In this paper, we provide evidence that anergized NK cells through secreted factors and direct cell–cell contact have the ability to induce differentiation of healthy dental pulp stem cells and stem cell of apical papillae as well as transformed oral squamous cancer stem cell (OSCSC) and Mia-Paca-2, poorly differentiated stem-like pancreatic tumors, resulting in their resistance to NK cell-mediated cytotoxicity. Induction of NK cell resistance and differentiation in the stem cells correlated with the increased expression of CD54, B7H1, and MHC class I, and mediated by the combination of membrane-bound or secreted IFN-γ and TNF-α from the NK cells since antibodies to both cytokines and not each one alone were able to inhibit differentiation or resistance to NK cells. Similarly, antibodies to both TNF-α and IFN-γ were required to prevent NK-mediated inhibition of cell growth, and restored the numbers of the stem cells to the levels obtained when stem cells were cultured in the absence of anergized NK cells. Interestingly, the effect of anti-IFN-γ antibody in the absence of anti-TNF-α antibody was more dominant for the prevention of increase in surface receptor expression since its addition abrogated the increase in CD54, B7H1, and MHC class I surface expression. Antibodies to CD54 or LFA-1 was unable to inhibit differentiation whereas antibodies to MHC class I but not B7H1 increased cytotoxicity of well-differentiated oral squamous carcinoma cells as well as OSCSCs differentiated by the IL-2 + anti-CD16 mAb-treated NK cells whereas it inhibited the cytotoxicity of NK cells against OSCSCs. Thus, NK cells may inhibit the progression of cancer by killing and/or differentiation of cancer stem cells, which severely halt cancer growth, invasion, and metastasis.
IFN-γ; NK; OSCSCs; OSCCs; MP2; cytotoxicity; regulatory NK
Cysteine cathepsins are lysosomal peptidases involved at different levels in the processes of the innate and adaptive immune responses. Some, such as cathepsins B, L, and H are expressed constitutively in most immune cells. In cells of innate immunity they play a role in cell adhesion and phagocytosis. Other cysteine cathepsins are expressed more specifically. Cathepsin X promotes dendritic cell maturation, adhesion of macrophages, and migration of T cells. Cathepsin S is implicated in major histocompatibility complex class II antigen presentation, whereas cathepsin C, expressed in cytotoxic T lymphocytes and natural killer (NK) cells, is involved in processing pro-granzymes into proteolytically active forms, which trigger cell death in their target cells. The activity of cysteine cathepsins is controlled by endogenous cystatins, cysteine protease inhibitors. Of these, cystatin F is the only cystatin that is localized in endosomal/lysosomal vesicles. After proteolytic removal of its N-terminal peptide, cystatin F becomes a potent inhibitor of cathepsin C with the potential to regulate pro-granzyme processing and cell cytotoxicity. This review is focused on the role of cysteine cathepsins and their inhibitors in the molecular mechanisms leading to the cytotoxic activity of T lymphocytes and NK cells in order to address new possibilities for regulation of their function in pathological processes.
cathepsins; cystatins; cytotoxicity; natural killer cells; cytotoxic T cells
Background: Colorectal carcinogenesis is believed to be a multi-stage process that originates with a localized adenoma, which linearly progresses to an intra-mucosal carcinoma, to an invasive lesion, and finally to metastatic cancer. This progression model is supported by tissue culture and animal model studies, but it is difficult to reconcile with several well-established observations, principally among these are that up to 25% of early stage (Stage I/II), node-negative colorectal cancer (CRC) develop distant metastasis, and that circulating CRC cells are undetectable in peripheral blood samples of up to 50% of patients with confirmed metastasis, but more than 30% of patients with no detectable metastasis exhibit such cells. The mechanism responsible for this diverse behavior is unknown, and there are no effective means to identify patients with pending, or who are at high risk for, developing metastatic CRC.
Novel findings: Our previous studies of human breast and prostate cancer have shown that cancer invasion arises from the convergence of a tissue injury, the innate immune response to that injury, and the presence of tumor stem cells within tumor capsules at the site of the injury. Focal degeneration of a capsule due to age or disease attracts lymphocyte infiltration that degrades the degenerating capsules resulting in the formation of a focal disruption in the capsule, which selectively favors proliferating or “budding” of the underlying tumor stem cells. Our recent studies suggest that lymphocyte infiltration also triggers metastasis by disrupting the intercellular junctions and surface adhesion molecules within the proliferating cell buds causing their dissociation. Then, lymphocytes and tumor cells are conjoined through membrane fusion to form tumor-lymphocyte chimeras (TLCs) that allows the tumor stem cell to avail itself of the lymphocyte's natural ability to migrate and breach cell barriers in order to intravasate and to travel to distant organs. Our most recent studies of human CRC have detected nearly identical focal capsule disruptions, lymphocyte infiltration, budding cells, and the formation of TLCs. Our studies have further shown that age- and type-matched node-positive and -negative CRC have a significantly different morphological and immunohistochemical profile and that the majority of lymphatic ducts with disseminated cells are located within the mucosa adjacent to morphologically normal appearing epithelial structures that express a stem cell-related marker.
New hypothesis: Based on these findings and the growth patterns of budding cells revealed by double immunohistochemistry, we further hypothesize that metastatic spread is an early event of carcinogenesis and that budding cells overlying focal capsule disruptions represent invasion- and metastasis-initiating cells that follow one of four pathways to progress: (1) to undergo extensive in situ proliferation leading to the formation of tumor nests that subsequently invade the submucosa, (2) to migrate with associated lymphocytes functioning as “seeds” to grow in new sites, (3) to migrate and intravasate into pre-existing vascular structures by forming TLCs, or (4) to intravasate into vascular structures that are generated by the budding cells themselves. We also propose that only node-positive cases harbor stem cells with the potential for multi-lineage differentiation and unique surface markers that permit intravasation.
Lymphocyte infiltration; tumor capsule; tumor invasion; tumor metastasis; stem cell.
Accumulated evidence from our laboratory indicates that conditioned or anergized NK cells have the ability to induce resistance of healthy stem cells and transformed cancer stem cells through both secreted factors and direct cell-cell contact by inducing differentiation. Cytotoxic function of NK cells is suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. Furthermore, decreased peripheral blood NK cell function has been documented in many cancer patients. We have previously shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells (OSCCs). In addition, human embryonic stem cells (hESCs), human mesenchymal stem cells (hMSCs), human dental pulp stem cells (hDPSCs) and induced human pluripotent stem cells (hiPSCs) were all significantly more susceptible to NK cell mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or gene deletion of COX2 significantly augmented NK cell function. Furthermore, the induction of resistance of the stem cells to NK cell mediated cytotoxicity and their subsequent differentiation is amplified when either the stem cells or the NK cells were cultured in the presence of monocytes. Therefore, we propose that the two stages of NK cell maturation namely CD16+CD56dimCD69- NK cells are important for the lysis of stem cells or poorly differentiated cells whereas the CD16dim/-CD56dim/+CD69+NK cells are important for differentiation and eventual regeneration of the tissues and the resolution of inflammation, thus functionally serving as regulatory NK cells (NKreg). CD16 receptor on the NK cells were found to be the receptor with significant potential to induce NK cell anergy, however, our recent data indicated that NKp46 but not NKp30 or NKp44 were also able to induce significant anergy in NK cells, although the levels were less when compared to CD16 receptor triggering. The concept of split anergy in NK cells and generation of NKreg and its contribution to cell differentiation, tissue repair and regeneration and in tumor resistance will be discussed in this review.
apoptosis; NFκB; NK; cancer stem cells; differentiation; Regulation.
It is a commonly held belief that infiltration of immune cells into tumor tissues and direct physical contact between tumor cells and infiltrated immune cells is associated with physical destructions of the tumor cells, reduction of the tumor burden, and improved clinical prognosis. An increasing number of studies, however, have suggested that aberrant infiltration of immune cells into tumor or normal tissues may promote tumor progression, invasion, and metastasis. Neither the primary reason for these contradictory observations, nor the mechanism for the reported diverse impact of tumor-infiltrating immune cells has been elucidated, making it difficult to judge the clinical implications of infiltration of immune cells within tumor tissues. This mini-review presents several existing hypotheses and models that favor the promoting impact of tumor-infiltrating immune cells on tumor invasion and metastasis, and also analyzes their strength and weakness.
immune cell; tumor progression; invasion; metastasis
Background: Our previous studies of human breast and prostate cancer have shown that aberrant immune cell infiltration is associated with focal tumor capsule disruption and tumor cell budding that facilitate invasion and metastasis. Our current study attempted to determine whether aberrant immune cell infiltration would have similar impact on colorectal cancer (CRC).
Materials and Methods: Tissue sections from 100 patients with primary CRC were assessed for the frequencies of focal basement membrane (BM) disruption, muscularis mucosa (MM) fragmentation, and tumor cell dissemination in epithelial structures adjacent and distal to infiltrating lymphoid aggregates using a panel of biomarkers and quantitative digital imaging.
Results: Our study revealed: (1) epithelial structures adjacent to lymphoid follicles or aggregates had a significantly higher (p<0.001) frequency of focally disrupted BM, dissociated epithelial cells in the stroma, disseminated epithelial cells within lymphatic ducts or blood vessels, and fragmented MM than their distal counterparts, (2) a majority of dissociated epithelial cells within the stroma or vascular structures were immediately subjacent to or physically associated with infiltrating immune cells, (3) the junctions of pre-invasive and invasive lesions were almost exclusively located at sites adjacent to lymphoid follicles or aggregates, (4) infiltrating immune cells were preferentially associated with epithelial capsules that show distinct degenerative alterations, and (5) infiltrating immune cells appeared to facilitate tumor stem cell proliferation, budding, and dissemination.
Conclusions: Aberrant immune cell infiltration may have the same destructive impact on the capsule of all epithelium-derived tumors. This, in turn, may selectively favor the proliferation of tumor stem or progenitor cells overlying these focal disruptions. These proliferating epithelial tumor cells subsequently disseminate from the focal disruption leading to tumor invasion and metastasis.
Colorectal cancer; tumor capsule; tumor invasion; metastasis, lymphocyte aggregates.
Because three-dimensional (3D) in vitro models are more accurate than 2D cell culture models and faster and cheaper than animal models, they have become a prospective trend in the biomedical and pharmaceutical fields, especially for personalized and targeted therapies. Because appropriate 3D models can be customized to mimic the in vivo microenvironment wherein various cell populations grow within an intricate but well organized extracellular matrix (ECM), they can accurately recapitulate physiological and pathophysiological progressions. The majority of cancers are carcinomas, which originate from epithelial cells, and dynamically interact with non-malignant cells including stromal cells (fibroblasts), vascular cells (endothelial cells and pericytes), immune cells (macrophages and mast cells), and the ECM. Employing a tumor monoclonal colony, tumor xenograft or patient cancer biopsy into an in vivo-like microenvironment, the native signaling pathways, cell-cell and cell-matrix interactions, and cell phenotypes are preserved and our fluorescent phenotypic 3D co-culture platforms can then accurately recapitulate the tumor in vivo scenario including tumor induced angiogenesis, tumor growth, and metastasis.
In this paper, we describe a robust and standardized method to co-culture a tumor colony or biopsy with different cell populations, e.g., endothelial cells, immune cells, pericytes, etc. The procedures for recovering cells from the co-culture for molecular analyses, imaging, and analyzing are also described. We selected ECM solubilized extract derived from Engelbreth-Holm-Swam sarcoma cells. Because the 3D co-culture platforms can provide drug chemosensitivity data within 9 days that is equivalent to the results generated from mouse tumor xenograft models in 50 days, the 3D co-culture platforms are more accurate, efficient, and cost-effective and may replace animal models in the near future to predict drug efficacy, personalize therapies, prevent drug resistance, and improve the quality of life.
3D co-culture platform; in vivo; tumor
Background: A great number of in vitro and in vivo studies have suggested that many pathways or factors can stimulate angiogenesis and lymphangiogenesis, which facilitate tumor progression and metastasis. However, the morphological and immunohistochemical profile of newly formed vasculatures has not been elucidated, making it difficult to differentiate them from the pre-existing ones, and to identify their unique molecular profiles for diagnosis and therapeutic interventions.
Experimental findings: As cytokeratin (CK)-19 is a well-recognized stem cell marker and CK-19-positive cells are frequently detected in the peripheral blood of patients with metastatic cancer, our recent studies have assessed the involvement of CK-19 in the formation of new vasculatures in primary colorectal cancer (CRC) tissues. Our studies showed that a subset of lymph node-positive cases harbored some isolated normal epithelial structures with distinct CK-19 immunostaining within an otherwise CK-19-negative background. These structures are exclusively located within or adjacent to lymphoid follicles and are often surrounded by tube-like structures expressing lymphatic endothelial marker D2-40. Similar structures are more frequently seen at the junctions between pre-invasive and invasive CRC with the following features: (1). they consist of a single layer of endothelial cells that express both D2-40 and CD34, (2). their endothelial walls are often incomplete with disseminated cells protruding into the adjacent stroma, and (3). they are exclusively associated with disseminated CK-19-positive cells
Hypothesis: Based on these findings, we propose that these tube-like structures represent newly formed vasculatures, which are derived by the convergence of aberrant lymphocyte infiltration and tumor stem cells. Because of their close physical proximity, tumor stem cells within the epithelial and stromal components contribute equally and coordinately to the morphogenesis of new vasculatures, which constitutes the basis for the unique morphologic and immunohistochemical features of newly formed vasculatures. Our hypothesis appears to be applicable to all epithelium-derived cancers.
CK-19; epithelium-derived cancers; tube-like structures
Bacterially induced cell death in human lymphocytes is an important virulence factor for pathogenic bacteria. Previously discovered mechanisms of bacterially induced cell death are predominantly based on the transfer of bacterial proteins to the target host cell, such as the toxins secreted through type I, II, and VI secretion systems or effector proteins injected through type III, IV, and Vb secretion systems. Here, we report a mechanism employed by the Gram-negative oral pathogen Fusobacterium nucleatum for cell death induction of human lymphocytes via two outer membrane proteins (OMPs), Fap2 and RadD, which share regions homologous to autotransporter secretion systems (type Va secretion systems). Genetic and physiological studies established that inactivation of the two OMPs led to significantly reduced ability to trigger cell death in Jurkat cells, while the corresponding double mutant was almost completely attenuated. Additional biochemical and molecular analyses demonstrated that cell-free F. nucleatum membranes are sufficient to induce cell death in Jurkat cells, suggesting that no active process or effector protein transfer was necessary to induce eukaryotic cell death.
Accumulated evidence indicates that cytotoxic function of immune effectors is largely suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. The aims of this review are to provide a rationale and a potential mechanism for immunosuppression in cancer and to demonstrate the significance of such immunosuppression in cellular differentiation and progression of cancer. To that end, we have recently shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells (OSCCs). In addition, human embryonic stem cells (hESCs), Mesenchymal Stem Cells (hMSCs), dental pulp stem cells (hDPSCs) and induced pluripotent stem cells (hiPSCs) were all significantly more susceptible to NK cell mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or targeted knock down of COX2 in primary monocytes in vivo significantly augmented NK cell function. Total population of monocytes and those depleted of CD16(+) subsets were able to substantially prevent NK cell mediated lysis of OSCSCs, MSCs and DPSCs. Taken together, our results suggest that stem cells are significant targets of the NK cell cytotoxicity. The concept of split anergy in NK cells and its contribution to tissue repair and regeneration and in tumor resistance and progression will be discussed in this review.
apoptosis; NFκB; NK; cancer stem cells; differentiation
The aims of this study are to demonstrate the increased lysis of stem cells but not their differentiated counterparts by the NK cells and to determine whether disturbance in cell differentiation is a cause for increased sensitivity to NK cell mediated cytotoxicity. Increased cytotoxicity and augmented secretion of IFN-γ were both observed when PBMCs or NK cells were co-incubated with primary UCLA oral squamous carcinoma stem cells (UCLA-OSCSCs) when compared to differentiated UCLA oral squamous carcinoma cells (UCLA-OSCCs). In addition, human embryonic stem cells (hESCs) were also lysed greatly by the NK cells. Moreover, NK cells were found to lyse human Mesenchymal Stem Cells (hMSCs), human dental pulp stem cells (hDPSCs) and human induced pluripotent stem cells (hiPSCs) significantly more than their differentiated counterparts or parental lines from which they were derived. It was also found that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or targeted knock down of COX2 in monocytes significantly augmented NK cell cytotoxicity and secretion of IFN-γ. Taken together, these results suggest that stem cells are significant targets of the NK cell cytotoxicity. However, to support differentiation of a subset of tumor or healthy untransformed primary stem cells, NK cells may be required to lyse a number of stem cells and/or those which are either defective or incapable of full differentiation in order to lose their cytotoxic function and gain the ability to secrete cytokines (split anergy). Therefore, patients with cancer may benefit from repeated allogeneic NK cell transplantation for specific elimination of cancer stem cells.
The mechanisms by which resin based materials induce adverse effects in patients have not been completely elucidated. Here we show that 2-hydroxyethyl methacrylate (HEMA) induces apoptotic cell death in oral keratinocytes. Functional loss and cell death induced by HEMA was significantly inhibited in the presence of N-acetyl cysteine (NAC) treatment. NAC also prevented HEMA mediated decrease in vascular endothelial growth factor secretion. The protective effect of NAC was partly related to its ability to induce NF-κB in the cells, since HEMA mediated inhibition of nuclear NF-κB expression and function was significantly blocked in the presence of NAC treatment. Moreover, blocking of nuclear translocation of NF-κB in oral keratinocytes sensitized these cells to HEMA mediated apoptosis. In addition, since NAC was capable of rescuing close to 50% of NF-κB knockdown cells from HEMA mediated cell death, there is, therefore, an NF-κB independent pathway of protection from HEMA mediated cell death by NAC. NAC mediated prevention of HEMA induced cell death in NF-κB knockdown cells was correlated with a decreased induction of c-Jun N-terminal kinase (JNK) activity since NAC inhibited HEMA mediated increase in JNK levels. Furthermore, the addition of a pharmacologic JNK inhibitor to HEMA treated cells prevented cell death and restored NF-κB knockdown cell function significantly. Therefore, NAC protects oral keratinocytes from the toxic effects of HEMA through NF-κB dependent and independent pathways. Moreover, our data suggest the potential involvement of JNK pathway in NAC mediated protection.
HEMA; NF-κB; dental pulp stromal cells; apoptosis; VEGF
The aim of this paper is to study the function of allogeneic and autologous NK cells against Dental Pulp Stem Cells (DPSCs) and Mesenchymal Stem Cells (MSCs) and to determine the function of NK cells in a three way interaction with monocytes and stem cells.
We demonstrate here that freshly isolated untreated or IL-2 treated NK cells are potent inducers of cell death in DPSCs and MSCs, and that anti-CD16 antibody which induces functional split anergy and apoptosis in NK cells inhibits NK cell mediated lysis of DPSCs and MSCs. Monocytes co-cultured with either DPSCs or MSCs decrease lysis of stem cells by untreated or IL-2 treated NK cells. Monocytes also prevent NK cell apoptosis thereby raising the overall survival and function of NK cells, DPSCs or MSCs. Both total population of monocytes and those depleted of CD16+ subsets were able to prevent NK cell mediated lysis of MSCs and DPSCs, and to trigger an increased secretion of IFN-γ by IL-2 treated NK cells. Protection of stem cells from NK cell mediated lysis was also seen when monocytes were sorted out from stem cells before they were added to NK cells. However, this effect was not specific to monocytes since the addition of T and B cells to stem cells also protected stem cells from NK cell mediated lysis. NK cells were found to lyse monocytes, as well as T and B cells.
By increasing the release of IFN-γ and decreasing the cytotoxic function of NK cells monocytes are able to shield stem cells from killing by the NK cells, resulting in an increased protection and differentiation of stem cells. More importantly studies reported in this paper indicate that anti-CD16 antibody can be used to prevent NK cell induced rejection of stem cells.
Genotoxic agents such as ionizing radiation trigger cell cycle arrest at the G1/S and G2/M checkpoints, allowing cells to repair damaged DNA before entry into mitosis. DNA damage-induced G1 arrest involves p53-dependent expression of p21 (Cip1/Waf-1), which inhibits cyclin-dependent kinases and blocks S phase entry. While much of the core DNA damage response has been well-studied, other signaling proteins that intersect with and modulate this response remain uncharacterized. In this study, we identify Suppressor of Cytokine Signaling (SOCS)-3 as an important regulator of radiation-induced G1 arrest. SOCS3-deficient fibroblasts fail to undergo G1 arrest and accumulate in the G2/M phase of the cell cycle. SOCS3 knockout cells phosphorylate p53 and H2AX normally in response to radiation, but fail to upregulate p21 expression. In addition, STAT3 phosphorylation is elevated in SOCS3-deficient cells compared to WT cells. Normal G1 arrest can be restored in SOCS3 KO cells by retroviral transduction of WT SOCS3 or a dominant-negative mutant of STAT3. Our results suggest a novel function for SOCS3 in the control of genome stability by negatively regulating STAT3-dependent radioresistant DNA synthesis, and promoting p53-dependent p21 expression.
SOCS3; Cell cycle; Signaling; p21; STAT3; DNA damage
Resin based materials are now widely used in dental restorations. While the use of these materials is aesthetically appealing in patients, they carry the risk of local and systemic adverse effects. The potential risks are direct damage to the cells and induction of immune-based hypersensitivity reactions. Dental Pulp Stromal Cells (DPSCs) and oral keratinocytes are the major cell types which may come in contact with dental resins such as 2-hydroxyethyl methacrylate (HEMA) after dental restorations. Here we show that N-acetyl cysteine (NAC) inhibits HEMA induced apoptotic cell death and restores the function of DPSCs and oral epithelial cells. NAC inhibits HEMA mediated toxicity through induction of differentiation in DPSCs since the genes for dentin sialoprotein (DSP), Osteopontin (OPN), Osteocalcin (OCN), and Alkaline Phosphatase (ALP) which are induced during differentiation are also induced by NAC. Unlike NAC, Vitamin E and C which known anti-oxidant compounds failed to prevent either HEMA mediated cell death or decrease in VEGF secretion by human DPSCs. More importantly, when added either alone or in combination with HEMA Vitamin E and Vitamin C did not increase the gene expression for OPN, and in addition Vitamin E inhibited the protective effect of NAC on DPSCs. NAC inhibited HEMA mediated decrease in NFκB activity thus, providing survival mechanism for the cells. Overall, the studies reported in this paper indicated that undifferentiated DPSCs have exquisite sensitivity to HEMA induced cell death, and their differentiation by NAC resulted in an increased NFκB activity which might have provided the basis for their increased protection from HEMA mediated functional loss and cell death.
NAC; NFκB; Dental Pulp Stromal Cells; differentiation; VEGF
Increased induction of interleukin 8 (IL-8) and intercellular adhesion molecule 1 (ICAM-1) by oral epithelial cells may play a role in the host defense mechanism in oropharyngeal candidiasis; however, little is known about the expression feature of these molecules on human gingival epithelial cells (HGECs) during Candida albicans infection. In this report we present evidence that neutralization with antibody against ICAM-1 inhibited both the adherence of C. albicans to HGECs and the Candida-induced production of IL-8, suggesting a role for ICAM-1 in recognition and signaling in HGECs to express IL-8 upon infection with C. albicans.
It is largely unknown why a variety of bacteria present in the oral cavity are capable of establishing themselves in the periodontal pockets of nonimmunocompromised individuals in the presence of competent immune effector cells. In this paper we present evidence for the immunosuppressive role of Fusobacterium nucleatum, a gram-negative oral bacterium which plays an important role in the generation of periodontal disease. Our studies indicate that the immunosuppressive role of F. nucleatum is largely due to the ability of this organism to induce apoptotic cell death in peripheral blood mononuclear cells (PBMCs) and in polymorphonuclear cells (PMNs). F. nucleatum treatment induced apoptosis of PBMCs and PMNs as assessed by an increase in subdiploid DNA content determined by DNA fragmentation and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end-labeling assays. The ability of F. nucleatum to induce apoptosis was abolished by either heat treatment or proteinase digestion but was retained after formaldehyde treatment, suggesting that a heat-labile surface protein component is responsible for bacterium-mediated cell apoptosis. The data also indicated that F. nucleatum-induced cell apoptosis requires activation of caspases and is protected by NF-κB. Possible mechanisms of F. nucleatum's role in the pathogenesis of periodontal disease are discussed.