The effect of manipulating apoptotic cell uptake on their immunogenicity

Abstract

Failure of the immune system to differentiate between apoptotic tumour cells and cells rendered apoptotic as part of homeostasis prevents a successful response being mounted against tumours. Apoptotic cells are cleared rapidly by professional phagocytes thus preventing the release of potentially inflammatory or immunogenic material into the surrounding environment. In addition, macrophages release immuno-suppressive cytokines such as tumour growth factor ? (TGF-?) and interleukin-10 (IL-10) that dampen the initiation of cytotoxic T cell (CTL) immunity and render T cells tolerant. It was hypothesised that manipulation of this clearance pathway, which is normally employed to prevent autoimmunity, may alleviate immuno-suppression of apoptotic tumour cells and promote the expansion of a tumour antigen-specific T cell response.
Milk fat globule – epidermal growth factor 8 (MFG-E8) is a glycoprotein secreted by activated macrophages and immature dendritic cells. It facilitates the uptake of apoptotic cells by acting as a bridging molecule between integrins on the phagocyte and phosphatidylserine (PS) on the apoptotic cell surface. Here, two recombinant dominant-negative MFG-E8 proteins were generated: one which is shown to inhibit PS-dependent apoptotic cell uptake by macrophages by over 40% (DN-MFG-E8), and a second which re-directed apoptotic cells through Fc? receptors and conferred enhanced phagocytosis by both macrophages and immature dendritic cells (DN-MFG-E8-Fc) in a dose-dependent manner.
Cross-presentation of cell-associated antigen by bone marrow-derived dendritic cells (BMDCs) was determined by CD8+ T cell proliferation assays in vitro and in vivo. Loading BMDCs with apoptotic cells via a PS-independent pathway or through Fc? receptors (Fc?Rs) reduced their ability to induce CD8+ T cell proliferation in vivo, suggesting the blockade of a mechanism which is intrinsic for DC maturation or migration. Similarly, the balance between activating and inhibitory Fc?Rs proved essential for effective DC maturation. Apoptotic cells treated with DN-MFG-E8-Fc protein resulted in upregulation of costimulatory molecules, CD86 and CD70, when BMDCs were deficient for the inhibitory Fc?R, Fc?RIIb.
Additionally, the immunosuppressive effect of apoptotic cells on antibody production proved dependent on the exposure of PS; whereby both DN-MFG-E8 and DN-MFG-E8-Fc proteins were shown to alleviate this suppression.

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Identifiers

ORCID for Aymen Al-Shamkhani: orcid.org/0000-0003-0727-4189

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