Microglia Are Mediators of Borrelia burgdorferi–Induced Apoptosis

Inflammation has long been implicated as a contributor to pathogenesis
in many CNS illnesses, including Lyme neuroborreliosis. Borrelia
burgdorferi is the spirochete that causes Lyme disease and it is known
to potently induce the production of inflammatory mediators in a variety
of cells. In experiments where B. burgdorferi was co-cultured in vitro
with primary microglia, we observed robust expression and release of
IL-6 and IL-8, CCL2 (MCP-1), CCL3 (MIP-1?), CCL4 (MIP-1?) and CCL5
(RANTES), but we detected no induction of microglial apoptosis. In
contrast, SH-SY5Y (SY) neuroblastoma cells co-cultured with B.
burgdorferi expressed negligible amounts of inflammatory mediators and
also remained resistant to apoptosis. When SY cells were co-cultured
with microglia and B. burgdorferi, significant neuronal apoptosis
consistently occurred. Confocal microscopy imaging of these cell
cultures stained for apoptosis and with cell type-specific markers
confirmed that it was predominantly the SY cells that were dying.
Microarray analysis demonstrated an intense microglia-mediated
inflammatory response to B. burgdorferi including up-regulation in gene
transcripts for TLR-2 and NF??. Surprisingly, a pathway that exhibited
profound changes in regard to inflammatory signaling was triggering
receptor expressed on myeloid cells-1 (TREM1). Significant transcript
alterations in essential p53 pathway genes also occurred in SY cells
cultured in the presence of microglia and B. burgdorferi, which
indicated a shift from cell survival to preparation for apoptosis when
compared to SY cells cultured in the presence of B. burgdorferi alone.
Taken together, these findings indicate that B. burgdorferi is not
directly toxic to SY cells; rather, these cells become distressed and
die in the inflammatory surroundings generated by microglia through a
bystander effect. If, as we hypothesized, neuronal apoptosis is the key
pathogenic event in Lyme neuroborreliosis, then targeting microglial
responses may be a significant therapeutic approach for the treatment of
this form of Lyme disease.
Author Summary

Lyme disease, which is transmitted to humans through the bite of a tick,
is currently the most frequently reported vector-borne illness in the
northern hemisphere. Borrelia burgdorferi is the bacterium that causes
Lyme disease and it is known to readily induce inflammation within a
variety of infected tissues. Many of the neurological signs and symptoms
that may affect patients with Lyme disease have been associated with B.
burgdorferi-induced inflammation in the central nervous system (CNS). In
this report we investigated which of the primary cell types residing in
the CNS might be functioning to create the inflammatory environment
that, in addition to helping clear the pathogen, could simultaneously be
harming nearby neurons. We report findings that implicate microglia, a
macrophage cell type in the CNS, as the key responders to infection with
B. burgdorferi. We also present evidence indicating that this organism
is not directly toxic to neurons; rather, a bystander effect is
generated whereby the inflammatory surroundings created by microglia in
response to B. burgdorferi may themselves be toxic to neuronal cells.

Citation: Myers TA, Kaushal D, Philipp MT (2009) Microglia Are Mediators
of Borrelia burgdorferi–Induced Apoptosis in SH-SY5Y Neuronal Cells.
PLoS Pathog 5(11): e1000659. doi:10.1371/journal.ppat.1000659

Editor: Jenifer Coburn, Medical College of Wisconsin, United States of
America

Received: May 11, 2009; Accepted: October 19, 2009; Published: November
13, 2009

Copyright: © 2009 Myers et al. This is an open-access article
distributed under the terms of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are credited.

Funding: This work was supported by grants NS048952 and RR00164 from the
National Institutes of Health. The funders had no role in study design,
data collection and analysis, decision to publish, or preparation of the
manuscript.

Competing interests: The authors have declared that no competing
interests exist.

Full text available at this URL

http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1000659

Microglia Are Mediators of Borrelia burgdorferi–Induced Apoptosis in
SH-SY5Y Neuronal Cells

Tereance A. Myers, Deepak Kaushal, Mario T. Philipp*

Division of Bacteriology & Parasitology, Tulane National Primate
Research Center, Tulane University Health Sciences Center, Louisiana,
United States of America

Abstract