Mechanism of immune evasion by Borrelia recurrentis

Louse-borne relapsing fever is caused by the spirochete bacterium Borrelia recurrentis, which is transmitted by body lice (not head lice). The disease was first described by Hippocrates (460–370 BC). Initial symptoms include a high fever lasting several days, followed by a fever-free interval. Typically, several recurrent episodes of fever follow. The disease can be treated with antibiotics. If left untreated, however, infection with Borrelia recurrentis is fatal in up to 20 percent of cases – particularly in regions where comprehensive medical care is not available.

Louse-borne relapsing fever is classified as a poverty-related neglected disease. During the last century, major epidemic outbreaks still occurred in Europe. Today, sporadic outbreaks are reported in countries around the Horn of Africa, including Eritrea, Ethiopia, Somalia, and South Sudan. According to individual studies, body lice infected with Borrelia are not currently found in Europe. Nevertheless, the disease attracted considerable attention in 2015, when increasing numbers of cases were diagnosed among refugees in several European countries.

A research team has now identified and characterized five closely related proteins that are crucial for the survival of Borrelia recurrentis in the human body. These so-called Chi proteins appear to have evolved from a common ancestor and are therefore considered homologous. By specifically binding to proteins in the blood, they prevent activation of a key component of the innate immune system: the human complement system. As a result, the Chi proteins stop the complement system from marking and destroying invading bacteria.

In addition, the Chi proteins can bind the enzyme precursor plasminogen present in the blood and convert it into active plasmin. The author explains: “We know from other pathogens that they use the body’s own plasmin to invade tissues. Together with the ability of the Chi proteins to block the complement system, this gives Borrelia recurrentis significant advantages in surviving and spreading after entering the human body.”

Based on these findings, the researchers have already developed diagnostic tests, and these proteins may also serve as potential targets for vaccine development. The author explains: “Fever of unknown origin occurs in many infectious diseases, so pathogen-specific tests make it possible to quickly initiate the appropriate antibiotic treatment against the causative agent. We have already made considerable progress in this area and are currently conducting studies in Kenya and Nigeria using serological tests developed in our laboratory.”

https://www.nature.com/articles/s41467-026-72359-y

https://sciencemission.com/Complement-inhibition-by-Borrelia-recurrentis