Peptide Identified As Natural Antibiotic Providing First Line Of Defense Against Bacterial Infection

The study, published by researchers at the University of California, San Diego (UCSD) School of Medicine and the VA San Diego Healthcare System in the Nov. 22, 2001 issue of the journal Nature, focused on peptides called cathelicidins (caths) which are found in various tissue in all mammals, including skin, lungs, intestines and circulating white blood cells. The research was conducted in mice and experimental culture systems.

Able to inhibit microbe growth in culture dishes, caths have never been shown, until now, to be natural antibiotics that play a part in mammalian innate immunity, the body’s immediate, first defense against invading pathogens.

“Although we’ve suspected for nearly 20 years that certain anti-microbial peptides contribute to the immune system’s first response in fighting infection, we’ve never had proof of the precise mechanism in living animals until now,” said senior author Richard Gallo, M.D., Ph.D., UCSD associate professor of medicine and pediatrics, and chief of the dermatology section at the VA. “Our findings show that humans and other mammals have the ability to make our own natural antibiotics.”

Gallo added that a potential application of the research is development of a blood test to identify mutated versions of the gene. This might allow doctors to design specific treatment for individuals more susceptible to bacterial infections than others. Another potential application, Gallo said, is use of caths as a model to develop new and safer drugs to fight infections.

First author Victor Nizet, M.D., an infectious disease specialist and UCSD assistant professor of pediatrics, added that the new discovery also gives researchers a new line of investigation regarding bacterial resistance.

“Overuse of pharmaceutical antibiotics often leads to bacterial resistance,” he said. “The natural antibiotic we studied, however, has continued to be effective in killing certain bacteria for tens of thousands of years. With further studies of its properties and the bacterial genes that determine sensitivity or resistance, we hope to gain insight into why and how some bacteria develop resistance to antibiotics while others don’t.”

The human immune response is a two-pronged attack. Innate immunity identifies infectious agents by their pattern or structure, and mounts a broad, rapid response. Phagocytic cells (such as neutrophils and macrophages) and natural killer cells are released to fight the infection. A second form of immune response, adaptive (or acquired) immunity, takes several days to gear up its more targeted attack. Adaptive immunity recognizes previous contact with a specific microbe and directs its defense against that specific invader with antibodies and T-cells . Because many infections spread quickly, a fully functioning innate response is crucial to the health of an infected individual.

Since both humans and mice have the anti-microbial caths, the UCSD/VA researchers investigated the protein in mice and experimental culture systems using two complementary scientific approaches – genetic engineering to make mice without caths and genetic selection for bacteria with altered cath resistance.

Gallo, a dermatologist who discovered that caths were in the skin during his days as a post-doctoral fellow at Harvard, was especially interested in finding out their natural function. Both normal mice and mice engineered without caths received skin injections of group A streptococcus (strep), chosen because the investigators observed that strep is highly sensitive to cath anti-microbial action and normal mice responded to strep infections by making more of the peptide.

A specialist in strep infections, Nizet noted that group A strep is the cause of common infections such as strep throat, and skin impetigo, but is increasingly associated with severe invasive infections including necrotizing fasciitis (the flesh-eating bacteria) and toxic shock syndrome.

When infected with group A strep, the mice designed to lack caths developed a rapidly growing lesion similar to that seen with invasive human infections. Their lesions were both larger and persisted longer than those in the normal mice infected with a similar dose of the bacteria.

In a second experiment, the researchers identified a mutant of group A strep that was resistant to killing by caths in a test tube. When this resistant strep was used to infect normal mice, a large and spreading infection was produced that was very similar to that observed in the cath-deficient mice, further proving that caths were critical to immune defense.

In continuing studies of caths’ role, Gallo said he plans to look at the peptide’s expression in various human diseases. For example, people with the common skin condition psoriasis have itchy red rashes, but don’t get many other skin infections.

“This may indicate large scale expression of caths,” he said. “On the other hand, people with other types of rashes get frequent infections, which could mean their cath level is low.”

The UCSD/VA researchers noted that caths are one of many peptides with antibacterial properties that also include genes called defensins. While their current studies focused on the vital role of caths as natural antibiotics, the defensins were not investigated.

“It’s possible that caths may work in concert with defensins, or even with cell-mediated innate immunity involving macrophages,” Gallo said, indicating that further studies need to elucidate these interrelationships.

Additional authors of the study were UCSD/VA post-doctoral fellows Takaaki Ohtake, M.D., Ph.D., Xavier Lauth, Ph.D., Janet Trowbridge, M.D., Ph.D., and Vasumati Pestonjamasp, Ph.D.; UCSD research associates Jennifer Rudisill, B.A. and Robert A. Dorschner, B.S.; and Joseph Piraino, B.S. and Kenneth Huttner, M.D., Division of Neonatology, Massachusetts General Hospital, Boston.

The research was funded by the National Institutes of Health and the Department of Veteran Affairs.