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Page 2 of 11 Davis et al. Neuroimmunol Neuroinflammation 2018;5:50 I http://dx.doi.org/10.20517/2347-8659.2018.60

cells. These findings advance our understanding of inflammatory signaling in C20 cells and support the value of this
cell line as a research tool.

Keywords: Interleukin-1β, chemokine, microglia, p38, nuclear factor-κB p65, neuroinflammation

INTRODUCTION
Microglia are resident macrophages in the central nervous system (CNS) and are essential to brain
[1]
physiology; but also, they are instrumental in response to injury and infection in the CNS (See Wolf et. al.
for review). Microglia constantly survey their local environment and respond to extracellular cues (e.g., ATP)
[2,3]
to maintain homeostasis . Other specific physiological functions of microglia include removal of dead
[5-7]
[4]
[2,3]
neurons and cellular debris , synaptic pruning , and regulation of synaptic connectivity and plasticity .
[8,9]
Microglia are also integral to innate immunity and are instrumental in neuroinflammation . For instance,
activated microglia release of pro-inflammatory mediators including, cytokines [e.g., interleukin (IL)-
1β, IL-6] [10,11] , chemokines (e.g., monocyte chemoattractant protein-1/CCL2, interferon-γ inducible protein
10/CXCL10) [11-13] and reactive oxygen species [14,15] . Microglia also modulate the inflammatory response
[17]
[16]
by releasing anti-inflammatory cytokines such as IL-10 and transforming growth factor (TGF)-β .
[18]
Controlled neuroinflammation is neuroprotective , however, excessive or chronic neuroinflammation is
[1]
neurotoxic, it contributes to neurodegeneration, and disrupts neuronal function . For instance, microglial
activation and neuroinflammation are present in neurodegenerative diseases, CNS infection and trauma, as
well as psychiatric disorders. Indeed, emerging evidence suggests pharmacological modulation of microglia
may be beneficial in treating certain CNS disorders [19-22] .
Much has been discovered about microglia function, including inflammatory signaling, using in vitro
approaches with primary cell cultures and transformed cell lines [23-25] . Significant insights have been
obtained about inflammatory signaling using either primary rat or mouse microglia [26-29] or transformed
cell lines such as BV-2 murine microglial cells [26,28-30] . While many of the findings have been observed in
[24]
primary human microglia [14,31,32] , not surprisingly, there have been differences observed . For example,
bacterial lipopolysaccharide (LPS) and interferon-γ (IFNγ) are potent inducers of inflammatory signaling
(e.g., cytokine/chemokine expression) in both mouse and human microglia [29,32-34] , whereas, IL-1β activates
human, but not mouse microglia [10,11,35,36] . The lack of responsiveness of mouse microglia to IL-1β stimulation
[36]
is likely a consequence of very low IL-1 receptor (IL-1R) expression .

Therefore, while there has been a wealth of knowledge obtained about rodent-derived microglia, it remains
critical to better understand inflammatory signaling in human microglia. Primary human microglia (fetal
and adult) remain a necessary tool, but they are relatively difficult and expensive to obtain and use, thus,
only a limited number of research groups have access, funds, and expertise to extensively use this approach.
Primary human microglia are commercially available (e.g., ScienCell Research Laboratories, cat. #HM-
1900) however, they are often in limited supply or not in stock. Therefore, there is increasing interest in
immortalized human microglial cell lines as sustainable tools for studying microglia function [37-39] . The
importance of human microglial cell lines extends beyond basic understanding of microglia function but
also these cell lines serve as a platform for investigating the effects of pharmacologic agents on microglia,
with an eye toward drug development.

Recently, a novel, immortalized human microglial cell line (C20) was introduced to the field; the
cells maintain microglial morphology, express multiple cell surface microglia markers, and express
[37]
proinflammatory cytokines following stimulation with tumor necrosis factor-α (TNFα) . We are
particularly interested in IL-1β-induced inflammatory signaling in microglia due to the established role of
IL-1β in neuroinflammation [40-43] . Interestingly, most studies utilize LPS and/or IFNγ to stimulate microglia