. Milky spot macrophages secrete chemokine ligands that promote the migration and colonization of ovarian cancer cells within the greater omentum.

The apron of the greater omentum of gastric cancer patients contains various lymphoid structures including lymph nodes

PurposeTo gain more insight into the pattern of peritoneal cancer dissemination and optimize cancer treatment, it is important to improve our understanding of the omental lymphatic system. Although omental milky spots (OMSs) are considered the only lymphoid structures in the omentum, clinical studies mention the presence of lymph nodes (LNs) as well. This discrepancy may be explained by the fact that OMSs are highly dynamic structures and may erroneously be mistaken for LNs. The aim of this study was to evaluate the lymphoid structures, and mainly the presence of lymph nodes, in the apron of the greater omentum. Basic proceduresIn this study, diagnostic samples of the greater omentum of 17 gastric cancer patients that were previously reported to contain LNs were re-evaluated for their presence. Paraffin embedded omental samples were stained with Picrosirius red, smooth muscle actin and CD20 and CD3, and microscopically re-examined according to predefined criteria to distinguish OMSs from LNs. Main findingsPathology records reported 47 LNs in 17 patients. Upon re-evaluation, 20/47 LNs could be classified as true LNs and were located in both the upper and lower quadrants of the greater omentum. The other 27 structures could not be classified as LNs or OMSs and were defined as intermediate lymphoid structures. ConclusionsThe omental apron of gastric cancer patients contains LNs and intermediate lymphoid structures, the latter most likely representing activated OMSs. These observations underline that our understanding of the lymphatic system of the greater omentum is incomplete and requires additional studies to gain further insight in its structure and function in both health and disease.

FRC-derived retinoic acid: The key to unlocking milky spots.

Milky spots of the omentum enable lymphocyte access to the peritoneal cavity. In this issue of JEM, Yoshihara and Okabe (2023. J. Exp. Med.https://doi.org/10.1084/jem.20221813) demonstrate how secretion of retinoic acid by fibroblastic reticular cells allows lymphocyte entry into milky spots and the peritoneal cavity.

Aldh1a2 + fibroblastic reticular cells regulate lymphocyte recruitment in omental milky spots.

Lymphoid clusters in visceral adipose tissue omentum, known as milky spots, play a central role in the immunological defense in the abdomen. Milky spots exhibit hybrid nature between secondary lymph organs and ectopic lymphoid tissues, yet their development and maturation mechanisms are poorly understood. Here, we identified a subset of fibroblastic reticular cells (FRCs) that are uniquely present in omental milky spots. These FRCs were characterized by the expression of retinoic acid-converting enzyme, Aldh1a2, and endothelial cell marker, Tie2, in addition to canonical FRC-associated genes. Diphtheria toxin-mediated ablation of Aldh1a2+ FRCs resulted in the alteration in milky spot structure with a significant reduction in size and cellularity. Mechanistically, Aldh1a2+ FRCs regulated the display of chemokine CXCL12 on high endothelial venules (HEVs), which recruit blood-borne lymphocytes from circulation. We further found that Aldh1a2+ FRCs are required for the maintenance of peritoneal lymphocyte composition. These results illustrate the homeostatic roles of FRCs in the formation of non-classical lymphoid tissues.

Великий чепець білих щурів: структурно-функціональна характеристика та його роль при перитоніті

The greater omentum is now recognized as an important organ in the fight against intra-abdominal infections, so it is often called the “policeman of the peritoneal cavity”. The greater omentum plays the most important role in the implementation of immune supervision of the antigenic condition of the peritoneal fluid and rapid response of damage to the gastrointestinal tract, which is often complicated by inflammation of the peritoneum – peritonitis. The purpose of the work was to study the peculiarities of the structure of the greater omentum of white rats and its role in peritonitis by means of a bibliographic analysis of the literature. Materials and methods. This bibliographic analysis is based on published articles, books, textbooks, monographs and dissertation abstracts. For the purposes of this systematic review, a literature search was carried out in the worldwide Internet, domestic sources of literature, scientific and electronic library of Poltava State Medical University using the following key words: “morphology”, “comparative anatomy”, “greater omentum”, “greater omentum of the rats”, “immune system”, “white rats”, “laboratory animals”, “rat anatomy”, “peritonitis”. Results and discussion. The greater omentum of white rats and humans are represented by two different structures: translucent membranous regions and regions rich in adipose tissue. According to the literature, the greater omentum of white rats is the main morphological characteristic (in miniature), homologous to the human one. The main function of the greater omentum, according to the opinion of many authors and ours too, is immunity. Immune structures of the greater omentum of white rats are represented by lymphoid nodules, known in the literature as milky spots, most of which are located near arteries and veins. In pathological conditions, the greater omentum acquires absolutely special properties, such as plasticity, the ability to fuse with a traumatic and inflamed surface, the ability to hemostasis and phagocytosis, the ability to germinate and revascularize, to absorb fluids and microparticles from the peritoneal cavity, and the ability to the immunological response. Conclusion. According to the literature, the greater omentum of white rats is a miniature likeness of the greater omentum of a person. However, unlike the latter, it consists of only one duplication of the visceral peritoneum, between the leaves of which there are blood vessels with deposits of adipose tissue and milk spots. Thus, the greater omentum of white rats is a perfectly acceptable object of experimental research with the subsequent legitimate extrapolation of their results to humans. The greater omentum occupies a central place in the peritoneal defense mechanisms due to its innate immune function, high absorbing capacity, and ability to attach to neighboring structures to close defects of abdominal organs and promote their healing, due to its expressive angiogenic activity

Pulmonary dust foci as rat pneumoconiosis lesion induced by titanium dioxide nanoparticles in 13-week inhalation study

BackgroundMost toxicological studies on titanium dioxide (TiO2) particles to date have concentrated on carcinogenicity and acute toxicity, with few studies focusing of pneumoconiosis, which is a variety of airspace and interstitial lung diseases caused by particle-laden macrophages. The present study examined rat pulmonary lesions associated with pneumoconiosis after inhalation exposure to TiO2 nanoparticles (NPs).MethodsMale and female F344 rats were exposed to 6.3, 12.5, 25, or 50 mg/m3 anatase type TiO2 NPs for 6 h/day, 5 days/week for 13 weeks using a whole-body inhalation exposure system. After the last exposure the rats were euthanized and blood, bronchoalveolar lavage fluid, and all tissues including lungs and mediastinal lymph nodes were collected and subjected to biological and histopathological analyses.ResultsNumerous milky white spots were present in the lungs after exposure to 25 and 50 mg/m3 TiO2 NPs. Histopathological analysis revealed that the spots were alveolar lesions, characterized predominantly by the agglomeration of particle-laden macrophages and the presence of reactive alveolar epithelial type 2 cell (AEC2) hyperplasia. We defined this characteristic lesion as pulmonary dust foci (PDF). The PDF is an inflammatory niche, with decreased vascular endothelial cells in the interstitium, and proliferating AEC2 transformed into alveolar epithelial progenitor cells. In the present study, the AEC2 in the PDF had acquired DNA damage. Based on PDF induction, the lowest observed adverse effect concentration for pulmonary disorders in male and female rats was 12.5 mg/m3 and 6.3 mg/m3, respectively. The no observed adverse effect concentration for male rats was 6.3 mg/m3. There was a sex difference in lung lesion development, with females showing more pronounced lesion parameters than males.ConclusionsInhalation exposure to TiO2 NPs caused PDF, an air-space lesion which is an alveolar inflammatory niche containing particle-laden macrophages and proliferating AEC2. These PDFs histopathologically resemble some pneumoconiosis lesions (pulmonary siderosis and hard metal pneumoconiosis) in workers and lung disease in smokers, suggesting that PDFs caused by exposure to TiO2 NPs in rats are an early pneumoconiosis lesion and may be a common alveolar reaction in mammals.Graphical

Abstract 2521: Impact of peritoneal lavage on omentum immune cell dynamics and metastasis

Abstract Peritoneal lavage is a routine procedure in which normal saline solution is administered into the abdominal cavity and the washings are cytologically examined for cancer cells. Peritoneal lavage often strips protective mesothelial linings and can promote formation of peritoneal adhesions. However, the impact of peritoneal lavage on the omentum, which is also exposed to this procedure, is unclear. The omentum is a fatty tissue that suspends from the stomach and contains highly vascularized immune cell structures called milky spots. Because circulating ovarian and gastric cancer cells have strong predilection for implanting in milky spots, it is important to understand the impact of peritoneal lavage on the omentum, as not all patients with these cancers undergo omentectomy. In this study, we administered clinically relevant amounts of normal saline solution to immunecompetent mice to mimic peritoneal lavage, and evaluated the absorption of saline solution and the gross architecture and immune cell composition of the omentum and other peritoneal fat tissues. As compared to other peritoneal fat tissues, the omentum showed the highest capability to absorb saline solution. Following administration of saline, a dramatic expansion in milky spots and increased vessel density were observed in the omentum. Immunophenotyping and functional studies revealed that these cellular changes were due in part to the expansion of a sub-population of peritoneal macrophages. Furthermore, studies using mouse allograft models of ovarian cancer revealed that saline administration stimulated the implantation of circulating cancer cells on to the omentum. Collectively, these findings raise the possibility that peritoneal lavage ‘primes’ the omentum to be conducive for colonization by occult cancer cells that circulate in the peritoneal fluid. Citation Format: Hironari Akasaka, Honami Naora. Impact of peritoneal lavage on omentum immune cell dynamics and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2521.

GENERAL STRUCTURE AND PRINCIPLES OF MORPHOMETRIC ANALYSIS OF GREATER OMENTUM IN WHITE RATS

The greater omentum is an organ belonging to the peripheral part of the immune system as it contains specialized lymphoid structures, known as milky spots. Moreover, the greater omentum plays an important role in the process of renewal and maintaining the quantitative constancy of the peritoneal fluid. According to the literature, the greater omentum of white rats in its main morphological features (in miniature) is homologous compared to a human one. The purpose of the study is to investigate the general structure and the principles of morphometric analysis of the greater omentum in white rats. Materials and methods. The experiment involved 15 white male rats of reproductive age, weighing from 278.08 to 346.47 g. After vivisection, carried out by an overdose of thiopental-sodium anesthesia, the abdominal cavity was opened providing an access to a complete examination of the internal organs in their natural positions; we took photographs and started a direct study of the greater omentum. Results and discussion. During a general examination of the contents of the peritoneal cavity of 15 test animals, we found out the greater omentum was not always present in its typical form; in 3 cases, it was hidden between the loops of the small intestine. Linear measurements of the size of the greater omentum were performed, the data obtained as a result of studying the corresponding sample of animals, were plotted in a frame of rectangular coordinates along the abscissa and ordinate axes, respectively; on the graph, they were visualized with parallel lines, and a visual diagram of the individual variability of the area of the greater omentum was obtained. Conclusion. When studying the greater omentum of a certain quantitative sample in white rats, it may turn out that in some cases it will be in a latent form, located mainly among the loops of the small intestine. The greater omentum of white rats is characterized by significant individual variability in shape and size that is not amenable to strict metric analysis, especially when determining its area, which is the main indicator of its surface contact with the peritoneal fluid, and, consequently, with its antigenic composition.

T cells in the peritoneum.

The peritoneal cavity is a fluid-packed area that houses most of the abdominal organs, including the omentum, a visceral adipose tissue with milky patches or groups of leukocytes organized in the same way to those observed in typical lymphoid tissues. A distinct population of leukocytes patrols the peritoneal cavity and travels in and out of the milky spots, facing antigens or pathogens in the peritoneal fluid and responding appropriately. T cells may play a crucial function in regulating adaptive immune responses to antigens in the peritoneal cavity to ensure tissue homeostasis and healing. When peritoneal homeostasis is interrupted by inflammation, infection, obesity, or tumor metastasis, the omentum's dedicated fibroblastic stromal cells and mesothelial cells control peritoneal leukocyte recruitment and activation in unique ways. T cells, which employ their T cell receptor to target specific antigens, are an important component of the acquired immune response since they are present in the peritoneal cavity. The peritoneum provides a different environment for T cells to respond to pathogens. This chapter outlines the anatomy relevant to T cell function and biology, such as antigen processing/presentation, T cell activation, and the many T cell subpopulations in the peritoneal cavity, as well as their role in cancer or other infection.

Omentum: Friend or foe in ovarian cancer immunotherapy?

Ovarian cancer often spreads out of the ovary before a patient is diagnosed and is the deadliest gynecological malignancy. The aggressiveness of ovarian cancer is determined by the progression in the form of peritoneal carcinomatosis, a stage with a poor prognosis and an untreatable condition in most patients. One of the first tumor nests or the origin of metastasis in the peritoneal cavity is the omentum. The omentum contains immune aggregates, called milky spots, embedded in adipose tissue, which support tumor growth by various mechanisms, including immunosuppressive immune cells and metabolic functions. In this sense, the abundance of blood vessels, omental resident macrophages, and chemokines, among other factors, are known to promote invasiveness, proliferation and resistance to cancer therapies. As a result, surgical practice employed in advanced-stage ovarian cancer almost constantly includes omentectomy. Paradoxically, the omentum is considered the "abdominal policeman" that contributes to peritoneal immunity by capturing antigens and pathogens from the peritoneal cavity and promoting effective immune responses against microbes. Why immunosurveillance against the metastatic tumor does not take place in the omentum? Could omental immune responses be activated with immunotherapeutic interventions? The omentum has largely been ignored in cancer immunology and immunotherapy, and the potential translational implications of this in ovarian cancer are still unclear. Here, we focus on the dual role of the omentum in ovarian cancer: its role in antitumor immune responses versus its activities fostering cancer progression.