How opsonins drive bacterial uptake: A phagocytes dinner

Abstract

The immune response against pathogenic bacteria involves a complex cascade of biological events aimed at clearing microorganisms and preventing infection. The first line of defense involves both humoral and cellular elements cooperating to achieve this goal. Among cellular elements, phagocytes, and especially neutrophils, are essential to chase, engulf, and chemically destroy bacteria in a process called phagocytosis. To initiate phagocytosis, contact between phagocytes and bacteria is necessary, so that phagocytic receptors on the cell membrane are engaged by specific bacterial ligands and instruct the cell to initiate the phagocytosis process. Ubiquitous host humoral proteins called opsonins, such as complement fragments and antibodies, greatly aid phagocytosis by depositing on the surface of the bacterium and enhancing its recognition via dedicated phagocytic receptors. Opsonins cooperate to enhance the specificity of recognition and the efficacy of phagocytosis. Staphylococcus aureus is a common human commensal and pathogen. Given the great global health and economic burden of S. aureus diseases, the concerning development of antibiotic-resistant strains, and the current lack of vaccines, new preventive and therapeutic strategies against S. aureus are urgently needed. Increased vulnerability of patients with neutrophil deficiencies to S. aureus infections, together with the evidence that S. aureus evolved several strategies specifically aimed at impairing neutrophils recognition and uptake, suggested neutrophil phagocytosis as a crucial defense mechanism against S. aureus infections. This thesis gathers three works aimed at expanding the scientific knowledge on the role of complement and antibodies in bacterial, specifically staphylococcal, phagocytosis by human neutrophils. Via a simple but informative phagocytosis assay based on the measurement of the uptake of fluorescent bacteria by cells via flow cytometry, we investigated the contribution that antibodies and complement proteins to bacterial phagocytosis in different experimental conditions. In Chapter 2 we investigated S. aureus phagocytosis induced by the opsonins naturally present human sera from healthy donors and analyzed the respective contributions of antibodies and complement. We concluded that the cooperation of complement and antibodies is crucial for the efficient uptake of bacteria. In Chapter 3 we used innovative techniques based on bioorthogonal chemistry to artificially label bacteria and beads with purified complement fragments. Thanks to this interdisciplinary work we unraveled a new phagocytic axis mediated by C3b and Complement receptor 1. Finally, in Chapter 4 we focused on one of the major anti-phagocytic virulence factors of S. aureus, Staphylococcal protein A (SpA). By investigating natural humoral immunity and antibodies we highlight a potentially important role for a specific type of antibodies, IgG3, in the humoral response against S. aureus.

In conclusion, studying opsonins and phagocytosis improves our understanding of the pathogenesis of S. aureus and other bacteria. Moreover, it provides us with new tools to influence and modulate the immune response and develop better anti-bacterial preventive and therapeutic strategies.