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Schindler et al. Microparticles in neuroimmune signaling

THP-1 cells released MPs. This is consistent with is released to activate other immune cells and trigger
previous publications showing the constitutive release an inflammatory response. Our data demonstrate
[15]
of MPs by resting cells. [29,49] LPS in combination with that the monocytic cell-derived MPs are able to act
IFN-γ was the only combination from the stimulants in an autocrine or paracrine manner. By recruiting
tested to induce the release of MPs above the levels additional monocytes, this activation pattern may then
released by the unstimulated control cells. This contribute to perpetuating the inflammation present
observation correlates well with other studies showing in neuroinflammatory diseases such as Alzheimer’s
that activation can significantly upregulate MP release disease. Peroxisome proliferator-activated receptor
by a variety of cell types, including THP-1 cells. [15,27] It is gamma (PPAR-γ) has been shown to be one of the
important to note that IFN-γ on its own did not enhance receptors targeted by MPs during autocrine activation
the release of MPs, which is consistent with previous of monocytes. Identifying the receptors mediating
[51]
studies demonstrating that IFN-γ is not capable of the cellular effects of MPs may provide additional
inducing significant monocytic cell cytotoxicity in the targets for attenuating the induced neuroinflammation.
absence of additional co-stimulatory molecule(s). [38,43]
TFAM, a novel DAMP, activates three different types
Next, we investigated whether the isolated MPs of cultured human mononuclear phagocytes, including
possessed cytotoxic properties by conducting THP-1 cells, peripheral blood monocytes, and primary
supernatant transfer experiments, which involved human microglia. It induces the expression of the
exposing THP-1 cells to MPs (10 μg protein/mL) pro-inflammatory cytokines IL-1β, IL-6, and IL-8.
[12]
isolated from THP-1 cells that had been stimulated Therefore, we decided to investigate an additional
with IFN-γ alone or in combination with LPS for 48 h. mechanism of action of TFAM by elucidating the role
Our data indicate that MPs derived from IFN-γ plus of MPs in TFAM-induced microglial toxicity towards
LPS-stimulated THP-1 cells possess the ability to neuronal cells. Moreover, to our knowledge, the role
induce monocytic cell toxicity, while MPs derived from of DAMPs such as TFAM or HMGB1 as triggers of MP
cells stimulated with IFN-γ only lack this ability. The release has not yet been investigated. Similar to the
cytotoxicity of THP-1 cells induced by MPs derived results obtained for the IFN-γ plus LPS-derived MPs,
from IFN-γ plus LPS-stimulated THP-1 donor cells was the MPs derived from donor THP-1 cells stimulated
enhanced by co-stimulation with IFN-γ. This finding with TFAM plus IFN-γ induced toxicity of THP-1 cells
was expected, as IFN-γ is a critical regulatory molecule towards the SH-SY5Y neuronal cells, resulting in
involved in both innate and acquired immunity that decreased cell viability.
has been shown to modulate the immune response of
phagocytic cells. Stimulation with IFN-γ on its own MPs were also investigated for their ability to induce the
[50]
did not induce monocytic cell toxicity towards neuronal release of pro-inflammatory cytokines by THP-1 cells.
cells, reinforcing the idea that IFN-γ requires additional Only MPs derived from IFN-γ plus LPS-stimulated
co-stimulatory molecules to achieve significant THP-1 cells induced the release of MCP-1 above the
monocytic cell activation and secretion of cytotoxins. levels of the unstimulated control cells. MCP-1 is a
Since only the MTT assay was performed on SH-SY5Y potent chemotactic factor for innate immune cells that is
cells, it is not known whether the neuronal cell death produced by a variety of cell types including monocytes,
induced by supernatants from MP-stimulated THP-1 astrocytes, and microglial cells, either constitutively
cells involved mainly apoptotic or necrotic mechanisms. or following activation by pro-inflammatory cytokines
Further studies are needed to address this research and oxidative stress. Within the CNS, MCP-1 has
[52]
question.We also confirmed that the cytotoxic activity of been shown to facilitate the infiltration of peripheral
the IFN-γ plus LPS-derived MPs towards the neuronal blood monocytes across the blood-brain barrier and
cells was indirect, as direct exposure of SH-SY5Y cells thereby amplify the neuroinflammatory state observed
to MPs derived from either unstimulated or stimulated in neuropathologies with dysregulated microglial
[53]
THP-1 cells did not induce any toxic effects [Figure 7]. activation. The CNS neurotoxicity associated with
inflammatory mediators is often due to their action on
Further studies will be required to determine the microglia and astrocytes that leads to the secretion of
molecular content of the MPs isolated in these reactive oxygen species or cytotoxins such as TNF-α.
experiments and the mechanism of action of these These mediators, in turn, can induce apoptotic or necrotic
MPs including the receptors involved in inducing cell death of nearby neurons. [54-57] Yang et al. showed
[58]
the observed THP-1 cell toxicity towards the SH- that MCP-1 was not toxic towards cultures of primary
SY5Y neuronal cells. A number of studies have cortical neurons. In the presence of microglia, however,
shown that activated cells shed MPs containing MCP-1 was shown to cause neuronal death. Microglia
inflammatory mediators such as IL-1β, which, in turn, express receptors for MCP-1, and MCP-1 was found to
Neuroimmunology and Neuroinflammation ¦ Volume 3 ¦ October 28, 2016 239