Stress does increase inflammation throughout the body. This conclusion is supported by correlational research and manipulated variable research. It is known that caregivers of patients with Alzheimer’s disease display elevations in markers of systemic inflammation.   Persons experiencing stress at work display elevations on inflammatory markers. When researchers induce stress in individuals by having them talk about embarrassing events in front of a scowling audience, inflammatory markers in the blood spike. (See the post on depression is inflammation for references.)

Systemic inflammation is a causal factor in the development of cardiovascular disease and type 2 diabetes.   Systematic inflammation is a marker of an activated immune system. However, the mystery is that stress impairs the body’s capacity for fighting cancer, fighting viruses, and mounting an immune response after vaccination. A new study sheds light on how the immune system can be both activated in some ways but depressed in other ways. The explanation focuses on a recently discovered class of white blood cells called Myeloid-Derived Suppressor Cells.

Myeloid-Derived Suppressor Cells have been the focus in research on tumors. Myeloid-Derived Suppressor Cells get recruited into cancerous tumors where they prevent the attack of the killer T cells, the cells which would otherwise kill cancerous cells.   Advancements have been made in the treatment of cancer. Antibodies to molecules expressed by tumors which turn off killer T cells can be blocked by antibody treatment.   Many cancers can be cured with these antibodies. However, solid tumors often fail to respond to the antibody treatment. The efficacy of the antibody treatment is impaired because of the second obstacle preventing the killer T cells from eliminating the tumor: Myeloid-Derived Suppressor Cells. The ways in which Myeloid-Derived Suppressor Cells prevent killer T cells from doing their job have been identified. In the future, ways to subvert the Myeloid-Derived Suppressor Cells will be forthcoming so that the immune system will be able to eradicate cancer.

Besides influencing the cancer outcomes, Myeloid-Derived Suppressor Cells can impair the immune system in other ways as well. They decrease the capacity of killer T cells to kill cells infected with virus. A recent study by Jin et al. showed that stressing animals by restricting their movement increased the number of Myeloid-Derived Suppressor Cells in circulation. This study demonstrated that psychological stress does increase the population of Myeloid-Derived Suppressor Cells.   Assuming this finding applies to many diverse types of psychological stressors it clears up the mystery of how stress can increase inflammation while weakening the immune system’s capacity for responding to cancer and viral infected cells.

As discussed in Neuroscience for Psychologist and Other Mental Health Professionals in Chapter 2, the arms of the immune system can be divided into the innate system and the adaptive immune system. The innate system responds non-specifically to any molecule that looks foreign. The adaptive immune system responds to a very specific foreign protein. The adaptive immune system is recruited by vaccination. Cells responding to the proteins in the vaccine are increased. These cells will only attack when they once again see the same protein that was in the vaccine. The Myeloid-Derived Suppressor Cells seem to primarily impair the function of the adaptive system, the system needed to fight cancer and viruses. Thus, Myeloid-Derived Suppressor Cells may be a critical link for how stress increases cancer risk and impairs the body’s capacity of eradicating viruses.

Jin, J., Wang, X., Wang, Q., Guo, X., Cao, J., Zhang, X., Zhu, T., Zhang, D., Wang, W., Wang, J., Shen, B., Gao, X., Shi, Y., & Zhang, J. (2013). Chronic psychological stress induces the accumulation of myeloid-derived suppressor cells in mice. PLoS One, 8 (9), e74497.