Myeloid dendritic cells are specialized immune cells that function as the body’s vigilant sentinels. They are highly adept at recognizing potential threats, whether invading pathogens or abnormal cells. These cells play a significant role in orchestrating the immune system’s responses, constantly surveying various tissues to activate broader immune defenses when necessary.
What Are Myeloid Dendritic Cells?
Myeloid dendritic cells originate from hematopoietic stem cells in the bone marrow, developing from myeloid progenitor cells. They are named “dendritic” due to their distinctive star-shaped appearance, with multiple extended, tree-like projections.
These cells are widely distributed throughout the body, acting as a first line of defense in many tissues that regularly interact with the external environment. They are commonly found in the skin, where they are known as Langerhans cells, and within mucosal surfaces like those lining the nose, lungs, stomach, and intestines. Myeloid dendritic cells also reside in peripheral blood, interstitial tissues, and various lymphoid organs such as the spleen and lymph nodes. They are identified by specific surface markers, including high levels of CD11c, MHC-II, and co-stimulatory molecules like CD40, CD80, CD83, and CD86.
Their Role in Immune Surveillance
Myeloid dendritic cells are professional antigen-presenting cells (APCs) that initiate adaptive immune responses. Their primary function involves capturing, processing, and presenting antigens to T cells, thereby bridging the innate and adaptive branches of immunity. They possess high phagocytic and endocytic abilities, allowing them to efficiently engulf foreign substances or cellular debris.
Once an antigen is detected and captured in peripheral tissues, immature myeloid dendritic cells undergo a maturation process. This maturation involves changes in their surface molecules and an increased ability to stimulate T cells. Following antigen capture and maturation, these cells migrate from the peripheral tissues, such as the skin or mucosal linings, towards regional lymph nodes.
Upon reaching the lymph nodes, myeloid dendritic cells present the processed antigens on their surface, specifically through major histocompatibility complex (MHC) class II molecules, to naive T cells. This presentation, along with co-stimulatory signals, activates the T cells, leading to their proliferation and differentiation into effector T cells capable of targeting the specific threat.
Their Impact on Health and Disease
Myeloid dendritic cells play a significant role in combating infectious diseases by initiating robust immune responses against various pathogens. When viruses or bacteria invade the body, these cells quickly detect them through pattern recognition receptors, such as Toll-like receptors. They then process and present pathogen-derived antigens to T cells, leading to the eradication of infected cells or the neutralization of extracellular microbes.
In the context of cancer, myeloid dendritic cells are involved in anti-tumor immunity by attempting to recognize and eliminate cancerous cells. They can capture tumor-associated antigens and present them to T cells, aiming to trigger an immune attack against the tumor. However, cancer cells often develop strategies to evade this immune surveillance, such as by inhibiting the maturation or function of myeloid dendritic cells, or by creating an immunosuppressive microenvironment that prevents effective T cell activation.
Dysregulation or abnormal activation of myeloid dendritic cells can contribute to the development of autoimmune diseases. In these conditions, the immune system mistakenly attacks healthy tissues. Myeloid dendritic cells can contribute by presenting self-antigens to T cells in a way that breaks immune tolerance. This inappropriate activation can lead to chronic inflammation and tissue damage characteristic of autoimmune disorders.
Myeloid dendritic cells also participate in allergic responses, where they contribute to the immune system’s overreaction to harmless substances like pollen or certain foods. They can capture allergens and present them to T helper 2 (Th2) cells, which then promote the production of antibodies and other immune mediators that trigger allergic symptoms.