The activation of immune cells, called macrophages, play a major role in the immune response to infections. A study, published in the journal Science Signaling, found that the amino acid arginine is not just a nutrient, but that it also controls the immune response by controlling signaling pathways in macrophages.
Arginine, also called L-arginine, is a constituent of many proteins. Foods, rich in arginine, are meat, fish, dairy products, grains, nuts, and seeds. Arginine can also be synthesized by our own bodies. It helps the kidneys to remove waste products. Arginine also helps to make creatine, a protein involved in muscle power and muscle building. Arginine is converted in the body to the powerful neurotransmitter nitric oxide, which fights infection and dilates blood vessels. Because of its vasodilating properties, arginine may improve blood pressure, blood flow and sexual function. Arginine also speeds up wound healing. Until now, its role in immunity has not been completely understood.
Control of immunity
Poor nutrition or starvation can make you more susceptible to infectious diseases. Researchers found that mouse macrophages need arginine to respond to bacterial toxins. Normally, bacterial toxins, called lippopolysaccharides, are released by bacteria. These toxins stimulate macrophages to initiate an immune response. The researchers found that when macrophages were deprived of arginine, they were unable to start an immune response in the presence of bacterial toxins. The arginine-deprived macrophages did not release tumor necrosis factor (TNF), which they use to kill bacteria. Starved mice, just like arginine-deprived macrophages, were unable to mount a successful immune response to the bacterial toxin. When starved mice were supplemented with arginine, they successfully responded to the bacterial toxins by making TNF. In both cases, the researchers showed that arginine affected specific signaling pathways in macrophages (Mieulet, V. et al.).
The importance of arginine in the spread of an infection is shown by other studies. When macrophages were infected with mycobacteria, the infected macrophages started making an enzyme that broke down arginine. The destruction of arginine prevented uninfected macrophages from mounting an immune response and the infection spread (Quails, J.E. et al.).
Arginine side effects
Although arginine deficiency is rare in healthy individuals, it has been documented in people suffering from trauma or cancer. Some people use arginine supplements to boost their immune system and critically ill patients are often given a nutritional formula, rich in arginine. In clinical trials, lasting up to three months, arginine supplementation caused only minor side effects such as abdominal pain, bloating, diarrhea, and gout. But supplementation with arginine can also be detrimental to some individuals. People with autoimmune disorders such as rheumatoid arthritis or inflammatory bowl disease and people with asthma may suffer an exacerbation of their symptoms.
Mieulet, V. TPL-2-Mediated Activation of MAPK Downstream of TLR4 Signaling is Coupled to Arginine Availability. Science Signaling (2010) 3: ra61
Quails, J.E. Arginine Usage in Mycobacteria-Infected Macrophages Depends on Autocrine-Paracrine Cytokine Signaling. Science Signaling (2010) 3: ra62