An estimated 10-20% of Lyme disease patients who are treated with antibiotics continue to suffer persisting symptoms 6 to 12 months later. This is known as post-treatment Lyme disease syndrome, because it may affect various organs. Symptoms include chronic pain, overwhelming fatigue, and neurological or psychiatric symptoms. It is not known why this occurs in a subset of treated patients, but an aberrant immune response has long been suspected. In a new study led by Dr. John Aucott of Johns Hopkins University, elevated CCL19 was identified as associated with persistent symptoms. During acute infection, all patients diagnosed with Lyme disease (defined by physician-diagnosed erythema migrans rash) had elevated CCL19. However, by one month after diagnosis and treatment, those with serum concentrations greater than 112 pg/ml were 13 times more likely to develop post-treatment symptoms at 6 and 12 months later. Other patients who recovered, or who had symptoms without functional impairment had lower CCL19 levels after one month. CCL19 is produced by activated dendritic cells, which are sentinel cells that recruit immune cells to the site of an infection. The authors propose that sustained, elevated CCL19 is linked to continued inflammation and may account for ongoing symptoms, but point out that further studies are needed to understand the mechanistic basis and to identify other relevant immune pathways.