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Cell-surface calreticulin initiates clearance of viable or apoptotic cells through trans-activation of LRP on the phagocyte
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2005 (Engelska)Ingår i: Cell, ISSN 0092-8674, E-ISSN 1097-4172, Vol. 123, nr 2, s. 321-334Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Apoptotic-cell removal is critical for development, tissue homeostasis, and resolution of inflammation. Although many candidate systems exist, only phosphatidylserine has been identified as a general recognition ligand on apoptotic cells. We demonstrate here that calreticulin acts as a second general recognition ligand by binding and activating LDL-receptor-related protein (LRP) on the engulfing cell. Since surface calreticulin is also found on viable cells, a mechanism preventing inadvertent uptake was sought. Disruption of interactions between CD47 (integrin-associated protein) on the target cell and SIRPalpha (SHPS-1), a heavily glycosylated transmembrane protein on the engulfing cell, permitted uptake of viable cells in a calreticulin/LRP-dependent manner. On apoptotic cells, CD47 was altered and/or lost and no longer activated SIRPalpha. These changes on the apoptotic cell create an environment where "don't eat me" signals are rendered inactive and "eat me" signals, including calreticulin and phosphatidylserine, congregate together and signal for removal.

Ort, förlag, år, upplaga, sidor
2005. Vol. 123, nr 2, s. 321-334
Nyckelord [en]
Animals, Antibodies; Monoclonal/metabolism, Antigens; CD47/metabolism, Apoptosis/genetics/*immunology, Calreticulin/*metabolism, Cell Line, Embryo, Fibroblasts/cytology/metabolism, Heterozygote, Humans, Jurkat Cells, Macrophages/immunology, Mice, Mice; Knockout, Microscopy; Video, Models; Biological, Neutrophils/cytology/metabolism, Phagocytes/*immunology, Phagocytosis/genetics/immunology, Receptors; Immunologic/genetics/*metabolism, Trans-Activation (Genetics)
Identifikatorer
URN: urn:nbn:se:umu:diva-12284DOI: 10.1016/j.cell.2005.08.032PubMedID: 16239148OAI: oai:DiVA.org:umu-12284DiVA, id: diva2:151955
Tillgänglig från: 2008-01-11 Skapad: 2008-01-11 Senast uppdaterad: 2018-06-09Bibliografiskt granskad
Ingår i avhandling
1. Mechanisms involved in macrophage phagocytosis of apoptotic cells
Öppna denna publikation i ny flik eller fönster >>Mechanisms involved in macrophage phagocytosis of apoptotic cells
2009 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Efficient removal of apoptotic cells is critical for development, tissue remodelling, maintenance of homeostasis, and response to injury. Phagocytosis of apoptotic cells is mediated by many phagocytic receptors, soluble bridging molecules, and pro-phagocytic ligands on the surface of apoptotic cells. Macrophage phagocytosis in general is controlled by stimulatory and inhibitory mechanisms. An example of the latter mechanism is that mediated by the cell surface glycoprotein CD47, which by binding to the inhibitory receptor Signal Regulatory Protein alpha (SIRPα) on macrophages, is known to inhibit phagocytosis of viable host cells. The studies of the present thesis aimed at investigating possible changes to CD47 on apoptotic cells, which could influence their elimination by macrophages.

The endoplasmatic protein calreticulin (CRT), in conjunction with Low density lipoprotein Receptorrelated Protein 1 (LRP1) on the phagocyte, can act as a receptor for collectin family members and mediate uptake of apoptotic cells. However, CRT itself was found to also be expressed on the surface of many viable cell types, and the CRT expression increased on apoptotic cells. By using antibodies to LRP1 or receptor‐associated protein (RAP), an antagonist blocking LRP1 ligand binding, we found that CRT on target cells could interact in trans with LRP1 on a phagocyte and stimulate phagocytosis. CD47 on the target cell inhibited LRP1‐mediated phagocytosis of viable cells (e.g. lymphocytes or erythtocytes), but not that of apoptotic cells. The inability of CD47 on apoptotic cells to inhibit LRP1‐ mediated phagocytosis could be explained in two ways: 1) Some apoptotic cell types (fibroblasts and neutrophils, but not Jurkat T cells) lost CD47 from the cell surface, or 2) CD47 is evenly distributed on the surface of viable cells, while it was redistributed into patches on apoptotic cells, segregated away from areas of the plasma membrane where the pro‐phagocytic ligands CRT and phoaphatidylserine (PS) were concentrated. Apoptotic murine thymocytes also showed a patched distribution of CD47, but no significant loss of the receptor. However, both PS‐independent and PS‐dependent macrophage phagocytosis of apoptotic CD47‐/‐ thymocytes was less efficient than uptake of apoptotic wild‐type (wt) thymocytes. This contradictory finding was explained by the fact that CD47 on apoptotic thymocytes did no longer inhibit phagocytosis, but rather mediated binding of the apoptotic cell to the macrophage. These effects could in part be dependent on the apoptotic cell type, since uptake of experimentally senescent PS+ wt or CD47‐/‐ erythrocytes by macrophage in vitro, or by dendritic cells (DC) in vivo, were the same. In vivo, PS+ erythrocytes were predominantly trapped by marginal zone macrophages and by CD8+ CD207+ DCs in the splenic marginal zone. DCs which had taken up PS+ erythrocytes showed a slight increase in expression levels of CD40, CD86 and MHC class II. These findings suggest that PS+ erythrocytes may be recognized by splenic macrophages and DCs in ways similar to that reported for apoptotic T cells. Uptake of senescent erythrocytes by DCs may serve as an important mechanism to maintain self‐tolerance to erythrocyte antigens, and defects in this function may facilitate development of AIHA.

Glucocorticoids are used to treat inflammatory conditions and can enhance macrophage uptake of apoptotic cells. We found that the glucocorticoid dexamethasone time‐ and dose‐dependently stimulated macrophage cell surface LRP1 expression. Dexamethasone‐stimulated macrophages also showed enhanced phagocytosis of apoptotic thymocytes and unopsonized viable CD47‐/‐ erythrocytes.

In summary, LRP1 can mediate phagocytosis of both viable and apoptotic cells by binding CRT on the target cell. Macrophage expression of LRP1 is increased by glucocorticoids, which could be one explanation for the anti‐inflammatory role of glucocorticoids. While CD47 on viable cells efficiently inhibits phagocytosis in macrophages, CD47 on apoptotic cells does not and can sometimes even promote their removal.

Ort, förlag, år, upplaga, sidor
Umeå: Umeå university, 2009. s. 50
Serie
Umeå University medical dissertations, ISSN 0346-6612 ; 1307
Nyckelord
macrophages, dendritic cells, phagocytosis, apoptotic cells, senescent erythrocytes, CD47, SIRPalpha, LRP1, calreticulin
Nationell ämneskategori
Immunologi
Forskningsämne
immunologi
Identifikatorer
urn:nbn:se:umu:diva-29869 (URN)978-91-7264-890-6 (ISBN)
Disputation
2009-12-17, BiA201 Biologihuset, Umeå Universitet, Umeå, 00:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2009-11-27 Skapad: 2009-11-25 Senast uppdaterad: 2012-03-06Bibliografiskt granskad

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