Fas-FasL interaction involved in pathogenesis of ocular toxoplasmosis in mice

Abstract

Ocular toxoplasmosis is a potentially blinding intraocular inflammation. The intent of this study was to investigate the role of Fas-FasL interaction in a murine model of acquired ocular toxoplasmosis induced by intracameral inoculation of Toxoplasma gondii. Intraocular inflammation, Fas and FasL expression on lymphocytes and on ocular tissues, the occurrence of apoptosis, and the frequency of CD8(+) and CD4(+) T cells in the infected eyes were analyzed in C57BL/6 (B6) mice. Susceptibility to parasite-induced intraocular inflammation was observed in Fas-deficient (B6-lpr) and FasL-deficient (B6-gld) mice. Inoculation of 5,000 T. gondii tachyzoites induced significant intraocular inflammation associated with increase of Fas and FasL expression in the inoculated eyes of wild-type B6 mice. Flow cytometry demonstrated a significant increase of Fas and FasL expression on the splenocytes from naive mice incubated in vitro with the parasite and on the splenocytes harvested from the infected mice at day 8 after parasite inoculation. Apoptosis of inflammatory cells and cells in ocular tissues was seen, and a greater frequency of CD8(+) than CD4(+) T cells was observed in the infected eyes. The intensity of intraocular inflammation was greater in B6-lpr and B6-gld mice than in wild-type B6 mice (P < 0.05). The results suggest that Fas-FasL interaction associated with apoptosis is involved in the pathogenesis of acquired ocular toxoplasmosis in mice.

FIG. 1

Clinical scores of different groups of mice inoculated with 5,000 strain PLK tachyzoites.

FIG. 2

Clinical score at day 8 of each group of mice inoculated with 5,000 strain PLK tachyzoites.

FIG. 3

(A) Uninfected eye; (B to D) ocular inflammation at day 8 of B6 mice (B) and significantly more intensively inflammatory reaction of the eyes of B6-lpr (C) and B6-gld (D) mice intraocularly infected with 5,000 tachyzoites. (Magnification, ×20.)

FIG. 4

Fas (A) and FasL (C) expression on normal B6 mouse retina (arrows) and strikingly increased Fas (B) and FasL (D) expression on inflammatory retina of the infected eye at day 8 after intraocular infection with 5,000 parasites. R, retina; U, uvea; V, vitreous humor. (Magnification, ×120.)

FIG. 5

Compared with the control isotype antibodies (light line), increased expression (bold line) of Fas and FasL on normal B6 mouse splenocytes incubated for 48 h with T. gondii PLK (2:1) (a and b) and on infected B6 mouse splenocytes and harvested at day 8 after intraocular inoculation with 5,000 tachyzoites (c and d).

FIG. 6

No apoptosis was detected in the anterior chamber and vitreous humor and in tissues such as the cornea, lens and retina of the eye (A and B). Apoptosis (dark blue) of infiltrating cells was seen in the anterior chamber of B6 mice at 48 h (C) and in the vitreous humor at day 4 (D), and apoptotic cells in the lens (E) and retina (F) were detected at day 8 after intraocular inoculation of 5,000 parasites. There were fewer apoptotic cells in the eyes of either B6-lpr (G) or B6-gld (H) mice at day 4 after infection. Ac, anterior chamber; C, cornea; Cb, ciliary body; L, lens; R, retina; U, uvea; V, vitreous humor. (Magnification, ×200.)

FIG. 7

CD8⁺ and CD4⁺ T cells in infected eyes at day 8. A greater frequency of CD8⁺ (A) than of CD4⁺ (B) T cells was observed at day 8. (Magnification, ×40.)