Longitudinal characterisation of phagocytic and neutralisation functions of anti-Spike antibodies in plasma of patients after SARS-CoV-2 infection.

Phagocytic responses by effector cells to antibody or complement-opsonised viruses have been recognized to play a key role in anti-viral immunity. These include antibody dependent cellular phagocytosis mediated via Fc-receptors, phagocytosis mediated by classically activated complement-fixing IgM or IgG1 antibodies and antibody independent phagocytosis mediated via direct opsonisation of viruses by complement products activated via the mannose-binding lectin pathway. Limited data suggest these phagocytic responses by effector cells may contribute to the immunological and inflammatory responses in SARS-CoV-2 infection, however, their development and clinical significance remain to be fully elucidated. In this cohort of 62 patients, acutely ill individuals were shown to mount phagocytic responses to autologous plasma-opsonised SARS-CoV-2 Spike protein-coated microbeads as early as 10 days post symptom onset. Heat inactivation of the plasma prior to use as an opsonin caused 77-95% abrogation of the phagocytic response, and pre-blocking of Fc-receptors on the effector cells showed only 18-60% inhibition. These results suggest that SARS-CoV-2 can provoke early phagocytosis, which is primarily driven by heat labile components, likely activated complements, with variable contribution from anti-Spike antibodies. During convalescence, phagocytic responses correlated significantly with anti-Spike IgG titers. Older patients and patients with severe disease had significantly higher phagocytosis and neutralisation functions when compared to younger patients or patients with asymptomatic, mild, or moderate disease. A longitudinal study of a subset of these patients over 12 months showed preservation of phagocytic and neutralisation functions in all patients, despite a drop in the endpoint antibody titers by more than 90%. Interestingly, surface plasmon resonance showed a significant increase in the affinity of the anti-Spike antibodies over time correlating with the maintenance of both the phagocytic and neutralisation functions suggesting that improvement in the antibody quality over the 12 months contributed to the retention of effector functions.

Bacterial molecular mimicry in autoimmune diseases

Bacterial molecular mimicry in autoimmune diseases is one of the leading mechanisms by which microorganisms may induce autoimmunity and survive in the host. The main purpose of the current study was to determine the main microbes that elicit autoimmune reactions through molecular mimicry and identify the most relevant approaches to investigate this mechanism. A classic example is the M protein of Streptococcus pyogenes, which induces antibody cross-reactivity with a cardiac protein and causes rheumatic fever. Another notable example is the protein from Porphyromonas gingivalis that closely resembles the human heat shock protein and accelerates atherosclerotic. There is evidence that antibodies against Helicobacter pylori CagA interact with different parts of smooth muscle and endothelial cells enhancing atherosclerotic vascular disease. Recently, one cause of infertility has been associated with Staphylococcus aureus molecular mimicry that triggers an antibody response that cross-reacts with human spermatozoa proteins. Further examples of bacterial molecular mimicry are associated with Chlamydia pneumoniae, Escherichia coli, Yersinia, and Salmonella. From the literature, the most widely used methods in this field are Basic Local Alignment Search Tool (BLAST), serological assays, and phage display. The subjects of particular concern are vaccine cross-reactivity and immunosuppressive drugs side-effects, therefore alternative approaches are needed. Such an approach is phage display where therapeutic antibody fragments obtained by this technique have been used in the treatment of autoimmune diseases by neutralizing the pathological effects of autoantibodies. Phage display libraries are constructed from the antibody repertoires of autoimmune disease patients. Antibody fragments without the Fc domain can not interact with Fc receptors and proteins of the complement system and trigger autoimmune diseases. Another approach is to block the Fc receptors. In conclusion, this review highlights key aspects of bacterial molecular mimicry to better understand the factors associated with autoimmune diseases and encourage further research in this field.

Cotton Rat Placenta Anatomy and Fc Receptor Expression and Their Roles in Maternal Antibody Transfer

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and viral pneumonia in infants and young children worldwide. Currently no vaccine is available to prevent RSV infection, but virus-neutralizing monoclonal antibodies can be given prophylactically, emphasizing the protective potential of antibodies. One concept of RSV vaccinology is mothers' immunization to induce high antibody titers, leading to passive transfer of high levels of maternal antibody to the fetus through the placenta and to the neonate through colostrum. Cotton rats are an excellent small animal model for RSV infection and have been used to test maternal immunization. To mechanistically understand antibody transfer in the cotton rat model, we characterized the cotton rat placenta and Fc receptor localization. Placentas from cotton rats at midgestation (approximately day 14) and at late gestation (approximately day 25) and neonatal (younger than 1 wk) gastrointestinal tracts were collected for light microscopy, immunohistochemistry, and transmission electron microscopy. The cotton rat placenta is hemotrichorial and has 5 distinct layers: decidua, junctional zone, labyrinth, chorionic plate, and yolk sac. Consistent with the transfer of maternal antibodies, the majority of the Fc receptors are present in the yolk sac endoderm and fetal capillary endothelium of the chorionic plate, involving 10% of the cells within the labyrinth. In addition, Fc receptors are present on duodenal and jejunal enterocytes in cotton rats, similar to humans, mice, and rats. These findings provide the structural basis for the pre- and postnatal transfer of maternal antibodies described in cotton rats.