Cell origin and niche availability dictate the capacity of peritoneal macrophages to colonize the cavity and omentum

The relationship between macrophages of the peritoneal cavity and the adjacent omentum remains poorly understood. Here, we describe two populations of omental macrophages distinguished by CD102 expression and use an adoptive cell transfer approach to investigate whether these arise from peritoneal macrophages, and whether this depends upon inflammatory status, the origin of peritoneal macrophages and availability of the omental niches. We show that whereas established resident peritoneal macrophages largely fail to migrate to the omentum, monocyte-derived resident cells readily migrate and form a substantial component of omental CD102(+) macrophages in the months following resolution of peritoneal inflammation. In contrast, both populations had the capacity to migrate to the omentum in the absence of endogenous peritoneal and omental macrophages. However, inflammatory macrophages expanded more effectively and more efficiently repopulated both CD102(+) and CD102(−) omental populations, whereas established resident macrophages partially reconstituted the omental niche via recruitment of monocytes. Hence, cell origin determines the migration of peritoneal macrophages to the omentum and predisposes established resident macrophages to drive infiltration of monocyte-derived cells

Selective effects of NF-jB1 deficiency in CD4 1 T cells on Th2 and TFh induction by alum-precipitated protein vaccines

NF-jB1-dependent signaling directs the development of CD4 1 Th2 cells during allergic airway inflammation and protective responses to helminth infection. Here, we show that IL-4 and IL-13 production is NF-jB1-dependent in mouse OVA-specific CD

Stromal Cells Covering Omental Fat-Associated Lymphoid Clusters Trigger Formation of Neutrophil Aggregates to Capture Peritoneal Contaminants

The omentum is a visceral adipose tissue rich in fat-associated lymphoid clusters (FALCs) that collects peritoneal contaminants and provides a first layer of immunological defense within the abdomen. Here, we investigated the mechanisms that mediate the capture of peritoneal contaminants during peritonitis. Single-cell RNA sequencing and spatial analysis of omental stromal cells revealed that the surface of FALCs were covered by CXCL1+ mesothelial cells, which we termed FALC cover cells. Blockade of CXCL1 inhibited the recruitment and aggregation of neutrophils at FALCs during zymosan-induced peritonitis. Inhibition of protein arginine deiminase 4, an enzyme important for the release of neutrophil extracellular traps, abolished neutrophil aggregation and the capture of peritoneal contaminants by omental FALCs. Analysis of omental samples from patients with acute appendicitis confirmed neutrophil recruitment and bacterial capture at FALCs. Thus, specialized omental mesothelial cells coordinate the recruitment and aggregation of neutrophils to capture peritoneal contaminants

Ontogeny of stromal organizer cells during lymph node development

The development of secondary lymphoid organs, such as lymph nodes (LNs), in the embryo results from the reciprocal action between lymphoid tissue inducer (LTi) cells and stromal cells. However, the initial events inducing LN anlagen formation before the LTi stromal cells cross-talk interactions take place are not fully elucidated. In this study, we show that the inguinal LN anlagen in mouse embryos developed from mesenchymal cells surrounding the lymph sacs, spherical structures of endothelial cells that bud from veins. Using inguinal and mesenteric LNs (mLNs), we provide evidence supporting a two-step maturation model for stromal cells: first, ICAM-1(−)VCAM-1(−) mesenchymal precursor cells become ICAM-1(int)VCAM-1(int) cells, in a process independent of LTi cells and lymphotoxin β receptor (LTβR) signaling. The second step involves the maturation of ICAM-1(int)VCAM-1(int) cells to ICAM-1(high)VCAM-1(high) mucosal addressin cell adhesion molecule-1(+) organizer cells and depends on both LTi cells and LTβR. Addition of αLTβR agonist to LN organ cultures was sufficient to induce ICAM-1(int)VCAM-1(int) cells to mature. In LtβR(−/−) embryos, both inguinal and mLN stromal cells showed a block at the ICAM-1(int)VCAM-1(int) stage, and, contrary to inguinal LNs, mLNs persist longer and contained LTi cells, which correlated with the sustained gene expression of Il-7, Cxcl13, and, to a lesser degree, Ccl21. Taken together, these results highlight the importance of the signals and cellular interactions that induce the maturation of stromal cells and ultimately lead to the formation of lymphoid tissues

Adipose tissue macrophage heterogeneity and the role of Tim4⁺ macrophages in lipid homeostasis

Resident macrophages are essential for the maintenance of tissue homeostasis as they participate in clearance of apoptotic cells and tissue remodelling and repair. In recent years, there has been an increased interest in the study of adipose tissue macrophages (ATMs). In lean individuals, ATMs are important for the control of insulin sensitivity, thermogenesis, angiogenesis and adipose tissue development. In obesity, the number and phenotype of ATMs is altered, and is associated with chronic low grade systemic and local inflammation. These “pro-inflammatory” changes are postulated to contribute to the manifestation of metabolic syndrome. These findings have suggested that the pool of ATMs is heterogeneous and may change, especially during obesity. To date, the characterisation of ATMs has been limited largely to the F4/80/CD11b markers, however the hypothesis of this thesis is that ATMs have distinct phenotype and function that could influence, in different ways, tissue homeostasis. This thesis aims to characterise and phenotype ATM subsets in order to better understand their potential specific role in the tissue. During the course of this research, a novel population of Tim4+ resident ATMs were identified. An additional aim of this thesis was to elucidate their role in adipose tissue homeostasis. Partial bone marrow chimeras were used to identify macrophage origin. The main AT depots were shielded from irradiation and a donor BM was injected intravenously. After 8 weeks, the origin of macrophages was analysed using flow cytometry. Tim4, a phosphatidylserine receptor mediating phagocytosis of apoptotic cells and a marker found on resident macrophages in other tissues, was used for the first time in adipose tissue. Four subsets of ATMs were identified: F4/80highCD11c-Tim4+, F4/80highCD11c- Tim4-; F4/80lowCD11c+Tim4-; F4/80lowCD11c-Tim4-. Interestingly, this newly described F4/80highTim4+ ATM subset showed the lowest non-host chimerism compared to the other ATMs, suggesting this is a main self-replenishing resident ATM population. To study the impact of obesity on ATM turnover, partial chimeric mice were fed HFD for 8 weeks. This increased the number of macrophages in AT. However, the different subsets of ATMs were differentially affected by the diet. Indeed, only a small proportion of Tim4+ ATMs derived from the bone marrow. In contrast, replenishment of the 3 other subsets was almost fully dependent on the arrival of monocyte-derived cells from the bone marrow. TIMD4, the gene encoding for Tim4, has been highlighted in genetic studies as being linked with dyslipidaemia. This suggests that Tim4+ ATMs might play a role in lipid homeostasis. Further characterisation of Tim4 ATMs demonstrated that these Tim4+ ATMs are highly charged in neutral lipid, and also have an increased lysosomal activity (shown by lysotracker staining) compared to the other ATM subsets. Using blocking anti-Tim4 antibodies in vivo, I found that Tim4 contributed markedly to free fatty acid (FFA) release into the plasma after short-term and long term HFD feeding. In addition, in vitro and in vivo experiments demonstrated that Tim4 could be required for the uptake of neutral lipids and their integration into lysosomes for degradation, though this seems to be dependent on the nature of the lipid. Collectively, these results indicate that Tim4 plays a crucial role in the control of lipid trafficking under conditions when dietary lipid is in excess. Tim4 allows uptake of lipids by Tim4+ ATMs and subsequent release of FFA into the circulation. Finally, the presence of Tim4+ lipid laden ATMs was demonstrated in the human omentum. This finding may lead to the discovery of new targets to improve metabolic health in obese patients. This work stresses the importance of resident ATM population in body lipid homeostasis as they could be involved in coping with lipid availability in the body and influence the amount of FFA in the plasma

Publisher Correction: IL-17 signalling is critical for controlling subcutaneous adipose tissue dynamics and parasite burden during chronic murine Trypanosoma brucei infection.

Correction to: Nature Communications https://doi.org/10.1038/s41467-023-42918-8, published online 03 November 2023

Lymphotoxin-β Receptor Signaling through NF-κB2-RelB Pathway Reprograms Adipocyte Precursors as Lymph Node Stromal Cells

SummaryLymph node development during embryogenesis involves lymphotoxin-β receptor engagement and subsequent differentiation of a poorly defined population of mesenchymal cells into lymphoid tissue organizer cells. Here, we showed that embryonic mesenchymal cells with characteristics of adipocyte precursors present in the microenvironment of lymph nodes gave rise to lymph node organizer cells. Signaling through the lymphotoxin-β receptor controlled the fate of adipocyte precursor cells by blocking adipogenesis and instead promoting lymphoid tissue stromal cell differentiation. This effect involved activation of the NF-κB2-RelB signaling pathway and inhibition of the expression of the key adipogenic factors Pparγ and Cebpα. In vivo organogenesis assays show that embryonic and adult adipocyte precursor cells can migrate into newborn lymph nodes and differentiate into a variety of lymph node stromal cells. Thus, we propose that adipose tissues act as a source of lymphoid stroma for lymph nodes and other lymphoid structures associated with fat

Control of innate-like B cell location for compartmentalised IgM production

Natural IgM are crucial for early protection against infection and play an important homeostatic function by clearing dead cells. The production of IgM is ensured by a population of B cells with innate-like properties: their response is rapidly activated by innate signals early during the onset of infection. The main reservoir of innate-like B cells (IBCs) are the serous cavities, but their maintenance and activation depends on their relocation to a variety of lymphoid tissues. Recent advances indicate that fat-associated lymphoid clusters (FALCs) and milky spots contribute to local IgM secretion and play a central role in the localisation and regulation of IBC function

Inflammation-induced formation of fat-associated lymphoid clusters

Fat-associated lymphoid clusters (FALCs) are a type of lymphoid tissue associated with visceral fat. Here we found that the distribution of FALCs was heterogeneous, with the pericardium containing large numbers of these clusters. FALCs contributed to the retention of B-1 cells in the peritoneal cavity through high expression of the chemokine CXCL13, and they supported B cell proliferation and germinal center differentiation during peritoneal immunological challenges. FALC formation was induced by inflammation, which triggered the recruitment of myeloid cells that expressed tumor-necrosis factor (TNF) necessary for signaling via the TNF receptors in stromal cells. Natural killer T cells (NKT cells) restricted by the antigen-presenting molecule CD1d were likewise required for the inducible formation of FALCs. Thus, FALCs supported and coordinated the activation of innate B cells and T cells during serosal immune responses