Photo: Siddharth Kankaria / Research Matters
When our body’s defensive immune responses end up injuring our own tissues and organs while fighting infections, it results in a clinical condition called sepsis. It is one of the leading causes of global mortality, with an estimated 90,000 deaths every year in India alone. Once it kicks off, sepsis or “septic shock”, commonly results in tissue damage, multiple organ failure and eventually death in high-risk patients. Fungal, viral and parasitic infections can all cause sepsis, with bacteria being the most common culprits. Conventionally, sepsis is treated using expensive antibiotics with poor shelf lives. Now, a new study by researchers at the Indian Institute of Science, Bangalore, has proposed a cost-effective treatment for sepsis.
Bacterial infections caused by E. coli, Salmonella and Chlamydia can often lead to sepsis, generally targeting Intensive Care Unit patients in hospitals. Certain chemicals, called endotoxins, present in these bacteria interact with specific receptor proteins on the body’s immune cells. This interaction initiates a cascade of events and kicks up severe inflammation to counter the infection. More than half of sepsis cases can be attributed to bacterial endotoxins that require rapid anti-microbial treatment. An endotoxin neutralising glycoprotein, called bactericidal/permeability-increasing protein (BPI), has shown to possess anti-inflammatory properties against sepsis and is used for treatment. However, due to its poor stability, high cost of production and limitations on storage, it has not been widely used.
The study, led by Prof. Dipshikha Chakravortty from the Molecular Pathogenesis Lab, IISc has devised a cost-effective method for delivering therapeutic BPI in high-risk patients. It involves a genetic engineering approach to synthesize a stable form of BPI by expressing a fusion protein of mouse BPI, called mBPI, and vesicle protein on the surface of gas vesicle nanoparticles (GVNPs) derived from a species of Halobacterium, to produce mBPIN-GVNPs. Gas vesicle nanoparticles are hollow and buoyant with a stable membrane, naturally produced by salt-loving microbes like Halobacterium. These GVNPs prove to be an ideal delivery system as they as relatively easy and inexpensive to produce, in addition to being inert and safe.
“In brief, the gene encoding mouse BPI was tagged along with gene encoding GVNP vesicle protein. This leads to the expression of the fusion protein on the surface of GVNP”, explains Mr. Arjun Balakrishnan, a member of the research team and lead author of this study. Studies have shown that GVNPs do not induce inflammation independently and are non-pathogenic in mice. Moreover, expressing BPI on the surface of these GVNPs improves the former’s stability thus increasing the shelf life. “Previously, we were working on Halobacterial vesicles expressing Salmonella protein for immunizing mice against it. From that study, we understood that these vesicles by itself will not provoke any immune response and the proteins expressed in these vesicles are highly stable”, elaborates Mr. Balakrishnan.
Results indicated that mBPI bound to GVNPs was more stable, more available and effective compared to unbound mBPI. “With this discovery, we have made the existing treatment cost-effective. The highly stable protein expressed on GVNPs and its ease of purifying reduces the cost and limitations associated with storage and production”, says Mr. Balakrishnan. The results also suggest that mBPIN-GVNPs may need to be present in the circulatory system by pre-treatment for complete clearance of endotoxins like Lipopolysaccharides (LPS). This reduces the interaction between endotoxin and immune cell receptors, thus lowering the severity of sepsis. The study is first of its kind that validates LPS neutralizing function of mBPI in a mouse model for septic shock. “With further studies exploring the pathways leading to endotoxin induced sepsis, clinical tests in higher animals and proper approvals in place, we may proceed for human trials. As of now, the therapeutic potential of mBPIN-GVNPs seems promising from the standpoint of both safety and efficacy”, signs off Mr. Balakrishnan.