Fat cells protect skin against infections

In a surprising discovery, scientists have found that fat cells below the skin help protect us from bacteria.
Researchers at the University of California, San Diego School of Medicine and colleagues have found that dermal fat cells, known as adipocytes, produce antimicrobial peptides that help fend off invading bacteria and other pathogens.
"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," said Richard Gallo, professor and chief of dermatology at UC San Diego School of Medicine.
"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," Gallo said.
Ling Zhang, the first author of the paper, exposed mice to Staphylococcus aureus, a common bacterium and major cause of skin and soft tissue infections in humans.

Within hours they detected a major increase in both the number and size of fat cells at the site of infection.
More importantly, these fat cells produced high levels of an antimicrobial peptide (AMP) called cathelicidin antimicrobial peptide or CAMP.
AMPs are molecules used by the innate immune response to directly kill invasive bacteria, viruses, fungi and other pathogens.

"AMPs are our natural first line defense against infection. They are evolutionarily ancient and used by all living organisms to protect themselves," said Gallo.
"However, in humans it is becoming increasingly clear that the presence of AMPs can be a double-edged sword, particularly for CAMP. Too little CAMP and people experience frequent infections.
"The best example is atopic eczema (a type of recurring, itchy skin disorder). These patients can experience frequent Staph and viral infections. But too much CAMP is also bad. Evidence suggests excess CAMP can drive autoimmune and other inflammatory diseases like lupus, psoriasis and rosacea," Gallo said.

The scientists confirmed the findings by analysing S aureus infections in mice unable to either effectively produce adipocytes or whose fat cells did not express sufficient antimicrobial peptides in general and CAMP in particular.
In all cases, they found the mice suffered more frequent and severe infections.
Further tests confirmed that human adipocytes also produce cathelicidin, suggesting the immune response is similar in both rodents and humans.
Interestingly, obese subjects were observed to have more CAMP in their blood than subjects of normal weight.