Conclusions
Retinal photoreceptors are susceptible to mitochondrial oxidative stress/mtDNA damage in robust TLR4-mediated innate immune response.
Methods
On day 5 after injection of complete Freund's adjuvant containing heat-killed Mycobacterium tuberculosis (CFA), retinas were submitted to polymerase chain reaction (PCR) array focused on the TLR signaling, or apoptosis, pathway. CFA-mediated TLR4 activation, oxidative stress, and mtDNA damage were determined in B10.RIII and knockout (KO) mice (recombination activation gene [Rag] 1(KO), TLR4(KO), myeloid differentiation primary response gene 88 [MyD88](KO), tumor necrosis factor [TNF]-α(KO), or caspase 7(KO) mice) using quantitative real-time PCR, enzyme-linked immunosorbent assay, Western blot analysis, and immunohistochemistry. The mycobacterial DNA load on the retina, brain, liver, and spleen was determined by real-time PCR after intracardiac perfusion.
Purpose
Herein the authors investigated whether the activation of Toll-like receptors (TLRs) in the innate immune response causes retinal photoreceptor oxidative stress and mitochondrial DNA (mtDNA) damage.
Results
PCR array demonstrated the upregulation of TLRs and their signaling molecules in retinas of CFA-injected mice compared with those of control animals without inflammatory cell infiltration in the retina and uvea. Mycobacterial DNA was detected in the retinas of CFA-injected mice. Retinas of CFA-injected animals showed oxidative stress and mtDNA damage, primarily in the photoreceptor inner segments. Upregulated TLR4 was localized with CD11b(+)MHCII(+) cells but not with GFAP(+) astrocytes. This oxidative stress/damage was similar in CFA-injected Rag1(KO) mice compared with wild-type controls. Such damage was absent in the retinas of CFA-injected TLR4(KO), MyD88(KO), and TNF-α(KO) mice. CFA-mediated inducible nitric oxide synthase expression in the retina was significantly decreased in TNF-α(KO) mice. Conclusions: Retinal photoreceptors are susceptible to mitochondrial oxidative stress/mtDNA damage in robust TLR4-mediated innate immune response.