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

*Essential notice: bioRxiv publishes preliminary scientific reports that usually are not peer-reviewed and, subsequently, shouldn’t be thought to be conclusive, guide clinical practice/health-related behavior, or treated as established information.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. Each m and S types of IgA contain fragment antigen-binding (Fab) and Fc regions. 

Notably, sIgA is the primary line of defense against pathogens within the host’s mucosal epithelium; nevertheless, sIgA interactions with CD89, a human FcαRI, and M4 remain poorly understood. There may be also limited data on IgA effector functions and host-pathogen interplay.

M proteins resembling M4 and M22 are group A streptococcus (GAS) virulence aspects 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; nevertheless, how these proteins interact with host ligands stays unknown. Though most published studies have focused on mIgA, one study revealed an overlapping interface of M4 and CD89, which raises the likelihood that M4 interferes with IgA effector functions.

About 12% of asymptomatic GAS carriers are healthy children, in whom mucosal sites function a reservoir for pathogens, including S. pyogenes. While some GAS infections are mild, resembling tonsillitis, others are life-threatening and might result in sepsis. Every yr, about 1.78 million recent GAS-related invasive infections occur and result in over 160,000 deaths, which makes streptococcal bacteria considered one of the highest ten causes of mortality worldwide. 

Concerning the study

In the current study, researchers determine the sIgA-M4 cryo-EM structure to a median resolution of three.1 Å. This allowed the researchers to refine the positions of essential and side chain atoms for many amino acid residues on this complex. The unexpected stoichiometry of the sIgA-CD89 complex at a resolution of three.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 evaluation and surface plasmon resonance (SPR) binding assays were also used to research the contribution of every 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; subsequently, M4 sure FcAB positions on sIgA otherwise from FcCD. Surface-associated M4 had two orientations, one sure one side of SIgA (FcAB) where it held all sIgA at an analogous orientation and removed from the bacterium surface, whereas the FcCD-FcαR binding site remained open. Accordingly, the IgA-binding region on the anticipated model had a root mean square deviation (RMSD) of 0.640 when aligned to the sIgA-M4 structure.

Each mIgA and sIgA sure two CD89 copies in vitro; nevertheless, the orientation and spacing of sure CD89 differed in CD89-mIgA and CD89-sIgA cryo-EM structures. Previous studies modeled that two copies of CD89 in sIgA were 99Å apart; nevertheless, the researchers of the present study observed this distance to be 108Å, thus indicating the necessity for extra studies to guage the host’s CD89 function.

Each sIgA may comprise unique CD89 binding sites with different accessibility and orientations relative to sure antigens, which can influence CD89 clustering to affect IgA effector functions. This may increasingly explain why different IgAs in numerous locations elicit various outcomes when encountering CD89 or other FcαRs and that additional aspects like CD11b/CD18 could be involved in sIgA-CD89 signaling. 

Conclusions

The present study raised the likelihood that sIgA has shaped GAS evolution, providing different selective pressures than mIgA. Nevertheless, further studies are needed to research GAS-sIgA interactions within 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 vital role in antimicrobial response and supply 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 those bacteria utilizes the nasopharynx as its primary human reservoir, where encounters with sIgA have the potential to modulate virulence and host response. 

*Essential notice: bioRxiv publishes preliminary scientific reports that usually are not peer-reviewed and, subsequently, shouldn’t be thought to be conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:

• Preliminary scientific report.
  Liu, Q. & Stadtmueller, B. M. (2023). The Structures of Secretory IgA in complex with Streptococcus pyogenes M4 and human CD89 provide insights on mucosal host-pathogen interactions. bioRxiv. doi:10.1101/2023.04.21.53787