Microglia are the primary immune cells of the central nervous system, where they act as the first line of defense by constantly surveying the environment for signs of trouble. These cells play a housekeeping role by clearing away cellular debris and are involved in responding to injury and pathogens. To study these cells outside of a living organism, scientists use a “cell line.” A cell line is a population of cells that can be maintained in a laboratory setting, providing a consistent and accessible model for research. This allows for controlled experiments that would not be possible within the complex environment of the brain.
The Creation and Purpose of Microglia Cell Lines
The development of microglia cell lines addresses challenges associated with studying these cells directly from brain tissue. Primary microglia, cells taken directly from an organism, are difficult to isolate in large numbers, have a limited lifespan in a laboratory dish, and can show variability from one preparation to another. These limitations make it challenging to conduct large-scale, repeatable experiments, so scientists created cell lines to provide a virtually unlimited supply of genetically similar cells.
To create a cell line, primary microglia undergo a process called immortalization, which grants them the ability to divide and multiply indefinitely. This is often achieved through genetic modification, where genes associated with cell proliferation, known as oncogenes, are introduced into the cells. Common methods involve using a retrovirus to deliver genes like v-myc or the SV40 large T antigen, which override the normal cellular machinery that limits cell division.
Some cell lines are created by introducing genes that extend the natural life of the cells, such as the human telomerase reverse transcriptase (hTERT) gene. This approach can slow the rate of proliferation compared to viral oncogenes. This may help the cells retain characteristics closer to those of primary cells.
Commonly Used Microglia Cell Lines
Several distinct microglia cell lines are widely used in research. One of the most common is the BV-2 cell line, derived from neonatal mouse microglia. These cells were immortalized using a retrovirus carrying the v-raf/v-myc oncogene and are known for their rapid growth and robust response to inflammatory stimuli. BV-2 cells are frequently used as a substitute for primary microglia in studies of neuroinflammation.
Another murine-derived line is the SIM-A9 cell line. SIM-A9 cells were spontaneously immortalized, meaning they naturally developed the ability to proliferate without intentional genetic modification. They retain microglial markers, such as Iba1 and CD68, for many passages in culture and exhibit phagocytic activity, which is the ability to engulf other cells or particles.
For researchers seeking a human-derived model, the HMC3 cell line is a common choice. Established from human embryonic brain tissue, these immortalized cells are used to study human-specific microglial responses. HMC3 cells express microglial surface markers and have been used to investigate phagocytosis and cytokine release in the context of neurodegenerative diseases. However, the origin of some human cell lines has been questioned, underscoring the need for careful validation.
Applications in Studying Neurological Disorders
Microglia cell lines help advance the understanding of neurological disorders where neuroinflammation is a contributing factor. These models allow researchers to dissect the roles microglia play in disease progression. By exposing the cells to disease-related proteins or toxins in a lab, scientists can observe how microglia react, what inflammatory molecules they release, and how they might contribute to neuron damage.
In Alzheimer’s disease research, cell lines like HMC3 and BV-2 are used to study how microglia respond to amyloid-beta plaques. Experiments show that exposure to amyloid-beta can trigger these cells to release pro-inflammatory cytokines, which are signaling molecules that can be toxic to nearby neurons. This helps scientists investigate the mechanisms that link inflammation to the neuronal loss seen in Alzheimer’s patients.
These cell lines are also applied to Parkinson’s disease research. Researchers use them to explore how microglia contribute to neuronal death, often by exposing them to substances known to be toxic to the neurons affected in Parkinson’s. In multiple sclerosis research, cell lines help in understanding demyelination, the destruction of the protective sheath around nerve fibers. They are also used for high-throughput screening, where thousands of potential drug compounds can be rapidly tested for their ability to suppress harmful microglial activation.
Contrasting Cell Lines with Primary Microglia
While cell lines are powerful tools, they must be contrasted with primary microglia. The greatest strength of primary microglia is their physiological relevance. Having developed within a living brain, they more accurately reflect the complex functions and states of microglia in the body, a complexity that cannot be fully replicated in a culture dish.
The main limitation of immortalized cell lines stems from the genetic modifications used to create them. These alterations can change the cells’ characteristics, so they may not perfectly mimic the behavior of their primary counterparts. For example, a cell line may not express all the same genes or respond to stimuli in the same way as primary microglia.
Over time in culture, cell lines can also undergo “phenotypic drift,” where their characteristics gradually change and introduce potential variability. For these reasons, findings from cell lines are often validated using primary cells. This step helps ensure the conclusions hold true in a more biologically accurate model.