Intravital microscopy visualizes innate immune crosstalk and function in tissue microenvironment

Significant advances have been made in the field of intravital microscopy (IVM) on myeloid cells due to the growing number of validated fluorescent probes and reporter mice. IVM provides a visualization platform to directly observe cell behavior and deepen our understanding of cellular dynamics, heterogeneity, plasticity, and cell-cell communication in native tissue environments. This review outlines the current studies on the dynamic interaction and function of innate immune cells with a focus on those that are studied with IVM and covers the advances in data analysis with emerging artificial intelligence-based algorithms. Finally, the prospects of IVM on innate immune cells are discussed.

Activation of Mitogen-Activated Protein Kinase Regulates Eotaxin-Induced Eosinophil Migration

Eotaxin is a potent eosinophil chemoattractant that plays an important role in regulating eosinophil tissue levels both in healthy individuals and in diseases associated with significant eosinophil infiltrates, such as the allergic inflammation observed in asthma. Here, we demonstrate that treatment of eosinophils with eotaxin induces the phosphorylation of the mitogen-activated protein kinases (MAPKs) p42 and p44, leading to kinase activation. Blockade of MAPK activation by the MAPK kinase inhibitor PD98059 leads to a dramatic decrease in eotaxin-induced eosinophil rolling in vivo and chemotaxis in vitro. This blockade in the leukocyte migration process is consistent with the observed inhibition of actin polymerization and rearrangement within the eosinophil following treatment with MAPK inhibitor. It is suggested, therefore, that the intrinsic mechanism of eotaxin-induced eosinophil rolling and migration involves activation of the p42/p44 MAPK, possibly through regulation of the cytoskeletal rearrangements necessary for chemotaxis.

Characterization of Eosinophil Adhesion to TNF-α-Activated Endothelium Under Flow Conditions: α4 Integrins Mediate Initial Attachment, and E-Selectin Mediates Rolling

The multistep model of leukocyte adhesion reveals that selectins mediate rolling interactions and that integrins mediate firm adhesion processes. In this study, the interaction between eosinophils and TNF-α-activated HUVEC (second or third passage) was studied under flow conditions (0.8 and 3.2 dynes/cm2). Especially the role of α4 integrins on eosinophils and E-selectin on HUVEC was studied. Inhibition of the integrin α4 chain on eosinophils reduced the number of firmly adhered resting eosinophils to TNF-α-stimulated endothelium by 43% whereas the percentage rolling cells increased 2.2-fold compared with untreated control eosinophils. Blocking of E-selectin on the endothelium reduced the number of adherent eosinophils by only 23% and 16%. In this situation, however, hardly any rolling adhesion was observed, and the few rolling cells showed a low rolling velocity. Blocking both α4 integrin on eosinophils and E-selectin on HUVEC reduced the number of adhered eosinophils by 95%. P-selectin did not significantly participate in eosinophil adhesion to TNF-α-activated HUVEC. Inhibition of both α4 integrins and β2 integrins on eosinophils resulted in a reduction of adhered cells by 65% and a 3-fold increase in percentage rolling cells. Taken together, these results clearly show that resting eosinophils preferentially use constitutively active α4 integrins (α4β1, α4β7) for the first attachment to TNF-α-activated HUVEC. In addition, α4 integrins and E-selectin work synergistically in eosinophil adherence to TNF-α-activated HUVEC. Although E-selectin is important for eosinophil rolling under these conditions, P-selectin plays only a minor role.

A Comparison of C3a and C5a-Mediated Stable Adhesion of Rolling Eosinophils in Postcapillary Venules and Transendothelial Migration In Vitro and In Vivo

The comparative ability of the complement anaphylatoxins C3a and C5a to mediate leukocyte adhesion and transendothelial migration in vivo and in vitro was investigated. Superfusion of IL-1β-stimulated rabbit mesentery with C3a resulted in a rapid and stable adhesion of rolling eosinophils, but not neutrophils, to postcapillary venules. However, C3a failed to evoke subsequent transmigration of the adherent eosinophils. In contrast, C5a induced both the rapid activation-dependent firm adhesion and transmigration of eosinophils and neutrophils through venular endothelium. C3a induced selective shedding of L-selectin and an increase in αMβ2 integrin expression on eosinophils but not neutrophils, while C5a induced shedding of L-selectin and up-regulation of αMβ2 integrin on both eosinophils and neutrophils. Both C3a- and C5a-dependent adhesion to venular endothelium was blocked by ex vivo treatment of eosinophils with anti-α4 and anti-β2 integrin mAbs. In vitro, both C3a (but not C3adesArg) and C5a (including C5adesArg)-dependent transmigration of eosinophils across IL-1β-stimulated endothelial monolayer was mediated by α4β1 and αMβ2 integrins. Overall these studies suggest that C3a is eosinophil-specific chemotactic mediator that influences selectively eosinophil adhesion but not transmigration in vivo. C5a in contrast is a complete activator of integrin-dependent adhesion as well as transmigration of eosinophils and neutrophils.

Sequence Motifs in the Integrin α4 Cytoplasmic Tail Required for Regulation of In Vivo Expansion of Murine Lymphoma Cells

The binding of integrins to cognate ligands is tightly controlled by intracellular signals. Conversely, integrin occupancy generates biochemical signals inside the cell. The present study examined whether concepts of integrin function established by in vitro analysis apply to regulation of receptor function in complex biologic settings in vivo using a mouse model of tumor metastasis. Integrin α4 subunits were truncated at amino acid Gln1014 (A4-1014), preserving the conserved GFFKR motif, and at position Glu1021 (A4-1021). In vitro adhesion assays revealed that cytoplasmic tail truncations did not affect constitutive ligand binding of α4 integrins, while agonist-induced adhesion was abolished by the A4-1014, but not by the A4-1021, mutation. Inducible ligand binding of α4 integrins was dependent on cytoskeletal function, whereas constitutive adhesion was not. In vivo metastasis formation assays demonstrated that expansion of murine T lymphoma cells in spleen is strongly inhibited by the wild-type α4 subunit and the A4-1021 mutant. In contrast, the in vivo phenotype of α4 integrin expression in lymphoma cells was completely abrogated by the A4-1014 mutation. Cross-linking of α4 integrins in vitro inhibited proliferation and induced apoptosis of LB cells expressing wild-type α4 subunits or the A4-1021 mutant, but not of LB-A4-1014 cells. In summary, these results demonstrate that sequence motifs regulating cytoskeleton-dependent α4 integrin activation in vitro are essential for the control of LB lymphoma cell expansion both in vitro and in vivo.

Important Contributions of P-Selectin Glycoprotein Ligand-1-Mediated Secondary Capture to Human Monocyte Adhesion to P-Selectin, E-Selectin, and TNF-α-Activated Endothelium Under Flow In Vitro

In this study, an in vitro flow model and a blocking mAb to P-selectin glycoprotein ligand-1 (PSGL-1) were used to define the role of PSGL-1 in monocyte attachment and rolling on E- and P-selectin and in attachment and accumulation on 6-h TNF-α-activated HUVEC. KPL1, an adhesion-blocking mAb directed against the tyrosine sulfate motif of PSGL-1, abolished monocyte-adhesive interactions with P-selectin, but only partially blocked monocyte interaction with E-selectin. Further analysis showed that on E-selectin, KPL1 blocked only secondary (i.e., monocyte/monocyte) interactions, but did not block primary (i.e., monocyte/E-selectin) interactions, with secondary adhesion accounting for 90% of the total adhesive interactions on either E- or P-selectin. On cytokine-activated HUVEC, monocytes initially attached and formed linear strings of adherent cells, which involved both primary and secondary adhesion. PSGL-1 or L-selectin mAb reduced string formation, and the combination of PSGL-1 and L-selectin mAb prevented monocyte strings and inhibited 86% of accumulation. Monocyte attachment and rolling on purified adherent monocytes were also critically dependent on PSGL-1 on the adherent monocytes. These studies document that secondary interactions between monocytes, mediated by PSGL-1, are crucial for monocyte initial attachment, rolling, and accumulation on activated endothelium under laminar shear flow.

Contribution of Endothelial Selectins and α4 Integrins to Eosinophil Trafficking in Allergic and Nonallergic Inflammatory Reactions in Skin

The role of endothelial selectins in mediating eosinophil recruitment was assessed using the trafficking of 111In-labeled blood eosinophils in mouse skin. An intradermal injection of chemoattractants (leukotriene B4, macrophage inflammatory protein-1α, and eotaxin) resulted in a rapid accumulation of 111In eosinophils that was reduced 49 to 91% by anti-P-selectin mAb. An anti-E-selectin mAb was ineffective, although a combined E- and P-selectin blockade resulted in >95% inhibition of all responses. The accumulation of a pulse of 111In eosinophils at sites of active cutaneous anaphylaxis (ACA) at 4 to 8 h and at 20 to 24 h after Ag challenge was completely dependent upon E- and P-selectin in combination, but not in isolation. In contrast, at 20 to 24 h after Ag challenge in a delayed-type hypersensitivity (DTH) reaction in skin, 111In eosinophil accumulation was largely independent of endothelial selectins, even when L-selectin was also blocked. An anti-α4 integrin mAb significantly reduced 111In eosinophil trafficking in both allergic reactions but was slightly more effective in the DTH reaction compared with the ACA reaction. These results show that P-selectin and to a lesser extent E-selectin mediate eosinophil recruitment in skin in acute inflammatory reactions. In allergic, late-onset inflammatory reactions, neither P- nor E-selectin alone are sufficient to mediate eosinophil accumulation; when combined, they are essential for trafficking in ACA but are less important in the DTH reaction. Whether α4 integrin-based strategies will be more effective than selectin-based strategies at inhibiting eosinophil recruitment in human disease remains to be determined.

Intrinsic Differences in L-Selectin Expression Levels Affect T and B Lymphocyte Subset-Specific Recirculation Pathways

Lymphocyte migration into lymphoid organs is regulated by tissue-specific adhesion molecules such as L-selectin and the α4β7 integrin. Whether L-selectin also regulates lymphocyte subset-specific migration into specific lymphoid tissues was examined in this study by comparing the migration of CD4+ T cells, CD8+ T cells, and B cells from L-selectin-deficient and wild-type mice. T cells were the predominant lymphocyte subset entering PLN, MLN, Peyer’s patches, and spleen during short term (1-h) migration assays. However, both B cell and CD4+ and CD8+ T cell entries into PLN, MLN, and Peyer’s patches were dramatically impaired (73–98%) by loss of L-selectin. Lymphocyte expression of α4β7 integrin did not compensate for the loss of L-selectin, since both B and T cells predominantly migrated into the spleen in the absence of L-selectin. The more efficient migration of T cells into peripheral lymphoid tissues relative to that of B cells was partly explained by the finding that T cells expressed L-selectin at 50 to 100% higher levels than B cells. In addition, a 50% reduction in L-selectin expression by lymphocytes from hemizygous L-selectin+/− mice resulted in a 50 to 70% decrease in short term lymphocyte migration into peripheral lymphoid tissues relative to that of wild-type lymphocytes. Thus, the differential migration of T and B lymphocyte subsets to lymphoid tissues is regulated in part by subset-specific differences in L-selectin expression levels.

Human Eotaxin Induces α4 and β2 Integrin-Dependent Eosinophil Accumulation in Rat Skin In Vivo: Delayed Generation of Eotaxin in Response to IL-4

The CC chemokine eotaxin, originally purified from bronchoalveolar lavage fluid of sensitized guinea pigs following allergen challenge, is a potent eosinophil-selective chemoattractant. In the present study, we have used 111In-eosinophils and human eotaxin to characterize the profile of chemokine-induced eosinophil accumulation in vivo in rat skin. Intradermally injected eotaxin caused a dose-dependent accumulation of 111In-eosinophils. Time course studies indicated that the response was rapid, since all the accumulation occurred within the first 1 to 2 h of eotaxin injection. The i.v. administration of anti-intercellular adhesion molecule-1, anti-vascular cell adhesion molecule-1, or anti-α4 integrin mAbs significantly inhibited the eosinophil accumulation induced by 100 pmol of human eotaxin by 73, 43, and 67%, respectively. Further, when 111In-eosinophils were pretreated in vitro with anti-α4 integrin or anti-β2 integrin mAbs, or with a combination of both mAbs, eotaxin-induced responses in vivo were reduced by 52, 49, and 68%, respectively. Eosinophil accumulation induced by intradermal IL-4, but not that induced by TNF-α or leukotriene B4, appeared to be mediated in part by endogenously generated eotaxin. Anti-eotaxin Abs significantly inhibited (54%) the later phases (24–28 h) but not the early phase (0–4 h) of the response to IL-4. This was consistent with eotaxin mRNA expression peaking at 18 h after IL-4 injection. Our findings show that human eotaxin is a potent inducer of eosinophil accumulation in vivo, this response being dependent on α4 integrin/vascular cell adhesion molecule-1 and β2 integrin/intercellular adhesion molecule-1 adhesion pathways. Further, the eosinophil accumulation in response to IL-4 is partly mediated by endogenously generated eotaxin.