A common pathogenic bacterium known as Chlamydia pneumoniae can lead to pneumonia or sinus infections. Now, researchers have shown that this microbe can remain in the eye as well as the brain over periods of years, which could worsen cognitive decline in Alzheimer's disease. This is the first report that has shown that C. pneumoniae can get into the retina and activate immune signals, which may cause inflammation, neuronal death, and cognitive problems. These findings, which were reported in Nature Communications, may also lead to new preventive treatments.
"Seeing Chlamydia pneumoniae consistently across human tissues, cell cultures, and animal models allowed us to identify a previously unrecognized link between bacterial infection, inflammation, and neurodegeneration," said senior study author Professor Maya Koronyo-Hamaoui, Ph.D., of Cedars-Sinai Health Sciences.
In this work, the investigators studied retinal tissued donated by 104 individuals, some of whom were Alzheimer’s patients while others had mild cognitive impairment, and some had normal cognitive function. The scientists found that there were much higher levels of C. pneumoniae in retina and brain samples from Alzheimer's patients compared to those with normal cognition. As bacterial levels rose, the brain changes and cognitive decline that were observed also worsened.
"The eye is a surrogate for the brain, and this study shows that retinal bacterial infection and chronic inflammation can reflect brain pathology and predict disease status, supporting retinal imaging as a noninvasive way to identify people at risk for Alzheimer’s,” noted Koronyo-Hamaoui.
There were also higher levels of C. pneumoniae in individuals who carry a genetic risk factor for Alzheimer’s: a gene variant known as APOE4.
Additional work conducted in mouse models and human neurons in culture has indicated that C. pneumoniae infections can cause inflammation to increase, neurons to die, and cognitive functions to decline. This seems to confirm that the bacterium can negatively affect disease processes. These infections also caused an increase in the levels of an aberrant protein that has long been linked to Alzheimer’s, known as amyloid-beta.
Several infectious diseases have been linked to cognitive decline before, like herpes simplex; and people who have had shingles vaccines or flu vaccines tend to have lower rates of dementia or Alzheimer's, respectively.
"This discovery raises the possibility of targeting the infection-inflammation axis to treat Alzheimer's," added co-corresponding study author Timothy Crother, Ph.D., also of Cedars-Sinai, among other appointments.
The researchers noted that these findings could lead to better ways to decrease the neuroinflammation that has been linked to neurodegeneration, and may be a noninvasive way to help treat or even prevent the disorder.