Eluding the immune system's frontline defense: Secreted complement evasion factors of pathogenic Gram-positive cocci.

The human complement system is an important part of the innate immune response in the fight against invasive bacteria. Complement responses can be activated independently by the classical pathway, the lectin pathway, or the alternative pathway, each resulting in the formation of a C3 convertase that produces the anaphylatoxin C3a and the opsonin C3b by specifically cutting C3. Other important features of complement are the production of the chemotactic C5a peptide and the generation of the membrane attack complex to lyse intruding pathogens. Invasive pathogens like Staphylococcus aureus and several species of the genus Streptococcus have developed a variety of complement evasion strategies to resist complement activity thereby increasing their virulence and potential to cause disease. In this review, we focus on secreted complement evasion factors that assist the bacteria to avoid opsonization and terminal pathway lysis. We also briefly discuss the potential role of complement evasion factors for the development of vaccines and therapeutic interventions.

Intranasal immunization with Ag85B peptide 25 displayed on Lactococcus lactis using the PilVax platform induces antigen-specific B- and T-cell responses.

Mycobacterium tuberculosis (Mtb) remains a global epidemic despite the widespread use of Bacillus Calmette-Guérin (BCG). Consequently, novel vaccines are required to facilitate a reduction in Mtb morbidity and mortality. PilVax is a peptide delivery strategy for the generation of highly specific mucosal immune responses and is based on the food-grade bacterium Lactococcus lactis that is used to express selected peptides engineered within the Streptococcus pyogenes M1T1 pilus, allowing for peptide amplification, stabilization and enhanced immunogenicity. In the present study, the dominant T-cell epitope from the Mtb protein Ag85B was genetically engineered into the pilus backbone subunit and expressed on the surface of L. lactis. Western blot and flow cytometry confirmed formation of pilus containing the peptide DNA sequence. B-cell responses in intranasally vaccinated mice were analyzed by ELISA while T-cell responses were analyzed by flow cytometry. Serum titers of peptide-specific immunoglobulin (Ig) G and IgA were detected, confirming that vaccination produced antibodies against the cognate peptide. Peptide-specific IgA was also detected across several mucosal sites sampled. Peptide-specific CD4+ T cells were detected at levels similar to those of mice immunized with BCG. PilVax immunization resulted in an unexpected increase in the numbers of CD3+ CD4- CD8- [double negative (DN)] T cells in the lungs of vaccinated mice. Analysis of cytokine production following stimulation with the cognate peptide showed the major cytokine producing cells to be CD4+ T cells and DN T cells. This study provides insight into the antibody and peptide-specific cellular immune responses generated by PilVax vaccination and demonstrates the suitability of this vaccine for conducting a protection study.