Macrophage response to apoptotic cells varies with the apoptotic trigger and is not altered by a deficiency in LRP expression

Complement Protein C1q Enhances Macrophage Foam Cell Survival and Efferocytosis

In the atherosclerotic lesion, macrophages ingest high levels of damaged modified low-density lipoproteins (LDLs), generating macrophage foam cells. Foam cells undergo apoptosis and, if not efficiently cleared by efferocytosis, can undergo secondary necrosis, leading to plaque instability and rupture. As a component of the innate immune complement cascade, C1q recognizes and opsonizes modified forms of LDL, such as oxidized or acetylated LDL, and promotes ingestion by macrophages in vitro. C1q was shown to be protective in an atherosclerosis model in vivo. Therefore, this study aimed to investigate whether ingestion of modified LDL in the presence of C1q alters macrophage foam cell survival or function. In an unbiased transcriptome analysis, C1q was shown to modulate expression of clusters of genes involved in cell death and apoptosis pathways in human monocyte-derived macrophages ingesting modified LDL; this was validated by quantitative PCR in human and murine macrophages. C1q downregulated levels and activity of active caspase-3 and PARP-1 in human and mouse macrophages during ingestion of modified LDL. This led to a measurable increase in survival and decrease in cell death, as measured by alamarBlue and propidium iodide assays, respectively. C1q opsonization also increased phagocytosis and efferocytosis in macrophage foam cells. These data suggest that C1q promotes macrophage survival during ingestion of excess cholesterol, as well as improves foam cell efferocytic function. This may be important in slowing disease progression and provides insight into the protective role of C1q in early atherosclerosis.

Interactions of apoptotic cells with macrophages in radiation-induced bystander signaling

Nontargeted effects that result in ongoing cellular and tissue damage show genotype-dependency in murine models with CBA/Ca, but not C57BL/6, exhibiting sensitivity to induced genomic instability. In vivo, radiation exposure is associated with genotype-dependent macrophage activation, and these cells are a source of bystander signaling involving cytokines and reactive oxygen and nitrogen species. The mechanisms responsible for macrophage activation and production of damaging bystander signals after irradiation are unclear. Macrophages from CBA/Ca exhibit an M1 (proinflammatory) phenotype compared to the M2 (anti-inflammatory) phenotype of C57BL/6 macrophages. Using the murine RAW264.7 macrophage-like cell line, we show that the ability of macrophages to interact with apoptotic cells and their responses to interaction varies significantly according to macrophage phenotype. Nonstimulated and M2 macrophages induce anti-inflammatory markers arginase and TGFβ after engulfment of apoptotic cells. In contrast, M1 macrophages do not induce anti-inflammatory responses, but express the proinflammatory markers NOS2, IL-6, TNFα, superoxide and NO, able to contribute to a damaging microenvironment. Macrophages stimulated with both inflammatory and anti-inflammatory agents prior to exposure to apoptotic cells induce a mixed response. The results indicate a complex cross-talk between macrophages and apoptotic cells and demonstrate that phagocytic clearance of apoptotic cells induced by genotoxic stress can produce microenvironmental responses consistent with the induction of a chromosomal instability phenotype in sensitive CBA/Ca mice with M1 macrophage activation, but not in resistant C57BL/6 mice with M2 macrophage activation. Modulation of macrophage phenotypes may represent a novel approach for reducing the nontargeted effects of radiation.

Apoptotic cells can deliver chemotherapeutics to engulfing macrophages and suppress inflammatory cytokine production

Immunosuppression via cell-cell contact with apoptotic cells is a well studied immunological phenomenon. Although the original studies of immune repression used primary cells, which undergo spontaneous cell death or apoptosis in response to irradiation, more recent studies have relied on chemotherapeutic agents to induce apoptosis in cell lines. In this work, we demonstrate that Jurkat cells induced to die with actinomycin D suppressed inflammatory cytokine production by macrophages, whereas cells treated with etoposide did not. This immune repression mediated by actinomycin D-treated cells did not require phagocytosis or cell-cell contact and thus occurs through a different mechanism from that seen with primary apoptotic neutrophils. Moreover, cells induced to die with etoposide and then treated for a short time with actinomycin D also suppressed macrophage responses, indicating that suppression was mediated by actinomycin D independent of the mechanism of cell death. Finally, phagocytosis of actinomycin D-treated cells caused apoptosis in macrophages, and suppression could be blocked by inhibition of caspase activity in the target macrophage. Together, these data indicate that apoptotic cells act as "Trojan horses," delivering actinomycin D to engulfing macrophages. Suppression of cytokine production by macrophages is therefore due to exposure to actinomycin D from apoptotic cells and is not the result of cell-receptor interactions. These data suggest that drug-induced death may not be an appropriate surrogate for the immunosuppressive activity of apoptotic cells. Furthermore, these effects of cytotoxic drugs on infiltrating immune phagocytes may have clinical ramifications for their use as antitumor therapies.

Phosphatidylserine exposure by Toxoplasma gondii is fundamental to balance the immune response granting survival of the parasite and of the host

Phosphatidylserine (PS) exposure on the cell surface indicates apoptosis, but has also been related to evasion mechanisms of parasites, a concept known as apoptotic mimicry. Toxoplasma gondii mimics apoptotic cells by exposing PS, inducing secretion of TGF-beta1 by infected activated macrophages leading to degradation of inducible nitric oxide (NO) synthase, NO production inhibition and consequently persisting in these cells. Here PS⁺ and PS⁻ subpopulation of tachyzoites were separated and the entrance mechanism, growth and NO inhibition in murine macrophages, and mice survival and pathology were analyzed. Infection index in resident macrophages was similar for both PS subpopulations but lower when compared to the total T. gondii population. Growth in resident macrophages was higher for the total T. gondii population, intermediate for the PS⁺ and lower for the PS⁻ subpopulation. Production of NO by activated macrophages was inhibited after infection with the PS⁺ subpopulation and the total populations of tachyzoites. However, the PS⁻ subpopulation was not able to inhibit NO production. PS⁺ subpopulation invaded macrophages by active penetration as indicated by tight-fitting vacuoles, but the PS⁻ subpopulation entered macrophages by phagocytosis as suggested by loose-fitting vacuoles containing these tachyzoites. The entrance mechanism of both subpopulations was confirmed in a non-professional phagocytic cell line where only the PS⁺ tachyzoites were found inside these cells in tight-fitting vacuoles. Both subpopulations of T. gondii killed mice faster than the total population. Clear signs of inflammation and no tachyzoites were seen in the peritoneal cavity of mice infected with the PS⁻ subpopulation. Moreover, mice infected with the PS⁺ subpopulation had no sign of inflammation and the parasite burden was intense. These results show that PS⁺ and PS⁻ subpopulations of T. gondii are necessary for a successful toxoplasma infection indicating that both subpopulations are required to maintain the balance between inflammation and parasite growth.

Investigations on the C1q-calreticulin-phosphatidylserine interactions yield new insights into apoptotic cell recognition

Both C1q and calreticulin (CRT) are involved in the recognition of apoptotic cells. CRT was initially characterized as a receptor for the C1q collagen-like fragment (CLF), whereas C1q was shown to bind apoptotic cells through its globular region (GR). Using purified CRT and recombinant CRT domains, we now provide unambiguous experimental evidence that, in addition to its CLF, the C1q GR also binds CRT and that both types of interactions are mediated by the CRT globular domain. Surface plasmon resonance analyses revealed that the C1q CLF and GR domains each bind individually to immobilized CRT and its globular domain with K(D) values of (2.6-8.3) × 10(-7) M. Further evidence that CRT binds to the C1q GR was obtained by electron microscopy. The role of CRT in the recognition of apoptotic HeLa cells by C1q was analyzed. The C1q GR partially colocalized with CRT on the surface of early apoptotic cells, and siRNA (small interfering RNA)-induced CRT deficiency resulted in increased apoptotic cell binding to C1q. The interaction between CRT and phosphatidylserine (PS), a known C1q ligand on apoptotic cells, was also investigated. The polar head of PS was shown to bind to CRT with a 10-fold higher affinity (K(D)=1.5 × 10(-5) M) than that determined for C1q, and, accordingly, the C1q GR-PS interaction was impaired in the presence of CRT. Together, these observations indicate that CRT, C1q, and PS are all closely involved in the uptake of apoptotic cells and strongly suggest a combinatorial role of these three molecules in the recognition step.