Bosutinib Stimulates Macrophage Survival, Phagocytosis, and Intracellular Killing of Bacteria

Host-acting compounds are emerging as potential alternatives to combating antibiotic resistance. Here, we show that bosutinib, an FDA-approved chemotherapeutic for treating chronic myelogenous leukemia, does not possess any antibiotic activity but enhances macrophage responses to bacterial infection. In vitro, bosutinib stimulates murine and human macrophages to kill bacteria more effectively. In a murine wound infection with vancomycin-resistant Enterococcus faecalis, a single intraperitoneal bosutinib injection or multiple topical applications on the wound reduce the bacterial load by approximately 10-fold, which is abolished by macrophage depletion. Mechanistically, bosutinib stimulates macrophage phagocytosis of bacteria by upregulating surface expression of bacterial uptake markers Dectin-1 and CD14 and promoting actin remodeling. Bosutinib also stimulates bacterial killing by elevating the intracellular levels of reactive oxygen species. Moreover, bosutinib drives NF-κB activation, which protects infected macrophages from dying. Other Src kinase inhibitors such as DMAT and tirbanibulin also upregulate expression of bacterial uptake markers in macrophages and enhance intracellular bacterial killing. Finally, cotreatment with bosutinib and mitoxantrone, another chemotherapeutic in clinical use, results in an additive effect on bacterial clearance in vitro and in vivo. These results show that bosutinib stimulates macrophage clearance of bacterial infections through multiple mechanisms and could be used to boost the host innate immunity to combat drug-resistant bacterial infections.

Enterococcus faecalis suppresses Staphylococcus aureus-induced NETosis and promotes bacterial survival in polymicrobial infections

Enterococcus faecalis is an opportunistic pathogen that is frequently co-isolated with other microbes in wound infections. While E. faecalis can subvert the host immune response and promote the survival of other microbes via interbacterial synergy, little is known about the impact of E. faecalis-mediated immune suppression on co-infecting microbes. We hypothesized that E. faecalis can attenuate neutrophil-mediated responses in mixed-species infection to promote survival of the co-infecting species. We found that neutrophils control E. faecalis infection via phagocytosis, ROS production, and degranulation of azurophilic granules, but it does not trigger neutrophil extracellular trap formation (NETosis). However, E. faecalis attenuates Staphylococcus aureus-induced NETosis in polymicrobial infection by interfering with citrullination of histone, suggesting E. faecalis can actively suppress NETosis in neutrophils. Residual S. aureus-induced NETs that remain during co-infection do not impact E. faecalis, further suggesting that E. faecalis possess mechanisms to evade or survive NET-associated killing mechanisms. E. faecalis-driven reduction of NETosis corresponds with higher S. aureus survival, indicating that this immunomodulating effect could be a risk factor in promoting the virulence polymicrobial infection. These findings highlight the complexity of the immune response to polymicrobial infections and suggest that attenuated pathogen-specific immune responses contribute to pathogenesis in the mammalian host.

Macrophage ACE2 is necessary for SARS-CoV-2 replication and subsequent cytokine responses that restrict continued virion release

Macrophages are key cellular contributors to the pathogenesis of COVID-19, the disease caused by the virus SARS-CoV-2. The SARS-CoV-2 entry receptor ACE2 is present only on a subset of macrophages at sites of SARS-CoV-2 infection in humans. Here, we investigated whether SARS-CoV-2 can enter macrophages, replicate, and release new viral progeny; whether macrophages need to sense a replicating virus to drive cytokine release; and, if so, whether ACE2 is involved in these mechanisms. We found that SARS-CoV-2 could enter, but did not replicate within, ACE2-deficient human primary macrophages and did not induce proinflammatory cytokine expression. By contrast, ACE2 overexpression in human THP-1-derived macrophages permitted SARS-CoV-2 entry, processing and replication, and virion release. ACE2-overexpressing THP-1 macrophages sensed active viral replication and triggered proinflammatory, antiviral programs mediated by the kinase TBK-1 that limited prolonged viral replication and release. These findings help elucidate the role of ACE2 and its absence in macrophage responses to SARS-CoV-2 infection.

HDAC7 is an immunometabolic switch triaging danger signals for engagement of antimicrobial versus inflammatory responses in macrophages

The immune system must be able to respond to a myriad of different threats, each requiring a distinct type of response. Here, we demonstrate that the cytoplasmic lysine deacetylase HDAC7 in macrophages is a metabolic switch that triages danger signals to enable the most appropriate immune response. Lipopolysaccharide (LPS) and soluble signals indicating distal or far-away danger trigger HDAC7-dependent glycolysis and proinflammatory IL-1β production. In contrast, HDAC7 initiates the pentose phosphate pathway (PPP) for NADPH and reactive oxygen species (ROS) production in response to the more proximal threat of nearby bacteria, as exemplified by studies on uropathogenic Escherichia coli (UPEC). HDAC7-mediated PPP engagement via 6-phosphogluconate dehydrogenase (6PGD) generates NADPH for antimicrobial ROS production, as well as D-ribulose-5-phosphate (RL5P) that both synergizes with ROS for UPEC killing and suppresses selective inflammatory responses. This dual functionality of the HDAC7-6PGD-RL5P axis prioritizes responses to proximal threats. Our findings thus reveal that the PPP metabolite RL5P has both antimicrobial and immunomodulatory activities and that engagement of enzymes in catabolic versus anabolic metabolic pathways triages responses to different types of danger for generation of inflammatory versus antimicrobial responses, respectively.

Ancestral SARS-CoV-2, but not Omicron, replicates less efficiently in primary pediatric nasal epithelial cells

Children typically experience more mild symptoms of Coronavirus Disease 2019 (COVID-19) when compared to adults. There is a strong body of evidence that children are also less susceptible to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection with the ancestral viral isolate. However, the emergence of SARS-CoV-2 variants of concern (VOCs) has been associated with an increased number of pediatric infections. Whether this is the result of widespread adult vaccination or fundamental changes in the biology of SARS-CoV-2 remain to be determined. Here, we use primary nasal epithelial cells (NECs) from children and adults, differentiated at an air–liquid interface to show that the ancestral SARS-CoV-2 replicates to significantly lower titers in the NECs of children compared to those of adults. This was associated with a heightened antiviral response to SARS-CoV-2 in the NECs of children. Importantly, the Delta variant also replicated to significantly lower titers in the NECs of children. This trend was markedly less pronounced in the case of Omicron. It is also striking to note that, at least in terms of viral RNA, Omicron replicated better in pediatric NECs compared to both Delta and the ancestral virus. Taken together, these data show that the nasal epithelium of children supports lower infection and replication of ancestral SARS-CoV-2, although this may be changing as the virus evolves.

Children typically experience more mild symptoms of COVID-19 when compared to adults; why is this? This study uses nasal epithelial cells from children and adults to show that the ancestral SARS-CoV-2 and Delta, but not the Omicron variant, replicate less efficiently in pediatric nasal epithelial cells.

Endothelial cells are not productively infected by SARS-CoV-2

Objectives

Thrombotic and microvascular complications are frequently seen in deceased COVID‐19 patients. However, whether this is caused by direct viral infection of the endothelium or inflammation‐induced endothelial activation remains highly contentious.

Methods

Here, we use patient autopsy samples, primary human endothelial cells and an in vitro model of the pulmonary epithelial–endothelial cell barrier.

Results

We show that primary human endothelial cells express very low levels of the SARS‐CoV‐2 receptor ACE2 and the protease TMPRSS2, which blocks their capacity for productive viral infection, and limits their capacity to produce infectious virus. Accordingly, endothelial cells can only be infected when they overexpress ACE2, or are exposed to very high concentrations of SARS‐CoV‐2. We also show that SARS‐CoV‐2 does not infect endothelial cells in 3D vessels under flow conditions. We further demonstrate that in a co‐culture model endothelial cells are not infected with SARS‐CoV‐2. Endothelial cells do however sense and respond to infection in the adjacent epithelial cells, increasing ICAM‐1 expression and releasing pro‐inflammatory cytokines.

Conclusions

Taken together, these data suggest that in vivo, endothelial cells are unlikely to be infected with SARS‐CoV‐2 and that infection may only occur if the adjacent pulmonary epithelium is denuded (basolateral infection) or a high viral load is present in the blood (apical infection). In such a scenario, whilst SARS‐CoV‐2 infection of the endothelium can occur, it does not contribute to viral amplification. However, endothelial cells may still play a key role in SARS‐CoV‐2 pathogenesis by sensing adjacent infection and mounting a pro‐inflammatory response to SARS‐CoV‐2.

An alloy of zinc and innate immunity: Galvanising host defence against infection

Innate immune cells such as macrophages and neutrophils initiate protective inflammatory responses and engage antimicrobial responses to provide frontline defence against invading pathogens. These cells can both restrict the availability of certain transition metals that are essential for microbial growth and direct toxic concentrations of metals towards pathogens as antimicrobial responses. Zinc is important for the structure and function of many proteins, however excess zinc can be cytotoxic. In recent years, several studies have revealed that innate immune cells can deliver toxic concentrations of zinc to intracellular pathogens. In this review, we discuss the importance of zinc status during infectious disease and the evidence for zinc intoxication as an innate immune antimicrobial response. Evidence for pathogen subversion of this response is also examined. The likely mechanisms, including the involvement of specific zinc transporters that facilitate delivery of zinc by innate immune cells for metal ion poisoning of pathogens are also considered. Precise mechanisms by which excess levels of zinc can be toxic to microorganisms are then discussed, particularly in the context of synergy with other antimicrobial responses. Finally, we highlight key unanswered questions in this emerging field, which may offer new opportunities for exploiting innate immune responses for anti-infective development.

Frontline Science: LPS-inducible SLC30A1 drives human macrophage-mediated zinc toxicity against intracellular Escherichia coli

TLR-inducible zinc toxicity is an antimicrobial mechanism utilized by macrophages, however knowledge of molecular mechanisms mediating this response is limited. Here, we show that E. coli exposed to zinc stress within primary human macrophages reside in membrane-bound vesicular compartments. Since SLC30A zinc exporters can deliver zinc into the lumen of vesicles, we examined LPS-regulated mRNA expression of Slc30a/SLC30A family members in primary mouse and human macrophages. A number of these transporters were dynamically regulated in both cell populations. In human monocyte-derived macrophages, LPS strongly up-regulated SLC30A1 mRNA and protein expression. In contrast, SLC30A1 was not LPS-inducible in macrophage-like PMA-differentiated THP-1 cells. We therefore ectopically expressed SLC30A1 in these cells, finding that this was sufficient to promote zinc-containing vesicle formation. The response was similar to that observed following LPS stimulation. Ectopically expressed SLC30A1 localized to both the plasma membrane and intracellular zinc-containing vesicles within LPS-stimulated THP-1 cells. Inducible overexpression of SLC30A1 in THP-1 cells infected with the Escherichia coli K-12 strain MG1655 augmented the zinc stress response of intracellular bacteria and promoted clearance. Furthermore, in THP-1 cells infected with an MG1655 zinc stress reporter strain, all bacteria contained within SLC30A1-positive compartments were subjected to zinc stress. Thus, SLC30A1 marks zinc-containing compartments associated with TLR-inducible zinc toxicity in human macrophages, and its ectopic over-expression is sufficient to initiate this antimicrobial pathway in these cells. Finally, SLC30A1 silencing did not compromise E. coli clearance by primary human macrophages, suggesting that other zinc exporters may also contribute to the zinc toxicity response.

For when bacterial infections persist: Toll-like receptor-inducible direct antimicrobial pathways in macrophages

Macrophages are linchpins of innate immunity, responding to invading microorganisms by initiating coordinated inflammatory and antimicrobial programs. Immediate antimicrobial responses, such as NADPH-dependent reactive oxygen species (ROS), are triggered upon phagocytic receptor engagement. Macrophages also detect and respond to microbial products through pattern recognition receptors (PRRs), such as TLRs. TLR signaling influences multiple biological processes including antigen presentation, cell survival, inflammation, and direct antimicrobial responses. The latter enables macrophages to combat infectious agents that persist within the intracellular environment. In this review, we summarize our current understanding of TLR-inducible direct antimicrobial responses that macrophages employ against bacterial pathogens, with a focus on emerging evidence linking TLR signaling to reprogramming of mitochondrial functions to enable the production of direct antimicrobial agents such as ROS and itaconic acid. In addition, we describe other TLR-inducible antimicrobial pathways, including autophagy/mitophagy, modulation of nutrient availability, metal ion toxicity, reactive nitrogen species, immune GTPases (immunity-related GTPases and guanylate-binding proteins), and antimicrobial peptides. We also describe examples of mechanisms of evasion of such pathways by professional intramacrophage pathogens, with a focus on Salmonella, Mycobacteria, and Listeria. An understanding of how TLR-inducible direct antimicrobial responses are regulated, as well as how bacterial pathogens subvert such pathways, may provide new opportunities for manipulating host defence to combat infectious diseases.

Salmonella employs multiple mechanisms to subvert the TLR-inducible zinc-mediated antimicrobial response of human macrophages

We aimed to characterize antimicrobial zinc trafficking within macrophages and to determine whether the professional intramacrophage pathogen Salmonella enterica serovar Typhimurium (S Typhimurium) subverts this pathway. Using both Escherichia coli and S Typhimurium, we show that TLR signaling promotes the accumulation of vesicular zinc within primary human macrophages. Vesicular zinc is delivered to E. coli to promote microbial clearance, whereas S. Typhimurium evades this response via Salmonella pathogenicity island (SPI)-1. Even in the absence of SPI-1 and the zinc exporter ZntA, S Typhimurium resists the innate immune zinc stress response, implying the existence of additional host subversion mechanisms. We also demonstrate the combinatorial antimicrobial effects of zinc and copper, a pathway that S. Typhimurium again evades. Our use of complementary tools and approaches, including confocal microscopy, direct assessment of intramacrophage bacterial zinc stress responses, specific E. coli and S Typhimurium mutants, and inductively coupled plasma mass spectroscopy, has enabled carefully controlled characterization of this novel innate immune antimicrobial pathway. In summary, our study provides new insights at the cellular level into the well-documented effects of zinc in promoting host defense against infectious disease, as well as the complex host subversion strategies employed by S Typhimurium to combat this pathway.-Kapetanovic, R., Bokil, N. J., Achard, M. E. S., Ong, C.-L. Y., Peters, K. M., Stocks, C. J., Phan, M.-D., Monteleone, M., Schroder, K., Irvine, K. M., Saunders, B. M., Walker, M. J., Stacey, K. J., McEwan, A. G., Schembri, M. A., Sweet, M. J. Salmonella employs multiple mechanisms to subvert the TLR-inducible zinc-mediated antimicrobial response of human macrophages.

Phenotypic and genotypic characterization of monoclonal anti-digoxin antibodies

Eleven hybridoma cell lines secreting monoclonal anti-digoxin antibody have been produced. They are primarily gamma heavy chain and kappa light chain molecules. Affinity constants for digoxin range from 2 X 10(6) to 3.5 X 10(8) liters/mole. Fine specificity analysis using a series of digoxin congeners demonstrates that an unsaturated lactone ring attached to the aglycone at the C-17 position is necessary for hapten recognition. The impact of other changes in digoxin's structure on antibody binding were also studied. DNA hybridization analysis demonstrates that there are at least three different variable region gene arrangements used to produce the heavy chains of the different hybridoma antibodies. Correlations between antigen binding characteristics and antibody V-gene arrangements are demonstrable.

Monoclonal anti-(T,G)-A--L antibodies: characterization of fine specificity and idiotype expression

The technique of polyethylene glycol mediated cell fusion was used to establish 22 monoclonal cell lines secreting anti-(T,G)-A--L antibody. Cell lines were derived from C3H.SW and B10 mice and produced antibody with kappa light chains and predominantly gamma 1, heavy chains. Fine-specificity analysis demonstrated that 15 cell lines made antibodies that also recognize a determinant present on GAT, GT (9:1) and GT (1:1), whereas little, if any, serum antibody demonstrates this cross-reaction. Fourteen antibodies, derived from both B10 and C3H.SW mice, bear idiotypic determinants defined by Lewis anti-[B10 anti-(T,G)-A--L], but only two, both from C3H.SW mice, react with Lewis anti-[C3H.SW anti-(T,G)-A--L]. Adsorption studies indicate that no hybridoma tested bore the complete set of idiotypic determinants defined by either serum.

Idiotypic analysis of hybridoma antibodies to branched synthetic polymer (Tyr, Glu)-Ala-Lys: idiotypic relationship with antibodies to linear random polymer (Glu60, Ala30, Tyr10)

The expression of three anti-GAT idiotypes, CGAT, Gte, and GA-1, on 17 C57BL/10 and four C3H.SW hybridoma anti-(T,G)-A--L antibodies was analyzed. These hybridoma anti-(T,G)-A--L antibodies exhibited two patterns of fine antigen binding specificity. The majority of the hybridoma antibodies bound the (T,G)-A--L, GT, and GAT polymers but not the GA polymer, and were designated as GT-reactive hybridoma antibodies. A minor population of hybridoma anti(T,G)-A--L antibodies bound to (T,G)-A--L but not to GT, GAT, or GA, i.e., (T,G)-A--L-specific. A complete correlation between fine antigen binding pattern and the expression of CGAT idiotype was demonstrated. None of the 21 hybridoma anti-(T,G)-A--L antibodies expressed the GA-1 idiotype. All of the GT-reactive and none of the GT-nonreactive hybridoma anti-(%,G)-A--L antibodies expressed the CGAT idiotype. Furthermore, the Gte idiotype was found on the majority of CGAT+-bearing C57BL/10 hybridoma anti-(T,G)-A--L antibodies. These results indicate that C57BL/10 anti-(T,G)-A--L antibody repertoire can be grouped into a minimum of three families; i.e., CGAT+ Gte+, CGAT+ Gte-, and CGAT- Gte- families, with the CGAT+ Gte+ family as the major compartment. This is confirmed by the high percentage idiotype binding of serum anti-(T,G)-A--L antibodies with anti-CGAT idiotypic antisera. Finally, anti-idiotypic antisera made against CGAT+ hybridoma anti-GAT or anti-(T,G)-A--L antibodies crossreact extensively with other CGAT+ hybridoma anti-GAT and anti-(T,G)-A--L antibodies. However, additional experiments demonstrated that CGAT+ hybridoma anti-(T,G)-A--L antibodies also possess private idiotypes.

Idiotypic analysis of hybridoma antibodies to branched synthetic polymer (Tyr, Glu)-Ala-Lys: idiotypic relationship with antibodies to linear random polymer (Glu60, Ala30, Tyr10)

The expression of three anti-GAT idiotypes, CGAT, Gte, and GA-1, on 17 C57BL/10 and four C3H.SW hybridoma anti-(T,G)-A--L antibodies was analyzed. These hybridoma anti-(T,G)-A--L antibodies exhibited two patterns of fine antigen binding specificity. The majority of the hybridoma antibodies bound the (T,G)-A--L, GT, and GAT polymers but not the GA polymer, and were designated as GT-reactive hybridoma antibodies. A minor population of hybridoma anti(T,G)-A--L antibodies bound to (T,G)-A--L but not to GT, GAT, or GA, i.e., (T,G)-A--L-specific. A complete correlation between fine antigen binding pattern and the expression of CGAT idiotype was demonstrated. None of the 21 hybridoma anti-(T,G)-A--L antibodies expressed the GA-1 idiotype. All of the GT-reactive and none of the GT-nonreactive hybridoma anti-(%,G)-A--L antibodies expressed the CGAT idiotype. Furthermore, the Gte idiotype was found on the majority of CGAT+-bearing C57BL/10 hybridoma anti-(T,G)-A--L antibodies. These results indicate that C57BL/10 anti-(T,G)-A--L antibody repertoire can be grouped into a minimum of three families; i.e., CGAT+ Gte+, CGAT+ Gte-, and CGAT- Gte- families, with the CGAT+ Gte+ family as the major compartment. This is confirmed by the high percentage idiotype binding of serum anti-(T,G)-A--L antibodies with anti-CGAT idiotypic antisera. Finally, anti-idiotypic antisera made against CGAT+ hybridoma anti-GAT or anti-(T,G)-A--L antibodies crossreact extensively with other CGAT+ hybridoma anti-GAT and anti-(T,G)-A--L antibodies. However, additional experiments demonstrated that CGAT+ hybridoma anti-(T,G)-A--L antibodies also possess private idiotypes.