Macrophage Response to Apoptotic Cells Varies with the Apoptotic Trigger and Is Not Altered by a Deficiency in LRP Expression

Abstract

Rapid engulfment of apoptotic cells in the absence of inflammation is required for maintenance of normal tissue homeostasis. The low-density lipoprotein receptor-related protein-1 (LRP/CD91) is a receptor mediating interactions between macrophages and apoptotic cells, but recent reports have challenged the requirement of this surface protein in this process. To explore the role of LRP in the recognition of apoptotic cells, target cells were generated with two distinct inducers of apoptotic cell death, etoposide and actinomycin-D. Jurkat T cells rendered apoptotic with etoposide exposed phosphatidylserine (PtdSer) and triggered engulfment by murine bone marrow-derived macrophages (BMDM), however they failed to suppress lipopolysaccharide-driven inflammatory cytokine secretion or, correspondingly, NFĸB-dependent or TNFα promoter-driven transcriptional activity in transfected RAW264.7 macrophages. In contrast, induction of apoptosis in either Jurkat cells or HeLa epithelial cells with actinomycin-D resulted in diminution of proinflammatory signaling from RAW264.7 cells and BMDM. Treatment of actinomycin-treated Jurkat cells with Q-VD-OPh, an irreversible inhibitor of caspase activity, blocked apoptosis, as assessed by the inhibition of PtdSer exposure; however, the cells maintained anti-inflammatory activity. Anti-inflammatory signaling mediated by actinomycin-treated cells was not affected by a macrophage-specific deletion in LRP. Moreover, the presence of LRP on macrophages did not alter the efficiency of engulfment of apoptotic cells in vitro or in vivo. These data demonstrate that the method of induction of apoptosis of target cells influences subsequent macrophage responsiveness, and that LRP is not required for engulfment of apoptotic cells regardless of the method of induction.

Apoptotic Cells Can Deliver Chemotherapeutics to Engulfing Macrophages and Suppress Inflammatory Cytokine Production

“…1C). These results are consistent with published studies (12) and indicate that cells that die in response to different drugs can evoke distinct effects on inflammatory responses by engulfing macrophages.…”

“…Recent work has demonstrated that although phagocytosed equally well by macrophages, Jurkat T cells killed by etoposide are proinflammatory, whereas those killed by actinomycin D are immunosuppressive (12). However, the mechanisms by which actinomycin D-killed cells suppressed the immune response of the engulfing cells could not be determined.…”

“…This not only serves to shrink the tumor mass but also provides abundant dead tumor cells, which can be captured and cross-presented by infiltrating antigen-presenting cells to prime antitumor immune responses (11). Recently, it has been reported that different chemotherapeutic drugs can induce either pro-or anti-inflammatory forms of apoptosis (12). Specifically, cells treated with actinomycin D, an antibiotic derived from Streptomyces that binds DNA and inhibits transcription, are antiinflammatory, whereas cells that die after treatment with etoposide, a topoisomerase inhibitor derived from the herbaceous plant Podophyllum peltatum, are proinflammatory.…”

“…Macrophages-We first characterized apoptotic cell death in Jurkat T cells treated with actinomycin D and etoposide, which have been reported to induce anti-inflammatory and inflammatory cell death, respectively (12). After 16 h of treatment, we could define three populations based on FACS analysis with AV and PI staining: live cells that were AV-and PI-negative, apoptotic cells that were AV-positive and PI-negative, and necrotic cells that were AV-and PI-positive (Fig.…”

Apoptosis, Cell Death and Inflammation

“…The low-density lipoprotein receptor-related protein-1 (LRP/CD91) mediates macrophage-apoptotic cell interactions but its role was recently challenged. Kozmar et al [18] provide new data showing that the operative molecular pathway involved in apoptosis induction might affect the ensuing macrophage recognition of apoptotic cells and the resulting inflammatory response, thus indicating that monocyte recognition of apoptotic cells is more complex than previously thought.…”