SIgA structures bound to Streptococcus pyogenes M4 and human CD89 provide insights into host-pathogen interactions

Area of Science:

• Immunology
• Structural Biology
• Microbiology

Background:

• Immunoglobulin A (IgA) exists as monomeric IgA in serum and Secretory IgA (SIgA) in mucosal secretions.
• Host IgA Fc receptors (FcαRs), like human FcαR1/CD89, mediate IgA effector functions.
• The pathogen Streptococcus pyogenes utilizes surface proteins, such as M4, to bind IgA, potentially interfering with host immunity.

Purpose of the Study:

• To elucidate the structural basis of SIgA interactions with host FcαR1/CD89 and the bacterial M4 protein.
• To understand how these interactions influence host-pathogen dynamics at mucosal surfaces.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of M4-SIgA and CD89-SIgA complexes.
• Supporting experimental data and computational modeling were employed.

Main Results:

• Cryo-EM revealed distinct SIgA-binding stoichiometries for M4 and CD89.
• SIgA bound to M4 on S. pyogenes adopts orientations that leave host FcαR binding sites accessible.
• This suggests a mechanism for bacterial immune evasion.

Conclusions:

• The findings highlight unappreciated functional consequences of SIgA binding to both host and bacterial FcαRs.
• This research contributes to understanding host-microbe co-evolution and IgA effector functions.
• The results may inform strategies to improve outcomes for Group A Streptococcus infections.