An improved method for examining the corneal endothelium during graft rejection in the rat

Differences in leukocyte phenotype and interferon-γ expression in stroma and endothelium during corneal graft rejection

Critical to the success of human corneal transplants is prevention of corneal endothelial rejection, yet little is known about the endothelial infiltrate. To examine the endothelium, a method for removal and processing this layer as a flat sheet was used and the infiltrate was compared with stroma and epithelium. LEW or PVG strain rat corneas were transplanted to PVG strain recipients. Clinical changes after transplantation were monitored by slit lamp and animals sacrificed at a range of time points during rejection. Clinically defined rejection, accompanied by an epithelial rejection line and endothelial cell infiltration, occurred between days 10 and 15. There was some infiltration of leukocytes in the stroma of isografts at these time points, but significantly more in allografts (p<0.003 for all subsets). There was no infiltration of isograft endothelium at any time and no infiltration of allograft endothelium on day 10. On day 15, there were similar numbers of all major subsets except B cells in the stroma, while on the endothelium macrophages, MHC class II(+) cells and CD8(+) cells predominated (p<0.001 CD4(+) vs CD8(+) cells). T cells and NK cells predominated in the epithelial rejection line. While TNF-alpha and IFN-gamma-producing cells were numerous in stroma and epithelium, no IFN-gamma-producing cells were found on endothelium. Distinct differences in infiltrative profile within layers of the cornea suggest that the mechanisms of rejection may also differ. The restricted endothelial cell profile and lack of IFN-gamma suggests that the anti-endothelial response may be modulated by the anterior chamber environment.

MHC class I and II antigens as targets of rejection in penetrating keratoplasty in low- and high-risk mouse eyes

PURPOSE

To determine the extent to which expression of major histocompatibility complex (MHC) class I and II molecules contributes to rejection of orthotopic corneal transplants in mice.

METHODS

Full-thickness corneas, prepared from eyes of normal C57BL/6 (B6) and BALB/c mice and from B6 mice in which the class II gene I-A or the beta2-microglobulin (beta2mu) gene was disrupted, were placed orthotopically in low- or high-risk eyes of BALB/c (fully incompatible), BALB.B [minor histoincompatible (H) only], and bm12 (class II only disparate) recipients. BALB/c grafts were placed in low-risk eyes of normal B6 and B6 mice with disrupted H-2 DMalpha genes. Graft survival was judged by clinical examination.

RESULTS

Recipient-identical class II, but not class I, molecules on fully allogeneic corneas grafted to low-risk beds promoted graft rejection. Allogeneic class II molecules on fully allogeneic corneas placed in high-risk beds promoted graft rejection more strongly than did allogeneic class I molecules. Neither allogeneic class I molecules nor recipient-identical class II molecules on grafts placed in high-risk beds contributed to graft outcome. Mice deficient in H-2 DMalpha failed to reject fully incompatible cornea grafts.

CONCLUSIONS

On corneal allografts, where minor H antigens are the major barriers to acceptance, allogeneic class II molecules promote rejection if the graft is placed in high-risk eyes, whereas recipient-type class II molecules promote rejection if the graft is placed in low-risk eyes. Allogeneic class I molecules make a minor contribution to rejection only if the grafts are placed in high-risk eyes.

Cornea Graft Endothelial Cells Undergo Apoptosis by Way of an Alternate (Caspase-Independent) Pathway

PURPOSE

To look for apoptosis pathways involved in corneal endothelial cell death during acute graft rejection and to evaluate the potential role of nitric oxide in this process.

MATERIALS AND METHODS

Corneal buttons from Brown-Norway rats were transplanted into Lewis rat corneas. At different time intervals after transplantation, apoptosis was assessed by diamino-2-phenylindol staining and annexin-V binding on flat-mount corneas, and by terminal transferase dUTP nick end labeling (TUNEL), caspase-3 dependent and leukocyte elastase inhibitor (LEI)/LDNase II caspase-independent pathways on sections. Inducible nitric oxide synthase (NOS-II) expression and the presence of nitrotyrosine were assayed by immunohistochemistry.

RESULTS

Graft endothelial cells demonstrated nuclear fragmentation and LEI nuclear translocation, annexin-V binding, and membranes bleb formation. Apoptosis associated with caspase-3 activity or TUNEL-positive reaction was not observed at any time either in the graft or in the recipient corneal endothelial cells. During 14 days posttransplantation, the recipient corneal endothelial cells remained unaltered and their number unchanged in all studied corneas. NOS-II was expressed in infiltrating cells present within the graft. This expression was closely associated with the presence of nitrotyrosine in endothelial and infiltrating cells.

CONCLUSION

During the time course of corneal graft rejection, graft endothelial cells undergo apoptosis. Apoptosis is caspase 3 independent and TUNEL negative and is, probably, carried out by an alternative pathway driven by an LEI/L-Dnase II. Peroxynitrite formation may be an additional mechanism for cell toxicity and programmed cell death of the graft endothelial cells during the rejection process in this model.

Factors affecting rejection of second corneal transplants in rats1

BACKGROUND

Second and subsequent corneal transplants in the same eye are more prone to rejection reactions and failure than first grafts. This may be a result of local changes or systemic sensitization to antigen shared by the first and second donors. Because HLA typing is not routine in corneal transplantation, a clear correlation between accelerated rejection and specific sensitization has not been established.

METHODS

PVG (RT1), Lewis (LEW; RT1), or AO (RT1) strain corneas were transplanted to PVG strain rats, followed by a LEW strain cornea in the ipsilateral or contralateral eye 6 weeks later. Graft survival was evaluated by slit lamp biomicroscopy. Proliferation of recipient lymph node cells was tested against allogeneic, syngeneic and third-party stimulator cells after the second transplantation.

RESULTS

A second allograft in the ipsilateral or contralateral eye was rejected in an accelerated fashion that was not donor MHC specific. Rejection was not significantly accelerated in the ipsilateral eye compared with the contralateral eye. There was a secondary lymphocyte proliferation response to third party (AO strain) in animals previously exposed only to the LEW strain.

CONCLUSIONS

Systemic sensitization to donor antigens, rather than local changes induced by first transplantation, contributed to accelerated rejection of a second graft. Accelerated rejection is not dependent on MHC compatibility between the grafts. It could be caused by shared "public" MHC determinants, by minor antigens shared by the first and second donors, or by cross-reactivity of T cells to epitopes on AO and LEW grafts. HLA mismatching of first and second donors may not prolong second graft survival.

Development of organised conjunctival leucocyte aggregates after corneal transplantation in rats

AIM

To investigate the development of lymphoid aggregates in the conjunctiva after corneal transplantation in rats.

METHODS

LEW or PVG strain corneas were transplanted orthotopically to PVG rats. Cornea and conjunctiva were examined clinically for up to 42 days. Eyes were removed with attached conjunctiva on days 10 and 15 after transplantation (before and during rejection), together with normal eyes, fixed, paraffin embedded, and examined immunohistochemically.

RESULTS

Clinically, the temporal half of the upper palpebral conjunctiva of recipients of 10/19 allografts and 1/10 isografts developed pronounced swelling, correlating with inflammation and rejection. Histologically, the swelling comprised leucocytic aggregates with an altered overlying epithelium. Aggregates contained granulocytes, macrophages, and cells expressing major histocompatibility complex (MHC) class II, CD4, and CD8, all more numerous in allograft associated conjunctiva. Class II+ cells were more abundant at the surface, whereas macrophages and T cells were more numerous in the deeper stroma. There were few B cells. There was greater CD54 expression by vascular endothelium in allograft associated aggregates. Cells expressing TNFalpha and IFNgamma but not IL1beta were present in stromal and superficial areas.

CONCLUSIONS

Corneal transplantation in rats induces the development of organised conjunctival leucocytic aggregates in a fixed location that are significantly more pronounced in recipients of allografts compared with isografts and show characteristics of a Th1 type immune response. These aggregates have characteristics of conjunctiva associated lymphoid tissue and may be sites of presentation of graft antigens and lymphocyte proliferation at the ocular surface.