Resident macrophage-dependent immune cell scaffolds drive anti-bacterial defense in the peritoneal cavity.

Peritoneal immune cells reside unanchored within the peritoneal fluid in homeostasis. Here, we examined the mechanisms that control bacterial infection in the peritoneum using a mouse model of abdominal sepsis following intraperitoneal Escherichia coli infection. Whole-mount immunofluorescence and confocal microscopy of the peritoneal wall and omentum revealed that large peritoneal macrophages (LPMs) rapidly cleared bacteria and adhered to the mesothelium, forming multilayered cellular aggregates composed by sequentially recruited LPMs, B1 cells, neutrophils, and monocyte-derived cells (moCs). The formation of resident macrophage aggregates (resMφ-aggregates) required LPMs and thrombin-dependent fibrin polymerization. E. coli infection triggered LPM pyroptosis and release of inflammatory mediators. Resolution of these potentially inflammatory aggregates required LPM-mediated recruitment of moCs, which were essential for fibrinolysis-mediated resMφ-aggregate disaggregation and the prevention of peritoneal overt inflammation. Thus, resMφ-aggregates provide a physical scaffold that enables the efficient control of peritoneal infection, with implications for antimicrobial immunity in other body cavities, such as the pleural cavity or brain ventricles.

Cholinergic macrophages promote the resolution of peritoneal inflammation.

The non-neural cholinergic system plays a critical role in regulating immune equilibrium and tissue homeostasis. While the expression of choline acetyltransferase (ChAT), the enzyme catalyzing acetylcholine biosynthesis, has been well documented in lymphocytes, its role in the myeloid compartment is less understood. Here, we identify a significant population of macrophages (Mϕs) expressing ChAT and synthesizing acetylcholine in the resolution phase of acute peritonitis. Using Chat-GFP reporter mice, we observed marked upregulation of ChAT in monocyte-derived small peritoneal Mϕs (SmPMs) in response to Toll-like receptor agonists and bacterial infections. These SmPMs, phenotypically and transcriptionally distinct from tissue-resident large peritoneal macrophages, up-regulated ChAT expression through a MyD88-dependent pathway involving MAPK signaling. Notably, this process was attenuated by the TRIF-dependent TLR signaling pathway, and our tests with a range of neurotransmitters and cytokines failed to induce a similar response. Functionally, Chat deficiency in Mϕs led to significantly decreased peritoneal acetylcholine levels, reduced efferocytosis of apoptotic neutrophils, and a delayed resolution of peritonitis, which were reversible with exogenous ACh supplementation. Intriguingly, despite B lymphocytes being a notable ChAT-expressing population within the peritoneal cavity, Chat deletion in B cells did not significantly alter the resolution process. Collectively, these findings underscore the crucial role of Mϕ-derived acetylcholine in the resolution of inflammation and highlight the importance of the non-neuronal cholinergic system in immune regulation.

A Liver Capsular Network of Monocyte-Derived Macrophages Restricts Hepatic Dissemination of Intraperitoneal Bacteria by Neutrophil Recruitment.

The liver is positioned at the interface between two routes traversed by pathogens in disseminating infection. Whereas blood-borne pathogens are efficiently cleared in hepatic sinusoids by Kupffer cells (KCs), it is unknown how the liver prevents dissemination of peritoneal pathogens accessing its outer membrane. We report here that the hepatic capsule harbors a contiguous cellular network of liver-resident macrophages phenotypically distinct from KCs. These liver capsular macrophages (LCMs) were replenished in the steady state from blood monocytes, unlike KCs that are embryonically derived and self-renewing. LCM numbers increased after weaning in a microbiota-dependent process. LCMs sensed peritoneal bacteria and promoted neutrophil recruitment to the capsule, and their specific ablation resulted in decreased neutrophil recruitment and increased intrahepatic bacterial burden. Thus, the liver contains two separate and non-overlapping niches occupied by distinct resident macrophage populations mediating immunosurveillance at these two pathogen entry points to the liver.

Loss of B1 and marginal zone B cells during ovarian cancer.

Recent advances in immunotherapy have not addressed the challenge presented by ovarian cancer. Although the peritoneum is an "accessible" locus for this disease there has been limited characterization of the immunobiology therein. We investigated the ID8-C57BL/6J ovarian cancer model and found marked depletion of B1 cells from the ascites of the peritoneal cavity. There was also selective loss of the B1 and marginal zone B cell subsets from the spleen. Immunity to antigens that activate these subsets validated their loss rather than relocation. A marked influx of myeloid-derived suppressor cells correlated with B cell subset depletion. These observations are discussed in the context of the housekeeping burden placed on innate B cells during ovarian cancer and to foster consideration of B cell biology in therapeutic strategies to address this challenge.

Farnesol remodels the peritoneal cavity immune environment influencing Candida albicans pathogenesis during intra-abdominal infection.

Candida albicans is a lifelong member of the mycobiome causing mucosal candidiasis and life-threatening, systemic, and intra-abdominal disease in immunocompromised and transplant patients. Despite the clinical importance of intra-abdominal candidiasis with mortality rates between 40% and 70%, the contribution of fungal virulence factors and host immune responses to disease has not been extensively studied. Secretion of the quorum-sensing molecule, farnesol, acts as a virulence factor for C. albicans during systemic infection, while inducing local, protective innate immune responses in oral models of infection. Previously, we reported that farnesol recruits macrophages to the peritoneal cavity in mice, suggesting a role for farnesol in innate immune responses. Here, we expand on our initial findings, showing that farnesol profoundly alters the peritoneal cavity microenvironment promoting innate inflammation. Intra-peritoneal injection of farnesol stimulates rapid local death of resident peritoneal cells followed by recruitment of neutrophils and inflammatory macrophages into the peritoneal cavity and peritoneal mesothelium associated with an early increase in chemokines followed by proinflammatory cytokines. These rapid inflammatory responses to farnesol significantly increase morbidity and mortality of mice with intra-abdominal candidiasis associated with increased formation of peritoneal adhesions, despite similar rates of fungal clearance from the peritoneal cavity and retro-peritoneal organs. C. albicans ddp3Δ/ddp3Δ knockout and reconstituted strains recapitulate these findings. This indicates that farnesol may be detrimental to the host during intra-abdominal infections. Importantly, our results highlight a need to understand how C. albicans virulence factors modulate the host immune response within the peritoneum, an exceedingly common site of Candida infection.

CD11c identifies microbiota and EGR2-dependent MHCII+ serous cavity macrophages with sexually dimorphic fate in mice.

The murine serous cavities contain a rare and enigmatic population of short-lived F4/80lo MHCII+ macrophages but what regulates their development, survival, and fate is unclear. Here, we show that mature F4/80lo MHCII+ peritoneal macrophages arise after birth, but that this occurs largely independently of colonization by microbiota. Rather, microbiota specifically regulate development of a subpopulation of CD11c+ cells that express the immunoregulatory cytokine RELM-α, are reliant on the transcription factor EGR2, and develop independently of the growth factor CSF1. Furthermore, we demonstrate that intrinsic expression of RELM-α, a signature marker shared by CD11c+ and CD11c- F4/80lo MHCII+ cavity macrophages, regulates survival and differentiation of these cells in the peritoneal cavity in a sex-specific manner. Thus, we identify a previously unappreciated diversity in serous cavity F4/80lo MHCII+ macrophages that is regulated by microbiota, and describe a novel sex and site-specific function for RELM-α in regulating macrophage endurance that reveals the unique survival challenge presented to monocyte-derived macrophages by the female peritoneal environment.

The peritoneal cancer index as a predictor of complete cytoreduction at primary and interval cytoreductive surgery in advanced ovarian cancer.

OBJECTIVE: The peritoneal cancer index quantitatively assesses cancer distribution and tumor burden in the peritoneal cavity. The aim of this study is to evaluate the association between the peritoneal cancer index and completeness of surgical cytoreduction for ovarian cancer and to identify a cut-off above which complete cytoreduction is unlikely. METHODS: This is a single-center prospective cohort observational study. A total of 100 consecutive patients who underwent ovarian cancer surgery were included. Peritoneal cancer index scores prior to and after surgery were calculated, and a cut-off value for incomplete cytoreduction was identified using a receiver operator characteristic (ROC) curve. Surgical complexity, blood loss, length of surgery, and complications were analyzed and associations with the peritoneal cancer index score were evaluated. RESULTS: The overall median peritoneal cancer index score was 9.5 (range 0-36). The median age of the patients was 61 years (range 24-85). The most common stage was III (13% stage II, 53% stage III, 34% stage IV) and the most common histologic sub-type was high-grade serous (76% high-grade serous, 8% low-grade serous, 5% clear cell, 4% serous borderline, 2% endometrioid, 2% adult granulosa cell, 2% adenocarcinoma, 1% carcinosarcoma). Complete cytoreduction was achieved in 82% of patients, with a median score of 9 (range 0-30). The remaining 18% had a median score of 28.5 (range 0-36). The best predictor of incomplete cytoreduction was the peritoneal cancer index score, with an area under the curve (AUC) of 0.928 (95% CI 0.85 to 1.00). ROC curve analysis determined a peritoneal cancer index cut-off score of 20. Major complications occurred in 15% of patients with peritoneal cancer index scores >20 and in 2.5% of patients with scores ≤20, which was statistically significant (p=0.014). CONCLUSIONS: In our study we found that a peritoneal cancer index score of ≤20 was associated with a high likelihood of complete cytoreduction. Incorporating the peritoneal cancer index into routine surgical practice and research may impact treatment plans.

Revisiting the Use of Normal Saline for Peritoneal Washing in Ovarian Cancer.

The omentum is the predominant site of ovarian cancer metastasis, but it is difficult to remove the omentum in its entirety. There is a critical need for effective approaches that minimize the risk of colonization of preserved omental tissues by occult cancer cells. Normal saline (0.9% sodium chloride) is commonly used to wash the peritoneal cavity during ovarian cancer surgery. The omentum has a prodigious ability to absorb fluid in the peritoneal cavity, but the impact of normal saline on the omentum is poorly understood. In this review article, we discuss why normal saline is not a biocompatible solution, drawing insights from clinical investigations of normal saline in fluid resuscitation and from the cytopathologic evaluation of peritoneal washings. We integrate these insights with the unique biology of the omentum and omental metastasis, highlighting the importance of considering the absorptive ability of the omentum when administering agents into the peritoneal cavity. Furthermore, we describe insights from preclinical studies regarding the mechanisms by which normal saline might render the omentum conducive for colonization by cancer cells. Importantly, we discuss the possibility that the risk of colonization of preserved omental tissues might be minimized by using balanced crystalloid solutions for peritoneal washing.

Expression of Gal-9 on Dendritic Cells and Soluble Forms of TIM-3/Gal-9 in Patients Suffering from Endometriosis.

Immune system dysregulation is clinically evident in the pathogenesis of endometriosis (EMS). Changes in the dendritic cells (DCs) activity or phenotype may be involved in the implantation and growth of endometrial tissue outside the uterus in the disease. The TIM-3/Gal-9 axis is implicated in the development of immune tolerance. However, the knowledge about the exact role of this pathway in the EMS is extremely poor. In the present study, we evaluated the expression of Gal-9 on myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) in the peripheral blood (PB) and peritoneal fluid (PF) of both EMS patients (n = 82) and healthy subjects (n = 10) via flow cytometry. We also investigated the concentrations of soluble Gal-9 and TIM-3 in the plasma and PF of EMS patients and the control group using ELISA. We showed significantly elevated percentages of mDCs-Gal-9+ and pDCs-Gal-9+, and significantly higher concentrations of the soluble form of Gal-9 and TIM-3 in the PF of EMS patients than in circulation. Our results led us to conclude that the accumulation of Gal-9 expressing mDCs and pDCs in the PF and high sTIM-3/Gal-9 production in the peritoneal cavity could represent the hallmark of immune regulation in EMS patients, which may augment the inflammatory process and development/maintenance of local immunosuppression.

Immune Niche Within the Peritoneal Cavity.

There are numbers of leukocytes present in peritoneal cavity, not only protecting body cavity from infection but also contributing to peripheral immunity including natural antibody production in circulation. The peritoneal leukocytes compose unique immune compartment, the functions of which cannot be replaced by other lymphoid organs. Atypical lymphoid clusters, called "milky spots", that are located in visceral adipose tissue omentum have the privilege of immune niche in terms of differentiation, recruitment, and activation of peritoneal immunity, yet mechanisms underlying the regulation are underexplored. In this review, I discuss the emerging views of peritoneal immune system in the contexts of its development, organization, and functions.

Phenotypic and functional alterations of peritoneal macrophages in lupus-prone mice.

BACKGROUND: Several studies have demonstrated the contribution of innate immune cells, including macrophages, in promoting systemic lupus erythematosus (SLE). Macrophages, one of the most abundant cell populations in the peritoneal cavity, are considered multifunctional cells with phenotypic plasticity. However, the functional properties of peritoneal macrophages in steady-state and during the progression of SLE remain poorly defined. METHODS AND RESULTS: Using the [NZB × NZW]F1 (BWF1) murine model of SLE, we analyzed the phenotype and function of peritoneal macrophages during the disease's onset. We found a higher frequency of peritoneal macrophages and B1a cells in BWF1-diseased mice than age-matched controls. Additionally, macrophages from diseased animals expressed lower levels of CD206, MHC-II, and Sirpα. RNAseq analysis identified 286 differentially expressed genes in peritoneal macrophages from diseased-BWF1 mice compared to control mice. Functional experiments demonstrate that peritoneal macrophages from diseased-BWF1 mice secrete higher levels of pro-inflammatory cytokines when activated with TLR7 and TLR9 agonists, and they were less efficient in suppressing the activation and proliferation of peritoneal LPS-activated B cells. These data demonstrate that peritoneal macrophages from BWF1-diseased mice present phenotypic and functional alterations shifting to a more pro-inflammatory state. CONCLUSIONS: The increase of macrophages with an altered phenotype and function together with the accumulation of B1a cells in the peritoneal cavity of diseased-BWF1 mice may promote the progression of the disease. Advancing awareness of the role and phenotype of peritoneal macrophages in SLE may contribute to a better understanding of these types of diseases and the development of novel therapies.

Comparison of the effect of general anesthesia and spinal anesthesia technique combined with general anesthesia on intraabdominal volume during gynecological laparoscopy.

PURPOSE: We aimed to investigate the effect of spinal anesthesia which will be performed simultaneously with general anesthesia on the site of operation with the same pressure. MATERIAL AND METHOD: This study was conducted as a randomized, prospective clinical study on 40 patients who were randomly divided into two groups. Twenty women underwent general anesthesia (Group GA) and 20 women underwent spinal anesthesia with general anesthesia (Group SGA). For all cases, preoperative height, weight, waist circumference, body mass index (kg/m2 ), the distance between both spina iliaca anterior superior, the distance of the intersection of both ribs with an imaginary line drawn over the anterior axillary line, suprapubic bone-umbilical, umbilical-xiphoid, and suprapubic bone-xiphoid distance from the midline of the abdomen were measured. Moreover, while the patient was lying in the neutral position on the operating table, the height of the highest point of the abdomen to the operating table was also measured. These measurements were repeated at intra-abdominal pressure (IAP) 14 and 25 mmHg. The amount of intra-abdominal insufflated CO2 was also recorded at IAP 14 and 25 mmHg. RESULTS: When the intra-abdominal insufflation volumes of both groups were compared at 14 and 25 mmHg, respectively, there was no statistical difference (p: 0.54, p: 0.40). When 14 and 25 mmHg were compared in all cases, a statistically significant difference was observed in other measurements except in xiphoid-umbilical distance (p < 0.05). CONCLUSION: We found that spinal anesthesia combined with GA had no effect on the abdominal volume and anthropometric measurements in laparoscopic procedures.

Surgeons' requirements for a surgical support system to improve laparoscopic access.

Creating surgical access is a critical step in laparoscopic surgery. Surgeons have to insert a sharp instrument such as the Veress needle or a trocar into the patient's abdomen until the peritoneal cavity is reached. They solely rely on their experience and distorted tactile feedback in that process, leading to a complication rate as high as 14% of all cases. Recent studies have shown the feasibility of surgical support systems that provide intraoperative feedback regarding the insertion process to improve laparoscopic access outcomes. However, to date, the surgeons' requirements for such support systems remain unclear. This research article presents the results of an explorative study that aimed to acquire data about the information that helps surgeons improve laparoscopic access outcomes. The results indicate that feedback regarding the reaching of the peritoneal cavity is of significant importance and should be presented visually or acoustically. Finally, a solution should be straightforward and intuitive to use, should support or even improve the clinical workflow, but also cheap enough to facilitate its usage rate. While this study was tailored to laparoscopic access, its results also apply to other minimally invasive procedures.

Clinical Value of the PD-1/PD-L1/PD-L2 Pathway in Patients Suffering from Endometriosis.

The interaction between dendritic cells (DCs) and T cells mediated by the programmed cell death 1 (PD-1)/programmed cell death ligand 1 (PD-L1)/programmed cell death ligand 2 (PD-L2) pathway is the most important point in regulating immunological tolerance and autoimmunity. Disturbances in the quantity, maturity, and activity of DCs may be involved in the implantation and growth of endometrial tissue outside the uterus in endometriosis (EMS). However, little is known about the role of the immune checkpoint pathways in EMS. In our study, we examined the expression of PD-L1/PD-L2 on myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) in the peripheral blood (PB) and peritoneal fluid (PF) of both EMS patients (n = 72) and healthy subjects (n = 20) via flow cytometry. The concentration of soluble PD-L1 and PD-L2 in the plasma and PF of EMS patients and the control group were determined using ELISA. We demonstrated an elevated percentage of mDCs, mDCs and pDCs with the PD-L1or PD-L2 expression, and a higher concentration of the soluble forms of PD-L1 and PD-L2 in the PF than in the plasma of EMS patients. We conclude that the peritoneal cavity environment and the PD-1/PD-L1/PD-L2 axis may play an important role in the modulation of immune response and the development and/or progression of EMS.

Metabolite profiles in the peritoneal cavity of endometriosis patients and mouse models.

RESEARCH QUESTION: Which metabolites are altered in the peritoneal cavity of women with endometriosis? Could the mouse endometriosis model simulate these alterations? DESIGN: Thirteen women with endometriosis and seven women with other benign gynaecological diseases, who underwent laparoscopic surgery, were included in this study. None had received hormonal therapy for 3 months before surgery. For the animal experiments, six and five mice were included in the endometriosis and control groups, respectively. Peritoneal fluid from the patients and peritoneal lavage fluid from the mice was collected and analysed. Non-targeted metabolomics via liquid chromatography with tandem mass spectrometry was used to identify the altered metabolites in the peritoneal fluid of endometriosis patients and mouse models. MetaboAnalyst 4.0 was used to visualize the data. RESULTS: Several metabolites in the peritoneal cavity were significantly altered in both humans and mice with endometriosis. Concentrations of lysophosphatidylcholine (LysopC) (P=0.017 in patients and P=0.041 in the mouse model) and derivatives of phosphoethanolamine (1-arachidonoyl-sn-glycero-3-phosphoethanolamine in patients, P=0.027; 1-oleoyl-sn-glycero-3-phosphoethanolamine in patients, P=0.0086; and phosphorylethanolamine in the mouse model, P=0.0027) were significantly up-regulated in both, whereas concentrations of acylcarnitines (l-palmitoylcarnitine, P=0.047; and stearoylcarnitine, P=0.029) and kynurenine (P=0.045) were significantly increased only in humans. The human and mouse samples shared three altered enriched metabolite sets. CONCLUSIONS: Women with endometriosis show an altered metabolic state in the abdominal cavity. The endometriosis mouse model shared half of the significantly altered metabolite sets found in the abdominal cavity of humans.

Diagnostic performance of ultrasound in assessing the extension of the disease in patients with suspicion of malignant ovarian tumor: correlation between ultrasound parameters and Fagotti's score.

BACKGROUND: A radical surgical approach represents the mainstay treatment for gynecological malignancy, and preoperative staging of ovarian cancer is crucial. Ultrasound evaluation is widely recognized as the gold standard technique for the characterization of ovarian masses due to a high sensitivity for malignancy. In addition, its accuracy in defining intra-abdominal ovarian cancer spread has been previously proposed. PRIMARY OBJECTIVE: To analyze the agreement between preoperative ultrasound examination and laparoscopic findings in assessing the extension of intra-abdominal disease using six parameters as described by Fagotti's score. STUDY HYPOTHESIS: When performed by expert examiners, ultrasound can be an accurate technique to assess tumor spread in ovarian cancer and therefore to predict surgical resectability. TRIAL DESIGN: This is a single-center prospective observational study. Patients with clinical and/or radiological suspicion of advanced ovarian or peritoneal cancer will be assessed with preoperative ultrasound and assigned a score based on the six Fagotti's laparoscopic score parameters. Each parameter will then be correlated with laparoscopic findings. MAJOR INCLUSION/EXCLUSION CRITERIA: Eligible patients include women 18-75 years of age with clinical and/or imaging suggestive of advanced ovarian or peritoneal cancer, and an ECOG performance status 0-3. PRIMARY ENDPOINTS: Sensitivity and specificity of ultrasound in detecting carcinomatosis, using the parameters of Fagotti's score as a reference standard. Agreement between preoperative ultrasound examination and laparoscopic findings in assessing the extension of intra-abdominal disease as described in Fagotti's score. SAMPLE SIZE: 240 patients. ESTIMATE DATES FOR COMPLETING ACCRUAL AND PRESENTING RESULTS: The accrual started in January 2019. Enrollment should be completed approximately by October 2020 and the results will be analyzed by December 2020. TRIAL REGISTRATION: The study received the Ethical Committee approval on July 19 2018 (Protocol 28967/18 ID:2172).

CD83 expression regulates antibody production in response to influenza A virus infection.

BACKGROUND: CD83 is known to regulate lymphocyte maturation, activation, homeostasis, and antibody response to immunization and infection. While CD83 has a major part in B cell function, its role in influenza A virus infection has not yet been investigated. METHODS: We investigated the role of CD83 using C57BL/6J wild type mice and CD83 knockout (KO) mice after intraperitoneal administration of the influenza A/WSN/1933 virus. We analyzed cells of the peritoneal cavity, splenocytes, and cells of the bone marrow with FACS to investigate CD83 expression and cell population change in response to the virus infection. ELISA was performed with sera and peritoneal cavity fluids to detect A/WSN/1933 virus-specific IgG and the subclasses of IgG. RESULTS: FACS analysis data showed a transient but distinct induction of CD83 expression in the peritoneal B cells of wild type mice. CD83 KO mice exhibited a delayed recovery of B cells in the bone marrow after influenza virus infection and overall, a smaller T cell population compared to wild type mice. The peritoneal cavity and serum of the wild type mice contained a high titer of IgG within 14 days after infection, whereas the CD83 KO mice had a very low titer of IgG. CONCLUSIONS: These results show the importance of CD83 in lymphocytes homeostasis and antibody production during influenza A virus infection.

Intestinal-peritoneal Coelioscopy for the Diagnosis of a Mesothelioma in a Pet Chicken.

An adult female chicken, from a small backyard flock, was presented to the Ontario Veterinary College Avian and Exotics Service for evaluation of dyspnea and recurrent ascites. An antemortem diagnostic evaluation included a coelomocentesis, coelomic ultrasound, and a coelioscopy procedure. A sample of the fluid collected during the coelomocentesis was submitted for analysis and was determined to be a nonspecific modified proteinaceous transudate. The coelomic ultrasound examination identified numerous coalescing fluid-filled and solid nodules throughout the coelom. However, no site of origin of the nodules could be identified. A coelioscopy of the intestinal-peritoneal cavity was performed by a ventral midline approach, and biopsies collected during the procedure were submitted for histologic examination. The pathologic diagnosis of the biopsy samples was a disseminated neoplasia, presumptively coelomic adenocarcinoma. The chicken received palliative treatment which included periodic coelomocentesis, meloxicam, antibiotics, and deslorelin following the diagnosis of a disseminated neoplasia. Three months following initial presentation the patient was euthanatized. A postmortem examination with histopathology confirmed the tissue biopsy results of coelomic neoplasia. Further immunohistochemistry supported mesothelioma as the definitive diagnosis. This case documents the usefulness of intestinal-peritoneal coelioscopy in identifying neoplasia as the cause of ascites in a pet chicken as well as describing the clinical features and progression of a mesothelioma in this species.

The peritoneal "soil" for a cancerous "seed": a comprehensive review of the pathogenesis of intraperitoneal cancer metastases.

Various types of tumors, particularly those originating from the ovary and gastrointestinal tract, display a strong predilection for the peritoneal cavity as the site of metastasis. The intraperitoneal spread of a malignancy is orchestrated by a reciprocal interplay between invading cancer cells and resident normal peritoneal cells. In this review, we address the current state-of-art regarding colonization of the peritoneal cavity by ovarian, colorectal, pancreatic, and gastric tumors. Particular attention is paid to the pro-tumoral role of various kinds of peritoneal cells, including mesothelial cells, fibroblasts, adipocytes, macrophages, the vascular endothelium, and hospicells. Anatomo-histological considerations on the pro-metastatic environment of the peritoneal cavity are presented in the broader context of organ-specific development of distal metastases in accordance with Paget's "seed and soil" theory of tumorigenesis. The activity of normal peritoneal cells during pivotal elements of cancer progression, i.e., adhesion, migration, invasion, proliferation, EMT, and angiogenesis, is discussed from the perspective of well-defined general knowledge on a hospitable tumor microenvironment created by the cellular elements of reactive stroma, such as cancer-associated fibroblasts and macrophages. Finally, the paper addresses the unique features of the peritoneal cavity that predispose this body compartment to be a niche for cancer metastases, presents issues that are topics of an ongoing debate, and points to areas that still require further in-depth investigations.