Unmasking SARS-CoV-2's coagulation conspiracy: the role of Spike.

The recent study by Ryu et al. demonstrates that the SARS-CoV-2 Spike protein binds fibrin(ogen), impacting coagulation and immune responses in mice. In this spotlight article we focus on the described effects on natural killer (NK) cell responses as well as the suggested translation into potential new therapeutic targets against COVID-19.

Amplification of autoimmune organ damage by NKp46-activated ILC1s.

In systemic lupus erythematosus, loss of immune tolerance, autoantibody production and immune complex deposition are required but not sufficient for organ damage1. How inflammatory signals are initiated and amplified in the setting of autoimmunity remains elusive. Here we set out to dissect layers and hierarchies of autoimmune kidney inflammation to identify tissue-specific cellular hubs that amplify autoinflammatory responses. Using high-resolution single-cell profiling of kidney immune and parenchymal cells, in combination with antibody blockade and genetic deficiency, we show that tissue-resident NKp46+ innate lymphoid cells (ILCs) are crucial signal amplifiers of disease-associated macrophage expansion and epithelial cell injury in lupus nephritis, downstream of autoantibody production. NKp46 signalling in a distinct subset of group 1 ILCs (ILC1s) instructed an unconventional immune-regulatory transcriptional program, which included the expression of the myeloid cell growth factor CSF2. CSF2 production by NKp46+ ILCs promoted the population expansion of monocyte-derived macrophages. Blockade of the NKp46 receptor (using the antibody clone mNCR1.15; ref. 2) or genetic deficiency of NKp46 abrogated epithelial cell injury. The same cellular and molecular patterns were operative in human lupus nephritis. Our data provide support for the idea that NKp46+ ILC1s promote parenchymal cell injury by granting monocyte-derived macrophages access to epithelial cell niches. NKp46 activation in ILC1s therefore constitutes a previously unrecognized, crucial tissue rheostat that amplifies organ damage in autoimmune hosts, with broad implications for inflammatory pathologies and therapies.

Proteomic and transcriptomic profiling of brainstem, cerebellum and olfactory tissues in early- and late-phase COVID-19.

Neurological symptoms, including cognitive impairment and fatigue, can occur in both the acute infection phase of coronavirus disease 2019 (COVID-19) and at later stages, yet the mechanisms that contribute to this remain unclear. Here we profiled single-nucleus transcriptomes and proteomes of brainstem tissue from deceased individuals at various stages of COVID-19. We detected an inflammatory type I interferon response in acute COVID-19 cases, which resolves in the late disease phase. Integrating single-nucleus RNA sequencing and spatial transcriptomics, we could localize two patterns of reaction to severe systemic inflammation, one neuronal with a direct focus on cranial nerve nuclei and a separate diffuse pattern affecting the whole brainstem. The latter reflects a bystander effect of the respiratory infection that spreads throughout the vascular unit and alters the transcriptional state of mainly oligodendrocytes, microglia and astrocytes, while alterations of the brainstem nuclei could reflect the connection of the immune system and the central nervous system via, for example, the vagus nerve. Our results indicate that even without persistence of severe acute respiratory syndrome coronavirus 2 in the central nervous system, local immune reactions are prevailing, potentially causing functional disturbances that contribute to neurological complications of COVID-19.

Post-COVID exercise intolerance is associated with capillary alterations and immune dysregulations in skeletal muscles.

The SARS-CoV-2 pandemic not only resulted in millions of acute infections worldwide, but also in many cases of post-infectious syndromes, colloquially referred to as "long COVID". Due to the heterogeneous nature of symptoms and scarcity of available tissue samples, little is known about the underlying mechanisms. We present an in-depth analysis of skeletal muscle biopsies obtained from eleven patients suffering from enduring fatigue and post-exertional malaise after an infection with SARS-CoV-2. Compared to two independent historical control cohorts, patients with post-COVID exertion intolerance had fewer capillaries, thicker capillary basement membranes and increased numbers of CD169+ macrophages. SARS-CoV-2 RNA could not be detected in the muscle tissues. In addition, complement system related proteins were more abundant in the serum of patients with PCS, matching observations on the transcriptomic level in the muscle tissue. We hypothesize that the initial viral infection may have caused immune-mediated structural changes of the microvasculature, potentially explaining the exercise-dependent fatigue and muscle pain.

Distinct tissue niches direct lung immunopathology via CCL18 and CCL21 in severe COVID-19.

Prolonged lung pathology has been associated with COVID-19, yet the cellular and molecular mechanisms behind this chronic inflammatory disease are poorly understood. In this study, we combine advanced imaging and spatial transcriptomics to shed light on the local immune response in severe COVID-19. We show that activated adventitial niches are crucial microenvironments contributing to the orchestration of prolonged lung immunopathology. Up-regulation of the chemokines CCL21 and CCL18 associates to endothelial-to-mesenchymal transition and tissue fibrosis within these niches. CCL21 over-expression additionally links to the local accumulation of T cells expressing the cognate receptor CCR7. These T cells are imprinted with an exhausted phenotype and form lymphoid aggregates that can organize in ectopic lymphoid structures. Our work proposes immune-stromal interaction mechanisms promoting a self-sustained and non-resolving local immune response that extends beyond active viral infection and perpetuates tissue remodeling.

Assessing and improving the validity of COVID-19 autopsy studies - A multicentre approach to establish essential standards for immunohistochemical and ultrastructural analyses.

BACKGROUND: Autopsy studies have provided valuable insights into the pathophysiology of COVID-19. Controversies remain about whether the clinical presentation is due to direct organ damage by SARS-CoV-2 or secondary effects, such as overshooting immune response. SARS-CoV-2 detection in tissues by RT-qPCR and immunohistochemistry (IHC) or electron microscopy (EM) can help answer these questions, but a comprehensive evaluation of these applications is missing. METHODS: We assessed publications using IHC and EM for SARS-CoV-2 detection in autopsy tissues. We systematically evaluated commercially available antibodies against the SARS-CoV-2 proteins in cultured cell lines and COVID-19 autopsy tissues. In a multicentre study, we evaluated specificity, reproducibility, and inter-observer variability of SARS-CoV-2 IHC. We correlated RT-qPCR viral tissue loads with semiquantitative IHC scoring. We used qualitative and quantitative EM analyses to refine criteria for ultrastructural identification of SARS-CoV-2. FINDINGS: Publications show high variability in detection and interpretation of SARS-CoV-2 abundance in autopsy tissues by IHC or EM. We show that IHC using antibodies against SARS-CoV-2 nucleocapsid yields the highest sensitivity and specificity. We found a positive correlation between presence of viral proteins by IHC and RT-qPCR-determined SARS-CoV-2 viral RNA load (N= 35; r=-0.83, p-value <0.0001). For EM, we refined criteria for virus identification and provide recommendations for optimized sampling and analysis. 135 of 144 publications misinterpret cellular structures as virus using EM or show only insufficient data. We provide publicly accessible digitized EM sections as a reference and for training purposes. INTERPRETATION: Since detection of SARS-CoV-2 in human autopsy tissues by IHC and EM is difficult and frequently incorrect, we propose criteria for a re-evaluation of available data and guidance for further investigations of direct organ effects by SARS-CoV-2. FUNDING: German Federal Ministry of Health, German Federal Ministry of Education and Research, Berlin University Alliance, German Research Foundation, German Center for Infectious Research.

COVID-19 associated myopathy.

PURPOSE OF REVIEW: The global spread of severe acute respiratory syndrome coronavirus 2 resulted in many cases of acute and postacute muscular symptoms. In this review, we try to decipher the potential underlying pathomechanisms and summarize the potential links between viral infection and muscle affection. RECENT FINDINGS: Disregarding single case studies that do not allow safe conclusions due to the high number of infections, histopathological evidence of myositis has only been reported in deceased individuals with severe COVID-19. Postacute myalgia and weakness seem to occur in a subset of patients up to one year after initial infection, reminiscent of postinfectious syndromes (PIS) described in prior epidemics and pandemics of the past. SUMMARY: COVID-19 associated myopathy likely comprises different entities with heterogeneous pathomechanisms. Individual factors such as disease severity and duration, age, sex, constitutional susceptibilities, and preexisting conditions are important to consider when formulating a diagnosis. Persisting symptoms show overlapping features with PIS or postintensive care syndrome. In lack of strong evidence for a direct infection of myocytes, inflammatory myopathies associated with COVID-19 are presumably immune-mediated. Differential diagnosis of rheumatological and nonmuscular neurological origin coinciding with the infection need to be considered, due to the extremely high numbers of newly occurring infections the last 2 years.

What SARS-CoV-2 does to our brains.

Neurological symptoms in SARS-CoV-2-infected patients have been reported, but their cause remains unclear. In theory, the neurological symptoms observed after SARS-CoV-2 infection could be (1) directly caused by the virus infecting brain cells, (2) indirectly by our body's local or systemic immune response toward the virus, (3) by coincidental phenomena, or (4) a combination of these factors. As indisputable evidence of intact and replicating SARS-CoV-2 particles in the central nervous system (CNS) is currently lacking, we suggest focusing on the host's immune reaction when trying to understand the neurocognitive symptoms associated with SARS-CoV-2 infection. In this perspective, we discuss the possible immune-mediated mechanisms causing functional or structural CNS alterations during acute infection as well as in the post-infectious context. We also review the available literature on CNS affection in the context of COVID-19 infection, as well as observations from animal studies on the molecular pathways involved in sickness behavior.

Daratumumab for treatment-refractory antibody-mediated diseases in neurology.

BACKGROUND AND PURPOSE: A fraction of patients with antibody-mediated autoimmune diseases remain unresponsive to first-/second-line and sometimes even to escalation immunotherapies. Because these patients are still affected by poor outcome and increased mortality, we investigated the safety and efficacy of the plasma cell-depleting anti-CD38 antibody daratumumab in life-threatening, antibody-mediated autoimmune diseases. METHODS: In this retrospective, single-center case series, seven patients with autoantibody-driven neurological autoimmune diseases (autoimmune encephalitis, n = 5; neurofascin antibody-associated chronic inflammatory demyelinating polyneuropathy associated with sporadic late onset nemaline myopathy, n = 1; seronegative myasthenia gravis, n = 1) unresponsive to a median of four (range = 4-9) immunotherapies were treated with four to 20 cycles of 16 mg/kg daratumumab. RESULTS: Daratumumab allowed a substantial clinical improvement in all patients, as measured by modified Rankin Scale (mRS; before treatment: mRS =5, n = 7; after treatment: median mRS =4, range = 0-5), Clinical Assessment Scale in Autoimmune Encephalitis (from median 21 to 3 points, n = 5), Inflammatory Neuropathy Cause and Treatment disability score (from 7 to 0 points, n = 1), and Quantitative Myasthenia Gravis score (from 16 to 8 points, n = 1). Daratumumab induced a substantial reduction of disease-specific autoreactive antibodies, total IgG (serum, 66%, n = 7; cerebrospinal fluid, 58%, n = 5), and vaccine-induced titers for rubella (50%) and tetanus toxoid (74%). Treatment-related toxicities Grade 3 or higher occurred in five patients, including one death. CONCLUSIONS: Our findings suggest that daratumumab provided a clinically relevant depletion of autoreactive long-lived plasma cells, identifying plasma cell-targeted therapies as promising escalation therapy for highly active, otherwise treatment-refractory autoantibody-mediated neurological diseases.