Interaction of Calreticulin with CD40 Ligand, TRAIL and Fas Ligand

Immunogenic cell death: The cornerstone of oncolytic viro-immunotherapy

According to the World Health Organization, cancer is one of the leading global health concerns, causing nearly 10 million deaths in 2020. While classical chemotherapeutics produce strong cytotoxicity on cancer cells, they carry limitations of drug resistance and off-target effects and sometimes fail to elicit adequate antitumor protection against tumor relapse. Additionally, most cancer cells have developed various ways to escape immune surveillance. Nevertheless, novel anticancer strategies such as oncolytic viro-immunotherapy can trigger immunogenic cell death (ICD), which can quickly grasp the attention of the host defense machinery, resulting in an ensuing antitumor immune response. Specifically, oncolytic viruses (OVs) can infect and destroy targeted cancer cells and stimulate the immune system by exposing pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) to promote inflammatory reactions, and concomitantly prime and induce antitumor immunity by the release of neoantigens from the damaged cancer cells. Thus, OVs can serve as a novel system to sensitize tumor cells for promising immunotherapies. This review discusses the concept of ICD in cancer, centralizing ICD-associated danger signals and their consequence in antitumor responses and ICD induced by OVs. We also shed light on the potential strategies to enhance the immunogenicity of OVs, including the use of genetically modified OVs and their combination with ICD-enhancing agents, which are helpful as forthcoming anticancer regimens.

In vitro Treatment of a Murine Mammary Adenocarcinoma Cell Line with Recombinant Trypanosoma cruzi Calreticulin Promotes Immunogenicity and Phagocytosis

American Trypanosomiasis, a parasitic disease produced by Trypanosoma cruzi (T. cruzi), endemic in Latin America, infects about 6 million people. During the chronic stage of the infection, approximately 30% of infected people will develop Chagas Disease, the clinical manifestation. Few decades ago it was reported that, during the chronic stage, the parasite interferes with the development of solid tumors. However, the identification of parasite molecules responsible for such effects remained elusive. Years later, we described T.cruzi Calreticulin (TcCalr), an endoplasmic reticulum resident chaperone that infective trypomastigotes translocate to the parasite exterior, where it displays anticomplement activities. Most likely, at least some of these activities are related with the antitumor properties of TcCalr, as shown in in vitro, ex vivo, in ovum, and in vivo models. In this context we, we have seen that in vivo subcutaneous peritumoral inoculation of rTcCalr enhances local infiltration of T cells and slows tumor development. Based on these precedents, we propose that in vitro treatment of a mammary adenocarcinoma (TA3 cell line) with rTcCalr, will enhance tumor immunogenicity. In agreement with this proposal, we have shown that: i). rTcCalr binds to TA3 cells in a concentration-dependent fashion, ii). C1q binds to TA3 cells in an rTcCalr-dependent fashion, confirmed by the reversion attained using anti-TcS (a central TcCalr domain that binds C1) F(ab')₂ antibody fragments, iii). incubation of TA3 cells with rTcCalr, promotes cell phagocytosis by murine macrophages and, iv). rTcCalr decreases the membrane expression of MHC class II, m-Dectin-1, Galectin-9 and PD-L1, while increasing the expression of Rae-1γ. In synthesis, herein we show that in vitro treatment of a murine mammary adenocarcinoma with rTcCalr enhances phagocytosis and modulates the expression of a variety of membrane molecules that correlates with increased tumor immunogenicity.

ERK1/2 and the Bcl-2 Family Proteins Mcl-1, tBid, and Bim Are Involved in Inhibition of Apoptosis During Persistent Chlamydia psittaci Infection

Chlamydia psittaci is an obligate intracellular pathogen that can cause zoonosis. Persistent C. psittaci infection can inhibit apoptosis in host cells, thus extending their survival and enabling them to complete their growth cycle. In this study, the antiapoptotic effects of persistent C. psittaci infection, induced by treatment with IFN-γ, were found to be associated with both the death receptor and the mitochondrial pathways of apoptosis. These effects were mediated by Bcl-2 family members, as evidenced by the decreased expression of proapoptotic proteins, such as tBid and Bim. Simultaneously, the antiapoptotic protein Mcl-1 was upregulated by persistent C. psittaci infection. Increased phosphorylation of ERK1/2 was observed; however, the expression of Bad, unlike that of other proapoptotic proteins, did not seem to be involved in this process. In summary, persistent chlamydial infection exerts antiapoptotic effects through both the death receptor and the mitochondrial pathways, in a process that is regulated by the ERK1/2 and apoptotic proteins of the Bcl-2 family.

Serum calreticulin as a novel biomarker of juvenile idiopathic arthritis disease activity

OBJECTIVE

This study aimed to investigate the relations between calreticulin (CRT) serum level and both disease activity and severity parameters in juvenile idiopathic arthritis (JIA).

MATERIAL AND METHODS

In this study, 60 children with JIA and 50 age-and-sex-matched healthy subjects were enrolled. The assessment of the disease activity was done using juvenile arthritis disease activity score 27 (JADAS-27). The assessment of disease severity was done via gray-scale ultrasonography (US) and power Doppler US (PDUS). Enzyme-linked immunosorbent assay (ELISA) was used to assay the serum level of human CRT.

RESULTS

The mean serum CRT levels in JIA patients was 8.6±1.2 ng/mL and showed a highly significant increase (p=0.001) as compared to the mean serum levels in the controls (5.02±0.77 ng/mL). There were statistically significant positive correlations between the serum CRT levels and disease duration, tender joint count, swollen joint count, visual analog scale, erythrocyte sedimentation rate, JADAS-27, C-reactive protein, rheumatoid factor titer, and ultrasonographic grading for synovitis and neovascularization.

CONCLUSION

Elevated serum CRT levels in JIA patients and its correlations with JIA disease activity and severity parameters signified that CRT might be used as a novel biomarker for disease activity and severity in JIA.

Mapping the Ca(2+) induced structural change in calreticulin

Calreticulin is a highly conserved multifunctional protein implicated in many different biological systems and has therefore been the subject of intensive research. It is primarily present in the endoplasmatic reticulum where its main functions are to regulate Ca(2+) homeostasis, act as a chaperone and stabilize the MHC class I peptide-loading complex. Although several high-resolution structures of calreticulin exist, these only cover three-quarters of the entire protein leaving the extended structures unsolved. Additionally, the structure of calreticulin is influenced by the presence of Ca(2+). The conformational changes induced by Ca(2+) have not been determined yet as they are hard to study with traditional approaches. Here, we investigated the Ca(2+)-induced conformational changes with a combination of chemical cross-linking, mass spectrometry, bioinformatics analysis and modelling in Rosetta. Using a bifunctional linker, we found a large Ca(2+)-induced change to the cross-linking pattern in calreticulin. Our results are consistent with a high flexibility in the P-loop, a stabilization of the acidic C-terminal and a relatively close interaction of the P-loop and the acidic C-terminal.

BIOLOGICAL SIGNIFICANCE

The function of calreticulin, an endoplasmatic reticulin chaperone, is affected by fluctuations in Ca(2+)concentration, but the structural mechanism is unknown. The present work suggests that Ca(2+)-dependent regulation is caused by different conformations of a long proline-rich loop that changes the accessibility to the peptide/lectin-binding site. Our results indicate that the binding of Ca(2+) to calreticulin may thus not only just be a question of Ca(2+) storage but is likely to have an impact on the chaperone activity.

Calreticulin Binds to Fas Ligand and Inhibits Neuronal Cell Apoptosis Induced by Ischemia-Reperfusion Injury

Background. Calreticulin (CRT) can bind to Fas ligand (FasL) and inhibit Fas/FasL-mediated apoptosis of Jurkat T cells. However, its effect on neuronal cell apoptosis has not been investigated. Purpose. We aimed to evaluate the neuroprotective effect of CRT following ischemia-reperfusion injury (IRI). Methods. Mice underwent middle cerebral artery occlusion (MCAO) and SH-SY5Y cells subjected to oxygen glucose deprivation (OGD) were used as models for IRI. The CRT protein level was detected by Western blotting, and mRNA expression of CRT, caspase-3, and caspase-8 was measured by real-time PCR. Immunofluorescence was used to assess the localization of CRT and FasL. The interaction of CRT with FasL was verified by coimmunoprecipitation. SH-SY5Y cell viability was determined by MTT assay, and cell apoptosis was assessed by flow cytometry. The measurement of caspase-8 and caspase-3 activity was carried out using caspase activity assay kits. Results. After IRI, CRT was upregulated on the neuron surface and bound to FasL, leading to increased viability of OGD-exposed SH-SY5Y cells and decreased activity of caspase-8 and caspase-3. Conclusions. This study for the first time revealed that increased CRT inhibited Fas/FasL-mediated neuronal cell apoptosis during the early stage of ischemic stroke, suggesting it to be a potential protector activated soon after IRI.

Agarose gel shift assay reveals that calreticulin favors substrates with a quaternary structure in solution

Here we present an agarose gel shift assay that, in contrast to other electrophoresis approaches, is loaded in the center of the gel. This allows proteins to migrate in either direction according to their isoelectric points. Therefore, the presented assay enables a direct visualization, separation, and prefractionation of protein interactions in solution independent of isoelectric point. We demonstrate that this assay is compatible with immunochemical methods and mass spectrometry. The assay was used to investigate interactions with several potential substrates for calreticulin, a chaperone that is involved in different biological aspects through interaction with other proteins. The current analytical assays used to investigate these interactions are mainly spectroscopic aggregation assays or solid phase assays that do not provide a direct visualization of the stable protein complex but rather provide an indirect measure of interactions. Therefore, no interaction studies between calreticulin and substrates in solution have been investigated previously. The results presented here indicate that calreticulin has a preference for substrates with a quaternary structure and primarily β-sheets in their secondary structure. It is also demonstrated that the agarose gel shift assay is useful in the study of other protein interactions and can be used as an alternative method to native polyacrylamide gel electrophoresis.

Calreticulin protects rat microvascular endothelial cells against microwave radiation-induced injury by attenuating endoplasmic reticulum stress

OBJECTIVE

This study was designed to evaluate whether exogenous CRT was beneficial for alleviating MR-induced injury by suppressing ER stress in rat MMECs.

METHODS

MMECs were pretreated with CRT (25 pg/mL) for 12 hours, followed by the exposure to 2.856 GHz radiation at a mean power density of 30 mW/cm(2) for six minutes. MR-induced injury in MMECs was evaluated by LDH leakage, apoptosis, and cell viability analysis. The expression of GRP78, CRT, CHOP, Bcl-2, and Bax were examined by Western blot analysis to reflect ER stress response and ER stress-related apoptosis.

RESULTS

MR induced marked MMECs injury, as shown by increased LDH leakage and apoptosis rate and decreased cell viability. MR also induced excessive ER stress, characterized by increased expression of GRP78 and CRT, and ER stress-related apoptotic signaling as well, as shown by the upregulation of CHOP and Bax and the downregulation of Bcl-2. Exogenous CRT pretreatment remarkably attenuated MR-induced cell apoptosis and LDH leakage, ER stress, and activation of the ER stress-related apoptotic signaling.

CONCLUSIONS

Exogenous CRT attenuates MR-induced ER stress-related apoptosis by suppressing CHOP-mediated apoptotic signaling pathways in MMECs.

The ever-expanding immunomodulatory role of calreticulin in cancer immunity

Calreticulin is a pleiotropic molecule that normally resides in the lumen of the endoplasmic reticulum (ER). Here, it has various functions, ranging from regulation of calcium homeostasis to ensuring proper protein folding. More recently, calreticulin gained special interest for its extracellular functions, where it has direct immunomodulatory activity. In this respect, calreticulin activates dendritic cells and macrophages. In addition, certain anti-cancer therapies induce the translocation of calreticulin from the ER to the cell surface of dying cancer cells, where calreticulin dictates the immunogenicity of these cells. Interestingly, treatment with tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) also induces membrane calreticulin exposure on cancer cells. As shown here, calreticulin directly interacts with TRAIL and its receptor-signaling complex, as well as with other TNF family members. Of note, TRAIL is a well known immunomodulatory molecule, and is expressed on the surface of natural killer T-cells. Therefore, calreticulin may have an as yet unrecognized wide(r) impact on immunity, with the TNF-ligand family modulating virtually all aspects of the immune response.

Targeted delivery of CD40L promotes restricted activation of antigen-presenting cells and induction of cancer cell death

Background

Stimulation of CD40 can augment anti-cancer T cell immune responses by triggering effective activation and maturation of antigen-presenting cells (APCs). Although CD40 agonists have clinical activity in humans, the associated systemic activation of the immune system triggers dose-limiting side-effects.

Methods

To increase the tumor selectivity of CD40 agonist-based therapies, we developed an approach in which soluble trimeric CD40L (sCD40L) is genetically fused to tumor targeting antibody fragments, yielding scFv:CD40L fusion proteins. We hypothesized that scFv:CD40L fusion proteins would have reduced CD40 agonist activity similar to sCD40L but will be converted to a highly agonistic membrane CD40L-like form of CD40L upon anchoring to cell surface exposed antigen via the scFv domain.

Results

Targeted delivery of CD40L to the carcinoma marker EpCAM on carcinoma cells induced dose-dependent paracrine maturation of DCs ~20-fold more effective than a non-targeted control scFv:CD40L fusion protein. Similarly, targeted delivery of CD40L to the B cell leukemia marker CD20 induced effective paracrine maturation of DCs. Of note, the CD20-selective delivery of CD40L also triggered loss of cell viability in certain B cell leukemic cell lines as a result of CD20-induced apoptosis.

Conclusions

Targeted delivery of CD40L to cancer cells is a promising strategy that may help to trigger cancer-localized activation of CD40 and can be modified to exert additional anti-cancer activity via the targeting domain.

Calreticulin as a hydrophilic chimeric molecular adjuvant enhances IgG responses to the spike protein of severe acute respiratory syndrome coronavirus

Fragment 450-650 of the spike (S) protein (S450-650) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) contains epitopes capable of being recognized by convalescent sera of SARS patients. Vaccination of mice with recombinant S450-650 (rS450-650) can induce Abs against SARS-CoV, although the titer is relatively low. In the present study, a fusion protein linking a fragment (residues 39-272) of murine calreticulin (CRT) to S450-650 in a prokaryotic expression system was created. Compared with target antigen alone, the recombinant fusion product (rS450-650-CRT) has much improved hydrophilicity and immunogenicity. The S450-650-specific IgG Abs of BALB/c mice subcutaneously immunized with rS450-650-CRT were in substantially higher titer (approximately fivefold more). Furthermore, the fusion protein, but not rS450-650 alone, was able to elicit S450-650-specific IgG responses in T cell deficient nude mice. Given that rCRT/39-272 can drive the maturation of bone-marrow-derived dendritic cells, directly activate macrophages and B cells, and also elicit helper T cell responses in vivo, we propose that fragment 39-272 of CRT is an effective molecular adjuvant capable of enhancing target Ag-specific humoral responses in both a T cell-dependent and independent manner. Fusion protein rS450-650-CRT is a potential candidate vaccine against SARS-CoV infection.

Modulation of cellular immunity by antibodies against calreticulin

Although caltreticulin (CRT) is mainly a residential ER protein, it is also expressed on the membrane surface of various types of cells exhibiting multiple functions. We report here that intraperitoneal administration of a soluble recombinant CRT fragment (rCRT/39-272) led to a substantial decrease in delayed type hypersensitivity (DTH) responses in BALB/c mice and EAE in C57BL/6 mice. In the recall response against keyhole limpet hemocyanin (KLH) in vitro, draining lymph node cells from the rCRT/39-272-treated mice produced less IFN-γ but more IL-4 as compared with the cells from the control group. The immunomodulating effect of intraperitoneally administered rCRT/39-272 was attributed to anti-CRT Abs thereby induced, because, in passive transfer experiments, the CRT-specific antiserum could suppress DTH in BALB/c mice. B-cell-deficient μMT mice were not susceptible to rCRT/39-272-mediated DTH suppression. Furthermore, CRT appears on the surface of murine T cells soon after activation and remains detectable (at relatively low level) by flow cytometry for approximately 5 days in vitro. Anti-CRT Abs were able to inhibit AKT phosphorylation, proliferation, and cytokine production by activated murine T cells. We propose that cell surface CRT could play a role in the function of effector T cells and may be considered a target for immunological manipulation.

Prerequisites for the antitumor vaccine-like effect of chemotherapy and radiotherapy

For a long time, anticancer therapies were believed to work (and hence convey a therapeutic benefit) either by killing cancer cells or by inducing a permanent arrest in their cell cycle (senescence). In both scenarios, the efficacy of anticancer regimens was thought to depend on cancer cell-intrinsic features only. More recently, the importance of the tumor microenvironment (including stromal and immune cells) has been recognized, along with the development of therapies that function by modulating tumor cell-extrinsic pathways. In particular, it has been shown that some chemotherapeutic and radiotherapeutic regimens trigger cancer cell death while stimulating an active immune response against the tumor. Such an immunogenic cell death relies on the coordinated emission of specific signals from dying cancer cells and their perception by the host immune system. The resulting tumor-specific immune response is critical for the eradication of tumor cells that may survive therapy. In this review, we discuss the molecular mechanisms that underlie the vaccine-like effects of some chemotherapeutic and radiotherapeutic regimens, with particular attention to the signaling pathways and genetic elements that constitute the prerequisites for immunogenic anticancer therapy.

CD91 interacts with mannan-binding lectin (MBL) through the MBL-associated serine protease-binding site

CD91 plays an important role in the scavenging of apoptotic material, possibly through binding to soluble pattern-recognition molecules. In this study, we investigated the interaction of CD91 with mannan-binding lectin (MBL), ficolins and lung surfactant proteins. Both MBL and L-ficolin were found to bind CD91. The MBL-CD91 interaction was time- and concentration-dependent and could be inhibited by known ligands of CD91. MBL-associated serine protease 3 (MASP-3) also inhibited binding between MBL and CD91, suggesting that the site of interaction is located at or near the MASP-MBL interaction site. This was confirmed by using MBL mutants deficient for MASP binding that were unable to interact with CD91. These findings demonstrate that MBL and L-ficolin interact with CD91, strongly suggesting that they have the potential to function as soluble recognition molecules for scavenging microbial and apoptotic material by CD91.

Extracellular calreticulin is present in the joints of patients with rheumatoid arthritis and inhibits FasL (CD95L)-mediated apoptosis of T cells

OBJECTIVE

The binding of FasL (CD95L) to its receptor, Fas (CD95), induces apoptosis. Studies have shown that in patients with rheumatoid arthritis (RA), T lymphocytes are resistant to FasL-induced apoptosis in vivo but are susceptible to FasL-induced apoptosis in vitro. Dysfunction in this mechanism may be an important contributor to the pathophysiology of RA. Thus, the present study was undertaken to determine which factors might inhibit FasL-Fas binding in vivo and those that would inhibit apoptosis of T lymphocytes in an in vitro model system.

METHODS

Human Jurkat T cells rendered apoptotic by FasL exposure were analyzed by flow cytometry. Necrosis was determined according to measurement of lactate dehydrogenase release. Quantification of calreticulin in plasma and synovial fluid and of calreticulin-FasL binding was performed by enzyme-linked immunosorbent assay. Measurement of nitrite/nitrate in the plasma and synovial fluid was carried out by chemiluminescence assay.

RESULTS

Extracellular calreticulin was present at a significantly higher concentration in the plasma (median 10.3 ng/ml, interquartile range [IQR] 14.8 ng/ml) and synovial fluid (median 10.3 ng/ml, IQR 12.0 ng/ml) of RA patients (each P < 0.05) compared with the plasma (median 3.1 ng/ml, IQR 1.3 ng/ml) and synovial fluid (median 2.9 ng/ml, IQR 0.9 ng/ml) of patients with psoriatic arthritis and the plasma of healthy control subjects (median 2.9 ng/ml, IQR 0.9 ng/ml). Calreticulin concentrations in the synovial fluid correlated with the tender and swollen joint counts and the activity scores on the 28-joint Disease Activity Score assessment. Calreticulin also bound directly to FasL. In vitro, calreticulin (2-16 ng/ml) inhibited FasL-induced apoptosis of Jurkat T cells.

CONCLUSION

Calreticulin was present at higher concentrations in the plasma and synovial fluid of RA patients. Calreticulin had the capacity to bind directly to FasL and to inhibit FasL-mediated apoptosis of Jurkat T cells, and thus might play a role in inhibiting apoptosis of inflammatory T cells in RA.

TRAIL and Taurolidine induce apoptosis and decrease proliferation in human fibrosarcoma

BACKGROUND

Disseminated soft tissue sarcoma still represents a therapeutic dilemma because effective cytostatics are missing. Therefore we tested TRAIL and Tarolidine (TRD), two substances with apoptogenic properties on human fibrosarcoma (HT1080).

METHODS

Viability, apoptosis and necrosis were visualized by TUNEL-Assay and quantitated by FACS analysis (Propidiumiodide/AnnexinV staining). Gene expression was analysed by RNA-Microarray and the results validated for selected genes by rtPCR. Protein level changes were documented by Western Blot analysis. NFKB activity was analysed by ELISA and proliferation assays (BrdU) were performed.

RESULTS AND DISCUSSION

The single substances TRAIL and TRD induced apoptotic cell death and decreased proliferation in HT1080 cells significantly. Gene expression of several genes related to apoptotic pathways (TRAIL: ARHGDIA, NFKBIA, TNFAIP3; TRD: HSPA1A/B, NFKBIA, GADD45A, SGK, JUN, MAP3K14) was changed. The combination of TRD and TRAIL significantly increased apoptotic cell death compared to the single substances and lead to expression changes in a variety of genes (HSPA1A/B, NFKBIA, PPP1R15A, GADD45A, AXL, SGK, DUSP1, JUN, IRF1, MYC, BAG5, BIRC3). NFKB activity assay revealed an antipodal regulation of the several subunits of NFKB by TRD and TRD+TRAIL compared to TRAIL alone.

CONCLUSION

TRD and TRAIL are effective to induce apoptosis and decrease proliferation in human fibrosarcoma. A variety of genes seems to be involved, pointing to the NFKB pathway as key regulator in TRD/TRAIL-mediated apoptosis.

Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis

Anoikis, apoptotic cell death due to loss of cell adhesion, is critical for regulation of tissue homeostasis in tissue remodeling. Fibrogenesis is associated with reduced fibroblast apoptosis. The matricellular protein thrombospondin 1 (TSP1) regulates cell adhesion and motility during tissue remodeling and in fibrogenesis. The N-terminal domain of TSP1 binds to the calreticulin-LRP1 receptor co-complex to signal down-regulation of cell adhesion and increased cell motility through focal adhesion disassembly. TSP1 signaling through calreticulin-LRP1 activates cell survival signals such as PI3-kinase. Therefore, we tested the hypothesis that TSP1 supports cell survival under adhesion-independent conditions to facilitate tissue remodeling. Here, we show that platelet TSP1, its N-terminal domain (NoC1) as a recombinant protein, or a peptide comprising the calreticulin-LRP1 binding site [amino acids 17-35 (hep I)] in the N-terminal domain promotes fibroblast survival under anchorage-independent conditions. TSP1 activates Akt and decreases apoptotic signaling through caspase 3 and PARP1 in suspended fibroblasts. Inhibition of PI3K/Akt activity blocks TSP1-mediated anchorage-independent survival. Fibroblasts lacking LRP1 or expressing calreticulin lacking the TSP1 binding site do not respond to TSP1 with anchorage-independent survival. These data define a novel role for TSP1 signaling through the calreticulin/LRP1 co-complex in tissue remodeling and fibrotic responses through stimulation of anoikis resistance.-Pallero, M. A., Elzie, C. A., Chen, J., Mosher, D. F., Murphy-Ullrich, J. E. Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis.

Immunogenic cancer cell death: a key-lock paradigm

Physiological cell death, which occurs as a continuous byproduct of cellular turnover, is non-immunogenic or even tolerogenic, thereby avoiding autoimmunity. By contrast, cancer cell death elicited by radiotherapy and some chemotherapeutic agents such as anthracyclines is immunogenic. Recent data suggest that innate and cognate immune responses elicited by such anti-cancer agents are required for an optimal therapeutic outcome, underscoring the clinical relevance of immunogenic cell death. Here we discuss the concept that immunogenic death involves changes in the composition of the cell surface, as well as the release of soluble immunogenic signals that occur in a defined temporal sequence. This 'key' then operates on a series of receptors expressed by dendritic cells (DC, the 'lock') to allow for the presentation of tumor antigens to T cells and for the initiation of a productive immune response. Immunogenic cell death is characterized by the early cell surface exposure of chaperones including calreticulin and/or heat shock proteins, which determine the uptake of tumor antigens and/or affect DC maturation. Moreover, the late release of High mobility group box 1 (HMGB1), which acts on toll-like receptor 4 (TLR4), is required for optimal presentation of antigens from dying tumor cells. Nonetheless, numerous details on the molecular events that define immunogenicity remain to be defined, both at the level of the dying cancer cells and at the level of the responding innate effectors.

The co-translocation of ERp57 and calreticulin determines the immunogenicity of cell death

The exposure of calreticulin (CRT) on the plasma membrane can precede anthracycline-induced apoptosis and is required for cell death to be perceived as immunogenic. Mass spectroscopy, immunofluorescence and immunoprecipitation experiments revealed that CRT co-translocates to the surface with another endoplasmic reticulum-sessile protein, the disulfide isomerase ERp57. The knockout and knockdown of CRT or ERp57 inhibited the anthracycline-induced translocation of ERp57 or CRT, respectively. CRT point mutants that fail to interact with ERp57 were unable to restore ERp57 translocation upon transfection into crt(-/-) cells, underscoring that a direct interaction between CRT and ERp57 is strictly required for their co-translocation to the surface. ERp57(low) tumor cells generated by retroviral introduction of an ERp57-specific shRNA exhibited a normal apoptotic response to anthracyclines in vitro, yet were resistant to anthracycline treatment in vivo. Moreover, ERp57(low) cancer cells (which failed to expose CRT) treated with anthracyclines were unable to elicit an anti-tumor response in conditions in which control cells were highly immunogenic. The failure of ERp57(low) cells to elicit immune responses and to respond to chemotherapy could be overcome by exogenous supply of recombinant CRT protein. These results indicate that tumors that possess an intrinsic defect in the CRT-translocating machinery become resistant to anthracycline chemotherapy due to their incapacity to elicit an anti-cancer immune response.

Adiponectin binds C1q and activates the classical pathway of complement

The adipose-specific protein adiponectin binds to a number of target molecules, including damaged endothelium and the surface of apoptotic cells. However, the significance of this binding remains unclear. This study demonstrates the binding of purified C1q to recombinant adiponectin under physiological conditions, and the dependence of this upon Ca(++) and Mg(++). Binding was enhanced by metaperiodate-mediated destruction of glucosylgalactosyl sugars on adiponectin. Adiponectin was bound by the globular domain of the A chain of collagenase-digested C1q, and C1q binding induced deposition of C4 and C3 through activation of the classical complement pathway. After Western blotting, affinity-purified adiponectin from human serum bound C1q, whereas adiponectin in whole serum did not, unless pre-treated with metaperiodate. These results suggest adiponectin is member of the pattern-recognition family of defence collagens, able to bind target molecules and activate complement. It may therefore play an important role in innate immunity and autoimmune phenomena.