Homologous complement activation on drug-induced apoptotic cells from a human lung adenocarcinoma cell line

Development of monoclonal antibodies that specifically interact with necrotic lymphoma cells

The immune system has evolved mechanisms to sense not only microbes, but also necrotic cells. The pattern-recognition receptors in macrophages/dendritic cells that stimulate the acquired immune system are closely associated with danger signaling. In this study monoclonal antibodies (mAbs) that specifically interact with necrotic cells were developed. One IgG1 and two IgM mAbs were established, and they recognized a 80 kDa protein expressed in necrotic, but not live or apoptotic, cells. These mAbs, which serve as a probe for necrosis, facilitate analyses of the role of the immune complex that consists of necrotic cells and Ab and contributes to the formation of the inflammatory milieu induced by necrotic cell death.

Complement: coming full circle

The complement system has long been known to be a major element of innate immunity. Traditionally, it was regarded as the first line of defense against invading pathogens, leading to opsonization and phagocytosis or the direct lysis of microbes. However, from the second half of the twentieth century on, it became clear that complement is also intimately involved in the induction and "fine tuning" of adaptive B- and T-cell responses as well as lineage commitment. This growing recognition of the complement system's multifunctional role in immunity is consistent with the recent paradigm that complement is also necessary for the successful contraction of an adaptive immune response. This review aims at giving a condensed overview of complement's rise from a simple innate stop-and-go system to an essential and efficient participant in general immune homeostasis and acquired immunity.

ADAM10-mediated Release of Complement Membrane Cofactor Protein during Apoptosis of Epithelial Cells

Membrane cofactor protein CD46 controls complement activation on cells, is a receptor for several pathogens, and modulates immune responses by affecting CD8(+) T cells. Cells can release CD46 in an intact form on membrane vesicles and in a truncated form by a metalloproteolytic cleavage. The mechanism of shedding and its relationship to cell physiology has remained unclear. We have found using RNA interference analysis that a disintegrin and metalloproteinase (ADAM) 10 is responsible for the regulated shedding of the ectodomain of CD46 in apoptotic cells. The shedding of CD46 was initiated with staurosporine and UVB. Exposure of cell cultures to either UVB or staurosporine resulted in changes of cell morphology and detachment of cells from their matrices within 8-24 h. During this process CD46 was released both in apoptotic vesicles (vCD46) and proteolytically (sCD46) into the medium. Both the metalloproteinase inhibitor GM6001 and RNA interference of ADAM10 completely prevented the release of sCD46 and increased the expression of vCD46 on HaCaT cell vesicles, suggesting that ADAM10 releases sCD46 from the apoptotic vesicles. To explore whether the release of sCD46 is associated with apoptosis we analyzed the effects of caspase inhibitors. As expected, the inhibition of caspase activity attenuated the characteristic features of apoptosis and also decreased the release of sCD46. Our results reveal ADAM10 as an important regulator of CD46 expression during apoptosis. The ADAM10-mediated release of CD46 from apoptotic vesicles may represent a form of strategy to allow restricted complement activation to deal with modified self.

CD46 Plays a Key Role in Tailoring Innate Immune Recognition of Apoptotic and Necrotic Cells

Complement is the canonical innate immune system involved in host defense and tissue repair with the clearance of cell debris. In contrast to the robust armory mounted against microbial nonself-pathogens, complement is selectively activated on altered self (i.e. apoptotic and necrotic cells) to instruct the safe demise by poorly characterized mechanisms. Our data shed new light on the role of complement C1q in sensing nucleic acids (NA) rapidly exposed on apoptotic Jurkat T cell membranes and in driving C3 opsonization but without the lytic membrane attack complex. DNA/RNase-treated apoptotic cells failed to activate complement. We found that several other apoptotic cell models, including senescent keratinocytes, ionophore-treated sperm cells, and CMK-derived platelets, stained for cleaved caspase 3 were rapidly losing the key complement regulator CD46. CD46 from nuclear and membrane stores was found to cluster into blebs and shed into microparticles together with NA, phosphatidylserine, C1q, and factor H. Classical and alternative pathways of complement were involved in the recognition of H2O2-treated necrotic cells. Membrane attack complex was detected on necrotic cells possibly as a result of CD46 and CD59 shedding into soluble forms. Our data highlight a novel and universal paradigm whereby the complement innate immune system is using two synergistic strategies with the recognition of altered self-NA and missing self-CD46 signals to instruct and tailor the efficient removal of apoptotic and necrotic cells in immunoprivileged sites.

Predominant role of IgM-dependent activation of the classical pathway in the clearance of dying cells by murine bone marrow-derived macrophages in vitro

Soluble molecules including complement components have been shown to facilitate the clearance of dying cells by phagocytes, a process that is important in preventing tissue damage and autoimmunity. However, the extent to which complement is involved in this process and the relative contribution of each of the complement activation pathways is not fully understood. We examined the role of complement in the recognition/uptake of apoptotic thymocytes by murine bone marrow-derived macrophages (BMDM) in vitro using sera from gene-targeted mice. We found this process to be IgM- and complement-dependent, especially when the apoptotic cell-to-BMDM ratio was low, and the level of C3 deposition on apoptotic cells correlated closely with their uptake. The addition of C1q rectified the phagocytic defect seen in the presence of C1q-deficient serum in vitro but had no effect on the phagocytic defect observed with serum deficient in both IgM antibodies and C1q. Similarly, complement activation by IgM antibodies was essential for in vivo C3 deposition on apoptotic cells and their uptake by peritoneal macrophages. Hence, the efficient uptake of dying cells by BMDM requires IgM antibodies and complement.

Accelerated autoimmune disease in MRL/MpJ-Fas(lpr) but not in MRL/MpJ following immunization with high load of syngeneic late apoptotic cells

Numerous studies have shown that autoantigens may be clustered in the blebs of apoptotic cells. However, it is not yet clear in what circumstances apoptotic cells could be immunogenic rather than tolerogenic when interacting with macrophages, dendritic cells, and B cells. In order to further study this question we compared immunization of high load of syngeneic late apoptotic cells in two genetically close pro-autoimmune mice strains: MRL/MpJ and MRL/MpJ-Fas(lpr). We show that high apoptotic load could accelerate the generation of anti-dsDNA and anticardiolipin, and the extent of kidney disease, in MRL/MpJ-Fas(lpr) but could not generate autoimmunity in MRL/MpJ. Thus, in this model, a high load of apoptotic cells could augment the autoimmune response in established autoimmunity, but did not generate de novo autoimmune response in pro-autoimmune mice. Taken together with previous observations, apoptotic cell load may modify autoimmune disease generating either immune inhibition and down regulation of autoimmunity or immune stimulation and acceleration of an autoimmune disease.

Systemic lupus erythematosus and apoptosis: a question of balance

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by the presence of autoantibodies directed against a range of intracellular nucleoprotein targets. SLE patients are believed to develop an autoimmune response triggered by surface-exposed intracellular macromolecules translocated to the cell surface during apoptosis. Apoptosis-or programmed cell death-is a genetically controlled process initiated by two principal pathways. The extrinsic pathway is activated by the ligation of death receptors, and the intrinsic pathway emerges from mitochondria. As shown in fas-deficient mice and humans, the inability of the immune system to eliminate self-reactive lymphocytes by apoptosis can cause persistence of autoreactive cells and autoimmunity. However, as shown in complement deficiencies, increased apoptotic material and altered clearance of apoptotic cells is found in patients with SLE. These results suggest that what is found in rare individuals with genetic deficiencies that develop SLE or SLE-like disease may be found in the larger population of SLE patients as a common end point pattern of unbalanced process of both apoptosis and clearance of apoptotic material.

Opsonization of apoptotic cells. Implications for uptake and autoimmunity

As a part of innate immunity, soluble host proteins called opsonins, which include complement ligands and immunoglobulins, initially coat microorganisms that penetrate the mammalian sterile milieu. The main purpose of opsonization is to allow subsequent clearance of opsonized particles by specific receptors on the surface of leukocytes. Similarly, several proteins that may act as opsonins and have a role in uptake of apoptotic cells and bodies include thrombospondin I, the complement system, beta 2GPI, immunoglobulins, CRP, and some unidentified others. The surface changes that lead to opsonization include the appearance of phosphatidylserine that acts as an activator molecule for some known opsonins as the complement system and beta 2GPI. The consequence of altered opsonization is demonstrated by the development of autoimmunity in C1q deficient mice, and the pro-inflammatory response by macrophages ingesting apoptotic cell opsonized by an autoantibody.

Antibody-independent classical complement pathway activation and homologous C3 deposition in xeroderma pigmentosum cell lines

Of human malignantly transformed cell lines, xeroderma pigmentosum (XP) cell lines were found to be highly susceptible to homologous complement (C): cells were opsonized by C3 fragments on incubation with diluted normal human serum. C3 fragment deposition on XP cells was Ca2+-dependent and occurred on live cells but not UV-irradiated apoptotic cells. (Ca2+ is required for activation of the classical C pathway via C1q and the lactin pathway via mannose binding lectin (MBL), and the surface of apoptotic cells usually activates the alternative C pathway.) In this study we tested which of the pathways participates in XP cell C3 deposition. In seven cell lines that allowed C3 deposition (i), Clq was shown to be essential but MBL played no role in C activation, (ii) Cls but not MASP bound XP cells for activation, (iii) no antibodies recognizing XP cells were required for homologous C3 deposition, and (iv) the alternative pathway barely participated in C3 deposition. Furthermore, the levels of C-regulatory proteins for host cell protection against C, decay-accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46), were found to be relatively low in almost all XP cell lines compared with normal cells. These results indicate that XP cells activate the classical C pathway in an antibody-independent manner through the expression of a molecule which directly attracts C1q in a C-activating form, and that relatively low levels of DAF and MCP on XP cells facilitate effective C3 deposition. The possible relationship between the pathogenesis of XP and our findings is discussed.

Complement-dependent clearance of apoptotic cells by human macrophages

Apoptotic cells are rapidly engulfed by phagocytes, but the receptors and ligands responsible for this phenomenon are incompletely characterized. Previously described receptors on blood- derived macrophages have been characterized in the absence of serum and show a relatively low uptake of apoptotic cells. Addition of serum to the phagocytosis assays increased the uptake of apoptotic cells by more than threefold. The serum factors responsible for enhanced uptake were identified as complement components that required activation of both the classical pathway and alternative pathway amplification loop. Exposure of phosphatidylserine on the apoptotic cell surface was partially responsible for complement activation and resulted in coating the apoptotic cell surface with C3bi. In the presence of serum, the macrophage receptors for C3bi, CR3 (CD11b/CD18) and CR4 (CD11c/CD18), were significantly more efficient in the uptake of apoptotic cells compared with previously described receptors implicated in clearance. Complement activation is likely to be required for efficient uptake of apoptotic cells within the systemic circulation, and early component deficiencies could predispose to systemic autoimmunity by enhanced exposure to and/or aberrant deposition of apoptotic cells.