The immune system protects the body from infection and disease. Among its many components are T helper cells, specialized white blood cells that orchestrate immune responses. One particular subset, Th17 cells, are notable for their dual capacity. These cells can both defend the body against certain threats and, under different circumstances, contribute to inflammatory conditions and autoimmune disorders.
What Are Th17 Cells?
T helper cells are a type of lymphocyte that play a central role in adaptive immunity, coordinating responses by releasing signaling molecules called cytokines. Th17 cells represent a distinct lineage of these T helper cells, identified by their unique cytokine production profile. They differentiate from naive T cells, which are immature T cells, in response to specific cytokine environments. This differentiation process involves various factors that guide naive T cells toward the Th17 lineage.
The defining characteristic of Th17 cells is their production of interleukin-17 (IL-17), a pro-inflammatory cytokine. In addition to IL-17, Th17 cells also produce other related cytokines, such as IL-17F, IL-21, and IL-22. These cytokines act as messengers, influencing the behavior of other immune cells and various tissue cells throughout the body. These specific cytokines distinguish Th17 cells from other T helper cell subsets, such as Th1 and Th2 cells, which produce different sets of signaling molecules.
Their Protective Side
Th17 cells play an important role in the body’s defense against particular types of pathogens, primarily extracellular bacteria and fungi. They do this by producing inflammatory cytokines and other proteins that help clear these infections. For instance, Th17 cells recruit neutrophils, a type of white blood cell, to sites of infection. This recruitment helps to quickly contain and eliminate invading microorganisms.
Th17 cells also contribute to maintaining the integrity of various bodily barriers, particularly in the gut and skin. They help to strengthen these protective surfaces, preventing pathogens from gaining entry into the body. This barrier reinforcement is achieved through the release of specific cytokines that promote the production of antimicrobial peptides and enhance the function of epithelial cells. This protective function contributes to overall immune surveillance and defense.
Their Role in Autoimmunity and Inflammation
While beneficial in fighting infections, Th17 cells can also contribute to chronic inflammation and autoimmune diseases when their activity becomes dysregulated. Their cytokine production, especially IL-17, can lead to significant tissue damage when misdirected against the body’s own healthy tissues. This misdirection occurs when the immune system mistakenly identifies self-antigens as foreign threats, triggering an inappropriate immune response. The overproduction of IL-17 by Th17 cells promotes the recruitment of various immune cells to the affected areas, perpetuating the inflammatory cycle.
The presence of these pro-inflammatory cytokines can lead to a cycle of inflammation and tissue destruction. For example, IL-17 can induce the production of other inflammatory molecules and enzymes that break down connective tissues. This sustained inflammatory environment can cause considerable harm to organs and tissues, contributing to the progression of autoimmune conditions.
Diseases Linked to Th17 Cells
Th17 cells are implicated in the development and progression of several autoimmune and inflammatory diseases. In Multiple Sclerosis (MS), Th17 cells contribute to inflammation within the central nervous system, leading to demyelination, which damages the protective covering of nerve fibers. This damage disrupts nerve signals, causing a range of neurological symptoms.
Inflammatory Bowel Disease (IBD), encompassing Crohn’s disease and ulcerative colitis, also shows a strong link to Th17 cell activity. In these conditions, overactive Th17 cells contribute to chronic inflammation in the digestive tract, leading to symptoms like abdominal pain and severe diarrhea. The cytokines produced by Th17 cells exacerbate the inflammatory responses in the gut lining. Psoriasis, a chronic skin condition, is another disease where Th17 cells play a significant role, promoting skin inflammation and the excessive proliferation of skin cells, resulting in characteristic red, scaly patches.
Rheumatoid Arthritis (RA), an autoimmune disease affecting the joints, also involves Th17 cells contributing to joint inflammation and destruction. These cells produce IL-17, which can stimulate other cells in the joint to release enzymes that break down cartilage and bone. Th17 cells have also been connected to systemic lupus erythematosus and asthma.