Quantifying Phagocytosis – studies on the antibody response during invasive streptococcal infections

The interaction between our immune system and pathogens encompasses a complex spectrum from symbiosis to potentially life-threatening diseases. Throughout evolution, our immune system has evolved numerous strategies such as phagocytosis to combat infections and protect us from diseases. Phagocytosis involves the engulfment of pathogens by our immune cells and can be facilitated through opsonization, a phenomenon where antibodies bind to the prey. However, pathogens like Streptococcus pyogenes and Streptococcus dysgalactiae have developed countermeasures against it.

These pathogens cause significant global health burdens, yet vaccines are unavailable, and the development of natural immunity is not fully understood. Opsonic antibodies are considered crucial in immunity, but quantifying phagocytosis remains challenging due to a lack of standardized, reproducible methods across laboratories and systems. Addressing this gap is vital not only for enhancing the quantification of opsonic antibody response in streptococcal infection but also for benefiting various research fields where phagocytosis is a pivotal and widely measured functional outcome. This thesis aims to develop a universal and robust method for quantifying phagocytosis and assessing antibody function in streptococcal infections.

A novel method for quantifying phagocytosis is established. It is based on the principles of the Hill equation and collision theory (Paper I). This versatile, quantifiable, and robust approach enhances phagocytosis assessment and provides insights into opsonic capacity of antibodies. Additionally, a biophysical model is developed that predicts the binding of antibodies against the streptococcal M protein (Paper II). Together, the opsonic capacity of antibody responses in patients with invasive infections of S. dysgalactiae and S. pyogenes is subsequently evaluated (Paper III and IV). Although limited study populations, instances of non-opsonic antibodies are discovered for both pathogens, which can potentially be linked to recurrent invasive S. dysgalactiae infections. The results indicate that higher antibody titers do not always equate to opsonic responses, calling for a nuanced understanding of antibody function in streptococcal infections. Furthermore, patients with S. pyogenes infection unexpectedly developed opsonic antibodies effective against other strains as well. These findings contribute to the current understanding of natural immunity by suggesting not only type-specific antibodies may convey S. pyogenes immunity.

This thesis provides insights into the development of immunity against invasive S. pyogenes and S. dysgalactiae infections and introduces novel methods for enhancing the assessment of phagocytosis and antibody function. In conclusion, it advances our understanding of how to quantify antibody-mediated phagocytosis and its importance in the context of invasive streptococcal infections.