The complement system, consisting a set of plasma proteins, serves as a crucial arsenal in the body’s immune response. When the body faces infection and injury, the system activates and generates a wide range of effects that enhance immunity. This activation begins with two fragments of complement proteins, C3a and C5a. The two peptides, called anaphylatoxins, bind to specific receptors on cell surfaces to initiate their action. A deficiency or malfunction in the system results in an immunocompromised state. Conversely, excessive or inappropriate activation of the system is detrimental, as seen in some infections and autoimmune disorders. 

In a recent collaborative study published in Cell, researchers deciphered the structural and molecular details of anaphylatoxin binding to the complement receptors, leading to subsequent activation. 

The three complement receptors — C3aR, C5aR1 and C5aR2 — possess an extracellular segment housing a ligand-binding pocket where C3a or C5a fit. The binding of a ligand is essential for the receptor to initiate further signalling, marking its transition into an ​‘active’ state. The outcomes of this signalling pathway vary depending on the specific intracellular proteins with which the receptor pairs with.

Arun K Shukla, Professor, Indian Institute of Technology, Kanpur (IIT‑K), who is the lead and co-corresponding author of the study, expressed,

The molecular mechanism behind the activation of the complement receptors was unclear. Considering the paramount significance of these receptors in pathogenic infections and numerous inflammatory disorders, we decided to visualise them using cryo-electron microscopy.

Shukla’s team and collaborators undertook the monumental task of capturing structural snapshots depicting the complement receptors bound to their ligands and other intracellular signalling proteins. 

To achieve this, the research team crystallised the protein complexes, froze them to sub-zero temperatures, and imaged their structures using an electron microscope. Shirsha Saha, a PhD student at IIT‑K and one of the first authors of the study, highlights, ​“For the first time, we are visualising the ligand-binding pocket of complement receptors while simultaneously studying the binding of synthetic peptides.” These synthetic peptides, derived from partial sequences of C3a and C5a, serve as agonists of the complement receptors.

A key finding from the study is the identification of a synthetic peptide, EP141, a modified form of C3a, exhibiting selective binding to the C3aR receptor. More importantly, it directs a biased signalling involving intracellular partners of the receptor called as G‑proteins. Studies indicate that G‑protein coupled signalling through the C3aR receptor may confer immune protection. Peptide-based drugs capable of directing biased signalling through the complement receptors hold immense therapeutic potential, allowing the separation of the receptors’ protective functions from their harmful effects. 

“Until now, our understanding was limited to the inactive state of the [complement] receptors, lacking insights into their active state. This study bridges that gap, providing valuable perspectives expected to facilitate the design of improved molecules for therapeutic use,” says Arvind Sahu, Executive Director, Regional Centre for Biotechnology, Faridabad, who was not involved in the study. Sahu further acknowledges the necessity for additional studies to thoroughly explore and regulate the signalling mechanisms, a point also highlighted by the authors of the study.

The research article has also revealed that the complement receptors in mice and humans have different downstream signalling responses to the same ligand. Saha says, 

This discovery potentially explains why numerous drugs that perform exceptionally well in pre-clinical trials fail to elicit desired results in clinical trials. Our findings caution against relying solely on animal models for pre-clinical drug trials without implementing adequate controls.