Factors influencing the phagocytosis of Clostridium difficile by human polymorphonuclear leukocytes

Engineered antibodies targeted to bacterial surface integrate effector functions with toxin neutralization to provide superior efficacy against bacterial infections

Anti-bacterial monoclonal antibody (mAb) therapies either rely on toxin neutralization or opsonophagocytic killing (OPK). Toxin neutralization protects the host from toxin-induced damage, while leaving the organism intact. OPK inducing antibodies clear the bacteria but leave the released toxins unencountered. Infection site targeted anti-toxin antibodies (ISTAbs) that we report here addresses this binary paradigm by combining both functionalities into a single molecule. ISTAbs consist of cell wall targeting (CWT) domains of bacteriophage endolysins fused to toxin neutralizing mAbs (IgG). CWT governs specific binding to the surface of bacteria while the IgG variable domain neutralizes the toxins as they are released. The complex is then cleared by phagocytic cells. As proof of concept, we generated several ISTAb prototypes targeting major toxins from two Gram-positive spore forming pathogens that have a high clinical significance; Clostridium difficile , causative agent of the most common hospital-acquired infection, and Bacillus anthracis , a Category A select agent pathogen. Both groups of ISTAbs exhibited potent toxin neutralization, binding to their respective bacterial cells, and induction of opsonophagocytosis. In mice infected with B. anthracis , ISTAbs exhibit significantly higher efficacy than parental IgG in both pre- and post-challenge models. Furthermore, ISTAbs fully protected against B. anthracis infection in a nonhuman primate (NHP) aerosol challenge model. These findings establish that as a platform technology, ISTAbs are broadly applicable for therapeutic intervention against several toxigenic bacterial pathogens.

The effect of different C. difficile MLST strains on viability and activity of macrophages

Objectives

Clostridioides difficile is the most common infectious agent of nosocomial diarrhea. C. difficile infection (CDI) pathogenesis and disease severity depend on its toxins (toxins A, B and binary) and on the host's immune response, especially the innate immune system. The current study examined the efficacy of macrophage activity, macrophages viability and cytokine secretion levelsin response to different sequence type (ST) strains of C. difficile.

Methods

RAW 264.7 macrophages were exposed to six different strains of C. difficile as well as to both toxins A and B and macrophage viability was measured. The levels of four secreted cytokines were determined by RT-PCR and ELISA. Morphological changes to the macrophages were investigated by fluorescent microscopy.

Results

Strains ST37 and ST42 affected macrophages' vitality the most. Toxins A and B led to a significant reduction in macrophages' vitality at most time points. In addition, starting at 30-min post-exposure to 5 ng/μl of both toxins led to significant differences in macrophage viability versus at lower concentrations. Furthermore, cytokine secretion levels, including IL-12, IL-6 and TNF-α, increased dramatically when macrophages were exposed to strains ST42 or ST104. Finally, gene expression surveys point to increases in IL-12 gene expression in response to both ST42 and ST104.

Conclusions

C. difficile strains with higher toxins levels induced an increased activation of the innate immune system and may activate macrophages more profoundly resulting in secretion of higher levels of pro-inflammatory cytokines. However, higher toxin levels may also damage macrophages' normal skeletal structure, reducing macrophage viability.

Role of the leukocyte response in normal and immunocompromised host after Clostridium difficile infection

Clostridium difficile is the leading cause of healthcare-associated infections in the United States. Clinically, C. difficile-associated disease can present as asymptomatic colonization, self-limited diarrheal illness or severe colitis (that may result in death). This variability in disease course and outcomes suggests that host factors play an important role as key determinants of disease severity. Currently, there are several scoring indices to estimate severity of C. difficile-associated disease. Leukocytosis and renal failure are considered to be the most important predictors of C. difficile disease severity in hosts with a normal immune system. The degree of leukocytosis which is considered significant for severe disease and how it is scored vary amongst scoring indices. None of the scores have been prospectively validated, and while total WBC count is useful to estimate the magnitude of the host response in most patient populations, in immune-compromised patients like those receiving chemotherapy, solid organ transplant patients or hematopoietic stem cell transplants the WBC response can be variable or even absent making this marker of severity difficult to interpret. Other cellular subsets like neutrophils, eosinophils and lymphocytes provide important information about the host immune status and play an important role in the immune response against C. difficile infection. However, under the current scoring systems the role of these cellular subsets have been underestimated and only total white blood cell counts are taken into account. In this review we highlight the role of host leukocyte response to C. difficile challenge in the normal and immunocompromised host, and propose possible ways that would allow for a better representation of the different immune cell subsets (neutrophils, lymphocytes and eosinophils) in the current scoring indices.

Older Is Not Wiser, Immunologically Speaking: Effect of Aging on Host Response to Clostridium difficile Infections

Clostridium difficile infection (CDI) is the most common cause of antibiotic-associated diarrhea and a significant burden on the health care system. Aging has been identified in the literature as a risk factor for CDI as well as adverse outcome from CDI. Although this effect of advanced age on CDI could be partially explained by clinical factors associated with aging, biologic factors are important. Innate immune system, responsible for immediate response to acute infections, plays a major role in CDI pathogenesis. Impairment in function of innate immunity with aging, demonstrated in other infection models, may lead to worse outcome with CDI. C. difficile toxin-specific antibody response protects the host against initial and recurrent infections as shown in observational studies and clinical trial. Effect of aging on antibody response to CDI has not been demonstrated, but the results from vaccine studies in other infections suggest a negative effect on humoral immunity from aging. Although intestinal microbiota from healthy people confers resistance to CDI by preventing C. difficile colonization, changes in composition of microbiota with aging may affect that resistance and increase risk for CDI. There are also age-associated changes in physiology, especially of the gastrointestinal tract, that may play a role in CDI risk and outcomes. In this review, we will first discuss the epidemiology of CDI in the elderly people, then the alteration in innate immunity, humoral response, and microbiota that increases susceptibility to CDI and severe disease and lastly, the physiological and functional changes that may modify outcomes of infection.

Neutrophil-mediated inflammation in the pathogenesis of Clostridium difficile infections

Clostridium difficile is the most important cause of nosocomial infectious diarrhea in the western world. C. difficile infections are a major healthcare burden with approximately 500,000 new cases every year and an estimated annual cost of nearly $1 billion in the U.S. Furthermore, the infections are no longer restricted to health care facilities, and recent studies indicate spread of C. difficile infection to the community as well. The clinical spectrum of C. difficile infection ranges from asymptomatic colonization to severe diarrhea, fulminant colitis and death. This spectrum results from a complex interplay between bacterial virulence factors, the colonic microbiome and the host inflammatory response. The overall vigor of host inflammatory response is believed to be an important determinant of C. difficile disease severity, and a more robust immune response is associated with worse outcomes. Neutrophils are the primary cells that respond to C. difficile invasion and neutrophilic inflammation is the hallmark of C. difficile-associated disease. In this review, we will focus on the role of neutrophils (infiltration to infected tissue, pathogen clearance and resolution of inflammation) in the immuno-pathogenesis of C. difficile-associated disease (CDAD).

High temporal resolution of glucosyltransferase dependent and independent effects of Clostridium difficile toxins across multiple cell types

BACKGROUND

Clostridium difficile toxins A and B (TcdA and TcdB), considered to be essential for C. difficile infection, affect the morphology of several cell types with different potencies and timing. However, morphological changes over various time scales are poorly characterized. The toxins' glucosyltransferase domains are critical to their deleterious effects, and cell responses to glucosyltransferase-independent activities are incompletely understood. By tracking morphological changes of multiple cell types to C. difficile toxins with high temporal resolution, cellular responses to TcdA, TcdB, and a glucosyltransferase-deficient TcdB (gdTcdB) are elucidated.

RESULTS

Human umbilical vein endothelial cells, J774 macrophage-like cells, and four epithelial cell lines (HCT8, T84, CHO, and immortalized mouse cecal epithelial cells) were treated with TcdA, TcdB, gdTcdB. Impedance across cell cultures was measured to track changes in cell morphology. Metrics from impedance data, developed to quantify rapid and long-lasting responses, produced standard curves with wide dynamic ranges that defined cell line sensitivities. Except for T84 cells, all cell lines were most sensitive to TcdB. J774 macrophages stretched and increased in size in response to TcdA and TcdB but not gdTcdB. High concentrations of TcdB and gdTcdB (>10 ng/ml) greatly reduced macrophage viability. In HCT8 cells, gdTcdB did not induce a rapid cytopathic effect, yet it delayed TcdA and TcdB's rapid effects. gdTcdB did not clearly delay TcdA or TcdB's toxin-induced effects on macrophages.

CONCLUSIONS

Epithelial and endothelial cells have similar responses to toxins yet differ in timing and degree. Relative potencies of TcdA and TcdB in mouse epithelial cells in vitro do not correlate with potencies in vivo. TcdB requires glucosyltransferase activity to cause macrophages to spread, but cell death from high TcdB concentrations is glucosyltransferase-independent. Competition experiments with gdTcdB in epithelial cells confirm common TcdA and TcdB mechanisms, yet different responses of macrophages to TcdA and TcdB suggest different, additional mechanisms or targets in these cells. This first-time, precise quantification of the response of multiple cell lines to TcdA and TcdB provides a comparative framework for delineating the roles of different cell types and toxin-host interactions.

Immune responses to Clostridium difficile infection

Clostridium difficile is the causal agent of antibiotic-associated diarrhea and is a leading cause of hospital-acquired infections in the US. C. difficile has been known to cause severe diarrhea and colitis for more than 30 years, but the emergence of a newer, hypervirulent strain of C. difficile (BI/NAP1) has further compounded the problem, and recently both the number of cases and mortality associated with C. difficile-associated diarrhea have been increasing. One of the major drivers of disease pathogenesis is believed to be an excessive host inflammatory response. A better understanding of the host inflammation and immune mechanisms that modulate the course of disease and control host susceptibility to C. difficile could lead to novel (host-targeted) strategies for combating the challenges posed by this deadly infection. This review summarizes our current knowledge of the host inflammatory response during C. difficile infection.

Differential binding and internalization of Clostridium difficile toxin A by human peripheral blood monocytes, neutrophils and lymphocytes

Colitis due to Clostridium difficile infection is mediated by secreted toxins A and B and is characterized by infiltration by cells from the systemic circulation. The aim of our study was to investigate interactions between fluorescently labelled toxin A and peripheral blood monocytes, neutrophils and lymphocytes. Purified toxin A was labelled with Alexa Fluor® 488 (toxin A(488)) and incubated with isolated human peripheral blood mononuclear cells or washed whole blood cells for varying time intervals at either 37 or 4 °C/ice. The ability of trypan blue to quench cell surface-associated (but not cytoplasmic) fluorescence was also investigated. At 37 °C, toxin A(488) -associated fluorescence in monocytes peaked at 1 h (majority internalized), with subsequent loss associated with cell death. In contrast to monocytes, binding of toxin A(488) in neutrophils was greater on ice than at 37 °C. Studies using trypan blue suggested that over 3 h at 37 °C, most of the toxin A(488)-associated fluorescence in neutrophils remained at the cell surface. Over 48 h (37 °C and ice/4 °C), there was minimal toxin A(488)-associated fluorescence in lymphocytes. These studies suggest major differences in interactions between toxin A and circulating cells that infiltrate the mucosa during colonic inflammation in C. difficile infection.

Toxins-useful biochemical tools for leukocyte research

Leukocytes are a heterogeneous group of cells that display differences in anatomic localization, cell surface phenotype, and function. The different subtypes include e.g., granulocytes, monocytes, dendritic cells, T cells, B cells and NK cells. These different cell types represent the cellular component of innate and adaptive immunity. Using certain toxins such as pertussis toxin, cholera toxin or clostridium difficile toxin, the regulatory functions of Gα_i, Gαs and small GTPases of the Rho family in leukocytes have been reported. A summary of these reports is discussed in this review.

Trans-10, cis-12 conjugated linoleic acid modulates phagocytic responses of canine peripheral blood polymorphonuclear neutrophilic leukocytes exposed to Clostridium difficile toxin B

Trans-10, cis-12 conjugated linoleic acid (t10c12-CLA) has been reported to enhance phagocyte function. Clostridium difficile toxin B (TcdB) has been known to inhibit Ras-homologous (Rho) guanosine triphosphatases (GTPases) which play essential roles in neutrophil immune functions. Here, we examined whether in vitro treatment with t10c12-CLA modulates the filamentous actin (F-actin) polymerization, phagocytic capacity, and oxidative burst activity (OBA) of canine peripheral blood polymorphonuclear neutrophilic leukocytes (PMNs) exposed to TcdB. Treatment with t10c12-CLA, but not linoleic acid, enhanced PMN F-actin polymerization, phagocytic capacity, and OBA, while TcdB suppressed these functions. t10c12-CLA reversed the suppressive effects of TcdB on these PMN functions. t10c12-CLA stimulated F-actin polymerization regardless of whether phagocytosis was stimulated by microspheres but only elevated OBA when microspheres were added. We asked whether the effects of t10c12-CLA were associated with changes in the activation of the Rho GTPase Cdc42. Treatment with t10c12-CLA augmented Cdc42 activity in both TcdB-treated and TcdB-naive PMNs during phagocytosis. Thus, t10c12-CLA up-regulates PMN phagocytic responses attenuated by TcdB. This effect is associated with an increase in actin polymerization and may involve the activation of Cdc42.

Passive immunisation of hamsters againstClostridium difficileinfection using antibodies to surface layer proteins

Clostridium difficile is a major cause of antibiotic-associated diarrhoea and the primary cause of pseudomembraneous colitis in hospitalised patients. We assessed the protective effect of anti-surface layer protein (SLP) antibodies on C. difficile infection in a lethal hamster challenge model. Post-challenge survival was significantly prolonged in the anti-SLP treated group compared with control groups (P=0.0281 and P=0.0283). The potential mechanism of action of the antiserum was shown to be through enhancement of C. difficile phagocytosis. This report indicates that anti-SLP antibodies can modulate the course of C. difficile infection and may therefore merit closer investigation for use as constituents of multi-component vaccines against C. difficile associated diarrhoea.

Modulation of polymorphonuclear cell interleukin-8 secretion by human monoclonal antibodies to type 8 pneumococcal capsular polysaccharide

Pneumococcal capsular polysaccharide (PS) vaccines induce type-specific immunoglobulin M (IgM), IgG, and IgA. Type-specific IgG to the PS is sufficient to confer protection against the homologous serotype of the pneumococcus, but the efficacies of type-specific IgM and IgA are less well understood. We examined the in vitro activities and efficacies in mice of two human monoclonal antibodies (MAbs) to type 8 PS, NAD (IgA) and D11 (IgM). MAb-mediated opsonophagocytic killing was evaluated after coculture of type 8 pneumococci with human polymorphonuclear cells (PMNs), type-specific or control MAbs, and human complement sources. The effects of the MAbs on PMN interleukin-8 (IL-8) and IL-6 secretion were determined in supernatants from cocultures containing pneumococci and PMNs by enzyme-linked immunosorbent assay. MAb efficacy was determined in an intratracheal model of type 8 infection in mice with classical complement pathway deficiency. Both MAbs were protective in 100% of infected mice. Neither MAb promoted a significant amount of killing of type 8 pneumococci compared to its isotype control MAb. Both type-specific MAbs mediated complement-dependent modulation of PMN IL-8 secretion, with increased secretion at effector/target (E:T) ratios of 500:1 and 50:1 and reduced secretion at 1:5. Trypan blue staining revealed that PMNs cocultured with D11 were less viable at an E:T ratio of 1:5 than PMNs cocultured with the control MAb. PMN IL-6 secretion was increased by both type-specific and control MAbs. These results suggest that certain type-specific IgM and IgAs might contribute to host defense by modulation of the inflammatory response to pneumococci.

Clostridium difficile-associated diarrhea and colitis: clinical manifestations, diagnosis, and treatment

PURPOSE

This review examines the pathogenesis, clinical manifestations, diagnosis, and current medical and operative strategies in the treatment of Clostridium difficile diarrhea and colitis. Prevention and future avenues of research are also investigated.

METHODS

A review of the literature was conducted with the use of MEDLINE.

RESULTS

C. difficile is a gram-positive, spore-forming bacterium capable of causing toxigenic colitis in susceptible patients, usually those receiving antibiotics. Overgrowth of toxigenic strains may result in a spectrum of disease, including becoming an asymptomatic carrier, diarrhea, self-limited colitis, fulminant colitis, and toxic megacolon. Diagnosis requires a high index of suspicion and depends on clinical data, laboratory stool studies (enzyme-linked immunoabsorbent assay and cytotoxin test), and endoscopy in selected cases. Protocols for treatment of primary and relapsing infections are provided in algorithm format. Discontinuation of antibiotics may be enough to resolve symptoms. Medical management with oral metronidazole or vancomycin is the first-line therapy for those with symptomatic colitis. Teicoplanin, Saccharomyces spp. and Lactobacillus spp., and intravenous IgG antitoxin are reserved for more recalcitrant cases. Refractory or relapsing infections may require vancomycin given orally or other newer modalities. Fulminant colitis and toxic megacolon warrant subtotal colectomy. Cost, in terms of extended hospital stay, medical and surgical management, and, in some cases, ward closure, is thought to be formidable. Review of perioperative antibiotic policies and analysis of hospital formularies may contribute to prevention and decreased costs.

CONCLUSION

C. difficile diarrhea and colitis is a nosocomial infection that may result in significant morbidity, mortality, and medical costs. Standard laboratory studies and endoscopic evaluation assist in the diagnosis of clinically suspicious cases. Appropriate perioperative antibiotic dosing, narrowing the antibiotic spectrum when treating infections, and discontinuing antibiotics at appropriate intervals prevent toxic sequelae.

Evaluation of formalin-inactivated Clostridium difficile vaccines administered by parenteral and mucosal routes of immunization in hamsters

Clostridium difficile produces toxins that cause inflammation, necrosis, and fluid in the intestine and is the most important cause of nosocomial antibiotic-associated diarrhea and colitis. We evaluated C. difficile antigens as vaccines to protect against systemic and intestinal disease in a hamster model of clindamycin colitis. Formalin-inactivated culture filtrates from a highly toxigenic strain were administered by mucosal routes (intranasal, intragastric, and rectal) with cholera toxin as a mucosal adjuvant. A preparation of culture filtrate and killed whole cells was also tested rectally. The toxoid was also tested parenterally (subcutaneously and intraperitoneally) and by a combination of three intranasal immunizations followed by a combined intranasal-intraperitoneal boost. Serum antibodies against toxins A and B and whole-cell antigen were measured by enzyme-linked immunosorbent assay, neutralization of cytotoxic activity, and bacterial agglutination. The two rectal immunization regimens induced low antibody responses and protected only 20% of hamsters against death and 0% against diarrhea. The intragastric regimen induced high antibody responses but low protection, 40% against death and 0% against diarrhea. Hamsters immunized by the intranasal, intraperitoneal, and subcutaneous routes were 100% protected against death and partially protected (40, 40, and 20%, respectively) against diarrhea. Among the latter groups, intraperitoneally immunized animals had the highest serum anticytotoxic activity and the highest agglutinating antibody responses. Hamsters immunized intranasally and revaccinated intraperitoneally were 100% protected against both death and diarrhea. Protection against death and diarrhea correlated with antibody responses to all antigens tested. The results indicate that optimal protection against C. difficile disease can be achieved with combined parenteral and mucosal immunization.

Effects of Clostridium difficile toxin B on human monocytes and macrophages: possible relationship with cytoskeletal rearrangement

Toxin B from Clostridium difficile is cytopathic in vitro for various types of cells, including polymorphonuclear cells, lymphocytes, and monocytes. Since intestine lamina propria is rich in macrophages, we studied the effect of toxin B on human monocytes and on human macrophages generated in vitro by long-term culture of purified circulating blood monocytes. Upon addition of toxin B, human monocytes exhibited few modifications whereas macrophages adopted a stellate morphology, with rounding up of the perikaryon. Toxin B made microfilaments of actin disappear and induced an important reorganization of vimentin and a redistribution of tubulin. Membrane area increased by approximately 16%. Toxin B did not affect the viability of human mononuclear phagocytes and did not exert any significant lytic effect. It profoundly altered the phagocytic function of macrophages. When activated by gamma interferon in the presence of toxin B, monocytes were more cytotoxic for U-937 target cells than control monocytes activated in absence of toxin. Finally, the combined treatment of monocytes with gamma interferon and toxin B increased significantly the secretion of tumor necrosis factor alpha, whereas toxin B alone was unable to induce tumor necrosis factor production. These results suggest that morphological and functional alterations induced in human mononuclear phagocytes by toxin B from C. difficile are due to the disorganization of the cytoskeleton and the resulting impairment of the membrane traffic equilibrium.

Clostridium difficile toxin A and its effects on cells

Clostridium difficile toxin A in its native form is a high molecular weight (520-540 K) aggregate with five major biological activities. It is lethal, enterotoxic, cytotoxic and cytotonic, and induces hemagglutination of rabbit red blood cells. Possibly these activities are contained in separate components. A major subunit of c. 230-310 K has been defined but lower molecular weight components cannot be excluded. The major component has been cloned, and sequence analysis indicated a complicated pattern of repeating sequences in the C-terminal third of the molecule. This review deals mainly with the effects of toxin A on cultured cells. Most mammalian cells are sensitive to toxin A whose major effect is to stop cell division irreversibly. The toxin binds via its repeat sequences to a trisaccharide receptor expressed on rabbit red cells and on brush border membranes from hamster intestine. This receptor seems to be functional in the hemagglutination reaction and the enterotoxicity. Its role in the cytotoxic effect of the toxin is not clear, but no other receptor structure has as yet been identified. In order to exert its cytotoxic (antiproliferative) effect toxin A must first be internalized by endocytosis. Thus a latency period of at least 30 min after toxin binding to cells is consistently observed, and all cytotoxic effects can be prevented by blocking the endocytosis pathway. The first microscopically visible signs of cytotoxicity consist in retraction and rounding of intoxicated cells. In addition the nucleus becomes polarized to one side of the cell while other cell organelles are not significantly affected. These morphological changes seem to be the consequence of a cytoskeletal rearrangement, mainly involving some components of the microfilament system. Inhibition of macromolecular syntheses as well as permeabilization of the plasma membrane may follow the early cytoskeletal effects and finally lead to cell death. Attempts to identify metabolic pathways of significance in the cytotoxicity suggest that the cytosolic level of Ca2+ is not important, thus excluding certain mechanisms for cell killing. In this respect the cytotoxic mode of action of toxin A clearly differs from that of toxin B. However, the biochemical basis for the antiproliferative effect of toxin A remains unknown.

Detection of capsule in strains of Clostridium difficile of varying virulence and toxigenicity

Nine toxigenic and six non-toxigenic strains of Clostridium difficile, of varying virulence in the hamster model of antibiotic-associated colitis, were examined for the presence of a capsule. Antibody stabilisation of the capsule with heterologous and/or homologous antiserum fixed in glutaraldehyde, or direct fixation in glutaraldehyde/diamine, were used with added ruthenium red to stain the capsular glycocalyx. All strains possessed a capsule which was either loose-knit or compact, sometimes with attached globular masses. Better capsule preservation was achieved in some strains with glutaraldehyde/diamine/ruthenium red fixative than with homologous or heterologous antibody stabilisation. The possession of a capsule following in vitro growth does not appear to correlate with the virulence status of these strains of C. difficile.

Demonstration of capsules in Clostridium difficile

In four strains of Clostridium difficile the formation of capsules was demonstrated by light and electron microscopy.

Clostridium difficile toxins A and B inhibit human immune response in vitro

Two Clostridium difficile toxins isolated from strain VPI 10463 were tested for their effect on different human T-cell proliferation systems. In mitogen- and antigen-driven T-cell proliferation systems, toxins inhibited the proliferative response in a dose-dependent fashion. In interleukin-2-driven culture systems, no effect of toxins could be found on preactivated T cells. We suspected that monocytes were the influenced cells, since in antigen- and mitogen-driven systems monocytes were necessary for the proliferative response, whereas the interleukin-2-driven system was independent of monocytes. To prove this concept, purified monocytes were treated with toxins. The treatment was found to markedly reduce the capacity of monocytes to stimulate T-cell proliferation. No inhibition of the proliferative response was measured when, instead of monocytes, resting or preactivated T cells were treated with toxins. These experiments clearly show that C. difficile toxins interact with monocytes and not with T cells. The effect of toxins on cells of the immune system might be one factor in the development of pseudomembranous colitis.