SARS-CoV-2 infection triggers profibrotic macrophage responses and lung fibrosis.

COVID-19-induced "acute respiratory distress syndrome" (ARDS) is associated with prolonged respiratory failure and high mortality, but the mechanistic basis of lung injury remains incompletely understood. Here, we analyze pulmonary immune responses and lung pathology in two cohorts of patients with COVID-19 ARDS using functional single-cell genomics, immunohistology, and electron microscopy. We describe an accumulation of CD163-expressing monocyte-derived macrophages that acquired a profibrotic transcriptional phenotype during COVID-19 ARDS. Gene set enrichment and computational data integration revealed a significant similarity between COVID-19-associated macrophages and profibrotic macrophage populations identified in idiopathic pulmonary fibrosis. COVID-19 ARDS was associated with clinical, radiographic, histopathological, and ultrastructural hallmarks of pulmonary fibrosis. Exposure of human monocytes to SARS-CoV-2, but not influenza A virus or viral RNA analogs, was sufficient to induce a similar profibrotic phenotype in vitro. In conclusion, we demonstrate that SARS-CoV-2 triggers profibrotic macrophage responses and pronounced fibroproliferative ARDS.

Severe COVID-19 Is Marked by a Dysregulated Myeloid Cell Compartment.

Coronavirus disease 2019 (COVID-19) is a mild to moderate respiratory tract infection, however, a subset of patients progress to severe disease and respiratory failure. The mechanism of protective immunity in mild forms and the pathogenesis of severe COVID-19 associated with increased neutrophil counts and dysregulated immune responses remain unclear. In a dual-center, two-cohort study, we combined single-cell RNA-sequencing and single-cell proteomics of whole-blood and peripheral-blood mononuclear cells to determine changes in immune cell composition and activation in mild versus severe COVID-19 (242 samples from 109 individuals) over time. HLA-DRhiCD11chi inflammatory monocytes with an interferon-stimulated gene signature were elevated in mild COVID-19. Severe COVID-19 was marked by occurrence of neutrophil precursors, as evidence of emergency myelopoiesis, dysfunctional mature neutrophils, and HLA-DRlo monocytes. Our study provides detailed insights into the systemic immune response to SARS-CoV-2 infection and reveals profound alterations in the myeloid cell compartment associated with severe COVID-19.

Outbreak of SARS-CoV-2 B.1.1.7 Lineage after Vaccination in Long-Term Care Facility, Germany, February-March 2021.

One week after second vaccinations were administered, an outbreak of B.1.1.7 lineage severe acute respiratory syndrome coronavirus 2 infections occurred in a long-term care facility in Berlin, Germany, affecting 16/20 vaccinated and 4/4 unvaccinated residents. Despite considerable viral loads, vaccinated residents experienced mild symptoms and faster time to negative test results.

Delayed Antibody and T-Cell Response to BNT162b2 Vaccination in the Elderly, Germany.

We detected delayed and reduced antibody and T-cell responses after BNT162b2 vaccination in 71 elderly persons (median age 81 years) compared with 123 healthcare workers (median age 34 years) in Germany. These data emphasize that nonpharmaceutical interventions for coronavirus disease remain crucial and that additional immunizations for the elderly might become necessary.

Krueppel-Like Factor 4 Expression in Phagocytes Regulates Early Inflammatory Response and Disease Severity in Pneumococcal Pneumonia.

The transcription factor Krueppel-like factor (KLF) 4 fosters the pro-inflammatory immune response in macrophages and polymorphonuclear neutrophils (PMNs) when stimulated with Streptococcus pneumoniae, the main causative pathogen of community-acquired pneumonia (CAP). Here, we investigated the impact of KLF4 expression in myeloid cells such as macrophages and PMNs on inflammatory response and disease severity in a pneumococcal pneumonia mouse model and in patients admitted to hospital with CAP. We found that mice with a myeloid-specific knockout of KLF4 mount an insufficient early immune response with reduced levels of pro-inflammatory cytokines and increased levels of the anti-inflammatory cytokine interleukin (IL) 10 in bronchoalveolar lavage fluid and plasma and an impaired bacterial clearance from the lungs 24 hours after infection with S. pneumoniae. This results in higher rates of bacteremia, increased lung tissue damage, more severe symptoms of infection and reduced survival. Higher KLF4 gene expression levels in the peripheral blood of patients with CAP at hospital admission correlate with a favourable clinical presentation (lower sequential organ failure assessment (SOFA) score), lower serum levels of IL-10 at admission, shorter hospital stay and lower mortality or requirement of intensive care unit treatment within 28 days after admission. Thus, KLF4 in myeloid cells such as macrophages and PMNs is an important regulator of the early pro-inflammatory immune response and, therefore, a potentially interesting target for therapeutic interventions in pneumococcal pneumonia.

Induction of Krüppel-Like Factor 4 Mediates Polymorphonuclear Neutrophil Activation in Streptococcus pneumoniae Infection.

The recruitment and activation of polymorphonuclear neutrophils (PMNs) are of central importance for the elimination of pathogens in bacterial infections. We investigated the Streptococcus pneumoniae-dependent induction of the transcription factor Krüppel-like factor (KLF) 4 in PMNs as a potential regulator of PMN activation. We found that KLF4 expression is induced in human blood-derived PMNs in a time- and dose-dependent manner by wild-type S. pneumoniae and capsule knockout mutants. Unencapsulated knockout mutants induced stronger KLF4 expression than encapsulated wild types. The presence of autolysin LytA-competent (thus viable) pneumococci and LytA-mediated bacterial autolysis were required for KLF4 induction in human and murine PMNs. LyzMcre-mediated knockdown of KLF4 in murine blood-derived PMNs revealed that KLF4 influences pneumococci killing and increases the release of the proinflammatory cytokines tumor necrosis factor α and keratinocyte chemoattractant and decreases the release of the anti-inflammatory cytokine interleukin-10. Thus, S. pneumoniae induces KLF4 expression in PMNs, which contributes to PMN activation in S. pneumoniae infection.

Absence of the complement regulatory molecule CD59a leads to exacerbated neuropathology after traumatic brain injury in mice.

BACKGROUND: Complement represents a crucial mediator of neuroinflammation and neurodegeneration after traumatic brain injury. The role of the terminal complement activation pathway, leading to generation of the membrane attack complex (MAC), has not been thoroughly investigated. CD59 is the major regulator of MAC formation and represents an essential protector from homologous cell injury after complement activation in the injured brain. METHODS: Mice deleted in the Cd59a gene (CD59a-/-) and wild-type littermates (n = 60) were subjected to focal closed head injury. Sham-operated (n = 60) and normal untreated mice (n = 14) served as negative controls. The posttraumatic neurological impairment was assessed for up to one week after trauma, using a standardized Neurological Severity Score (NSS). The extent of neuronal cell death was determined by serum levels of neuron-specific enolase (NSE) and by staining of brain tissue sections in TUNEL technique. The expression profiles of pro-apoptotic (Fas, FasL, Bax) and anti-apoptotic (Bcl-2) mediators were determined at the gene and protein level by real-time RT-PCR and Western blot, respectively. RESULTS: Clinically, the brain-injured CD59a-/- mice showed a significantly impaired neurological outcome within 7 days, as determined by a higher NSS, compared to wild-type controls. The NSE serum levels, an indirect marker of neuronal cell death, were significantly elevated in CD59a-/- mice at 4 h and 24 h after trauma, compared to wild-type littermates. At the tissue level, increased neuronal cell death and brain tissue destruction was detected by TUNEL histochemistry in CD59a-/- mice within 24 hours to 7 days after head trauma. The analysis of brain homogenates for potential mediators and regulators of cell death other than the complement MAC (Fas, FasL, Bax, Bcl-2) revealed no difference in gene expression and protein levels between CD59a-/- and wild-type mice. CONCLUSION: These data emphasize an important role of CD59 in mediating protection from secondary neuronal cell death and further underscore the key role of the terminal complement pathway in the pathophysiology of traumatic brain injury. The exact mechanisms of complement MAC-induced secondary neuronal cell death after head injury require further investigation.

Peptidoglycan Recognition Protein 2 Regulates Neutrophil Recruitment Into the Lungs After Streptococcus pneumoniae Infection.

Peptidoglycan (PGN) recognition proteins (PGLYRPs) are a highly conserved group of host defense proteins in insects and mammals that sense bacterial cell wall PGN and act bactericidally or cleave PGN by amidase function. Streptococcus (S.) pneumoniae is one of the top five killers worldwide and causes, e.g., pneumonia, endocarditis, meningitis and sepsis. S. pneumoniae accounts for approximately 1.5-2 million deaths every year. The risk of antibiotic resistance and a general poor prognosis in young children and elderly people have led to the need for new treatment approaches. To the best of our knowledge, there is no report on the relevance of PGLYRP2 in lung infections. Therefore, we infected mice deficient for PGLYRP2 transnasally with S. pneumoniae and examined the innate immune response in comparison to WT animals. As expected, PGLYRP2-KO animals had to be sacrificed earlier than their WT counterparts, and this was due to higher bacteremia. The higher bacterial load in the PGLYRP2-KO mice was accomplished with lower amounts of proinflammatory cytokines in the lungs. This led to an abolished recruitment of neutrophils into the lungs, the spread of bacteria and the subsequent aggravated course of the disease and early mortality of the PGLYRP2-KO mice. These data suggest a substantial role of PGLYRP2 in the early defense against S. pneumoniae infection, and PGLYRP2 might also affect other infections in the lungs.

Peptidoglycan Recognition Protein 4 Limits Bacterial Clearance and Inflammation in Lungs by Control of the Gut Microbiota.

Streptococcus pneumoniae is the most frequent cause of community-acquired pneumonia. Endogenous host defense molecules such as peptidoglycan recognition protein 4 (PGLYRP4) might influence the course of this disease. To the best of our knowledge, there are no reports on the relevance of PGLYRP4 in pneumonia. Therefore, wild type (WT) and PGLYRP4-deficient (PGLYRP4KO) mice were analyzed in an in vivo and in vitro experimental setting to examine the influence of PGLYRP4 on the course of pneumococcal pneumonia. Furthermore, caecal 16S rRNA microbiome analysis was performed, and microbiota were transferred to germfree WT mice to assess the influence of microbiotal communities on the bacterial burden. Mice lacking PGLYRP4 displayed an enhanced bacterial clearance in the lungs, and fewer mice developed bacteremia. In addition, an increased recruitment of immune cells to the site of infection, and an enhanced bacterial killing by stronger activation of phagocytes could be shown. This may depend partly on the detected higher expression of complement factors, interferon-associated genes, and the higher pro-inflammatory cytokine response in isolated primary PGLYRP4KO vs. WT cells. This phenotype is underlined by changes in the complexity and composition of the caecal microbiota of PGLYRP4KO compared to WT mice. Strikingly, we provided evidence, by cohousing and stable transfer of the respective WT or PGLYRP4KO mice microbiota into germfree WT mice, that the changes of the microbiota are responsible for the improved clearance of S. pneumoniae lung infection. In conclusion, the deficiency of PGLYRP4, a known antibacterial protein, leads to changes in the gut microbiota. Thus, alterations in the microbiota can change the susceptibility to S. pneumoniae lung infection independently of the host genotype.

Peptidoglycan Recognition Protein 3 Does Not Alter the Outcome of Pneumococcal Pneumonia in Mice.

Pneumococci frequently cause community-acquired pneumonia, a disease with high mortality rates, particularly in young children and in the elderly. Endogenous antimicrobial peptides and proteins such as PGLYRP3 may contribute to the progression and outcome of this disease. Since increasing antibiotic resistant strains occur all over the world, these endogenous antimicrobial molecules are interesting new targets for future therapies. In this study, the expression pattern of PGLYRP3 was analyzed in alveolar epithelial cells, alveolar macrophages and neutrophils. Additionally, the function of PGLYRP3 during Streptococcus pneumoniae-induced pneumonia was investigated in a murine pneumococcal pneumonia model using PGLYRP3KO mice. PGLYRP3 is expressed in all selected cell types but pneumococcus-dependent induction of PGLYRP3 was observed only in neutrophils and alveolar macrophages. Interestingly, there were no significant differences in the bacterial loads within the lungs, the blood or the spleens, in the cytokine response, the composition of immune cells and the histopathology between wild type and PGLYRP3KO mice. Finally, we could neither observe significant differences in the clinical symptoms nor in the overall survival. Collectively, PGLYRP3 seems to be dispensable for the antibacterial defense during pneumococcal pneumonia.

Healing parameters in a rabbit partial tendon defect following tenocyte/biomaterial implantation.

Although rabbits are commonly used as tendon repair model, interpretative tools are divergent and comprehensive scoring systems are lacking. Hence, the aim was to develop a multifaceted scoring system to characterize healing in a partial Achilles tendon defect model. A 3 mm diameter defect was created in the midsubstance of the medial M. gastrocnemius tendon, which remained untreated or was filled with a polyglycolic-acid (PGA) scaffold + fibrin and either left cell-free or seeded with Achilles tenocytes. After 6 and 12 weeks, tendon repair was assessed macroscopically and histologically using self-constructed scores. Macroscopical scoring revealed superior results in the tenocyte seeded PGA + fibrin group compared with the controls at both time points. Histology of all operated tendons after 6 weeks proved extracellular matrix (ECM) disorganization, hypercellularity and occurrence of irregular running elastic fibres with no significance between the groups. Some inflammation was associated with PGA implantation and increased sulphated proteoglycan deposition predominantly with the empty defects. After 12 weeks defect areas became hard to recognize and differences between groups, except for the increased sulphated proteoglycans content in the empty defects, were almost nullified. We describe a partial Achilles tendon defect model and versatile scoring tools applicable for characterizing biomaterial-supported tendon healing.

Effect of pro-inflammatory and immunoregulatory cytokines on human tenocytes.

Tendon injury induces a local inflammatory response, characterized by the induction of pro-inflammatory cytokines. The aim of the present study was to analyze the effects of TNFalpha, IL-6 and IL-10 on key parameters of tendon homeostasis. Cultured primary human tenocytes were treated with the recombinant cytokines IL-6, IL-10, TNFalpha, or combinations of TNFalpha with IL-6 and IL-10 (10 ng/mL, 6, 24 h). Expression of type I collagen, elastin, MMP-1, TNFalpha, IL-1beta, IL-6, IL-10, and suppressors of cytokine signaling (SOCS1, 3) was analyzed with the use of RTD-PCR, immunocytochemistry, and Western blot analysis. In response to TNFalpha, tenocytes reduced their type I collagen deposition but increased their elastin gene expression and highly upregulated their expression for MMP-1, pro-inflammatory (TNFalpha, IL-1beta) and immunoregulatory (IL-6, IL-10) cytokines. TNFalpha stimulation augmented SOCS1, whereas SOCS3 expression in tenocytes was also induced by IL-6. The treatment of tenocytes with IL-6 and IL-10 had no effect on cytokine expression. Neither IL-6 nor IL-10 modulated the observed effects of TNFalpha significantly. These results indicate that TNFalpha strongly activates the tenocytes to amplify their own TNFalpha expression and, subsequently, that of other regulatory cytokines and matrix degrading enzymes. However, the impact of IL-6 and IL-10 on tenocytes remains unclear.