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Role of CD47 in regulating macrophage and dendritic cell uptake of calcium-induced experimentally senescent erythrocytes in vitro and in vivo
Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
(English)Manuscript (preprint) (Other academic)
Abstract [en]

During senescence, erythrocytes are rapidly eliminated by phagocytes in the spleen and liver,and the interplay between macrophages or dendritic cells (DCs) with lymphocytes in the spleen could be of importance to regulate self-tolerance and to prevent development of autoimmune hemolytic anemia (AIHA).The cell surface glycoprotein CD47 on the surface of host target cells can inhibit phagocytosis of normal circulating blood cells by binding to signal regulatory protein alpha (SIRPα) on macrophages and DCs. Here we investigated the clearance of Ca2+-ionophore-treated experimentally senescent erythrocytes (Ca2+-RBCs) in vivo, and if CD47 on Ca2+-RBCs regulated their uptake by macrophages in vitro or their clearance in vivo. Ca2+-RBCs exposed phosphatidylserine (PS) on their surface and were rapidly phagocytosed by macrophages in vitro. This uptake was dependent on heat-stable serum factors, but was not inhibited by CD47 on the erythrocytes. Following intravenous injection, a large fraction of PKH26-labeled Ca2+-RBCs were trapped by splenic marginal zone macrophages and DCs within 1 hour. In addition, at 20 hours after injection fluorescence from the injected cells could also be detected within the T cell area of the splenic white pulp and associated with both CD8+ and CD8- DCs. We found that DCs in wild-type (Wt) recipient mice showed equal uptake of transfused Wt and CD47-/- Ca2+-RBCs. DCs which had taken up Ca2+-RBCs showed a slight increase in expression levels of CD40, CD86 and MHC class II. In recipients of CD47-/- Ca2+-RBCs, DCs negative for RBC-uptake showed strongly increased expression of CD86, as compared with that in recipients of Wt Ca2+-RBCs. These findings suggest that PS+ erythrocytes may be recognized by splenic macrophages and DCs in ways similar to that reported for nucleated apoptotic 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.

National Category
Immunology
Research subject
Immunology
Identifiers
URN: urn:nbn:se:umu:diva-29861OAI: oai:DiVA.org:umu-29861DiVA: diva2:278321
Available from: 2009-11-25 Created: 2009-11-25 Last updated: 2010-06-24Bibliographically approved
In thesis
1. Mechanisms involved in macrophage phagocytosis of apoptotic cells
Open this publication in new window or tab >>Mechanisms involved in macrophage phagocytosis of apoptotic cells
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Umeå: Umeå university, 2009. 50 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1307
Keyword
macrophages, dendritic cells, phagocytosis, apoptotic cells, senescent erythrocytes, CD47, SIRPalpha, LRP1, calreticulin
National Category
Immunology
Research subject
Immunology
Identifiers
urn:nbn:se:umu:diva-29869 (URN)978-91-7264-890-6 (ISBN)
Public defence
2009-12-17, BiA201 Biologihuset, Umeå Universitet, Umeå, 00:00 (English)
Opponent
Supervisors
Available from: 2009-11-27 Created: 2009-11-25 Last updated: 2012-03-06Bibliographically approved

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Oldenborg, Per-Arne

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