Stimulation of TNF receptor type 2 expands regulatory T cells and ameliorates established collagen-induced arthritis in mice.

Tumor necrosis factor (TNF) and its receptors TNF receptor type 1 (TNFR1) and type 2 (TNFR2) have a central role in chronic inflammatory diseases. While TNFR1 mainly confers inflammation, activation of TNFR2 elicits not only pro-inflammatory but also anti-inflammatory effects. In this study, we wanted to investigate the anti-inflammatory therapeutic potential of selective activation of TNFR2 in mice with established collagen-induced arthritis. Mice with established arthritis induced by immunization with bovine collagen type II were treated with six injections of the TNFR2-specific agonist TNCscTNF80, given every second day. Two days after treatment cessation, the cell compositions of bone marrow, spleen and lymph nodes were analyzed. Mice were visually scored until day 30 after the start of therapy and the degree of joint inflammation was determined by histology. Treatment with TNCscTNF80 increased arthritis-induced myelopoiesis. Little effect was seen on the infiltration rate of inflammatory immature myeloid cells and on the reduction of lymphoid cells in secondary lymphoid organs. Upon treatment, frequency of regulatory T (Treg) cells in the CD4+ T-cell population was increased in both spleen and inguinal lymph nodes. In addition, the expression of TNFR2 on Treg cells was enhanced. The clinical score started to improve 1 week after cessation treatment and remained lower 30 days after initiation of therapy. The histological score also revealed amelioration of joint inflammation in TNCscTNF80-treated versus control mice. Activation of TNFR2 might provide a suitable therapeutic strategy in autoimmune arthritis by increasing the numbers of regulatory cell types, in particular Treg cells, and by attenuation of arthritis.

Chronic Inflammation Increases the Sensitivity of Mouse Treg for TNFR2 Costimulation.

TNF receptor type 2 (TNFR2) has gained attention as a costimulatory receptor for T cells and as critical factor for the development of regulatory T cells (Treg) and myeloid suppressor cells. Using the TNFR2-specific agonist TNCscTNF80, direct effects of TNFR2 activation on myeloid cells and T cells were investigated in mice. In vitro, TNCscTNF80 induced T cell proliferation in a costimulatory fashion, and also supported in vitro expansion of Treg cells. In addition, activation of TNFR2 retarded differentiation of bone marrow-derived immature myeloid cells in culture and reduced their suppressor function. In vivo application of TNCscTNF80-induced mild myelopoiesis in naïve mice without affecting the immune cell composition. Already a single application expanded Treg cells and improved suppression of CD4 T cells in mice with chronic inflammation. By contrast, multiple applications of the TNFR2 agonist were required to expand Treg cells in naïve mice. Improved suppression of T cell proliferation depended on expression of TNFR2 by T cells in mice repeatedly treated with TNCscTNF80, without a major contribution of TNFR2 on myeloid cells. Thus, TNFR2 activation on T cells in naïve mice can lead to immune suppression in vivo. These findings support the important role of TNFR2 for Treg cells in immune regulation.

Exogenous TNFR2 activation protects from acute GvHD via host T reg cell expansion.

Donor CD4(+)Foxp3(+) regulatory T cells (T reg cells) suppress graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (HCT [allo-HCT]). Current clinical study protocols rely on the ex vivo expansion of donor T reg cells and their infusion in high numbers. In this study, we present a novel strategy for inhibiting GvHD that is based on the in vivo expansion of recipient T reg cells before allo-HCT, exploiting the crucial role of tumor necrosis factor receptor 2 (TNFR2) in T reg cell biology. Expanding radiation-resistant host T reg cells in recipient mice using a mouse TNFR2-selective agonist before allo-HCT significantly prolonged survival and reduced GvHD severity in a TNFR2- and T reg cell-dependent manner. The beneficial effects of transplanted T cells against leukemia cells and infectious pathogens remained unaffected. A corresponding human TNFR2-specific agonist expanded human T reg cells in vitro. These observations indicate the potential of our strategy to protect allo-HCT patients from acute GvHD by expanding T reg cells via selective TNFR2 activation in vivo.

Induction of Suppressor Cells and Increased Tumor Growth following Chronic Psychosocial Stress in Male Mice.

To study the impact of psychosocial stress on the immune system, male mice were subjected to chronic subordinate colony housing (CSC), a preclinically validated mouse model for chronic psychosocial stress. CSC substantially affected the cell composition of the bone marrow, blood, and spleen by inducing myelopoiesis and enhancing the frequency of regulatory T cells in the CD4 population. Expansion of the myeloid cell compartment was due to cells identified as immature inflammatory myeloid cells having the phenotype of myeloid-derived suppressor cells of either the granulocytic or the monocytic type. Catecholaminergic as well as TNF signaling were implicated in these CSC-induced cellular shifts. Although the frequency of regulatory cells was enhanced following CSC, the high capacity for inflammatory cytokine secretion of total splenocytes indicated an inflammatory immune status in CSC mice. Furthermore, CSC enhanced the suppressive activity of bone marrow-derived myeloid-derived suppressor cells towards proliferating T cells. In line with the occurrence of suppressor cell types such as regulatory T cells and myeloid-derived suppressor cells, transplanted syngeneic fibrosarcoma cells grew better in CSC mice than in controls, a process accompanied by pronounced angiogenesis and clustering of immature myeloid cells in the tumor tissue. In addition, tumor implantation after CSC reinforced the CSC-induced increase in myeloid-derived suppressor cells and regulatory T cell frequencies while the CSC-induced cellular changes eased off in mice without tumor. Together, our data suggest a role for suppressor cells such as regulatory T cells and myeloid-derived suppressor cells in the enhanced tumor growth after chronic psychosocial stress.

TNF and regulatory T cells are critical for sepsis-induced suppression of T cells.

The immune system in sepsis is impaired as seen by reduced numbers and function of immune cells and impaired antigen-specific antibody responses. We studied T cell function in septic mice using cecal ligation and puncture (CLP) as a clinically relevant mouse model for sepsis. The proliferative response of CD4(+) and CD8(+) T cells was suppressed in septic mice. Adoptive transfer experiments demonstrated that the T cells were not intrinsically altered by CLP. Instead, the septic host environment was responsible for this T cell suppression. While CLP-induced suppression was dependent on TNF activity, neither the activation of TNF receptors type 1 nor TNF receptor type 2 alone was sufficient to generate sepsis-induced suppression showing that the two TNF receptors can substitute each other. Specific depletion of regulatory T (Treg) cells improved the impaired T cell proliferation in septic recipients demonstrating participation of Treg in sepsis-induced suppression. In summary, sepsis leads to TNF-dependent suppression of T cell proliferation in vivo involving induction of Treg cells.

Myeloid suppressor cells require membrane TNFR2 expression for suppressive activity.

TNF and TNF receptor type 2 (TNFR2) have been shown to be important for generation of myeloid-derived suppressor cells (MDSC). In order to analyze whether and how TNFR2 passes the effect of TNF on, myeloid cells from TNFR2-deficient mice were compared to respective cells from wild-type mice. Primary TNFR2-deficient myeloid cells showed reduced production of NO and IL-6 which was attributable to CD11b(+) CD11c(-) Ly6C(+) Ly6G(-) immature monocytic MDSC. TNFR2-deficient MDSC isolated from bone marrow were less suppressive for T cell proliferation compared to WT-derived MDSC. These differences on myeloid cells between the two mouse lines were still observed after co-culture of bone marrow cells from the two mouse lines together during myeloid cell differentiation, which demonstrated that the impaired functional capacity of TNFR2-deficient cells was independent of soluble factors but required membrane expression of TNFR2. Similarly, adoptive transfer of TNFR2-deficient bone marrow cells into wild-type hosts did not rescue the TNFR2-specific phenotype of bone marrow-derived myeloid cells. Therefore, membrane TNFR2 expression determines generation and function of monocytic MDSC.

Langerhans cells promote early germinal center formation in response to Leishmania-derived cutaneous antigens.

Efficient formation of early GCs depends on the close interaction between GC B cells and antigen-primed CD4(+) follicular helper T cells (TFH ). A tight and stable formation of TFH /B cell conjugates is required for cytokine-driven immunoglobulin class switching and somatic hypermutation of GC B cells. Recently, it has been shown that the formation of TFH /B cell conjugates is crucial for B-cell differentiation and class switch following infection with Leishmania major parasites. However, the subtype of DCs responsible for TFH -cell priming against dermal antigens is thus far unknown. Utilizing a transgenic C57BL/6 mouse model designed to trigger the ablation of Langerin(+) DC subsets in vivo, we show that the functionality of TFH /B cell conjugates is disturbed after depletion of Langerhans cells (LCs): LC-depleted mice show a reduction in somatic hypermutation in B cells isolated from TFH /B cell conjugates and markedly reduced GC reactions within skin-draining lymph nodes. In conclusion, this study reveals an indispensable role for LCs in promoting GC B-cell differentiation following cutaneous infection with Leishmania major parasites. We propose that LCs are key regulators of GC formation and therefore have broader implications for the development of allergies and autoimmunity as well as for future vaccination strategies.

Immune humanization of immunodeficient mice using diagnostic bone marrow aspirates from carcinoma patients.

Tumor xenografts in immunodeficient mice, while routinely used in cancer research, preclude studying interactions of immune and cancer cells or, if humanized by allogeneic immune cells, are of limited use for tumor-immunological questions. Here, we explore a novel way to generate cancer models with an autologous humanized immune system. We demonstrate that hematopoietic stem and progenitor cells (HSPCs) from bone marrow aspirates of non-metastasized carcinoma patients, which are taken at specialized centers for diagnostic purposes, can be used to generate a human immune system in NOD-scid IL2rγ(null) (NSG) and HLA-I expressing NSG mice (NSG-HLA-A2/HHD) comprising both, lymphoid and myeloid cell lineages. Using NSG-HLA-A2/HHD mice, we show that responsive and self-tolerant human T cells develop and human antigen presenting cells can activate human T cells. As critical factors we identified the low potential of bone marrow HSPCs to engraft, generally low HSPC numbers in patient-derived bone marrow samples, cryopreservation and routes of cell administration. We provide here an optimized protocol that uses a minimum number of HSPCs, preselects high-quality bone marrow samples defined by the number of initially isolated leukocytes and intra-femoral or intra-venous injection. In conclusion, the use of diagnostic bone marrow aspirates from non-metastasized carcinoma patients for the immunological humanization of immunodeficient mice is feasible and opens the chance for individualized analyses of anti-tumoral T cell responses.

TNFR2 maintains adequate IL-12 production by dendritic cells in inflammatory responses by regulating endogenous TNF levels.

Sepsis-induced immune reactions are reduced in TNF receptor 2 (TNFR2)-deficient mice as previously shown. In order to elucidate the underlying mechanisms, the functional integrity of myeloid cells of TNFR2-deficient mice was analyzed and compared to wild type (WT) mice. The capacity of dendritic cells to produce IL-12 was strongly impaired in TNF-deficient mice, mirroring impaired production of IL-12 by WT dendritic cells in sepsis or after LPS or TNF pre-treatment. In addition, TNFR2-deficient mice were refractory to LPS pre-treatment and also to hyper-sensitization by inactivated Propionibacterium acnes, indicating habituation to inflammatory stimuli by the immune response when TNFR2 is lacking. Constitutive expression of TNF mRNA in kidney, liver, spleen, colon and lung tissue, and the presence of soluble TNFR2 in urine of healthy WT mice supported the conclusion that TNF is continuously present in naïve mice and controlled by soluble TNFR2. In TNFR2-deficient mice endogenous TNF levels cannot be balanced and the continuous exposure to enhanced TNF levels impairs dendritic cell function. In conclusion, TNF pre-exposure suppresses secondary inflammatory reactions of myeloid cells; therefore, continuous control of endogenous TNF by soluble TNFR2 seems to be essential for the maintenance of adequate sensitivity to inflammatory stimuli.

LTßR expression on hematopoietic cells regulates acute inflammation and influences maturation of myeloid subpopulations.

Lymphotoxin beta-receptor (LTßR) is involved in the formation and maintenance of secondary lymphoid structures, as well as in the regulation of inflammatory responses. Because LTßR lymphoid structure formation continues to develop in infants, we compared two different chimera models: one using adult mice and the other using a transplantation model of neonatal mice. To elucidate the function of LTßR on lymphoid and non-lymphoid cells, we generated bone marrow chimeras on the wild type C57Bl/6 and the LTßR-deficient (LTßR(-/-)) background, and reconstituted the mice with bone marrow cells reciprocally. These chimeric mice were analyzed in the experimental model of acute dextran sulfate sodium-induced colitis. Interestingly, both models revealed not only equal reconstitution levels but also similar immunological responses: LTßR expression on stromal cells is essential for lymph node formation, whereas LTBR on hematopoietic cells is crucial for a decrease in inflammation. In addition, mice lacking LTßR on hematopoietic cells revealed (a) an increase of immature granulocytic cells in the spleen and (b) a reduced proportion of myeloid cells in peripheral blood and spleen expressing CD11b(+)Ly6C(+)Ly6G(-) (myeloid-derived suppressor cells expression profile). In conclusion, LTßR expression on hematopoietic cells seems to be involved in the down-regulation of acute inflammatory reactions paralleled by the appearance of immature myeloid cells.

Immature mouse granulocytic myeloid cells are characterized by production of ficolin-B.

Ficolins activate the lectin pathway of the complement system upon binding to carbohydrate patterns on pathogens. To characterize the producer cells of ficolin-B the expression of mouse ficolin-B, the orthologue of human M-ficolin, was studied in macrophages and dendritic cells during differentiation from bone marrow cells, in primary granulocytes, and during differentiation of granulocytes derived from ER-Hoxb8 cells. Expression of ficolin-B mRNA declined in all myeloid cell types to low levels during terminal differentiation. However, in contrast to macrophages and dendritic cells, ficolin-B expression was enhanced upon activation in granulocytes. High expression of ficolin-B was observed in primary immature neutrophilic CD11b(+) Ly-6C(int) Ly-6G(high) granulocytes when isolated from the bone marrow, in particular during sepsis. Ficolin-B was demonstrated in lysates of primary granulocytes, ER-Hoxb8-derived granulocytes, bone marrow-derived macrophages, and dendritic cells. Native ficolin-B from cell lysates and supernatants of granulocytes activated the lectin pathway as measured by binding to MASP-2 and inducing C4 deposition. Specific staining demonstrated intra-cellular or cell associated ficolin-B protein in activated immature granulocytes deposited in a granular fashion. This study shows that ficolin-B is stored in and set free from immature granulocytic myeloid cells indicating a role in the early infection-induced cellular response of these inflammatory cells.

Humanized mice, a new model to study the influence of drug treatment on neonatal sepsis.

Bacterial infection with group B Streptococcus (GBS) represents a prominent threat to neonates and fetuses in the Western world, causing severe organ damage and even death. To improve current therapeutic strategies and to investigate new approaches, an appropriate in vivo model to study the immune response of a human immune system is needed. Therefore, we introduced humanized mice as a new model for GBS-induced sepsis. Humanized mice feature deficiencies similar to those found in neonates, such as lower immunoglobulin levels and myeloid cell dysfunction. Due to the husbandry in specific-pathogen-free (SPF) facilities, the human immune cells in these mice also exhibit a naive phenotype which mimics the conditions in fetuses/neonates. Following infection, cytokine release and leukocyte trafficking from the bone marrow to the lymphoid organ (spleen) and into the peritoneum (site of infection) as well as bacterial spreading and clearance were traceable in the humanized mice. Furthermore, we investigated the effects of betamethasone and indomethacin treatment using this novel sepsis model. Although both drugs are commonly used in perinatal care, little is known about their effects on the neonatal immune system. Treatment of infected humanized mice not only induced the reduction of human leukocytes in the spleen but also increased the bacterial load in all analyzed organs, including the brain, which did not show infiltration of live GBS in untreated controls. These studies demonstrate the utility of the humanized mice as a new model to study an immature human immune response during bacterial infection and allow the investigation of side effects induced by various treatments.

An early reduction in Treg cells correlates with enhanced local inflammation in cutaneous leishmaniasis in CCR6-deficient mice.

Resistance to Leishmania major infection is dependent on the development of a cell-mediated Th1 immune response in resistant C57BL/6 mice whereas Th2-prone BALB/c mice develop non-healing lesions after infection. The chemokine receptor CCR6 is shared by anti-inflammatory regulatory T cells and pro-inflammatory Th17 cells. In a recent study we showed that C57BL/6 mice deficient in CCR6 exhibited enhanced footpad swelling and impaired T helper cell migration indicated by reduced recruitment of total T helper cells into the skin after infection and a reduced delayed type hypersensitivity reaction. Based on these findings we tested whether the lack of CCR6 alters Treg or Th17 cell responses during the course of Leishmania major infection. When we analyzed T cell subsets in the lymph nodes of CCR6-deficient mice, Th17 cell numbers were not different. However, reduced numbers of Treg cells paralleled with a stronger IFNγ response. Furthermore, the early increase in IFNγ-producing cells correlated with increased local tissue inflammation at later time points. Our data indicate an important role of CCR6 for Treg cells and a redundant role for Th17 cells in a Th1 cell-driven anti-parasitic immune response against Leishmania major parasites in resistant C57BL/6 mice.

Hypoferraemia during the early inflammatory response is dependent on tumour necrosis factor activity in a murine model of protracted peritonitis.

On the grounds of clinical, in vitro and in vivo studies, tumour necrosis factor (TNF) is considered to be one of the inflammatory cytokines that contributes to to the generation of hypoferraemia and anaemia of inflammation (AI). We used a recently described murine model for AI and hypoferraemia, based on sublethal caecal ligation and puncture (CLP) with ensuing protracted peritonitis, to investigate the contribution of TNF to the generation of hypoferraemia. During the early inflammatory response to CLP, a marked decrease in serum iron concentration occurs within 8 h. To determine whether TNF contributes to the generation of hypoferraemia at this time point, we studied TNF-deficient mice and wild-type mice that underwent CLP. The serum iron concentration was decreased in wild-type mice whereas TNF-deficient mice maintained normal serum iron levels following CLP. Hypoferraemia in wild-type mice was accompanied by the downregulation of ferroportin 1 (Fp1) in macrophages. In the macrophages of TNF-deficient mice, Fp1 was not downregulated following CLP. The initial expression of hepcidin was detectable at the mRNA level but not at the protein level by immunohisto-chemistry in wild-type and TNF-deficient mice. Therefore, hepcidin does not appear to be involved in the regulation of early hypoferraemia. TNF appears to regulate the expression of Fp1 by transcriptional control. Our results demonstrate that TNF mediates hypoferraemia during the early inflammatory response by regulating the expression of Fp1 in macrophages.

Mechanisms of immune complex-mediated experimental glomerulonephritis: possible role of the balance between endogenous TNF and soluble TNF receptor type 2.

In an experimental model of immune-complex-mediated glomerulonephritis, mice excreted increased levels of urinary protein starting three days after the induction. Mice lacking the TNF receptor type 2 (TNFR2) were protected from early proteinuria and enhanced mortality. Analysis of the molecular basis of the mechanisms of glomerulonephritis revealed that naïve mice continuously excrete soluble TNF-neutralizing TNFR2 in urine. Mice kept in a specific pathogen-free environment did not go on to develop early proteinuria or enhanced mortality, following induction of glomerulonephritis. TNFR2-deficient mice were protected from early proteinuria and enhanced mortality only when housed conventionally. Mice producing human TNFR2 that can be activated by mouse TNF, in addition to mouse TNFR2, did not demonstrate enhanced susceptibility to the lethal effects of glomerulonephritis, indicating that pro-inflammatory signalling via TNFR2 does not account for a sensitizing effect. Finally, we suggest that the protective effect seen in mice lacking TNFR2 results rather from environment-induced attenuation by low dose bacterial endotoxins than from missing pro-inflammatory signalling via the TNFR2.

Inflammation augments the development of experimental glomerulonephritis by accelerating proteinuria and enhancing mortality.

Proteinuria represents a parameter for a damaged filtration capacity of the kidney. We investigated how inflammation influences the development of experimental, immune complex-mediated glomerulonephritis by monitoring proteinuria. Mice pre-treated with LPS or TNF, one day before induction of glomerulonephritis, excreted high levels of protein in the urine immediately after the induction of glomerulonephritis, in contrast to non-treated mice where proteinuria increased steadily after day 3. Protein levels in the urine of pre-treated mice remained elevated over the 15-day observation time. The severity of proteinuria at later times correlated with the degree of tissue pathology and mortality in individual mice. Pre-treatment with inflammatory agents accelerated the development of proteinuria and induced more severe kidney damage.

Immune cell populations and cytokine production in spleen and mesenteric lymph nodes after laparoscopic surgery versus conventional laparotomy in mice.

PURPOSE: There is evidence that open as well as minimally invasive abdominal surgery impair post-operative innate and acquired immune function. To compare the impact of these approaches as well as the one of different peritoneal gas exposures on immune function, we investigated cellular as well as cytokine-based immune parameters in mesenteric lymph nodes and the spleen postoperatively. METHODS: Mice (n = 26) were randomly assigned to the 4 study groups: (1) sham controls undergoing anesthesia alone, (2) laparotomy, and (3) air, or (4) carbon dioxide pneumoperitoneum. Mice were sacrificed 48 h after the intervention, and their spleens and mesenteric lymph nodes were harvested. Cytokine production (TNF-α, IL-6, IL-10, and IFN-γ), splenic T cell subpopulations (cytotoxic T cells, T helper cells, and regulatory T cells) were analyzed. RESULTS: TNF-α production of splenocytes 16 h after ex vivo lipopolysaccharides (LPS) stimulation was significantly increased in the laparotomy group compared to all other groups. In contrast, TNF-α production of lymph node cells and IL-6 production of splenocytes after ex vivo LPS stimulation did not differ significantly between the groups. The numbers of regulatory T cells (Treg) in the spleen differed between groups. A significant reduction in Treg cell frequency was detected in the CO(2) insufflation group compared to the laparotomy and the air insufflation group. CONCLUSION: Our findings demonstrate a distinct difference in immune effector functions and cellular composition of the spleen with regard to splenic TNF-α production and increased numbers of Treg cells in the spleen. These findings are in line with a higher peritoneal inflammatory status consequent to peritoneal air rather than CO(2) exposure. Treg turned out to be key modulators of postoperative dysfunction of acquired immunity.

Functional analysis of mouse ficolin-B and detection in neutrophils.

Ficolins and mannan-binding lectin recognize pathogen-associated molecular patterns and initiate the lectin pathway of complement activation via the associated serine proteases. In contrast to human ficolins and mouse ficolin-A, mouse ficolin-B has been considered incapable of complement activation. Dose-dependent binding of recombinant ficolin-B to immobilized GlcNAc, acetylated BSA, acetylated LDL, and fetuin was detected with ficolin-B-specific monoclonal antibodies. Recombinant ficolin-B bound to immobilized acetylated bovine serum albumin interacted with recombinant human mannan-binding lectin-associated serine protease-2, which led to C4 cleavage, thus demonstrating the capability of ficolin-B to activate the lectin pathway. Ficolin-B-specific monoclonal antibodies identified natural ficolin-B protein in lysates of mouse granulocytes isolated from the bone marrow. These results identify mouse ficolin-B as a functional member of the ficolin family activating complement via the lectin pathway.

Sepsis leads to a reduced antigen-specific primary antibody response.

Immunosuppression, impaired cytokine production and high susceptibility to secondary infections are characteristic for septic patients, and for mice after induction of polymicrobial septic peritonitis by sublethal cecal ligation and puncture (CLP). Here, we demonstrate that CLP markedly altered subsequent B-cell responses. Total IgG and IgM levels, as well as the memory B-cell response, were increased in septic mice, but antigen-specific primary antibody production was strongly impaired. We found that two days after CLP, CD11b(+) splenocytes were activated as demonstrated by the increased expression of activation markers, expression of arginase and production of NO by immature myeloid cells. The in vivo clearance of a bacterial infection was not impaired. DCs demonstrated reduced IL-12 production and altered antigen presentation, resulting in decreased proliferation but enhanced IFN-γ production by CD4(+) cells. CD4(+) T cells from mice immunized on day 2 after CLP showed reduced Th1 and Th2 cytokine production. In addition, there was an increase in Treg cells. Interestingly, levels of immature B cells decreased but levels of mature B cells increased two days after CLP. However, adoptive transfer of naïve CD4(+) T cells, naïve B cells, or naïve DCs did not rescue the antigen-specific antibody response.

Ficolin-B marks apoptotic and necrotic cells.

Ficolins are a group of proteins consisting of a fibrinogen-like and a collagen-like domain. They play a role in innate immunity by activating the complement system via the lectin pathway upon binding to carbohydrate patterns on pathogens. Two types of ficolins have been identified in mice, ficolin A and ficolin B (FcnB). We show in this article that recombinant FcnB binds to late apoptotic cells and to apoptotic bodies as well as to necrotic cells but not to early apoptotic cells. This binding was calcium-dependent and could be competitively inhibited by acetylated BSA, a classical binding substrate of FcnB. In addition, DNA inhibited binding of FcnB to apoptotic and necrotic cells, indicating that DNA exposed by dying cells could also be a ligand for FcnB. Thus, FcnB may play a role in the removal of damaged host cells and maintenance of tissue homeostasis.