Replication of Mycobacterium tuberculosis in retinal pigment epithelium.
OBJECTIVE
Mycobacterium tuberculosis is an important cause of posterior uveitis in tuberculosis-endemic regions. Clinical and histopathologic evidence suggests that retinal pigment epithelium (RPE) can harbor M tuberculosis. However, the mechanism of M tuberculosis phagocytosis and its growth in RPE is not clear.


OBJECTIVE
To investigate M tuberculosis phagocytosis, replication, and cytopathic effects in RPE cells compared with macrophages.


METHODS
Human fetal RPE and monocytic leukemia macrophage (THP-1) cell lines were cultured, and RPE and THP-1 cells were exposed to avirulent M tuberculosis H37Ra. Mycobacteria were added to RPE and THP-1 cells with a 5:1 multiplicity of infection. Nonphagocytized M tuberculosis was removed after 12 hours of exposure (day 0). Cells were harvested at days 0, 1, and 5 to count live and dead cells and intracellular mycobacteria. Toll-like receptor 2 (TLR2) and TLR4 expression was determined by immunohistochemistry; intracellular bacillary load, following TLR2 and TLR4 blockade.


METHODS
Number of intracellular M tuberculosis, cell survival, and TLR2 and TLR4 expression in RPE and THP-1 cells following exposure to M tuberculosis.


RESULTS
At day 0, an equal number of intracellular M tuberculosis was observed per THP-1 and RPE cells (0.45 and 0.35 M tuberculosis per RPE and THP-1 cells, respectively). Mean (SD) number of intracellular M tuberculosis at day 5 was 1.9 (0.03) and 3.3 (0.01) per RPE and THP-1 cells, respectively (P < .001). Viability of infected RPE was significantly greater than that of THP-1 cells at day 5 (viable cells: 17 [8%] THP-1 vs 73% [4%] RPE; P < .05). Expression of TLR2 and TLR4 was detected in both cell types after 12 hours of exposure. Inhibition of TLR2 and TLR4 reduced intracellular M tuberculosis counts in RPE but not in THP-1 cells.


CONCLUSIONS
Mycobacterium tuberculosis is phagocytized by RPE to a similar extent as in macrophages. However, RPE cells are better able to control bacillary growth and RPE cell survival is greater than that of THP-1 cells following mycobacterial infection, suggesting that RPE can serve as a reservoir for intraocular M tuberculosis infection.
University of Southern California Eye Institute, Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles.
Hossein
H
Nazari
Department of Medicine, The Johns Hopkins University, Baltimore, Maryland3Department of International Health, The Johns Hopkins University, Baltimore, Maryland.
Petros C
PC
Karakousis
University of Southern California Eye Institute, Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles.
Narsing A
NA
Rao
D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002470:Cell Survival; D002478:Cells, Cultured; D015818:Eye Infections, Bacterial / Q000382:microbiology / Q000473:pathology; D005333:Fetus; D006801:Humans; D007951:Leukemia, Myeloid; D008264:Macrophages / Q000166:cytology / Q000382:microbiology; D009169:Mycobacterium tuberculosis / Q000254:growth & development* / Q000472:pathogenicity*; D010587:Phagocytosis / Q000502:physiology; D055213:Retinal Pigment Epithelium / Q000166:cytology / Q000382:microbiology*; D012680:Sensitivity and Specificity; D051195:Toll-Like Receptor 2 / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000378:metabolism; D014605:Uveitis / Q000382:microbiology / Q000503:physiopathology
C495342:TLR2 protein, human; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4