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Endogenous PMN sialidase activity exposes activation epitope on CD11b/CD18 which enhances its binding interaction
Chiguang Feng1, Lei Zhang, Lama Almulki
1Center for Vaccine Development, Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Abstract:
Diapedesis is a dynamic, highly regulated process by which leukocytes are recruited to inflammatory sites. We reported previously that removal of sialyl residues from PMNs enables these cells to become more adherent to EC monolayers and that sialidase activity within intracellular compartments of resting PMNs translocates to the plasma membrane following activation. We did not identify which surface adhesion molecules were targeted by endogenous sialidase. Upon activation, β2 integrin (CD11b/CD18) on the PMN surface undergoes conformational change, which allows it to bind more tightly to the ICAM-1 and ICAM-2 on the EC surface. Removal of sialyl residues from CD18 and CD11b, by exogenous neuraminidase or mobilization of PMN sialidase, unmasked activation epitopes, as detected by flow cytometry and enhanced binding to ICAM-1. One sialidase isoform, Neu1, colocalized with CD18 on confocal microscopy. Using an autoperfused microflow chamber, desialylation of immobilized ICAM-1 enhanced leukocyte arrest in vivo. Further, treatment with a sialidase inhibitor in vivo reversed endotoxin-induced binding of leukocytes to ICAM-1, thereby suggesting a role for leukocyte sialidase in the cellular arrest. These data suggest that PMN sialidase could be a physiologic source of the enzymatic activity that removes sialyl residues on β2 integrin and ICAM-1, resulting in their enhanced interaction. Thus, PMN sialidase may be an important regulator of the recruitment of these cells to inflamed sites.
Insights
Neutrophil sialidase activity removes sialyl residues from leukocyte adhesion molecules, enhancing their interaction and promoting leukocyte recruitment to inflammatory sites. This enzyme may be a key regulator of inflammation.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Leukocyte recruitment to inflammatory sites (diapedesis) is a critical immune process.
- Previous studies indicated that removing sialyl residues from neutrophils (PMNs) increases their adhesion to endothelial cells (ECs).
- The specific surface adhesion molecules targeted by endogenous PMN sialidase were previously unidentified.
Purpose of the Study:
- To identify the surface adhesion molecules targeted by endogenous PMN sialidase.
- To investigate the role of PMN sialidase in leukocyte adhesion and recruitment.
- To determine if PMN sialidase activity regulates the interaction between β2 integrin and ICAMs.
Main Methods:
- Flow cytometry to detect activation epitopes on β2 integrin (CD11b/CD18) after desialylation.
- Confocal microscopy to assess the colocalization of sialidase isoform Neu1 with CD18.
- Autoperfused microflow chamber for in vivo leukocyte arrest studies.
- In vivo administration of a sialidase inhibitor to assess its effect on leukocyte binding.
Main Results:
- Removal of sialyl residues from CD18 and CD11b unmasked activation epitopes and enhanced binding to ICAM-1.
- The sialidase isoform Neu1 was found to colocalize with CD18.
- Desialylation of ICAM-1 in vivo enhanced leukocyte arrest, and sialidase inhibition reversed endotoxin-induced leukocyte binding.
Conclusions:
- PMN sialidase is likely the physiologic source of enzymatic activity removing sialyl residues from β2 integrin and ICAM-1.
- This desialylation enhances the interaction between β2 integrin and ICAM-1, promoting leukocyte adhesion.
- PMN sialidase may serve as a critical regulator of leukocyte recruitment to inflamed tissues.
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