Active release of bound antibody by Streptococcus mutans

Area of Science:

• Microbiology
• Immunology
• Oral Biology

Background:

• Streptococcus mutans releases surface protein antigens, notably antigen P1, which is crucial for adherence to acquired pellicles.
• Antigen P1 is immunodominant, suggesting its importance in the host-pathogen interaction.

Purpose of the Study:

• To investigate the significance of Streptococcus mutans releasing surface antigens, specifically antigen P1.
• To determine if Streptococcus mutans actively sheds bound antibodies and the mechanisms involved.

Main Methods:

• Incubation of S. mutans NG8 with anti-P1 rabbit immunoglobulin G (IgG) and human colostral IgA.
• Analysis of antibody release under varying pH, time, and temperature conditions.
• Investigation of the role of cell viability and culture age in antibody release.
• Isolation and characterization of released IgG-associated immune complexes.
• Assay of secretory IgA (SIgA) inhibition of bacterial adherence before and after antibody release.

Main Results:

• Streptococcus mutans actively releases bound antibodies in a pH (optimal 6-8) and time (plateau at 1 hour) dependent manner at 37°C.
• Early-exponential-phase cells showed maximum IgG release, which was associated with antigen P1, forming immune complexes.
• Heat-killed cells bound antibodies but did not release them, indicating an active cellular process.
• Secretory IgA (SIgA) initially inhibited bacterial adherence, but this inhibition was lost after the bacteria released the bound SIgA.

Conclusions:

• Streptococcus mutans sheds surface-bound antibodies as immune complexes, potentially as a mechanism to evade the host immune system.
• This shedding process allows bacteria to overcome the neutralizing effects of naturally occurring antibodies in the oral environment.
• The active release of antibody-antigen complexes represents a novel immune evasion strategy for oral bacteria.