Could fat cells help protect us against infection?

Adipocytes, or fat cells, are important for maintaining the body's energy balance. But they may have another use; a study by researchers from the University of California-San Diego claims adipocytes under the skin protect the body against infection.

Researchers found that adipocytes, or fat cells, under the skin protect us against infection by producing antimicrobial peptides that stave off pathogens.

The researchers at the University of California-San Diego School of Medicine, and colleagues say they have demonstrated that adipocytes produce antimicrobial peptides that stave off pathogens.

The researchers publish their findings in the journal Science.

For their study, the team exposed mice to Staphylococcus aureus - a common bacterium that causes skin and soft tissue infections in humans, such as impetigo and cellulitis.

The researchers explain that they chose to study this bacterium in particular as past studies had identified its presence in the fat layer of the skin. As such, they wanted to see whether this fat aided the prevention of skin infections.

Adipocytes produced high levels of an antimicrobial peptide in response to S. aureus

Within a few hours of exposing the mice to S. aureus, the researchers witnessed a significant rise in the number and size of adipocytes at the infection site.

The most surprising finding, however, was that the adipocytes produced large amounts of cathelicidin antimicrobial peptide (CAMP) - an antimicrobial peptide (AMP) that forms a part of the innate immune system response to kill bacteria, viruses and other potentially harmful microbes.

AMPs are our natural first-line defense against infection. "They are evolutionarily ancient and used by all living organisms to protect themselves.

"It was thought that once the skin barrier was broken, it was entirely the responsibility of circulating [white] blood cells - like neutrophils and macrophages - to protect us from getting sepsis.

But it takes time to recruit these cells [to the wound site]. We now show that the fat stem cells are responsible for protecting us. That was totally unexpected. It was not known that adipocytes could produce antimicrobials, let alone that they make almost as much as a neutrophil."

Next, the researchers exposed mice that were either unable to produce adipocytes or unable to express satisfactory amounts of AMPs - particularly CAMP - to S. aureus.

The team found that both groups of mice developed more severe and frequent infections, which confirmed their previous findings that adipocytes play a role in warding off pathogens.

Obese individuals have higher CAMP levels

However, the research team describes the presence of AMPs - especially CAMP - as a "double-edged sword," noting that people with low levels of CAMP often experience infections.

The best example is atopic eczema, however, may be at increased risk of autoimmune and inflammatory diseases, such as lupus and psoriasis.

On analyzing human adipocytes, the researchers found they also produce CAMP. What is more, obese individuals - who have higher levels of adipocytes - were found to have higher levels of CAMP in their blood than people of a normal weight.

Defective AMP production by mature adipocytes can occur due to obesity or insulin resistance, resulting in greater susceptibility to infection, but too much cathelicidin may provoke an unhealthy inflammatory response:

"The key is that we now know this part of the immune response puzzle. It opens fantastic new options for study.

For example, current drugs designed for use in diabetics might be beneficial to other people who need to boost this aspect of immunity. Conversely, these findings may help researchers understand disease associations with obesity and develop new strategies to optimize care."

References:

UC-San Diego Health Sciences news release, accessed 31 December 2014 via Newswise.