Bridging The Gap
The immune system’s role in ear infection
April 23, 2021
When we last heard from SPARK Associate Director Peter Santa Maria, he shared his poignant story of attempting to provide care for children suffering from chronic ear infections and accompanying ear drum perforations in remote indigenous communities in Australia. A lack of access to care spurred Dr. Santa Maria to develop a topical growth factor treatment to regenerate the ear drum, which is now in human clinical trials.
That didn’t stop Dr. Santa Maria and his SPARK team from further exploring the chronic, persistent infection called chronic suppurative otitis media (CSOM) that causes the ear drum perforations, and investigating ways to treat it.
CSOM is a neglected tropical pediatric disease that affects more than 330 million people worldwide. It is characterized by a chronically discharging and infected middle ear, and is caused most commonly by the bacteria Pseudomonas aeruginosa. During his work, Dr. Santa Maria noticed that there was a lack of accurate animal models of CSOM. As we’ve learned, animal models are crucial for testing new therapeutics.
Dr. Santa Maria and team set out to develop an accurate model of CSOM in mice. The group wrote in a 2020 Science Advances paper, “The majority of what is known about CSOM is adapted from acute infection models and models that do not involve bacteria typically known to cause CSOM.” However, chronic, or persistent and long-lasting disease like CSOM, is often quite different from acute, or sudden and short illness.
Dr. Santa Maria said, “CSOM is a chronic persistent infection, and past models of grossly innoculating ears with bacteria on face value may seem appropriate. However, there are very different mechanisms that occur on the microbiology and immunological levels.”
Therefore the team used P. aeruginosa and replicated the chronic conditions to create a mouse model of the disease. Dr. Santa Maria said, “This animal model is the first to replicate the CSOM human infection. We recreated a model that persists for at least 6 months and mirror the human infection in microbiology and immunology.”
Next, the team turned to the human’s response to bacterial infection during CSOM, hypothesizing that targeting the host immune system could help treat the disease.
Let’s get a little background on the development of CSOM: in the mouse model, the team found that chronic infection relied on so-called P. aeruginosa persister cells – a set of inactive bacteria within a bacterial biofilm. If a person stops using antibiotics, the persister cells can become active again. Dr. Santa Maria explained, “If bacteria are too little, infection is not established. If bacteria are too much and are too aggressive, the infection leads to sepsis and death. The real story of chronic bacterial infections lies somewhere in the middle.”
The team used the mouse model to investigate how the immune system responds to the bacteria during this development. They looked at a type of immune cell called neutrophils, which have previously been implicated in chronic bacterial infection.
“When looking to create the CSOM infection in mice, the neutrophils are the first responders,” Dr. Santa Maria said. “We first began looking at how neutrophils react to these innoculations and witnessed, similar to other bacterial biofilm diseases that neutrophils were becoming more immature as the infection began to overwhelm the immune system.”
The team confirmed a key role for neutrophils “in the ineffective immune response to P. aeruginosa infection observed in CSOM,” they wrote in a recently published paper, through the accumulation of neutrophils at the infection site, particularly the immature cells.
“We found neutrophils are part of the problem and lead to the cycle of chronic infection in the ear,” Dr. Santa Maria said. Interestingly, “The bacteria use this to their advantage by using the DNA of the dead neutrophils to build their biofilm scaffolds,” he added.
Next, the team found that suppressing the neutrophils, using an inhibitor of the enzyme neutrophil elastase, decreased bacteria numbers and cleared the infection in the CSOM mouse model when combined with an antibiotic. Infection returned in animals treated with the antibiotic alone.
The work “directly highlights that neutrophil suppression itself can prevent the disease and identifies the neutrophil as a target in reducing infection burden in general,” Dr. Santa Maria said.
He added that neutrophil suppression could complement the team’s previous growth factor-based therapy to treat CSOM. “So far the topical growth factor treatment for tympanic regeneration is for chronic perforations where the infection is inactive. If we are able to convert these active CSOM infections through neutrophil suppression, or otherwise, it opens the potential for topical growth factor treatment.”
As far as the team’s next steps, Dr. Santa Maria said, “We have a number of potential therapeutic candidates going through the SPARK program which we are evaluating. We are completing in vivo safety and toxicity before moving towards preclinical IND enabling studies.”