Ocular immune privilege in the immunosuppressive intraocular microenvironment

Rejection Prophylaxis in Corneal Transplant

BACKGROUND

Graft rejection. Twenty to thirty percent of patients with corneal transplants experience at least one rejection episode in the first 5 years after transplantation. Prophylaxis through matching for human leukocyte antigens (HLA) is controversial. We herein report the results of the Functional ANtigen matChing in keratoplastY (FANCY) trial.

METHODS

FANCY was a randomized, double-blind, multicenter clinical trial. The primary objective was to evaluate superiority of HLA matching versus random graft assignment. The primary endpoint was rejection-free graft survival. We included both normal-risk and high-risk indications. The study is registered with ClinicalTrials. gov (NCT00810472).

RESULTS

721 patients were included, 639 patients were randomized. 474 patients underwent keratoplasty within the study; 165 patients received grafts outside the trial. One patient died and one patient was lost to follow up. We observed 33 graft rejections in the HLA matching arm (n = 224). The corresponding estimated cumulative incidence rate of immune reactions after two years was 15.7%. We observed 40 rejections in the control arm (n = 249). After two years this yields an estimated cumulative incidence rate of 17%.

CONCLUSION

In our heterogenous study group, HLA matching did not show a significant advantage compared to random graft assignment. The rejection rate in our sample was lower than expected. Therefore no definite conclusions can be drawn as to whether HLA matching is beneficial in corneal transplantation.

A latent ability to persist: differentiation in Toxoplasma gondii

A critical factor in the transmission and pathogenesis of Toxoplasma gondii is the ability to convert from an acute disease-causing, proliferative stage (tachyzoite), to a chronic, dormant stage (bradyzoite). The conversion of the tachyzoite-containing parasitophorous vacuole membrane into the less permeable bradyzoite cyst wall allows the parasite to persist for years within the host to maximize transmissibility to both primary (felids) and secondary (virtually all other warm-blooded vertebrates) hosts. This review presents our current understanding of the latent stage, including the factors that are important in bradyzoite induction and maintenance. Also discussed are the recent studies that have begun to unravel the mechanisms behind stage switching.

Efficacy of topical cyclosporine-loaded nanocapsules on keratoplasty rejection in the rat

PURPOSE

To evaluate the efficacy of a topical formulation of nanocapsules loaded with 1% cyclosporine A (CsA), which has been previously demonstrated to enhance CsA corneal penetration, compared to 1% CsA in migliol oil on a penetrating keratoplasty rejection model in the rat.

METHODS

Lewis rats received orthotopic corneal allografts from inbred Fisher donors. Rats were treated with 10 microliters of the following topical solutions four times daily for 30 days, starting one day before surgery: Group 1 (n = 9), 1% CsA-loaded nanocapsules; group 2 (n = 13), 1% CsA dissolved in migliol oil; group 3 (n = 12), migliol oil; group 4 (n = 13), no treatment. Rejection index, mean survival time and rejection percentage were calculated for each group, and CsA levels in blood were measured.

RESULTS

The rejection percentage was 84.6% for group 2, 91.7% for group 3, and 100% for groups 1 and 4, with no significant differences among groups. Mean graft survival time was 7.3 days for group 1, 15.5 days for group 2, 8.36 days for group 3, and 7.69 days for group 4, with significant differences between group 2 and the other groups. Systemic CsA levels were only detectable in group 2.

CONCLUSIONS

CsA formulated in migliol oil delayed corneal rejection onset, but blood levels were evident in this group. CsA loaded-nanocapsules showed no effect on rejection and the drug was not detectable in blood. These data, along with the current concepts on corneal graft rejection immunology, suggest that the immunomodulatory effect of topical CsA in the prevention of corneal graft rejection may be systemically-mediated.

Aqueous humor induces transforming growth factor-beta (TGF-beta)-producing regulatory T-cells

PURPOSE

The intraocular microenvironment is an immune-privileged site where immunogenic inflammation is suppressed. Suppression of immunogenic inflammation has been associated with immunosuppressive factors found in aqueous humor produced by ocular tissues. To further understand the mechanisms suppressing immunogenic inflammation in the eye, we have examined the production of lymphokines by primed T-cells activated in the presence of aqueous humor.

METHODS

Enriched in vivo primed T-cells were T-cell receptor-stimulated in the presence of fresh aqueous humor. The culture supernatant was assayed for IFN-gamma and IL-4 by sandwich ELISA. TGF-beta production by T-cells stimulated in the presence of aqueous humor was assayed by a TGF-beta bioassay of the culture supernatant and by quantitative RT-PCR for TGF-beta 1 mRNA expression. Aqueous humor-treated T-cells were assayed for their capacity to suppress IFN-gamma production by stimulated, primed T-cells.

RESULTS

Aqueous humor-enhanced proliferation but irreversibly suppressed production of both IFN-gamma and IL-4 by in vitro-activated, in vivo-primed T-cells. Aqueous humor induced in vivo primed T-cells to produce TGF-beta in vitro, and these TGF-beta-producing T-cells suppressed IFN-gamma production by other T-cells activated in co-cultures.

CONCLUSIONS

Aqueous humor alters the functional program of TCR-ligand-activated, primed T-cells, converting the cells to TGF-beta-producing regulatory cells.

Immunotolerance induced by intratesticular antigen priming: expression of TGF-beta, Fas and Fas ligand

The authors have previously reported that an injection of S-antigen (S-Ag) into rat testes prior to immunization induces systemic tolerance (designated orchidic tolerance) and protects the animals from experimental autoimmune uveoretinitis (EAU) and that the signal for orchidic tolerance induction emigrates from the testis within a few hours after antigen priming of the testis. In order to understand the mechanism by which the signal or signal carrier is generated, they determined in this study changes in immunoreactivity for transforming growth factor-beta (TGF-beta), IFN-gamma, IL-2, Fas and Fas ligand in the testis following an injection of S-Ag. Immunoreactivity for TGF-beta increased with time, reaching a maximum in six hours and declining thereafter. The time required for the maximum expression of TGF-beta coincided well with the time-dependent profile of orchidic tolerance signal generation within the testis. Little or no immunoreactivity was observed for IFN-gamma and IL-2 in normal (control) and S-Ag-injected testes. Immunoreactivity for Fas and Fas ligand was detected both in control and experimental testes and did not change appreciably with time following Ag-priming of the testis. Fas immunoreactivity was found in spermatids and virtually absent in the interstitial tissue, while Fas ligand immunoreactivity was primarily associated with the interstitial cells such as Leydig cells. Fas ligand immunoreactivity was very weak, if any, in the germ cells and Sertoli cells. These results suggest that TGF-beta and Fas ligand expressed in MHC-positive interstitial cells may play an important role in the generation of orchidic tolerance induction signal. A preliminary study showed that splenocytes preincubated with testis extracts and S-Ag, when transferred to naive rats, induced systemic tolerance in recipient animals. Inclusion of anti-TGF-beta or a carboxyl terminal peptide of Fas in the testis extract reduced the potency of incubated splenocytes to induce systemic tolerance in recipient rats. These results indicate that generation of the orchidic tolerance signal does not require the anatomical structure of the testis but is mediated by molecular entities such as TGF-beta and Fas ligand.

Transplantation immunity and tolerance in the eye: rejection and acceptance of orthotopic corneal allografts in mice

The eye has been considered for a long time as a privileged site, where the normal immune response is not manifested. Using a model of orthotopic corneal allografts in mice we demonstrate that both transplantation immunity and tolerance are expressed in the eye. In two strain combinations, one having genetic disparities at the major histocompatibility complex (MHC) and non-MHC antigens and the other with antigenic differences only in non-MHC antigens, survival of corneal allografts was evaluated in normal unmodified recipients, in recipients presensitized with a skin allograft, and in mice made specifically tolerant to the donor alloantigens. While the corneal allografts in unmodified recipients underwent rejection leading to graft failure in 40-50% of the recipients, all corneal allografts in presensitized recipients were rejected. On the contrary, corneal allografts performed in neonatally tolerant recipients enjoyed survival comparable to that of syngeneic grafts. No significant differences in corneal allograft survival were found between MHC antigen compatible and MHC antigen incompatible strain combinations. The results demonstrate that transplantation immunity and tolerance are efficiently expressed in the eye. Consequently, the local regulatory mechanisms must be responsible for the characteristics of immunity after intraocular immunization and for a high degree of acceptance of corneal allografts in normal recipients.