Differential roles of MyD88 and TRIF in hematopoietic and resident cells during murine gram-negative pneumonia

Bruton's Tyrosine Kinase in Neutrophils Is Crucial for Host Defense against Klebsiella pneumoniae

Humans with dysfunctional Bruton's tyrosine kinase (Btk) are highly susceptible to bacterial infections. Compelling evidence indicates that Btk is essential for B cell-mediated immunity, whereas its role in myeloid cell-mediated immunity against infections is controversial. In this study, we determined the contribution of Btk in B cells and neutrophils to host defense against the extracellular bacterial pathogen Klebsiella pneumoniae, a common cause of pulmonary infections and sepsis. Btk-/- mice were highly susceptible to Klebsiella infection, which was not reversed by Btk re-expression in B cells and restoration of natural antibody levels. Neutrophil-specific Btk deficiency impaired host defense against Klebsiella to a similar extent as complete Btk deficiency. Neutrophil-specific Btk deficiency abolished extracellular reactive oxygen species production in response to Klebsiella. These data indicate that expression of Btk in neutrophils is crucial, while in B cells, it is dispensable for in vivo host defense against K. pneumoniae.

Platelets in pediatric and neonatal sepsis: novel mediators of the inflammatory cascade

Sepsis, a dysregulated host response to infection, has been difficult to accurately define in children. Despite a higher incidence, especially in neonates, a non-specific clinical presentation alongside a lack of verified biomarkers has prevented a common understanding of this condition. Platelets, traditionally regarded as mediators of haemostasis and thrombosis, are increasingly associated with functions in the immune system with involvement across the spectrum of innate and adaptive immunity. The large number of circulating platelets (approx. 150,000 cells per microlitre) mean they outnumber traditional immune cells and are often the first to encounter a pathogen at a site of injury. There are also well-described physiological differences between platelets in children and adults. The purpose of this review is to place into context the platelet and its role in immunology and examine the evidence where available for its role as an immune cell in childhood sepsis. It will examine how the platelet interacts with both humoral and cellular components of the immune system and finally discuss the role the platelet proteome, releasate and extracellular vesicles may play in childhood sepsis. This review also examines how platelet transfusions may interfere with the complex relationships between immune cells in infection. IMPACT: Platelets are increasingly being recognised as important "first responders" to immune threats. Differences in adult and paediatric platelets may contribute to differing immune response to infections. Adult platelet transfusions may affect infant immune responses to inflammatory/infectious stimuli.

Regnase-1 Deficiency Restrains Klebsiella pneumoniae Infection by Regulation of a Type I Interferon Response

Excessive inflammation can cause tissue damage and autoimmunity, sometimes accompanied by severe morbidity or mortality. Numerous negative feedback mechanisms exist to prevent unchecked inflammation, but this restraint may come at the cost of suboptimal infection control. Regnase-1 (MCPIP1), a feedback regulator of IL-17 and LPS signaling, binds and degrades target mRNAs. Consequently, Reg1 deficiency exacerbates autoimmunity in multiple models. However, the role of Reg1 in bacterial immunity remains poorly defined. Here, we show that mice deficient in Reg1 are resistant to Klebsiella pneumoniae (KP). Reg1 deficiency did not accelerate bacterial eradication. Rather, Reg1-deficient alveolar macrophages had elevated Ifnb1 and enrichment of type I IFN genes. Blockade of IFNR during KP infection reversed disease improvement. Reg1 did not impact Ifnb1 stability directly, but Irf7 expression was affected. Thus, Reg1 suppresses type I IFN signaling restricting resistance to KP, suggesting that Reg1 could potentially be a target in severe bacterial infections. IMPORTANCE Klebsiella pneumoniae (KP) can cause life-threatening bacterial pneumonia and is the third most common cause of ventilator-associated pneumonia in the United States. Host inflammatory responses to infection are necessary to control disease, yet at the same time can cause collateral damage or immunopathology. During immune responses, many events are established within the infected tissue to limit unchecked inflammation. However, this restraint of immunity can impair infection control, and it is not fully understood how this balance is maintained during different infection settings. In this study we explored the possibility that a host-derived negative regulator of RNA, Regnase-1, limits immunity to KP by dampening inflammation. Indeed, mice with reduced Regnase-1 levels showed improved survival to KP infection, linked to regulation of type I interferons. Therefore, although restraint of Reg1 is beneficial to prevent immunopathology, temporary blockade of Reg1 could potentially be exploited to improve host defense during infectious settings such as KP.

Pulmonary Innate Immune Response Determines the Outcome of Inflammation During Pneumonia and Sepsis-Associated Acute Lung Injury

The lung is a primary organ for gas exchange in mammals that represents the largest epithelial surface in direct contact with the external environment. It also serves as a crucial immune organ, which harbors both innate and adaptive immune cells to induce a potent immune response. Due to its direct contact with the outer environment, the lung serves as a primary target organ for many airborne pathogens, toxicants (aerosols), and allergens causing pneumonia, acute respiratory distress syndrome (ARDS), and acute lung injury or inflammation (ALI). The current review describes the immunological mechanisms responsible for bacterial pneumonia and sepsis-induced ALI. It highlights the immunological differences for the severity of bacterial sepsis-induced ALI as compared to the pneumonia-associated ALI. The immune-based differences between the Gram-positive and Gram-negative bacteria-induced pneumonia show different mechanisms to induce ALI. The role of pulmonary epithelial cells (PECs), alveolar macrophages (AMs), innate lymphoid cells (ILCs), and different pattern-recognition receptors (PRRs, including Toll-like receptors (TLRs) and inflammasome proteins) in neutrophil infiltration and ALI induction have been described during pneumonia and sepsis-induced ALI. Also, the resolution of inflammation is frequently observed during ALI associated with pneumonia, whereas sepsis-associated ALI lacks it. Hence, the review mainly describes the different immune mechanisms responsible for pneumonia and sepsis-induced ALI. The differences in immune response depending on the causal pathogen (Gram-positive or Gram-negative bacteria) associated pneumonia or sepsis-induced ALI should be taken in mind specific immune-based therapeutics.

Caspase-11 contributes to pulmonary host defense against Klebsiella pneumoniae and local activation of coagulation

Klebsiella (K.) pneumoniae is a common cause of gram-negative pneumonia and sepsis. Caspase-11 is an intracellular receptor for lipopolysaccharide and regulates pyroptosis, a specific form of inflammatory cell death, which aids in host defense against intracellular gram-negative bacteria. Recently, caspase-11 has also been implicated in blood coagulation. Previously, we found that local fibrin formation contributes to protective immunity against Klebsiella infection of the lung. The aim of the present study was to determine the role of caspase-11 in host defense during K. pneumoniae-evoked pneumonia and sepsis. Therefore, we infected wild-type and caspase-11-deficient (Casp11^-/-) mice with a low-dose K. pneumoniae via the airways to induce a gradually evolving pneumosepsis. Casp11^-/- mice displayed increased bacterial numbers in the lung 12 h and 48 h after inoculation. Analysis of pulmonary IL-1α, IL-1β, and TNF levels showed reduced IL-1α levels in bronchoalveolar lavage fluid and increased TNF levels in the lung of Casp11^-/- mice at 48 h after inoculation. Lung γH2AX staining (marker for cell death), lung pathology and neutrophil influx in the lung, as well as bacterial dissemination and organ damage, however, were not altered in Casp11^-/- mice after Klebsiella infection. Strikingly, analysis of cross-linked fibrin and D-dimer (markers for coagulation) revealed significantly less fibrin formation in the lungs of Casp11^-/- mice at either time point after Klebsiella infection. These data reveal that caspase-11 contributes to protective immunity against K. pneumoniae possibly by activation of blood coagulation in the lung.

Klebsiella pneumoniae infection biology: living to counteract host defences

Klebsiella species cause a wide range of diseases including pneumonia, urinary tract infections (UTIs), bloodstream infections and sepsis. These infections are particularly a problem among neonates, elderly and immunocompromised individuals. Klebsiella is also responsible for a significant number of community-acquired infections. A defining feature of these infections is their morbidity and mortality, and the Klebsiella strains associated with them are considered hypervirulent. The increasing isolation of multidrug-resistant strains has significantly narrowed, or in some settings completely removed, the therapeutic options for the treatment of Klebsiella infections. Not surprisingly, this pathogen has then been singled out as an 'urgent threat to human health' by several organisations. This review summarises the tremendous progress that has been made to uncover the sophisticated immune evasion strategies of K. pneumoniae. The co-evolution of Klebsiella in response to the challenge of an activated immune has made Klebsiella a formidable pathogen exploiting stealth strategies and actively suppressing innate immune defences to overcome host responses to survive in the tissues. A better understanding of Klebsiella immune evasion strategies in the context of the host-pathogen interactions is pivotal to develop new therapeutics, which can be based on antagonising the anti-immune strategies of this pathogen.

ASC and NLRP3 impair host defense during lethal pneumonia caused by serotype 3 Streptococcus pneumoniae in mice

Streptococcus (S.) pneumoniae is the most common cause of community-acquired pneumonia. The Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome, consisting of NLRP3, ASC (the adaptor apoptosis-associated speck-like protein containing a CARD) and caspase-1, has been implicated in protective immunity during pneumonia induced by high doses of S. pneumoniae serotype 2. Here we investigated the role of the NLRP3 inflammasome in the host response during lethal airway infection with a low dose of serotype 3 S. pneumoniae. Mice were euthanized at predefined endpoints for analysis or observed in survival studies. In additional studies, Tlr2^-/- /Tlr4^-/- mice and Myd88^-/- mice incapable of Toll-like receptor signaling were studied. In stark contrast with existing literature, both Nlrp3^-/- and Asc^-/- mice showed a strongly improved host defense, as reflected by a markedly reduced mortality rate accompanied by diminished bacterial growth and dissemination. Host defense was unaltered in Tlr2^-/- /Tlr4^-/- mice and Myd88^-/- mice. These results show that the NLRP3 inflammasome impairs host defense during lethal pneumonia caused by serotype 3 S. pneumoniae. Our findings challenge the current paradigm that proximal innate detection systems are indispensable for an adequate host immune response against bacteria.

Epithelial Myeloid-Differentiation Factor 88 Is Dispensable during Klebsiella Pneumonia

Klebsiella pneumoniae is a common cause of pneumonia. Previous studies have documented an important role for Toll-like receptors (TLRs) expressed by myeloid cells in the recognition of K. pneumoniae and the initiation of a protective immune response. Lung epithelial cells also express TLRs and can participate in innate immune defense. The aim of this study was to examine the role of the common TLR adaptor protein myeloid-differentiation factor (MyD) 88 in lung epithelium during host defense against K. pneumoniae-induced pneumonia. To this end, we first crossed mice expressing cre recombinase under the control of the surfactant protein C (SftpCcre) or the club cell 10 kD (CC10cre) promoter with reporter mice to show that SftpCcre mice mainly express cre in type II alveolar cells, whereas CC10cre mice express cre almost exclusively in bronchiolar epithelial cells. We then generated mice with cell-targeted deletion of MyD88 in type II alveolar (SftpCcre-MyD88-lox) and bronchiolar epithelial (CC10cre-MyD88-lox) cells, and infected them with K. pneumoniae via the airways. Bacterial growth and dissemination were not affected by the loss of MyD88 in SftpCcre-MyD88-lox or CC10cre-MyD88-lox mice compared with control littermates. Furthermore, inflammatory responses induced by K. pneumoniae in the lung were not dependent on MyD88 expression in type II alveolar or bronchiolar epithelial cells. These results indicate that MyD88 expression in two distinct lung epithelial cell types does not contribute to host defense during pneumonia caused by a common human gram-negative pathogen.

Klebsiella pneumoniae: Going on the Offense with a Strong Defense

Klebsiella pneumoniae causes a wide range of infections, including pneumonias, urinary tract infections, bacteremias, and liver abscesses. Historically, K. pneumoniae has caused serious infection primarily in immunocompromised individuals, but the recent emergence and spread of hypervirulent strains have broadened the number of people susceptible to infections to include those who are healthy and immunosufficient. Furthermore, K. pneumoniae strains have become increasingly resistant to antibiotics, rendering infection by these strains very challenging to treat. The emergence of hypervirulent and antibiotic-resistant strains has driven a number of recent studies. Work has described the worldwide spread of one drug-resistant strain and a host defense axis, interleukin-17 (IL-17), that is important for controlling infection. Four factors, capsule, lipopolysaccharide, fimbriae, and siderophores, have been well studied and are important for virulence in at least one infection model. Several other factors have been less well characterized but are also important in at least one infection model. However, there is a significant amount of heterogeneity in K. pneumoniae strains, and not every factor plays the same critical role in all virulent Klebsiella strains. Recent studies have identified additional K. pneumoniae virulence factors and led to more insights about factors important for the growth of this pathogen at a variety of tissue sites. Many of these genes encode proteins that function in metabolism and the regulation of transcription. However, much work is left to be done in characterizing these newly discovered factors, understanding how infections differ between healthy and immunocompromised patients, and identifying attractive bacterial or host targets for treating these infections.

Lung epithelial MyD88 drives early pulmonary clearance of Pseudomonas aeruginosa by a flagellin dependent mechanism

Pseudomonas aeruginosa is a flagellated pathogen frequently causing pneumonia in hospitalized patients and sufferers of chronic lung disease. Here we investigated the role of the common Toll-like receptor (TLR) adaptor myeloid differentiation factor (MyD)88 in myeloid vs. lung epithelial cells in clearance of P. aeruginosa from the airways. Mice deficient for MyD88 in lung epithelial cells (Sftpccre-MyD88-lox mice) or myeloid cells (LysMcre-MyD88-lox mice) and bone marrow chimeric mice deficient for TLR5 (the receptor recognizing Pseudomonas flagellin) in either parenchymal or hematopoietic cells were infected with P. aeruginosa via the airways. Sftpccre-MyD88-lox mice demonstrated a reduced influx of neutrophils into the bronchoalveolar space and an impaired early antibacterial defense after infection with P. aeruginosa, whereas the response of LysMcre-MyD88-lox mice did not differ from control mice. The immune-enhancing role of epithelial MyD88 was dependent on recognition of pathogen-derived flagellin by epithelial TLR5, as demonstrated by an unaltered clearance of mutant P. aeruginosa lacking flagellin from the lungs of Sftpccre-MyD88-lox mice and an impaired bacterial clearance in bone marrow chimeric mice lacking TLR5 in parenchymal cells. These data indicate that early clearance of P. aeruginosa from the airways is dependent on flagellin-TLR5-MyD88-dependent signaling in respiratory epithelial cells.

Sepsis Induces Hematopoietic Stem Cell Exhaustion and Myelosuppression through Distinct Contributions of TRIF and MYD88

Toll-like receptor 4 (TLR4) plays a central role in host responses to bacterial infection, but the precise mechanism(s) by which its downstream signaling components coordinate the bone marrow response to sepsis is poorly understood. Using mice deficient in TLR4 downstream adapters MYD88 or TRIF, we demonstrate that both cell-autonomous and non-cell-autonomous MYD88 activation are major causes of myelosuppression during sepsis, while having a modest impact on hematopoietic stem cell (HSC) functions. In contrast, cell-intrinsic TRIF activation severely compromises HSC self-renewal without directly affecting myeloid cells. Lipopolysaccharide-induced activation of MYD88 or TRIF contributes to cell-cycle activation of HSC and induces rapid and permanent changes in transcriptional programs, as indicated by persistent downregulation of Spi1 and CebpA expression after transplantation. Thus, distinct mechanisms downstream of TLR4 signaling mediate myelosuppression and HSC exhaustion during sepsis through unique effects of MyD88 and TRIF.

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Activation of TLR4 by LPS causes HSC injury, myelosuppression, and neutropenia

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LPS-induced MYD88 activation leads to apoptosis and myelosuppression

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LPS causes HSC damage and exhaustion by TRIF activation

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HSC retain long-term memory of LPS injury

Endoplasmic reticulum chaperone gp96 in macrophages is essential for protective immunity during Gram-negative pneumonia

Klebsiella pneumoniae is among the most common Gram-negative bacteria that cause pneumonia. Gp96 is an endoplasmic reticulum chaperone that is essential for the trafficking and function of Toll-like receptors (TLRs) and integrins. To determine the role of gp96 in myeloid cells in host defence during Klebsiella pneumonia, mice homozygous for the conditional Hsp90b1 allele encoding gp96 were crossed with mice expressing Cre-recombinase under control of the LysM promoter to generate LysMcre-Hsp90b1-flox mice. LysMcre-Hsp90b1-flox mice showed absence of gp96 protein in macrophages and partial depletion in monocytes and granulocytes. This was accompanied by almost complete absence of TLR2 and TLR4 on macrophages. Likewise, integrin subunits CD11b and CD18 were not detectable on macrophages, while being only slightly reduced on monocytes and granulocytes. Gp96-deficient macrophages did not release pro-inflammatory cytokines in response to Klebsiella and displayed reduced phagocytic capacity independent of CD18. LysMcre-Hsp90b1-flox mice were highly vulnerable to lower airway infection induced by K. pneumoniae, as reflected by enhanced bacterial growth and a higher mortality rate. The early inflammatory response in Hsp90b1-flox mice was characterized by strongly impaired recruitment of granulocytes into the lungs, accompanied by attenuated production of pro-inflammatory cytokines, while the inflammatory response during late-stage pneumonia was not dependent on the presence of gp96. Blocking CD18 did not reproduce the impaired host defence of LysMcre-Hsp90b1-flox mice during Klebsiella pneumonia. These data indicate that macrophage gp96 is essential for protective immunity during Gram-negative pneumonia by regulating TLR expression.

The role of platelet MyD88 in host response during gram-negative sepsis

BACKGROUND

Beside their role in hemostasis, platelets serve as sentinel cells in host defense during infection. In sepsis, platelets have been implicated in both beneficial (antibacterial) and detrimental responses (thrombosis and organ damage). Toll-like receptors and their common adaptor, myeloid differentiation factor 88 (MyD88), are essential for pathogen recognition and protective immunity. Platelets express functional Toll-like receptors and MyD88, which participate in platelet responsiveness to bacterial agonists.

OBJECTIVE

Considering the pivotal involvement of platelets and MyD88 in the host response to bacteria, we studied the role of platelet MyD88 in gram-negative sepsis using intravenous and airway infections with the common human sepsis pathogen Klebsiella pneumoniae.

METHODS

Platelet-specific Myd88(-/-) mice were generated by crossing mice with a conditional Myd88 flox allele with mice expressing Cre recombinase controlled by the platelet factor 4 promoter. In a reverse approach, full Myd88(-/-) mice were transfused with wild-type platelets.

RESULTS

In both settings, platelet MyD88 did not impact on bacterial growth or dissemination. In addition, platelet MyD88 did not influence hallmark sepsis responses such as thrombocytopenia, coagulation or endothelial activation, or distant organ injury. Platelet MyD88 played no role in lung pathology during pneumonia-derived sepsis.

CONCLUSION

Despite known literature, platelet MyD88-dependent TLR signaling does not contribute to the host response during gram-negative sepsis.

TIR-Domain-Containing Adaptor-Inducing Interferon-β (TRIF) Mediates Antibacterial Defense during Gram-Negative Pneumonia by Inducing Interferon-x03B3

Klebsiella pneumoniae is an important cause of Gram-negative pneumonia and sepsis. Mice deficient for TIR-domain-containing adaptor-inducing interferon-β (TRIF) demonstrate enhanced bacterial growth and dissemination during Klebsiella pneumonia. We show here that the impaired antibacterial defense of TRIF mutant mice is associated with absent interferon (IFN)-x03B3; production in the lungs. IFN-x03B3; production by splenocytes in response to K. pneumoniae in vitro was critically dependent on Toll-like receptor 4 (TLR4), the common TLR adaptor myeloid differentiation primary response gene (MyD88) and TRIF. Reconstitution of TRIF mutant mice with recombinant IFN-x03B3; via the airways reduced bacterial loads in lungs and distant body sites to levels measured in wild-type mice, and partially restored pulmonary cytokine levels. The IFN-x03B3;-induced, improved, enhanced antibacterial response in TRIF mutant mice occurred at the expense of increased hepatocellular injury. These data indicate that TRIF mediates antibacterial defense during Gram-negative pneumonia, at least in part, by inducing IFN-x03B3; at the primary site of infection.

Platelet and endothelial cell P-selectin are required for host defense against Klebsiella pneumoniae-induced pneumosepsis

BACKGROUND

Sepsis is associated with activation of platelets and endothelial cells accompanied by enhanced P-selectin surface expression. Both platelet- and endothelial P-selectin have been associated with leukocyte recruitment and induction of inflammatory alterations. Klebsiella (K.) pneumoniae is a common human sepsis pathogen, particularly in the context of pneumonia.

METHODS

Wild-type (WT) and P-selectin-deficient (Selp(-/-) ) mice or bone marrow chimeric mice were infected with K. pneumoniae via the airways to induce pneumosepsis. Mice were sacrificed during early (12 h after infection) or late-stage (44 h) sepsis for analyses, or followed in a survival study.

RESULTS

Selp(-/-) mice displayed 10-1000-fold higher bacterial burdens in the lungs, blood and distant organs during late-stage sepsis. P-selectin deficiency did not influence leukocyte recruitment to the lungs, but was associated with decreased platelet-monocyte complexes and increased cytokine release. Bone marrow transfer studies revealed a role for both platelet and endothelial cell P-selectin as mice deficient in platelet or endothelial cell P-selectin displayed an intermediate phenotype in bacterial loads and survival compared with full wild-type or full knockout control mice.

CONCLUSION

Both platelet and endothelial cell P-selectin contribute to host defense during Klebsiella pneumosepsis.

Hematopoietic but not endothelial cell MyD88 contributes to host defense during gram-negative pneumonia derived sepsis

Klebsiella pneumoniae is an important cause of sepsis. The common Toll-like receptor adapter myeloid differentiation primary response gene (MyD)88 is crucial for host defense against Klebsiella. Here we investigated the role of MyD88 in myeloid and endothelial cells during Klebsiella pneumosepsis. Mice deficient for MyD88 in myeloid (LysM-Myd88(-/-)) and myeloid plus endothelial (Tie2-Myd88(-/-)) cells showed enhanced lethality and bacterial growth. Tie2-Myd88(-/-) mice reconstituted with control bone marrow, representing mice with a selective MyD88 deficiency in endothelial cells, showed an unremarkable antibacterial defense. Myeloid or endothelial cell MyD88 deficiency did not impact on lung pathology or distant organ injury during late stage sepsis, while LysM-Myd88(-/-) mice demonstrated a strongly attenuated inflammatory response in the airways early after infection. These data suggest that myeloid but not endothelial MyD88 is important for host defense during gram-negative pneumonia derived sepsis.

MyD88 signaling regulates both host defense and immunopathogenesis during pneumocystis infection

The immune response protects against Pneumocystis infection but is also a key component of Pneumocystis pneumonia (PcP)-related immunopathogenesis. Signaling through myeloid differentiation factor 88 (MyD88) is critical for activation of immune pathways downstream of TLRs and IL-1R. To determine whether MyD88 regulates normal host defense against Pneumocystis, nonimmunosuppressed wild-type (WT) and MyD88-deficient mice were infected. MyD88(-/-) mice had higher early Pneumocystis burdens than did WT mice but mounted an effective adaptive immune response and cleared Pneumocystis similarly to WT. However, MyD88(-/-) mice displayed a more intense and prolonged pulmonary immune response than did WT mice. To determine the role of MyD88 in the development of PcP-related immunopathogenesis, WT and MyD88(-/-) mice were rendered susceptible to PcP by depletion of CD4(+) T cells. At 4 wk postinfection, CD4-depleted WT and MyD88(-/-) mice harbored similar organism burdens, but MyD88(-/-) mice were protected from the PcP-related respiratory impairment observed in WT mice. Improved pulmonary physiology in MyD88(-/-) mice correlated with lower lung CCL2 levels and reduced cell recruitment. However, by 5 wk postinfection, the overall health of MyD88(-/-) mice began to deteriorate rapidly relative to WT, with accelerated weight loss, impaired lung function, and exacerbated alveolar inflammation. This physiological decline of MyD88(-/-) mice was associated with increased TNF-α and IFN-γ in the lung, and by the inability to control Pneumocystis burden. Thus, MyD88 is not required for resistance to Pneumocystis infection, but limits the adaptive immune response in immunocompetent mice. In the setting of active PcP, MyD88 signaling contributes to both immunopathogenesis and control of fungal burden.