FALC stromal cells define a unique immunological niche for the surveillance of serous cavities

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.

Fat-associated lymphoid clusters: Supporting visceral adipose tissue B cell function in immunity and metabolism

It is now widely understood that visceral adipose tissue (VAT) is a highly active and dynamic organ, with many functions beyond lipid accumulation and storage. In this review, we discuss the immunological role of this tissue, underpinned by the presence of fat-associated lymphoid clusters (FALCs). FALC's distinctive structure and stromal cell composition support a very different immune cell mix to that found in classical secondary lymphoid organs, which underlies their unique functions of filtration, surveillance, innate-like immune responses, and adaptive immunity within the serous cavities. FALCs are important B cell hubs providing B1 cell-mediated frontline protection against infection and supporting B2 cell-adaptative immune responses. Beyond these beneficial immune responses orchestrated by FALCs, immune cells within VAT play important homeostatic role. Dysregulation of immune cells during obesity and aging leads to chronic pathological "metabolic inflammation", which contributes to the development of cardiometabolic diseases. Here, we examine the emerging and complex functions of B cells in VAT homeostasis and the metabolic complications of obesity, highlighting the potential role that FALCs play and emphasize the areas where further research is needed.

Cardiac Fibroblastic Niches in Homeostasis and Inflammation

Fibroblasts are essential for building and maintaining the structural integrity of all organs. Moreover, fibroblasts can acquire an inflammatory phenotype to accommodate immune cells in specific niches and to provide migration, differentiation, and growth factors. In the heart, balancing of fibroblast activity is critical for cardiac homeostasis and optimal organ function during inflammation. Fibroblasts sustain cardiac homeostasis by generating local niche environments that support housekeeping functions and by actively engaging in intercellular cross talk. During inflammatory perturbations, cardiac fibroblasts rapidly switch to an inflammatory state and actively communicate with infiltrating immune cells to orchestrate immune cell migration and activity. Here, we summarize the current knowledge on the molecular landscape of cardiac fibroblasts, focusing on their dual role in promoting tissue homeostasis and modulating immune cell-cardiomyocyte interaction. In addition, we discuss potential future avenues for manipulating cardiac fibroblast activity during myocardial inflammation.

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.

Omentum-on-a-chip: A multicellular, vascularized microfluidic model of the human peritoneum for the study of ovarian cancer metastases

Epithelial ovarian cancer has the highest mortality rate of any gynecologic malignancy and most frequently metastasizes to the peritoneal cavity. Intraperitoneal metastases are highly associated with ascites, the pathologic accumulation of peritoneal fluid due to impaired drainage, increased peritoneal permeability, and tumor and stromal cytokine secretion. However, the relationship between ascites, vascular and mesothelial permeability, and ovarian cancer intraperitoneal metastases remains poorly understood. In this study, a vascularized in vitro model of the human peritoneal omentum and ovarian tumor microenvironment (TME) was employed to study stromal cell effects on tumor cell (TC) attachment and growth, as well as TC effects on vascular and mesothelial permeability in models of both early- and late-stage metastases. Control over the number of TCs seeded in the vascularized peritoneum revealed a critical cell density requirement for tumor growth, which was further enhanced by stromal adipocytes and endothelial cells found in the peritoneal omentum. This tumor growth resulted in both a physically-mediated decrease and cytokine-mediated increase in microvascular permeability, emphasizing the important and potentially opposing roles of tumor cells in ascites formation. This system provides a robust platform to elucidate TC-stromal cell interactions during intraperitoneal metastasis of ovarian cancer and presents the first in vitro vascularized model of the human peritoneum and ovarian cancer TME.

Fat-associated lymphoid clusters as expandable niches for ectopic liver development

BACKGROUND AND AIMS

Hepatocyte transplantation holds great promise as an alternative approach to whole-organ transplantation. Intraportal and intrasplenic cell infusions are primary hepatocyte transplantation delivery routes for this procedure. However, patients with severe liver diseases often have disrupted liver and spleen architectures, which introduce risks in the engraftment process. We previously demonstrated i.p. injection of hepatocytes as an alternative route of delivery that could benefit this subpopulation of patients, particularly if less invasive and low-risk procedures are required; and we have established that lymph nodes may serve as extrahepatic sites for hepatocyte engraftment. However, whether other niches in the abdominal cavity support the survival and proliferation of the transplanted hepatocytes remains unclear.

APPROACH AND RESULTS

Here, we showed that hepatocytes transplanted by i.p. injection engraft and generate ectopic liver tissues in fat-associated lymphoid clusters (FALCs), which are adipose tissue-embedded, tertiary lymphoid structures localized throughout the peritoneal cavity. The FALC-engrafted hepatocytes formed functional ectopic livers that rescued tyrosinemic mice from liver failure. Consistently, analyses of ectopic and native liver transcriptomes revealed a selective ectopic compensatory gene expression of hepatic function-controlling genes in ectopic livers, implying a regulated functional integration between the two livers. The lack of FALCs in the abdominal cavity of immunodeficient tyrosinemic mice hindered ectopic liver development, whereas the restoration of FALC formation through bone marrow transplantation restored ectopic liver development in these mice. Accordingly, induced abdominal inflammation increased FALC numbers, which improved hepatocyte engraftment and accelerated the recovery of tyrosinemic mice from liver failure.

CONCLUSIONS

Abdominal FALCs are essential extrahepatic sites for hepatocyte engraftment after i.p. transplantation and, as such, represent an easy-to-access and expandable niche for ectopic liver regeneration when adequate growth stimulus is present.

Histopathological Impact of Bleomycin on Lung Injury and Development of Mediastinal Fat-Associated Lymphoid Clusters in the Lymphoproliferative Mouse Model

The purpose of this study is to elucidate the impact of bleomycin on the degree of lung injury and development of mediastinal fat-associated lymphoid clusters (MFALCs) in the lymphoproliferative mouse model (MRL/MpJ-Faslpr/lpr “Lpr”) and its control strain (MRL/MpJ “MpJ”). We analyzed immune cells, the degree of proliferation, lymphatic vessels (LVs), and high endothelial venules (HEVs) in lungs and MFALCs in Lpr and MpJ mice on the 7th and 21st days following intranasal instillation of either bleomycin (BLM group) or PBS (PBS group). The BLM group showed a significant increase in the size of MFALCs, lung injury score, and positive area ratios of LVs, HEVs, and immune cells (especially macrophages, B- and T-lymphocytes) on both days 7 and 21. Interestingly, the lungs in the BLM group on day 21 showed higher collagen deposition and cellular infiltration in MpJ and Lpr, respectively. Moreover, significant positive correlations were observed between the size of MFALCs and lung injury. In conclusion, BLM could exert lung fibrosis or lymphoproliferative infiltration in chronic stages in MpJ and Lpr, respectively, and this varied effect could be due to the variations in the degree of immune cell proliferation and the development of LVs and HEVs among the studied strains.

Communication, construction, and fluid control: lymphoid organ fibroblastic reticular cell and conduit networks

Fibroblastic reticular cells (FRCs) are a crucial part of the stromal cell infrastructure of secondary lymphoid organs (SLOs). Lymphoid organ fibroblasts form specialized niches for immune cell interactions and thereby govern lymphocyte activation and differentiation. Moreover, FRCs produce and ensheath a network of extracellular matrix (ECM) microfibers called the conduit system. FRC-generated conduits contribute to fluid and immune cell control by funneling fluids containing antigens and inflammatory mediators through the SLOs. We review recent progress in FRC biology that has advanced our understanding of immune cell functions and interactions. We discuss the intricate relationships between the cellular FRC and the fibrillar conduit networks, which together form the basis for efficient communication between immune cells and the tissues they survey.

Nanomaterials and the Serosal Immune System in the Thoracic and Peritoneal Cavities

The thoracic and peritoneal cavities are lined by serous membranes and are home of the serosal immune system. This immune system fuses innate and adaptive immunity, to maintain local homeostasis and repair local tissue damage, and to cooperate closely with the mucosal immune system. Innate lymphoid cells (ILCs) are found abundantly in the thoracic and peritoneal cavities, and they are crucial in first defense against pathogenic viruses and bacteria. Nanomaterials (NMs) can enter the cavities intentionally for medical purposes, or unintentionally following environmental exposure; subsequent serosal inflammation and cancer (mesothelioma) has gained significant interest. However, reports on adverse effects of NM on ILCs and other components of the serosal immune system are scarce or even lacking. As ILCs are crucial in the first defense against pathogenic viruses and bacteria, it is possible that serosal exposure to NM may lead to a reduced resistance against pathogens. Additionally, affected serosal lymphoid tissues and cells may disturb adipose tissue homeostasis. This review aims to provide insight into key effects of NM on the serosal immune system.

Role of adipose-associated lymphoid tissues in the immunological homeostasis of the serosal surface

Although not typical lymphoid organs, analysis of the visceral adipose-associated lymphoid tissues has recently substantially expanded our knowledge about the immunological features of these elusive compartments. Recent data have highlighted their considerable complexity in cellular organization and interactions in several biological processes, including adaptive immune responses, tissue plasticity to accommodate mesenchymal stem cells and progenitors, and providing a suitable microenvironment for serosal tumor propagation. This review aims to present a comprehensive view of the adipose-associated lymphoid tissues in local and systemic immune responsiveness, with particular emphasis on the omental and mesenteric lymphoid tissues in the serosal defense of abdominal organs.