Leukocyte migration in the interstitial space of non-lymphoid organs

Journal name:
Nature Reviews Immunology
Volume:
14,
Pages:
232–246
Year published:
DOI:
doi:10.1038/nri3641
Published online

Abstract

Leukocyte migration through interstitial tissues is essential for mounting a successful immune response. Interstitial motility is governed by a vast array of cell-intrinsic and cell-extrinsic factors that together ensure the proper positioning of immune cells in the context of specific microenvironments. Recent advances in imaging modalities, in particular intravital confocal and multi-photon microscopy, have helped to expand our understanding of the cellular and molecular mechanisms that underlie leukocyte navigation in the extravascular space. In this Review, we discuss the key factors that regulate leukocyte motility within three-dimensional environments, with a focus on neutrophils and T cells in non-lymphoid organs.

At a glance

Figures

  1. Polarization of cytoskeletal dynamics governs leukocyte shape and migration.
    Figure 1: Polarization of cytoskeletal dynamics governs leukocyte shape and migration.

    A schematic representation of a polarized leukocyte migrating directionally in a three-dimensional environment using a pseudopodial amoeboid mode. A balance of expansive and contractile forces regulates the shape of the leukocyte. Contractile forces are localized at the rear and mid-body of the cell, whereas expansive forces at the leading edge give rise to protrusions. In the uropod, actomyosin contractility, driven by the small GTPase RHOA, helps to propel the cell body forward via cytosolic flow, and can disengage molecular clutch mechanisms that link the cell cortex to the extracellular matrix (ECM) via integrins. Polarization of CD44 to the uropod recruits ezrin, radixin and moesin (ERM) proteins that strengthen cortical integrity, ensuring that protrusion formation is not favoured at the rear of the cell. In the mid-body, actomyosin contractility can distort the shape of the nucleus in order for the cell to traverse restrictive pore sizes of the ECM. At the leading edge, pseudopodia intercalate the porous network of the ECM. The small GTPases RAC and CDC42 activate the ARP2/3 complex, which drives the polymerization of branched actin networks. The actin filaments are oriented such that the addition of monomers generates protrusive forces that extend the plasma membrane. Hydrostatic pressure generated by distal actomyosin contractility can extend the leading edge of cells into blebs, however, the principle of blebbing migration remains to be proven in leukocytes. In confined environments, pseudopodia can generate enough friction to sustain forward propulsion of the cell body, via intercalation of the ECM. Nascent adhesion sites in the pseudopodia, mainly integrin-based, can facilitate leukocyte motility, but are dispensable. MTOC, microtubule-organizing centre.

  2. The perivascular extravasation unit assists in neutrophil emigration.
    Figure 2: The perivascular extravasation unit assists in neutrophil emigration.

    a | Schematic representation of the perivascular extravasation unit (PVEU), consisting of endothelial cells, the basement membrane, pericytes, perivascular macrophages and mast cells. After firm adherence in post-capillary venules, neutrophils transmigrate through the endothelial cell monolayer. Following transendothelial migration, neutrophils crawl along pericytes that are negative for the chondroitin sulphate proteoglycan NG2 (NG2 pericyte). This abluminal crawling is dependent on intercellular adhesion molecule 1 (ICAM1), leukocyte function-associated antigen 1 (LFA1) and MAC1. Abluminally crawling neutrophils extravasate from regions of low extracellular matrix protein densities in the basement membrane (low expression regions). Perivascular macrophages may assist in the formation of the extravasation 'hot spots' due to their ability to produce large quantities of chemokines. Mast cells present in close proximity to the blood vessel wall may also respond to danger signals with the release of chemoattractive factors. b | High magnification schematic representing abluminal crawling of extravasating neutrophils along NG2 pericytes in an ICAM1-, LFA1- and MAC1-dependent manner. c | Cropped confocal image of dermal whole mount from a DPE-GFP mouse (which expresses green fluorescent protein (GFP) under the control of the Cd4 enhancers and promoter). The association of perivascular macrophages (green; marked by GFP expression) with pericytes (red; stained with α-smooth muscle actin-specific antibody) and endothelial cells (blue; stained with CD31-specific antibody) is shown. PSGL1, P-selectin glycoprotein ligand 1.

  3. Three-step cascade guides neutrophils to the site of sterile injury in the skin.
    Figure 3: Three-step cascade guides neutrophils to the site of sterile injury in the skin.

    A schematic representation of the individual phases of neutrophil attraction towards tissue injury sites. Step 1: following an insult, scarce 'scouting' neutrophils are attracted towards the injury site. Step 2: activated 'scouts' then amplify the response by producing leukotriene B4 (LTB4), which enables additional neutrophil recruitment from more distal regions. Neutrophils transmigrating from the vasculature can also be guided to the site of injury by NG2+ pericytes, which are present around capillaries and arterioles and express macrophage migration inhibitory factor. Neutrophil accumulation at the injury site modifies the extracellular matrix (ECM), resulting in an ECM-free region in the centre of the growing neutrophil cluster. Step 3: large numbers of neutrophils form stabilized clusters thereby containing tissue injury. At this stage, further attraction of neutrophils ceases. Monocytes are present in the periphery of the clusters. Some neutrophils migrate away from clusters to the draining lymphatics, whereas others may also enter the vasculature and then travel to distant organs. Tracks (arrows and lighter coloured cells) for a few neutrophils are shown to highlight the migration pattern of individual cells in the interstitial space. DAMPs, damage-associated molecular patterns; PAMPs, pathogen-associated molecular patterns.

  4. Context-dependent mechanisms of neutrophil attraction to injury sites.
    Figure 4: Context-dependent mechanisms of neutrophil attraction to injury sites.

    a | Model depicting the cascades of chemoattractive gradients, which mediate the centripetal motion of neutrophils towards injury sites via signal relay. Evidence was compiled from several studies in various models51, 53, 57, 60, 63, 66 to indicate the biochemical basis underlying the proposed multistep attraction cascade. The formation of distinct gradients includes damage-associated molecular patterns (DAMPs), for example, purinergic nucleotides or formylated peptides (such as fMLP), that are released directly at the injury site, chemokines produced by tissue-resident cells or attracted leukocytes, and lipid mediators, such as leukotriene B4 (LTB4), released by neutrophils themselves. Interpretation of these gradients by intracellular signalling molecules allows neutrophil guidance towards the damage focus. b | The production of hydrogen peroxide (H2O2) by epithelial cells establishes a chemotactic gradient for neutrophils during the wound-healing response in zebrafish63. c | The schematic depicts a mechanism for the recruitment of neutrophils to the site of sterile hepatic injury57. Neutrophils initially migrate along intravascular chemokine gradients in the liver sinusoids, followed by an abrupt switch to fMLP gradient sensing close to the necrotic focus. MIF, macrophage migration inhibitory factor; PAMP, pathogen-associated molecular pattern.

  5. CD8+ effector T cell behaviour in the tumour microenvironment.
    Figure 5: CD8+ effector T cell behaviour in the tumour microenvironment.

    Schematic representation of cytotoxic T lymphocyte (CTL) behaviour in the tumour microenvironment. a | CTLs enter the tumour bed through blood vessels in the periphery of solid cancers. Upon entering the tumour, T cells contact extracellular matrix (ECM) fibres, which potentially serve as haptotactic migration scaffolds. Migrating CTLs also can be repelled by dense ECM fibre bundles around tumour cell nests. CTLs scan both antigen-presenting cells (APCs) and tumour cells for the presence of cognate antigen. Serial contacts of CD8+ T cells with APCs can lead to their activation as shown by nuclear factor of activated T cells (NFAT) translocation into the nucleus. Regulatory T (TReg) cells potentially shorten the duration of these contacts, contributing to the development of tolerance. Activated T cells interact with cognate antigen-positive tumour cells, resulting in the formation of a lytic synapse. Release of cytoxic mediators into the cleft between a CTL and a tumour cell leads to tumour cell apoptosis. The apoptotic debris is phagocytosed by macrophages present in the tumour microenvironment. Tracks (arrows and lighter coloured cells) for a single CD8+ T cell are shown to highlight the migration pattern in the interstitial space. b | Schematic representation of potential migratory dynamics of CD8+ T cells undergoing Brownian walk movement, or — as recently demonstrated in infected brains — Lévy walk behaviour. Lévy walks facilitate the scanning of a larger tissue area by CD8+ T cells. FASL, FAS ligand; ICAM1, intercellular adhesion molecule 1; LFA1, lymphocyte function-associated antigen 1; TCR, T cell receptor.

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Affiliations

  1. Centenary Institute for Cancer Medicine and Cell Biology, Newtown, New South Wales 2042, Australia.

    • Wolfgang Weninger,
    • Maté Biro &
    • Rohit Jain
  2. Discipline of Dermatology, University of Sydney, New South Wales 2006, Australia.

    • Wolfgang Weninger
  3. Department of Dermatology, Royal Prince Alfred Hospital, Camperdown, New South Wales 2050, Australia.

    • Wolfgang Weninger
  4. Sydney Medical School, The University of Sydney, New South Wales 2006, Australia.

    • Maté Biro

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The authors declare no competing interests.

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Author details

  • Wolfgang Weninger

    Wolfgang Weninger received his training in clinical dermatology at the Department of Dermatology, University of Vienna Medical School, Austria (1992–1998). He then spent 4 years as a postdoctoral fellow at Harvard Medical School, USA. Between 2003 and 2007, he was a Faculty member at the Wistar Institute and the Department of Dermatology, University of Pennsylvania, Philadelphia, USA. In 2007, he was appointed Chair of the Discipline of Dermatology, Sydney Medical School, Australia, and Head of Department of Dermatology at Royal Prince Alfred Hospital, Australia. He also heads the Immune Imaging Program at the Centenary Institute, Australia. His research focuses on understanding the molecular basis of immune cell migration and interactions using advanced imaging technology.
    Wolfgang Weninger's homepage

  • Maté Biro

    Maté Biro received his Ph.D. at the Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany, in 2011. He previously studied physics (B.Sc.) and bioinformatics (M.Sc.) at Imperial College, London, UK, and did his masters research at Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. He also worked as a research associate at the Bioinformatics Institute of the A*STAR in Singapore. Since 2012, he has worked at the Centenary Institute and the University of Sydney, Australia. His research focuses on the regulation and dynamics of the cytoskeleton, notably during the migration of immune and cancer cells.
    Maté Biro's homepage

  • Rohit Jain

    Rohit Jain received his Masters in pharmaceutical biotechnology from the National Institute of Pharmaceutical Education and Research (NIPER), India, followed by his Ph.D. at the National Institute of Immunology, India. Since 2010, he has been a postdoctoral fellow at the Immune Imaging Program at the Centenary Institute, Australia. His ongoing research focuses on the study of host–pathogen interactions, with a particular emphasis on investigating pathogen-mediated immune evasion strategies using intravital imaging approaches.

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