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.

A catalytically inactive gelatinase B/MMP-9 mutant impairs homing of chronic lymphocytic leukemia cells by altering migration regulatory pathways.

We previously showed that MMP-9 overexpression impairs migration of primary CLL cells and MEC-1 cells transfected with MMP-9. To determine the contribution of non-proteolytic activities to this effect we generated MEC-1 transfectants stably expressing catalytically inactive MMP-9MutE (MMP-9MutE-cells). In xenograft models in mice, MMP-9MutE-cells showed impaired homing to spleen and bone marrow, compared to cells transfected with empty vector (Mock-cells). In vitro transendothelial and random migration of MMP-9MutE-cells were also reduced. Biochemical analyses indicated that RhoAGTPase and p-Akt were not downregulated by MMP-9MutE, at difference with MMP-9. However, MMP-9MutE-cells or primary cells incubated with MMP-9MutE had significantly reduced p-ERK and increased PTEN, accounting for the impaired migration. Our results emphasize the role of non-proteolytic MMP-9 functions contributing to CLL progression.

Evaluation of the potential therapeutic benefits of macrophage reprogramming in multiple myeloma.

Tumor-associated macrophages (TAM) are important components of the multiple myeloma (MM) microenvironment that support malignant plasma cell survival and resistance to therapy. It has been proposed that macrophages (MØ) retain the capacity to change in response to stimuli that can restore their antitumor functions. Here, we investigated several approaches to reprogram MØ as a novel therapeutic strategy in MM. First, we found tumor-limiting and tumor-supporting capabilities for monocyte-derived M1-like MØ and M2-like MØ, respectively, when mixed with MM cells, both in vitro and in vivo. Multicolor confocal microscopy revealed that MM-associated MØ displayed a predominant M2-like phenotype in the bone marrow of MM patient samples, and a high expression of the pro-M2 cytokine macrophage migration inhibitory factor (MIF). To reprogram the protumoral M2-like MØ present in MM toward antitumoral M1-like MØ, we tested the pro-M1 cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) plus blockade of the M2 cytokines macrophage colony-stimulating factor or MIF. The combination of GM-CSF plus the MIF inhibitor 4-iodo-6-phenyl-pyrimidine achieved the best reprogramming responses toward an M1 profile, at both gene and protein expression levels, as well as remarkable tumoricidal effects. Furthermore, this combined treatment elicited MØ-dependent therapeutic responses in MM xenograft mouse models, which were linked to upregulation of M1 and reciprocal downregulation of M2 MØ markers. Our results reveal the therapeutic potential of reprogramming MØ in the context of MM.

Whole-mount immunofluorescence imaging and isolation of mesothelium-bound immune cell aggregates during mouse peritoneal inflammation.

Resident peritoneal macrophages (resMØs) are crucial for repairing peritoneal injuries and controlling infections by forming mesothelium-bound resMØ-aggregates in the peritoneal wall and omentum. Here we present a protocol to analyze these structures in mouse models of peritoneal inflammation. We describe the dissection, fixation, immunofluorescent staining, and mounting of whole peritoneal wall and omentum samples and subsequent confocal microscopy imaging of resMØ-aggregates. We also detail the steps to isolate resMØ-aggregates for additional studies, including flow cytometry and electron-microscopy-based analysis. For complete details on the use and execution of this protocol, please refer to Vega-Pérez et al. (2021).1.

Mouse Tissue-Resident Peritoneal Macrophages in Homeostasis, Repair, Infection, and Tumor Metastasis.

Large peritoneal macrophages (LPMs) are long-lived, tissue-resident macrophages, formed during embryonic life, developmentally and functionally confined to the peritoneal cavity. LPMs provide the first line of defense against life-threatening pathologies of the peritoneal cavity, such as abdominal sepsis, peritoneal metastatic tumor growth, or peritoneal injuries caused by trauma, or abdominal surgery. Apart from their primary phagocytic function, reminiscent of primitive defense mechanisms sustained by coelomocytes in the coelomic cavity of invertebrates, LPMs fulfill an essential homeostatic function by achieving an efficient clearance of apoptotic, that is crucial for the maintenance of self-tolerance. Research performed over the last few years, in mice, has unveiled the mechanisms by which LPMs fulfill a crucial role in repairing peritoneal injuries and controlling microbial and parasitic infections, reflecting that the GATA6-driven LPM transcriptional program can be modulated by extracellular signals associated with pathological conditions. In contrast, recent experimental evidence supports that peritoneal tumors can subvert LPM metabolism and function, leading to the acquisition of a tumor-promoting potential. The remarkable functional plasticity of LPMs can be nevertheless exploited to revert tumor-induced LPM protumor potential, providing the basis for the development of novel immunotherapeutic approaches against peritoneal tumor metastasis based on macrophage reprogramming.

MMP-9 affects gene expression in chronic lymphocytic leukemia revealing CD99 as an MMP-9 target and a novel partner in malignant cell migration/arrest.

We previously showed that MMP-9 contributes to CLL pathology by regulating cell survival and migration and that, when present at high levels, MMP-9 induces cell arrest. To further explore the latter function, we studied whether MMP-9 influences the gene-expression profile in CLL. Microarray analyses rendered 131 differentially expressed genes in MEC-1 cells stably transfected with MMP-9 (MMP-9-cells) versus cells transfected with empty vector (Mock-cells). Ten out of twelve selected genes were also differentially expressed in MEC-1 cells expressing the catalytically inactive MMP-9MutE mutant (MMP-9MutE-cells). Incubation of primary CLL cells with MMP-9 or MMP-9MutE also regulated gene and protein expression, including CD99, CD226, CD52, and CD274. Because CD99 is involved in leukocyte transendothelial migration, we selected CD99 for functional and mechanistic studies. The link between MMP-9 and CD99 was reinforced with MMP-9 gene silencing studies, which resulted in CD99 upregulation. CD99 gene silencing significantly reduced CLL cell adhesion, chemotaxis and transendothelial migration, while CD99 overexpression increased cell migration. Mechanistic analyses indicated that MMP-9 downregulated CD99 via binding to α4ß1 integrin and subsequent inactivation of the Sp1 transcription factor. This MMP-9-induced mechanism is active in CLL lymphoid tissues, since CD99 expression and Sp1 phosphorylation was lower in bone marrow-derived CLL cells than in their peripheral blood counterparts. Our study establishes a new gene regulatory function for MMP-9 in CLL. It also identifies CD99 as an MMP-9 target and a novel contributor to CLL cell adhesion, migration and arrest. CD99 thus constitutes a new therapeutic target in CLL, complementary to MMP-9.

CCL20 Expression by Tumor-Associated Macrophages Predicts Progression of Human Primary Cutaneous Melanoma.

The chemokine axis CCR6/CCL20 is involved in cancer progression in a variety of tumors. Here, we show that CCR6 is expressed by melanoma cells. The CCR6 ligand, CCL20, induces migration and proliferation in vitro, and enhances tumor growth and metastasis in vivo Confocal analysis of melanoma tissues showed that CCR6 is expressed by tumor cells, whereas CCL20 is preferentially expressed by nontumoral cells in the stroma of certain tumors. Stromal CCL20, but not tumoral CCR6, predicted poor survival in a cohort of 40 primary melanoma patients. Tumor-associated macrophages (TAM), independently of their M1/M2 polarization profile, were identified as the main source of CCL20 in primary melanomas that developed metastasis. In addition to CCL20, TAMs expressed TNF and VEGF-A protumoral cytokines, suggesting that melanoma progression is supported by macrophages with a differential activation state. Our data highlight the synergistic interaction between melanoma tumor cells and prometastatic macrophages through a CCR6/CCL20 paracrine loop. Stromal levels of CCL20 in primary melanomas may be a clinically useful marker for assessing patient risk, making treatment decisions, and planning or analyzing clinical trials. Cancer Immunol Res; 6(3); 267-75. ©2018 AACR.