Histopathological and histochemical alterations in the early stages of corneal graft rejection

Corneal Graft Rejection

Penetrating keratoplasty is the most widely practiced type of transplantation in humans. Irreversible immune rejection of the transplanted cornea is the major cause of human allograft failure in the intermediate and late postoperative period. This immunological process causes reversible or irreversible damage to the grafted cornea in several cases despite the use of intensive immunosuppressive therapy. Corneal graft rejection comprises a sequence of complex immune responses that involves the recognition of the foreign histocompatibility antigens of the corneal graft by the host's immune system, leading to the initiation of the immune response cascade. An efferent immune response is mounted by the host immune system against these foreign antigens culminating in rejection and graft decompensation in irreversible cases. A variety of donor- and host-related risk factors contribute to the corneal rejection episode. Epithelial rejection, chronic stromal rejection, hyperacute rejection, and endothelial rejection constitute the several different types of corneal graft rejection that might occur in isolation or in conjunction. Corneal graft failure subsequent to graft rejection remains an important cause of blindness and hence the need for developing new strategies for suppressing graft rejection is colossal. New systemic pharmacological interventions recommended in corneal transplantation need further evaluation and detailed guidelines. Two factors, prevention and management, are of significant importance among all aspects of immunological graft rejection. Preventive aspects begin with the recipient selection, spread through donor antigenic activity, and end with meticulous surgery. Prevention of corneal graft rejection lies with reduction of the donor antigenic tissue load, minimizing host and donor incompatibility by tissue matching and suppressing the host immune response. Management of corneal graft rejection consists of early detection and aggressive therapy with corticosteroids. Corticosteroid therapy, both topical and systemic, is the mainstay of management. Addition of immunosuppressive to the treatment regimen helps in quick and long term recovery. Knowledge of the immunopathogenesis of graft rejection may allow a better understanding of the immunological process thus helping in its prevention, early detection and management.

Experimental corneal allograft rejection

The major findings regarding corneal allograft rejection in experimental animals are reviewed. The principal anatomic and biological feature of the cornea that determines the immunologic privilege of this tissue is its avascularity. The surgical trauma of transplantation compromises the immunologic privilege, putting corneal allografts at risk for immune rejection. During the past 50 yr, rabbits, rats, and mice have been used extensively in the study of the process of immunologically mediated corneal allograft rejection. It is clear that the inflammation and neovascularization of the graft that occurs following transplantation predisposes a corneal allograft to the classic cell-mediated immune rejection response. The antigenicity of cornea cells has been studied and has been found to be significantly lower compared to other cells and tissues. Rejection of acorneal allograft is acell-mediated process directed against major histocompatibility complex antigens involving both CD4+ T helper cells and CD8+ cytotoxic cells. The prevention of corneal allograft rejection depends on the development of topically applied compounds that can prevent inflammation and vascularization and inhibit the activation of T lymphocytes. Considerable progress has been made using immunomodulators, including blocking antibodies and soluble coreceptor blocking agents such as CTLA4-Ig. Combinations of antiangiogenic agents and immunomodulators hold great promise for preventing corneal allograft rejection in patients.

Hydrolase participation in allograft rejection in rat penetrating keratoplasty

BACKGROUND

A rat model of orthotopic corneal graft rejection was used to investigate the alterations in hydrolase activity within the corneal graft or within cellular infiltrates during acute rejection.

METHODS

The distribution of the lysosomal enzymes [acid phosphatase (AP), N-acetyl-beta-D-glucosaminidase (NAG), beta-glucuronidase (beta-Gluc), beta-galactosidase (beta-Gal), dipeptidylpeptidase II (DPPII)] and of the membrane-bound proteases [aminopeptidase M (APM), aminopeptidase A (APA), gamma-glutamyltransferase (GGT), alkaline phosphatase (ALP), dipeptidylpeptidase IV (DP-PIV)] were investigated by histochemical methods in the grafts at 3, 5, 8, 10 and 12 days following allogeneic transplantation. Serial sections of the grafts were also examined for RT1b, CD4, CD4+, CD8, CD11b/c and CD45, in order to determine hydrolase activity within infiltrating cells.

RESULTS

Allogeneic grafts were invaded by macrophages, CD4- and CD8-positive lymphocytes. In contrast, syngeneic grafts, performed as a control, contained occasional lymphocytes and focal aggregations of macrophages around suture sites. The allogeneic cellular infiltrate stained intensely for AP and ALP; moderately for beta-Gluc, NAG and beta-Gal; and mildly for GGT, DPPII and APM in grafts at all postoperative times. Serial sectioning indicated that the majority of the lysosomal hydrolases were located in macrophages; AP, APM and GGT were, however, observed in lymphocytes. Vessel ingrowth could be observed with enzyme staining for AP, beta-Gluc, NAG, ALP, APA and APM. Hydrolase activity in the corneal endothelium served as an indicator of endothelial function during the rejection process.

CONCLUSION

Changes in normal hydrolase activities in corneal grafts in the rat model indicate decreasing corneal function during the rejection process. Hydrolases released from infiltrating cells contribute to the morphological disruption and, possibly, to graft rejection.

Leflunomide therapy following penetrating keratoplasty in the rat

BACKGROUND

The isoxal derivative, leflunomide (LF), is a new potent immunosuppressive which has been shown to be effective in preventing autoimmune disorders and reactions leading to organ transplantation rejection. LF is thought to antagonise cytokine activity and thereby to interfere with T-helper-cell-dependent B- and T-lymphocyte proliferation.

METHODS

We used LF to treat corneal allograft rejection in the rat, comparing its effect with that of cyclosporin A (CSA). Corneal buttons were grafted from Lewis/Brown Norway rats to Lewis recipients. Animals were randomly assigned to the following treatment groups: I, untreated; II, CSA (10 mg/kg i.m.); III, LF (2.5 mg/kg p.o.); IV, LF (5 mg/kg p.o.); V, LF (10 mg/kg p.o.); VI, combined therapy (LF 10 mg/kg p.o. and CSA 10 mg/kg i.m). Treatment began on the first postoperative day and was continued until rejection occurred.

RESULTS

The mean graft rejection time in the untreated allogeneic group was 12 days. A significant delay in graft rejection was observed in all treatment groups compared with group I (P < 0.001). Further, the delay in graft rejection resulting from combined therapy (group VI) was statistically significant compared with all other groups (P < 0.001).

CONCLUSION

These results suggest that (a) LF when used alone is as effective as CSA in treating corneal allograft rejection in the rat, and (b) when LF and CSA are combined they are more effective than either drug alone in the prolongation of allograft survival.

Corneal allograft rejection

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Ocular injury induced by methyl ethyl ketone peroxide

Methyl ethyl ketone peroxide is a commonly used catalyst in various industries. We studied 19 eyes with a single exposure to methyl ethyl ketone peroxide that developed clinical patterns of mild injury, moderate injury, severe injury, or delayed keratitis. Delayed methyl ethyl ketone peroxide keratitis may cause exacerbations and remissions of corneal and limbal disease lasting more than 20 years with palpebral and bulbar hyperemia equal to the initial chemical exposure. With repeat exacerbation, further pannus may occur, which can be associated with a poorer outcome. Based on the capability of methyl ethyl ketone peroxide to change DNA to a new weak antigen, we suggest possible methods of therapy to prevent or limit delayed methyl ethyl ketone peroxide keratitis. This proposed type of injury has important implications in studying various limbal and corneal diseases. A major factor in the severity of ocular injury was the length of time from exposure to methyl ethyl ketone peroxide to obtaining a topical ocular local anesthetic to perform adequate lavage.

Pars planitis and autoimmune endotheliopathy

A migrating endothelial rejection line is the clinical criterion of specific immune reaction in corneal allotransplantation. This line when seen in otherwise intact corneas has been presumed to be an autoimmune endotheliopathy. We have recorded corneal changes similar to autoimmune endotheliopathy in four of ten patients with the diagnosis of pars planitis. These observations suggest that pars planitis may be an autoimmune process directed against the vitreous humor.

Corneal allograft rejection in bilateral penetrating keratoplasty: clinical and laboratory studies

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Composition of cellular infiltrates in rejected human corneal allografts

We utilized the leu series of monoclonal antibodies and an indirect immunoperoxidase technique to quantitate the cellular infiltrates in seven rejected human corneal allografts and compared them to nine identically processed corneas from patients with chronic herpetic stromal keratitis and to three control corneas. The cellular infiltrates in the rejected allografts were of mixed composition, comprised predominantly of leu-1 positive T-lymphocytes and leu-M3 positive macrophages. Both helper and cytotoxic-suppressor T-cells were identified in the stromal lesions. Statistically significant differences were noted in the absolute number of mononuclear cells and macrophages seen infiltrating the rejected corneal allografts compared to the herpetic corneas. We discuss the implications of these findings with regard to possible mechanisms underlying corneal allograft rejection in man.

Corneal allograft rejection. The role of the major histocompatibility complex

The greater success of corneal transplantation compared to other organ transplants has led to the concept that the cornea is a site of "immunological privilege." Corneal cells possess the antigens of the major histocompatibility complex responsible for allograft rejection in other tissues (i.e., HLA antigens). The avascularity of the cornea accounts for the relative protection of the donor cornea from the immunological surveillance of the recipient. As the roles and functions of the major histocompatibility complex are unravelled, the mechanisms responsible for host sensitization, lymphocyte activation and allograft rejection are becoming better understood. In particular, the HLA-DR antigen in humans is believed to play an integral part in allograft rejection. Langerhans cells in human corneal epithelium have been shown to bear this antigen. Evidence suggests that these cells or similar HLA-DR-bearing cells in the cornea play a major role in corneal allograft rejection. In light of these advances in transplantation immunobiology, new methods of suppressing and possibly preventing allograft rejection in corneal transplantation are presented.

Corneal transplant allograft reaction: possible predisposing factors

A series of 869 PKs was reviewed to determine the incidence of allograft reactions in the presence or absence of possible predisposing factors. The average follow-up time after PK was 46 months. The possible predisposing factors studied were mild and severe preoperative vascularization, postoperative neovascularization, synechiae, and inflammation. Other factors considered were bilateral grafts, repeat grafts, and recipient's age. Although there was a significantly higher incidence of allograft reactions in the group with possible predisposing factors compared with the group with none of these, a cause-and-effect relationship could not be established for each possible predisposing factor when considered individually. Most of these factors were usually encountered in combinations of two or more, creating a large number of small variables with less statistical significance in predicting first and second reactions. The incidence of second reactions was similar in the presence or absence of these factors, approximately 25%. Only postoperative synechiae and neovascularization had some significance in eventually leading to permanent graft rejection. The incidence of allograft reaction was also similar among patients with repeat grafts and bilateral grafts. There was an insignificant tendency for the incidence of reactions to decrease in older patients, whether possible predisposing factors were present or not. Final visual acuity was comparable between the group having allograft reactions and the total group. The allograft rejection reaction continues to be the most important cause of graft failure. The role of donor-recipient antigen matching was briefly reviewed.

Penetrating keratoplasty in the cat. A clinically applicable model

A series of 28 consecutive penetrating keratoplasties were performed on adult cats. Donor corneas (n = 14) were maintained in culture medium for 14--24 hours prior to transplantation. Rotational autografts (n = 7) were used to control for cell loss caused by culture maintenance as well as for the effects of surgery. Additional homografts (n = 7) were transplanted following removal of the corneal endothelium to study the extent of host corneal endothelial cell regeneration. Pre- and post-operative endothelial cell counts of the homografts made from specular micrographs demonstrated an average cell loss of 30% one month following surgery. A similar 30% average cell loss was present in the rotational autografts. Clinically, both homografts and autografts remained clear and were near normal in thickness. Homografts lacking endothelium exhibited persistent, severe edema that correlated with the inability of the host corneal endothelium to resurface the graft. Clinical and morphologic evidence of mild homograft rejection as observed in 15% of the animals that received normal homografts. Corneal endothelial cell loss following penetrating keratoplasty in the cat approximates that observed following the same procedure in the human. Additionally, regenerative capacity of the corneal endothelium in the cat, like that of the human, is limited. These features suggest that this cooperative, hardy animal is an excellent model in which to study many aspects of corneal transplantation that have direct application to the treatment of human corneal disease.

Presumed autoimmune corneal endotheliopathy

Two men (58 and 19 years of age) had an unusual recurrent, bilaterally symmetric disease process involving the cornea. It is characterized by stromal edema progressing centrally from the periphery in otherwise normal eyes. The corneal edema in each instance was closely associated with slowly moving linear keratic precipitates accompanied by the destruction of the endothelium, with minimal anterior chamber reaction. There was no history of herpetic keratitis or trauma, and serologic tests for syphilis were negative in both cases. A similar pattern of linear endothelial destruction has been reported heretofore only in association with corneal allograft rejection. This clinical pattern, the cytologic findings for the aqueous humor (macrophages and lymphocytes), and the rapid response to corticosteroid therapy suggested that an autoimmune process was the underlying cause of this disease.

Prevention of immune graft rejection after corneal transplantation

The effectiveness of fluorometholone was compared to dexamethasone phosphate and prednisolone acetate in preventing the immune corneal graft reaction in rabbits. Clear corneal grafts were obtained. Rejection was induced after skin from the corneal donor animal was grafted subcutaneously in the host animal and the animals were randomized into four treatment groups. Rejection occurred in eight of nine rabbits in the control group; one of eight in the dexamethasone phosphate group; one of eight in the fluorometholone group; and one of ten in the prednisolone acetate group. Histologic examination confirmed the above findings. In this animal study fluorometholone prevented immune graft rejection in a percentage similar to that of prednisolone acetate and dexamethasone phosphate.

Allogeneic corneal grafting in inbred strains of rats. Histology of graft reaction

Histological studies of interlamellar keratoplasty in different inbred strains of rats (CAP, LEW) without additional sensitization--i.e. first-set reactions--are described. The corneas of 65 eyes, after allogeneic or syngeneic grafting, were examined from the 3rd to the 90th day--every 2nd day at the beginning of the reaction and later every 5th day. After syngeneic grafting (LEW leads to LEW) a non-specific healing-reaction (only slight vascularization of the graft bed, edema, granulocytic infiltration) reached its climax on the 6th day and subsided by the 10th day. After allogeneic grafting (CAP leads to LEW; RtH-1-incompatible) the non-specific-healing reaction progressed into a second phase, namely the specific reaction: increasing infiltration of the host cornea and the graft with small lymphocytes, blast cells and macrophages, directly followed by severe vascularization, reaching its climax about the 14th day. A third, phagocytic phase succeeded the infiltration leading to elimination of the donor cells, but leaving the donor stroma undamaged. All these alterations had almost completely disappeared after 35 days. Thus, the corneal allograft reaction is discussed as a typical immunological reaction leading to the destruction of transplantation-antigen-bearing cells and permitting observation of the different reaction phases more clearly than in most other tissues.

Contributions of electron microscopy to the study of corneal pathology

Most of the electron microscopic studies of pathological corneas have been done only recently. Keratoplasty has been the most important source of specimens for ultramicroscopic investigation. With the introduction of electron microscopic techniques, we have been able to confirm many light microscopic studies in pathological corneas. This contribution has been most valuable in the identification of the sites and types of pathological changes in corneal dystrophies and degenerations. This review of electron microscopic studies describes the present concepts on the nature of the histological changes in dystrophies and degenerations of the anterior and posterior corneal layers and corneal stroma. It also includes a review of some corneal inflammatory conditions as well as metabolic disorders affecting its transparency.

HLA-compatible donor cornea for prevention of allograft reaction

Our series comprises 80 penetrating grafts. Of the recipient corneas 36 were vascularized. Of the donor corneas 48 were fresh and 32 were cryopreserved. As prophylaxis, we gave topical and systemic prednisolone, combined, in heavily vascularized cases, with azathioprine. Corneal donor-recipient pairs were divided into three groups according to HLA histocompatibility. In Group I (0-1 mismatch) the rate of allograft reaction was 1 in 27, in Group II (2-4 mismatches) it was 4 in 19, and in the untyped Group III 9 in 34. Even when the prognosis was poor, a well-matched donor cornea (Group I) greatly improved the chances of success; when the corneal bed was vascularized, the well-matched donor corneas had a failure rate of 1 in 13, whereas in untyped pairs it was 7 in 18. The importance of possible presensitization is discussed.