Dendritic cells are a unique type of immune cell acting as specialized scouts within the body’s defense system. These cells are found throughout various tissues, particularly at interfaces that regularly encounter the external environment, including areas like the skin, the delicate linings of the nose and lungs, and the extensive network of the gastrointestinal tract. They are named for their distinctive, branched projections, which help them interact with their surroundings and constantly monitor for potential threats.
Surveillance and Antigen Presentation
The primary function of dendritic cells involves continuously sampling their environment for molecules, known as antigens, that signal danger, such as viruses, bacteria, or cancerous cells. Dendritic cells efficiently internalize these antigens through various processes, including phagocytosis, macropinocytosis, and receptor-mediated endocytosis.
Once an antigen is engulfed, the dendritic cell processes it internally, breaking it down into smaller, recognizable fragments called peptides. Unlike other immune cells, dendritic cells preserve these antigenic fragments. They then display these processed antigens on their outer surface using specialized display platforms called Major Histocompatibility Complex (MHC) molecules. This display of antigens on their surface is known as antigen presentation, effectively preparing the evidence of a threat for other immune cells.
Orchestrating the T Cell Response
After successfully capturing and processing antigens in peripheral tissues, the dendritic cell matures and migrates from its surveillance outpost, traveling through lymphatic vessels to a nearby lymph node. Lymph nodes serve as centralized meeting points for various immune cells, coordinating responses.
Within the lymph node, the mature dendritic cell seeks out specific T cells that possess receptors capable of recognizing the presented antigen. This precise interaction involves the dendritic cell’s MHC-bound antigen engaging with the T cell receptor. Beyond antigen presentation, the dendritic cell provides additional co-stimulatory signals and releases signaling molecules called cytokines. These combined cues direct the T cell on the nature of the threat and shape the immune response needed to neutralize it. A single mature dendritic cell can engage and activate a large number of T cells, initiating a widespread and targeted immune attack.
The Role in Health and Disease
Dendritic cells maintain overall health through continuous immune surveillance. They consistently clear out daily threats, preventing illness and helping the body remain robust against challenges.
However, when dendritic cell function is disrupted, it can contribute to various health issues. In autoimmune conditions, for instance, dendritic cells may mistakenly present “self-antigens”—molecules from the body’s own healthy tissues—as foreign threats. This misidentification can lead to T cells launching an attack against the body’s own cells, as seen in diseases like multiple sclerosis or type 1 diabetes. Conversely, in allergies, dendritic cells can overreact to harmless substances from the environment, such as pollen or certain food proteins, triggering an exaggerated and unnecessary immune response. Dendritic cells also play a role in fostering “tolerance,” a process where they instruct the immune system not to respond aggressively to beneficial elements like gut bacteria or ingested food proteins.
Harnessing Dendritic Cells for Immunotherapy
Dendritic cells are a focal point for developing new immunotherapies, particularly in cancer treatment. This approach, often referred to as dendritic cell vaccines, aims to help a patient’s immune system recognize and attack malignant cells. One notable example is sipuleucel-T, an FDA-approved therapy for prostate cancer.
In this personalized treatment, a patient’s peripheral blood mononuclear cells, which contain dendritic cells, are collected through a process similar to blood donation. These cells are then transported to a laboratory where they are incubated with a specific cancer-associated antigen, such as prostatic acid phosphatase (PAP) for prostate cancer, often combined with a stimulant like granulocyte-macrophage colony-stimulating factor (GM-CSF). This exposure “loads” the dendritic cells with the tumor antigen and promotes their maturation, transforming them into antigen-presenting cells. Once “trained” in the lab, these activated dendritic cells are re-infused back into the patient, where they can then teach the patient’s T cells to identify and eliminate cancer cells.