Fat is bad. This is what doctors and nutritionists have preached for years, making fat public enemy number one in the battle against obesity. We now know that not all fat is bad. Unsaturated fats, like those found in fish and nuts, have many potential health benefits while saturated and trans fats should be avoided. For the first time, researchers have shown how fat cells in our skin can directly help protect against a bacterial infection. 

The skin is the largest organ in the human body and our armour against microbial pathogens. Beneath the top two layers of skin is a layer of adipose tissue, or fat. This layer of fat insulates our bodies, stores energy, and cushions the underlying tissues. The fat cells that make up the adipose tissue are known as adipocytes. We have long known that adipocytes have important metabolic functions, including producing a chemical signal called leptin that controls appetite. What is becoming more clear is that these cells also play a role in our body’s immune functions.

While studying how the skin protects itself against bacteria, a team led by Dr. Richard Gallo at the University of California, San Diego and their colleagues at UC Irvine and The Salk Institute came across an unexpected finding. They found that when mice were infected with the bacteria Staphylococcus aureus under the skin, the fat layer expanded dramatically. This expansion was caused by individual fat cells becoming larger in size and by increased transformation of immature pre-adipocytes into mature cells. The researchers then tested whether this expansion of the fat layer was important for containing the bacterial infection. They looked at mice with a genetic mutation that inhibits the generation of fat cells as well as normal mice treated with a chemical that prevents the maturation of fat cells. When infected with Staph, both groups of mice showed less expansion of their fat layer and higher susceptibility to infection, with more severe symptoms.

Having established a correlation between adipose tissue expansion and resistance to infection, the researchers next asked how a thicker layer of fat cells could protect against bacterial infections. One of the first lines of defense against microbial infection is antimicrobial peptides (AMPs). AMPs are small molecules produced by our immune system to kill bacteria, fungi, and some viruses. When the researchers compared the amounts of different AMP made by pre- and mature adipocytes, they found one AMP, in particular, that showed a big difference. Levels of the AMP cathelicidin were roughly 80 times higher in mature adipocytes compared to immature pre-adipocytes. This provided an important clue that cathelicidin might be the key factor behind fat’s ability to protect against infection. The researchers confirmed that mature fat cells make cathelicidin, which is then pumped out of the cell. As they predicted, the fat cell extracts, which contain high levels of cathelicidin, were able to stop the growth of Staph and another bacteria Pseudomonas aeruginosa.

Why is this cool? Because it shows us that our fat cells have a completely new function that we hadn’t even imagined before! Until now, fat cells were just thought to be messengers. When a cut on your finger gets infected, the fat cells underneath send out chemical signals to your immune system, which then deploy professional killer cells to the injury site. These professional killer cells are trained to recognize and destroy any and all pathogens. This new study has shown that in addition to sending out chemical messages, fat cells can also make weapons of their own – cathelicidin. As the fat layer expands after a skin infection, the amount of cathelicidin being produced and secreted also goes up. The spike in cathelicidin production at the site of injury can help to contain the infection until the professional killer cells arrive.

Now, I know what you’re thinking. If temporarily expanding your fat layer is protective against bacterial infections, then having more body fat or a higher body mass index (BMI) must be ultra-strong protection. Well, no. Obesity in both mice and humans has been associated with an increased risk for skin and soft tissue infections, such as those caused by Staph. Even though overweight mice and humans have slightly higher levels of cathelicidin in their bodies, those levels are still 40 to 100 times lower than the levels needed to kill Staph. So while a little increase in subcutaneous fat is good when you have an infection, too much of it all the time is definitely not a good thing.

References:
Zhang, L., Guerrero-Juarez, C., Hata, T., Bapat, S., Ramos, R., Plikus, M., & Gallo, R. (2015). Dermal adipocytes protect against invasive Staphylococcus aureus skin infection Science, 347 (6217), 67-71 DOI: 10.1126/science.1260972