Anticoagulation and Antiplatelet Therapy for Prevention of Venous and Arterial Thrombotic Events in Critically Ill Patients With COVID-19: COVID-PACT.

BACKGROUND: The efficacy and safety of prophylactic full-dose anticoagulation and antiplatelet therapy in critically ill COVID-19 patients remain uncertain. METHODS: COVID-PACT (Prevention of Arteriovenous Thrombotic Events in Critically-ill COVID-19 Patients Trial) was a multicenter, 2×2 factorial, open-label, randomized-controlled trial with blinded end point adjudication in intensive care unit-level patients with COVID-19. Patients were randomly assigned to a strategy of full-dose anticoagulation or standard-dose prophylactic anticoagulation. Absent an indication for antiplatelet therapy, patients were additionally randomly assigned to either clopidogrel or no antiplatelet therapy. The primary efficacy outcome was the hierarchical composite of death attributable to venous or arterial thrombosis, pulmonary embolism, clinically evident deep venous thrombosis, type 1 myocardial infarction, ischemic stroke, systemic embolic event or acute limb ischemia, or clinically silent deep venous thrombosis, through hospital discharge or 28 days. The primary efficacy analyses included an unmatched win ratio and time-to-first event analysis while patients were on treatment. The primary safety outcome was fatal or life-threatening bleeding. The secondary safety outcome was moderate to severe bleeding. Recruitment was stopped early in March 2022 (≈50% planned recruitment) because of waning intensive care unit-level COVID-19 rates. RESULTS: At 34 centers in the United States, 390 patients were randomly assigned between anticoagulation strategies and 292 between antiplatelet strategies (382 and 290 in the on-treatment analyses). At randomization, 99% of patients required advanced respiratory therapy, including 15% requiring invasive mechanical ventilation; 40% required invasive ventilation during hospitalization. Comparing anticoagulation strategies, a greater proportion of wins occurred with full-dose anticoagulation (12.3%) versus standard-dose prophylactic anticoagulation (6.4%; win ratio, 1.95 [95% CI, 1.08-3.55]; P=0.028). Results were consistent in time-to-event analysis for the primary efficacy end point (full-dose versus standard-dose incidence 19/191 [9.9%] versus 29/191 [15.2%]; hazard ratio, 0.56 [95% CI, 0.32-0.99]; P=0.046). The primary safety end point occurred in 4 (2.1%) on full dose and in 1 (0.5%) on standard dose (P=0.19); the secondary safety end point occurred in 15 (7.9%) versus 1 (0.5%; P=0.002). There was no difference in all-cause mortality (hazard ratio, 0.91 [95% CI, 0.56-1.48]; P=0.70). There were no differences in the primary efficacy or safety end points with clopidogrel versus no antiplatelet therapy. CONCLUSIONS: In critically ill patients with COVID-19, full-dose anticoagulation, but not clopidogrel, reduced thrombotic complications with an increase in bleeding, driven primarily by transfusions in hemodynamically stable patients, and no apparent excess in mortality. REGISTRATION: URL: https://www. CLINICALTRIALS: gov; Unique identifier: NCT04409834.

Validation of a Crisis Standards of Care Model for Prioritization of Limited Resources During the Coronavirus Disease 2019 Crisis in an Urban, Safety-Net, Academic Medical Center.

OBJECTIVES: The coronavirus disease 2019 pandemic has overwhelmed healthcare resources even in wealthy nations, necessitating rationing of limited resources without previously established crisis standards of care protocols. In Massachusetts, triage guidelines were designed based on acute illness and chronic life-limiting conditions. In this study, we sought to retrospectively validate this protocol to cohorts of critically ill patients from our hospital. DESIGN: We applied our hospital-adopted guidelines, which defined severe and major chronic conditions as those associated with a greater than 50% likelihood of 1- and 5-year mortality, respectively, to a critically ill patient population. We investigated mortality for the same intervals. SETTING: An urban safety-net hospital ICU. PATIENTS: All adults hospitalized during April of 2015 and April 2019 identified through a clinical database search. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Of 365 admitted patients, 15.89% had one or more defined chronic life-limiting conditions. These patients had higher 1-year (46.55% vs 13.68%; p < 0.01) and 5-year (50.00% vs 17.22%; p < 0.01) mortality rates than those without underlying conditions. Irrespective of classification of disease severity, patients with metastatic cancer, congestive heart failure, end-stage renal disease, and neurodegenerative disease had greater than 50% 1-year mortality, whereas patients with chronic lung disease and cirrhosis had less than 50% 1-year mortality. Observed 1- and 5-year mortality for cirrhosis, heart failure, and metastatic cancer were more variable when subdivided into severe and major categories. CONCLUSIONS: Patients with major and severe chronic medical conditions overall had 46.55% and 50.00% mortality at 1 and 5 years, respectively. However, mortality varied between conditions. Our findings appear to support a crisis standards protocol which focuses on acute illness severity and only considers underlying conditions carrying a greater than 50% predicted likelihood of 1-year mortality. Modifications to the chronic lung disease, congestive heart failure, and cirrhosis criteria should be refined if they are to be included in future models.

Changes in forced vital capacity over time in systemic sclerosis: application of group-based trajectory modelling.

OBJECTIVE: An accelerated rate of decline in forced vital capacity (FVC) affects >50% of patients with SSc but data on the variability and determinants of this change are scarce. We sought to identify trajectories of FVC and their associated variables in SSc patients over a 12-year period. METHODS: Clinical and pulmonary function data were retrospectively collected. SSc patients with three or more FVC values were included. Group-based modelling was used to cluster similar FVC patterns into trajectories. Baseline variables were associated with the trajectories using multinomial logistic regression. The effect of CYC on FVC was examined with each trajectory as a time-varying covariate. RESULTS: In 254 SSc patients we identified seven distinct FVC trajectories: very low slow decline (5.5%), very low improve (13.8%), low fast decline (9.5%), low stable (19.7%), low-normal improve (31.1%), normal improve (16.1%) and normal stable (4.3%). Younger age and the presence of pulmonary hypertension, Interstitial lung disease and shortness of breath at baseline significantly increased the odds of declining trajectories vs the reference trajectory (low-normal improve). CYC was associated with FVC improvement in the low fast decline trajectory. CONCLUSION: The course of FVC in SSc was highly variable, with improvement and stability experienced even by those with low baseline FVC. Trajectory modelling was able to identify SSc patients who were most likely to experience FVC decline and thus could be a useful tool for patient management as well as clinical trial design.

Where is Noninvasive Ventilation Actually Delivered for Acute Respiratory Failure?

PURPOSE: Few studies have examined locations of noninvasive ventilation (NIV) application for acute respiratory failure (ARF). We aimed to track actual locations of NIV delivery and related outcomes. METHODS: Observational cohort study based at 8 acute care hospitals in Massachusetts on adult patients admitted for ARF requiring ventilatory support during pre-determined time intervals. RESULTS: Of 1225 ventilator starts, 499 were NIV; 209 (42%) in intensive care units (ICU), 185 (37%) in emergency departments (ED), 91 (18%) on general wards, and 14 (3%) in other units. Utilization (% of all ventilator starts) (1), success (2) and in-hospital mortality (3) rates for patients initiated on NIV in ICU, ED, and general and other wards were (1) 38, 36, 73, and 52%, (2) 60, 77, 68, and 93% and (3) 25, 12, 17, and 0%, respectively (p < 0.05 for all). Patients with acute-on-chronic lung disease (ACLD) and acute pulmonary edema (APE) were begun on NIV most often in EDs and patients with 'de novo' ARF and neurologic disorders most often in ICU's. Approximately 2/3 of patients begun on NIV outside of ICUs were transferred within 72 h to ICUs, wards or other units. CONCLUSIONS: Most NIV starts occurred in ICUs and EDs but utilization rate was highest (>50%) on general wards where a fifth of NIV starts took place. Actual location depended on etiology of ARF as patients with ACLD and APE were started more often in EDs and "de novo" ARF in ICU. NIV failure and mortality rates were higher in ICUs related to the greater proportion of patients with "de novo" ARF.

Implementation of a Phenobarbital-based Pathway for Severe Alcohol Withdrawal: A Mixed-Method Study.

Rationale: Several institutions have implemented phenobarbital-based pathways for the treatment of alcohol withdrawal syndrome (AWS). However, little is known about the care processes, effectiveness, and safety of phenobarbital-based pathways for intensive care unit (ICU) patients. Objectives: To examine clinician acceptability and feasibility and patient outcomes after the implementation of a phenobarbital-based pathway for medical ICU (MICU) patients with severe AWS. Methods: We conducted a mixed-method study of a quality-improvement intervention designed to improve the workflow without deleterious effects on outcomes. We used semistructured, qualitative interviews and surveys of clinicians to assess the acceptability and feasibility of the phenobarbital-based pathway and a previous benzodiazepine-based pathway. We used a noninferiority interrupted-time-series analysis to compare mechanical ventilation rates before and after implementation among MICU patients within an urban safety-net hospital who were admitted with severe alcohol withdrawal. We explored several secondary outcomes, including physical restraint use and hospital length of stay. Results: Four themes related to clinician acceptability and feasibility of the phenobarbital-based pathway emerged: 1) designing a pathway that balanced standardization with clinical judgment promoted acceptability, 2) pathway simplicity promoted feasibility, 3) implementing pathway-driven care streamlined the workflow, and 4) ad hoc implementation strategies facilitated new pathway uptake. Two hundred thirty-three and 252 patients were initiated on the benzodiazepine- and phenobarbital-based pathways, respectively. The rate of mechanical ventilation decreased from 17.1% to 12.9% after implementation of the phenobarbital-based pathway, and an adjusted mean difference of -4.9% (95% upper confidence interval [CI]: 0.7%) corresponding to relative change in the 95% upper limit of 4%, which was below the a priori noninferiority margin, was shown. After implementation, use of physical restraints decreased from 51.6% to 32.4% (mean difference, -18.0%; 95% CI: -26.4% to -9.7%), and the hospital length of stay was shorter (8.6-6.8 d; mean difference, -1.8 d; 95% CI: -3.4 to -0.2 d). Conclusions: Clinicians believed that the phenobarbital-based pathway was more efficient and simpler to use, and patient mechanical ventilation rates were noninferior compared with the previous benzodiazepine-based pathway for the treatment of severe AWS.

Rationale and Design of the Awake Prone Position for Early Hypoxemia in COVID-19 Study Protocol: A Clinical Trial.

The unprecedented public health burdens of coronavirus disease (COVID-19) have intensified the urgency of identifying effective, low-cost treatments that limit the need for advanced life support measures and improve clinical outcomes. However, personal protective equipment and staffing shortages, disease virulence, and infectivity have created significant barriers to traditional clinical trial practices. We present the novel design of a pragmatic, adaptive, multicenter, international, prospective randomized controlled clinical trial evaluating the safety and effectiveness of awake prone positioning in spontaneously breathing patients with COVID-19 (APPEX-19 [Awake Prone Position for Early Hypoxemia in COVID-19]). Key innovations of this trial include 1) a novel smartphone-based communication process that facilitates rapid enrollment and intervention delivery while allowing social distancing and conservation of personal protective equipment, 2) Bayesian response-adaptive randomization to allow preferential assignment to the most effective intervention and expedite trial completion compared with frequentist designs, 3) remote electronic collection of patient-reported outcomes and electronic medical record data, and 4) pragmatic prospective use of patient-reported data and data collected as part of routine clinical care. Clinical trial registered with www.clinicaltrials.gov (NCT04344587).

Early administration of interleukin-6 inhibitors for patients with severe COVID-19 disease is associated with decreased intubation, reduced mortality, and increased discharge.

OBJECTIVE: The aim of this observational study was to determine the optimal timing of interleukin-6 receptor inhibitor (IL6ri) administration for coronavirus disease 2019 (COVID-19). METHODS: Patients with COVID-19 were given an IL6ri (sarilumab or tocilizumab) based on iteratively reviewed guidelines. IL6ri were initially reserved for critically ill patients, but after review, treatment was liberalized to patients with lower oxygen requirements. Patients were divided into two groups: those requiring ≤45% fraction of inspired oxygen (FiO2) (termed stage IIB) and those requiring >45% FiO2 (termed stage III) at the time of IL6ri administration. The main outcomes were all-cause mortality, discharge alive from hospital, and extubation. RESULTS: A total of 255 COVID-19 patients were treated with IL6ri (149 stage IIB and 106 stage III). Patients treated in stage IIB had lower mortality than those treated in stage III (adjusted hazard ratio (aHR) 0.24, 95% confidence interval (CI) 0.08-0.74). Overall, 218 (85.5%) patients were discharged alive. Patients treated in stage IIB were more likely to be discharged (aHR 1.43, 95% CI 1.06-1.93) and were less likely to be intubated (aHR 0.43, 95% CI 0.24-0.79). CONCLUSIONS: IL6ri administration prior to >45% FiO2 requirement was associated with improved COVID-19 outcomes. This can guide clinical management pending results from randomized controlled trials.

Human immunodeficiency virus infection alters tumor necrosis factor alpha production via Toll-like receptor-dependent pathways in alveolar macrophages and U1 cells.

Human immunodeficiency virus (HIV)-positive persons are predisposed to pulmonary infections, even after receiving effective highly active antiretroviral therapy. The reasons for this are unclear but may involve changes in innate immune function. HIV type 1 infection of macrophages impairs effector functions, including cytokine production. We observed decreased constitutive tumor necrosis factor alpha (TNF-alpha) concentrations and increased soluble tumor necrosis factor receptor type II (sTNFRII) in bronchoalveolar lavage fluid samples from HIV-positive subjects compared to healthy controls. Moreover, net proinflammatory TNF-alpha activity, as measured by the TNF-alpha/sTNFRII ratio, decreased as HIV-related disease progressed, as manifested by decreasing CD4 cell count and increasing HIV RNA (viral load). Since TNF-alpha is an important component of the innate immune system and is produced upon activation of Toll-like receptor (TLR) pathways, we hypothesized that the mechanism associated with deficient TNF-alpha production in the lung involved altered TLR expression or a deficit in the TLR signaling cascade. We found decreased Toll-like receptor 1 (TLR1) and TLR4 surface expression in HIV-infected U1 monocytic cells compared to the uninfected parental U937 cell line and decreased TLR message in alveolar macrophages (AMs) from HIV-positive subjects. In addition, stimulation with TLR1/2 ligand (Pam(3)Cys) or TLR4 ligand (lipopolysaccharide) resulted in decreased intracellular phosphorylated extracellular signal-regulated kinase and subsequent decreased transcription and expression of TNF-alpha in U1 cells compared to U937 cells. AMs from HIV-positive subjects also showed decreased TNF-alpha production in response to these TLR2 and TLR4 ligands. We postulate that HIV infection alters expression of TLRs with subsequent changes in mitogen-activated protein kinase signaling and cytokine production that ultimately leads to deficiencies of innate immune responses that predispose HIV-positive subjects to infection.

Comparison of Methods To Identify Advance Care Planning in Patients with Severe Chronic Obstructive Pulmonary Disease Exacerbation.

BACKGROUND: Advance care planning (ACP) is recommended for patients with chronic obstructive pulmonary disease (COPD). Yet, ACP documentation is often inaccessible at the time of impending respiratory failure, which may lead to unwanted and costly medical intensive care unit admissions. Electronic medical records (EMRs) contain directive fields and the ability to search for keywords and phrases, but these strategies to rapidly identify ACP have not been validated. OBJECTIVES: The aim of this study is to identify the percentage of patients with severe COPD exacerbation who have outpatient ACP documentation and validate two EMR-based methods of rapidly identifying ACP documentation. DESIGN: Retrospective cohort analysis. SETTING/SUBJECTS: Patients who required medical intensive care unit admission for exacerbation of COPD at an urban safety-net hospital between 2009 and 2014 were observed. MEASUREMENTS: We analyzed the sensitivity and specificity of two methods to rapidly identify outpatient ACP documentation: (1) documentation in the EMR directive field and (2) text string search of notes for key phrases, compared with a gold standard clinician review. RESULTS: Our cohort (n = 311) was racially diverse and severely ill with obstructive lung disease. One hundred thirty-two patients (43%) had ACP documentation by gold standard chart review. Compared with a gold standard chart review, a parsimonious text string search was both sensitive (95%) and specific (97%), while the directive box was specific (100%), but not sensitive (54%), for identifying outpatient ACP documentation. CONCLUSIONS: EMR directive fields may substantially underestimate ACP when used alone. As full clinician chart reviews are impractical in the emergent setting, text string searches may be a useful strategy to rapidly identify ACP discussions for clinical care and research.

The RNA uridyltransferase Zcchc6 is expressed in macrophages and impacts innate immune responses.

Alveolar macrophages orchestrate pulmonary innate immunity and are essential for early immune surveillance and clearance of microorganisms in the airways. Inflammatory signaling must be sufficiently robust to promote host defense but limited enough to prevent excessive tissue injury. Macrophages in the lungs utilize multiple transcriptional and post-transcriptional mechanisms of inflammatory gene expression to delicately balance the elaboration of immune mediators. RNA terminal uridyltransferases (TUTs), including the closely homologous family members Zcchc6 (TUT7) and Zcchc11 (TUT4), have been implicated in the post-transcriptional regulation of inflammation from studies conducted in vitro. In vivo, we observed that Zcchc6 is expressed in mouse and human primary macrophages. Zcchc6-deficient mice are viable and born in Mendelian ratios and do not exhibit an observable spontaneous phenotype under basal conditions. Following an intratracheal challenge with S. pneumoniae, Zcchc6 deficiency led to a modest but significant increase in the expression of select cytokines including IL-6, CXCL1, and CXCL5. These findings were recapitulated in vitro whereby Zcchc6-deficient macrophages exhibited similar increases in cytokine expression due to bacterial stimulation. Although loss of Zcchc6 also led to increased neutrophil emigration to the airways during pneumonia, these responses were not sufficient to impact host defense against infection.

Use and Outcomes of Noninvasive Ventilation for Acute Respiratory Failure in Different Age Groups.

BACKGROUND: The prevalence of chronic disease and do-not-intubate status increases with age. Thus, we aimed to determine characteristics and outcomes associated with noninvasive ventilation (NIV) use for acute respiratory failure (ARF) in different age groups. METHODS: A database comprising prospective data collected on site on all adult patients with ARF requiring ventilatory support from 8 acute care hospitals in Massachusetts was used. RESULTS: From a total of 1,225 ventilator starts, overall NIV utilization, success, and in-hospital mortality rates were 22, 54, and 18% in younger (18-44 y); 34, 65, and 13% in middle-aged (45-64 y); 49, 68, and 17% in elderly (65-79 y); and 47, 76, and 24% in aged (≥ 80 y) groups, respectively (P < .001, P = .08, and P = .11, respectively). NIV use for cardiogenic pulmonary edema and subjects with a do-not-intubate order increased significantly with advancing age (25, 57, 57, and 74% and 7, 12, 18, and 31%, respectively, in the 4 age groups [P < .001 and P = .046, respectively]). For subjects receiving NIV with a do-not-intubate order, success and in-hospital mortality rates were similar in different age groups (P = .27 and P = .98, respectively). CONCLUSIONS: NIV use and a do-not-intubate status are more frequent in subjects with ARF ≥ 65 y than in those <65 y, especially for subjects with cardiogenic pulmonary edema. However, NIV success and mortality rates were similar between age groups. (ClinicalTrials.gov registration NCT00458926.).

Use and outcomes of noninvasive positive pressure ventilation in acute care hospitals in Massachusetts.

BACKGROUND: This study determined actual utilization rates and outcomes of noninvasive positive pressure ventilation (NIV) at selected hospitals that had participated in a prior survey on NIV use. METHODS: This observational cohort study, based at eight acute care hospitals in Massachusetts, focused on all adult patients requiring ventilatory support for acute respiratory failure during predetermined time intervals. RESULTS: Of 548 ventilator starts, 337 (61.5%) were for invasive mechanical ventilation and 211 (38.5%) were for NIV, with an overall NIV success rate of 73.9% (ie, avoidance of intubation or death while on NIV or within 48 h of discontinuation). Causal diagnoses for respiratory failure were classified as (I) acute-on-chronic lung disease (23.5%), (II) acute de novo respiratory failure (17.9%), (III) neurologic disorders (19%), (IV) cardiogenic pulmonary edema (16.8%), (V) cardiopulmonary arrest (12.2%), and (VI) others (10.6%). NIV use and success rates for each of the causal diagnoses were, respectively, (I) 76.7% and 75.8%, (II) 37.8% and 62.2%, (III) 1.9% and 100%, (IV) 68.5% and 79.4%, (V) none, and (VI) 17.2% and 60%. Hospital mortality rate was higher in patients with invasive mechanical ventilation than in patients with NIV (30.3% vs 16.6%, P < .001). CONCLUSIONS: NIV occupies an important role in the management of acute respiratory failure in acute care hospitals in selected US hospitals and is being used for a large majority of patients with acute-on-chronic respiratory failure and acute cardiogenic pulmonary edema. NIV use appears to have increased substantially in selected US hospitals over the past decade. TRIAL REGISTRY: ClinicalTrials.gov; No.: NCT00458926; URL: www.clinicaltrials.gov.

Myeloid ZFP36L1 does not regulate inflammation or host defense in mouse models of acute bacterial infection.

Zinc finger protein 36, C3H type-like 1 (ZFP36L1) is one of several Zinc Finger Protein 36 (Zfp36) family members, which bind AU rich elements within 3' untranslated regions (UTRs) to negatively regulate the post-transcriptional expression of targeted mRNAs. The prototypical member of the family, Tristetraprolin (TTP or ZFP36), has been well-studied in the context of inflammation and plays an important role in repressing pro-inflammatory transcripts such as TNF-α. Much less is known about the other family members, and none have been studied in the context of infection. Using macrophage cell lines and primary alveolar macrophages we demonstrated that, like ZFP36, ZFP36L1 is prominently induced by infection. To test our hypothesis that macrophage production of ZFP36L1 is necessary for regulation of the inflammatory response of the lung during pneumonia, we generated mice with a myeloid-specific deficiency of ZFP36L1. Surprisingly, we found that myeloid deficiency of ZFP36L1 did not result in alteration of lung cytokine production after infection, altered clearance of bacteria, or increased inflammatory lung injury. Although alveolar macrophages are critical components of the innate defense against respiratory pathogens, we concluded that myeloid ZFP36L1 is not essential for appropriate responses to bacteria in the lungs. Based on studies conducted with myeloid-deficient ZFP36 mice, our data indicate that, of the Zfp36 family, ZFP36 is the predominant negative regulator of cytokine expression in macrophages. In conclusion, these results imply that myeloid ZFP36 may fully compensate for loss of ZFP36L1 or that Zfp36l1-dependent mRNA expression does not play an integral role in the host defense against bacterial pneumonia.

Cardiopulmonary exercise testing with right-heart catheterization in patients with systemic sclerosis.

OBJECTIVE: To examine the role of cardiopulmonary exercise testing with right-heart catheterization (CPET/RHC) in patients with systemic sclerosis (SSc) with potentially multifactorial exertional limitation. METHODS: This was a single-center retrospective cohort study of patients with SSc referred for CPET/RHC. RESULTS: A total of 19 patients with SSc [subtypes: 10 limited, 5 diffuse, 1 systemic lupus erythematosus (SLE)/SSc overlap, and 3 with no subtype specified in the medical record] underwent CPET/RHC testing from February 2003 to February 2008. Of these patients, the primary limitations to exercise were found to be ventilatory (n = 6), deconditioning/cardiovascular (n = 6), pulmonary vascular (PVL; n = 3), and exercise-induced left ventricular diastolic dysfunction (exercise-LVDD; n = 4). No prior physical examination, pulmonary function test, imaging, or echocardiographic data reliably predicted the etiology of exercise limitation determined by CPET/RHC. Vital capacity and ventilatory equivalent for CO(2) did not differ during CPET testing between PVL and exercise-LVDD, limiting the utility of CPET alone for discriminating these etiologies of dyspnea. Exercise alveolar-arterial oxygen gradient was elevated in subjects shown to have PVL [median 48 mm Hg (interquartile range 45.3, 62.0)] compared to those with exercise-LVDD [26.0 (IQR 10.6, 36.0)] and deconditioning [13.9 (IQR 4.0, 16.4); p = 0.02]. Major therapeutic changes occurred in 11/19 (58%) subjects after CPET/RHC testing. CONCLUSION: CPET/RHC testing in subjects with SSc and potentially multifactorial dyspnea adds potentially useful diagnostic information unavailable from noninvasive testing.

Macrophage apoptosis in response to high intracellular burden of Mycobacterium tuberculosis is mediated by a novel caspase-independent pathway.

We previously reported that macrophage exposure to attenuated strains of pathogenic mycobacteria at multiplicities of infection (MOI) < or = 10 triggers TNF-alpha-mediated apoptosis which reduces the viability of intracellular bacilli. Virulent strains were found to suppress macrophage apoptosis, and it was proposed that apoptosis is an innate defense against intracellular Mycobacterium tuberculosis analogous to apoptosis of virus-infected cells. The potential similarity of host cell responses to intracellular infection with mycobacteria and viruses suggests that M. tuberculosis might lyse infected macrophage when that niche is no longer needed. To investigate this question, we challenged murine macrophages with high intracellular bacillary loads. A sharp increase in cytolysis within 24 h was observed at MOI > or = 25. The primary death mode was apoptosis, based on nuclear morphology and phosphatidyl serine exposure, although the apoptotic cells progressed rapidly to necrosis. Apoptosis at high MOI differs markedly from low MOI apoptosis: it is potently induced by virulent M. tuberculosis, it is TNF-alpha-independent, and it does not reduce mycobacterial viability. Caspase inhibitors failed to prevent high MOI apoptosis, and macrophages deficient in caspase-3, MyD88, or TLR4 were equally susceptible as wild type. Apoptosis was reduced in the presence of cathepsin inhibitors, suggesting the involvement of lysosomal proteases in this novel death response. We conclude that the presence of high numbers of intracellular M. tuberculosis bacilli triggers a macrophage cell death pathway that could promote extracellular spread of infection and contribute to the formation of necrotic lesions in tuberculosis.