Stealthy leprosy pathogen evades immune response

gene activity of the vitamin D–dependent immune pathway used to help fight infection. When researchers neutralized the activity of hsa-mir-21 in macrophages, the cells were able to kill the bacteria again.

“The leprosy pathogen was able to effectively evade the host’s immune response by regulating critical immune system genes,” said senior investigator Dr. Robert Modlin, UCLA’s Klein Professor of Dermatology and chief of dermatology at the Geffen School of Medicine. “It’s like having the enemy sending a decoy message to your combat troops and telling them to lower their weapons.”

To test the significance of this micro-RNA with other infectious diseases, the researchers also introduced hsa-mir-21 to human macrophages that were then infected with tuberculosis in the lab. Researchers found that the micro-RNA similarly blocked the ability of the macrophages to kill the bacteria.

Researchers also demonstrated that immune activation of the leprosy-infected immune cells decreased the leprosy bacteria’s viability four-fold — but only when hsa-mir-21 activity was silenced. In fact, an over-expression of this micro-RNA blocked immune activity, resulting in a five-fold increase in bacterial viability.

“We were surprised at the devastating effects that even a single micro-RNA had on the ability of immune cells to fight the infections,” Liu said.

In addition, the team showed that this micro-RNA was found in human immune cells only 18 hours after the onset of leprosy infection. The presence of the micro-RNA so early in the infection suggests it might play a role in actual disease development, the researchers said.

Further investigation of this single micro-RNA in leprosy may provide a framework for analyzing other micro-RNAs to help determine their cumulative role in regulating the immune response.

The micro-RNAs are small, and therefore it is possible to develop treatments which neutralize them, the researchers said.

“We may find that a combination of vitamin D supplementation with a genetically targeted therapy could provide an optimal treatment approach to leprosy and possibly other chronic infectious diseases,” said Modlin, who also serves as vice chair for cutaneous medicine and dermatological research at UCLA and is a distinguished professor of medicine and of microbiology, immunology and molecular genetics.

“Vitamin D insufficiency has been associated with a number of infectious and autoimmune diseases, cardiovascular disease and cancers,” Modlin added. “Our study indicates that micro-RNAs can alter human vitamin D responses and contribute to disease pathology.”

Dr. Barry Bloom of Harvard University, who was not an author of this study but is part of the research team studying this field, agreed.

“Such a novel approach may be especially worth exploring in treatment of drug-resistant pathogens such as some forms of tuberculosis, where antimicrobial therapy is becoming increasingly problematic,” Bloom said.

Bloom, the former dean of the faculty at Harvard’s School of Public Health, is Harvard’s Distinguished University Service Professor and the Jack and Joan Jacobson Professor of Public Health in the School of Public Health’s department of immunology and infectious diseases and department of global health and population.

The study was funded by the National Institute of Allergy and Infectious Diseases and the National Institute of Arthritis, Skin and Musculoskeletal Diseases, both parts of the National Institutes of Health.

— Read more in Philip T Liu et al., “MicroRNA-21 targets the vitamin D–dependent antimicrobial pathway in leprosy,” Nature Medicine (29 Janaury 2012) (doi:10.1038/nm.2584)