Unlocking the secrets of our body's first line of defense against strep throat and more

By Neha MathurReviewed by Benedette Cuffari, M.Sc.Sep 10 2023

In a recent study posted to the bioRxiv* preprint server, researchers determined the cryogenic-electron microscopic (cryo-EM) structures of secretory immunoglobulin A (sIgA)-M4 and sigA-CD89 complexes.

Study: The Structures of Secretory IgA in complex with Streptococcus pyogenes M4 and human CD89 provide insights on mucosal host-pathogen interactions. Image Credit: Corona Borealis Studio / Shutterstock.com

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

Ig A is an antibody present in monomeric (m) and secretory (S) forms in serum and mucosal secretions of mammals, respectively. Both m and S forms of IgA contain fragment antigen-binding (Fab) and Fc regions. 

Notably, sIgA is the first line of defense against pathogens in the host's mucosal epithelium; however, sIgA interactions with CD89, a human FcαRI, and M4 remain poorly understood. There is also limited data on IgA effector functions and host-pathogen interplay.

M proteins such as M4 and M22 are group A streptococcus (GAS) virulence factors that bind IgA. These proteins adopt a coiled-coil structure that protrudes from the bacterium cell to bind extracellular matrix (ECM) and serum proteins and modulate the host immune response.

Previous studies have shown that GAS strains M4 and M22 use a 29-amino acid-long residue to bind IgA; however, how these proteins interact with host ligands remains unknown. Even though most published studies have focused on mIgA, one study revealed an overlapping interface of M4 and CD89, which raises the possibility that M4 interferes with IgA effector functions.

About 12% of asymptomatic GAS carriers are healthy children, in whom mucosal sites serve as a reservoir for pathogens, including S. pyogenes. While some GAS infections are mild, such as tonsillitis, others are life-threatening and can lead to sepsis. Every year, about 1.78 million new GAS-related invasive infections occur and lead to over 160,000 deaths, which makes streptococcal bacteria one of the top ten causes of mortality worldwide. 

About the study

In the present study, researchers determine the sIgA-M4 cryo-EM structure to an average resolution of 3.1 Å. This allowed the researchers to refine the positions of main and side chain atoms for most amino acid residues in this complex. The unexpected stoichiometry of the sIgA-CD89 complex at a resolution of 3.2 Å was also examined. 

Alphafold2-multimer was used to model the full-length M4 and align this protein to the M4-sIgA structure to create schematic representations of the complex on a bacterium surface. Mutational analysis and surface plasmon resonance (SPR) binding assays were also used to investigate the contribution of each M4 residue on sIgA binding.

Study findings

CD89 and M4 engaged sIgA through five common amino acid residues and exhibited distinct binding stoichiometry. These observations suggest that while this 'hot spot' was conserved on s and mIgAs, its accessibility to different host receptors and microbial proteins was variable.

Sterically enforced 1:1 M4: sIgA stoichiometry demonstrated that sIgA adopted an asymmetric structure; therefore, M4 bound Fc_AB positions on sIgA differently from Fc_CD. Surface-associated M4 had two orientations, one bound one side of SIgA (Fc_AB) where it held all sIgA at a similar orientation and far from the bacterium surface, whereas the Fc_CD-FcαR binding site remained open. Accordingly, the IgA-binding region on the predicted model had a root mean square deviation (RMSD) of 0.640 when aligned to the sIgA-M4 structure.

Both mIgA and sIgA bound two CD89 copies in vitro; however, the orientation and spacing of bound CD89 differed in CD89-mIgA and CD89-sIgA cryo-EM structures. Previous studies modeled that two copies of CD89 in sIgA were 99Å apart; however, the researchers of the current study observed this distance to be 108Å, thus indicating the need for additional studies to evaluate the host's CD89 function.

Each sIgA may comprise unique CD89 binding sites with different accessibility and orientations relative to bound antigens, which may influence CD89 clustering to impact IgA effector functions. This may explain why different IgAs in different locations elicit varying outcomes when encountering CD89 or other FcαRs and that additional factors like CD11b/CD18 might be involved in sIgA-CD89 signaling. 

Conclusions

The current study raised the possibility that sIgA has shaped GAS evolution, providing different selective pressures than mIgA. Nevertheless, further studies are needed to investigate GAS-sIgA interactions in the mucosa, where M4/M22 might block host FcαR effector functions to confer a survival advantage for the bacteria.

The concept that host sIgA effector functions play a crucial role in antimicrobial response and provide selective pressure extends beyond GAS. Accordingly, S. pyogenes, S. aureus, and S. pneumoniae, three distinct pathogenic bacterial species, were found to bind sIgA-expressed specific sIgA binding proteins. Each of these bacteria utilizes the nasopharynx as its primary human reservoir, where encounters with sIgA have the potential to modulate virulence and host response. 

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.