The appendage domain of the AP-2 subunit is not required for assembly or invagination of clathrin-coated pits.

Coated pits contain a resident membrane molecule(s) that binds clathrin AP-2 with high affinity. AP-2 binding to this site is likely to be the first step in coated pit assembly because this subunit functions as a template for the polymerization of clathrin into flat polygonal lattices. Integral membrane proteins involved in receptor mediated endocytosis cluster in the newly assembled pits as they invaginate and bud from the membrane. The AP-2 subunit is a multi-domain, molecular complex that can be separated by proteolysis into a brick-shaped core and ear-like appendage domains. We have used this property to identify the domain involved in the various stages of coated pit assembly and budding. We found that the core of AP-2 is the domain that binds both to membranes and to triskelions during assembly. Triskelions are perfectly capable of forming lattices on the membrane bound cores. Clathrin lattices bound only to core domains were also able to invaginate normally. Limited proteolysis was also useful for further characterizing the AP-2 binding site. Elastase treatment of the inside membrane surface released a peptide fraction that is able to bind AP-2 in solution and prevent it from interacting with membranes. Affinity purification of binding activity yielded a collection of peptides that was dominated by a 45-kD species. This is the candidate peptide for containing the AP-2-binding site. Therefore, the appendage domain does not directly participate in any of the assembly or invagination events required for coated pit function.

Clathrin-coated pits contain an integral membrane protein that binds the AP-2 subunit with high affinity.

Coated pits will assemble onto purified plasma membranes that are attached to a poly-L-lysine coated substratum (Moore, M. S., Mahaffey, D. T., Brodsky, F. M., and Anderson, R. G. W. (1987) Science 236, 558-563; Mahaffey, D. T., Moore, M. S., Brodsky, F. M., and Anderson, R. G. W. (1989) J. Cell Biol. 108, 1615-1624). To better understand the assembly reaction, we have purified both clathrin triskelion and AP-2 subunits from bovine brain and assayed for their ability to bind to the cytoplasmic surface of attached membranes. Two types of membranes were analyzed: those washed with a high pH buffer that selectively removes triskelions and those washed with a high salt buffer that removes both the AP-2 and the triskelion subunits. We found that purified AP-2 subunits bind with high affinity (Kd approximately 3 x 10(-8) M) to salt stripped membranes. Binding is saturable and abolished by treating membranes with less than 20 micrograms/ml of elastase. When membranes were treated with elastase before the salt wash and then salt washed and assayed for AP-2 binding, normal binding was seen, which indicates that the presence of clathrin-coated pits protects the binding site from the protease. Membranes that had rebound AP-2 did not bind purified triskelions, even though high pH buffer-washed membranes that bear endogenous AP-2 bound triskelions with high affinity (Kd approximately 3 x 10(-9) M) and supported lattice assembly. We conclude that coated pit assembly is initiated by the binding of AP-2 to an integral membrane protein but that the AP-2 complex must be activated by an unknown process before the coated pit lattice will assemble.

Phagocytosis of particulate antigens by corneal epithelial cells stimulates interleukin-1 secretion and migration of Langerhans cells into the central cornea.

Under normal physiological conditions, the central corneal epithelium is devoid of Ia+ Langerhans cells. However, a variety of stimuli can induce the migration of peripheral Langerhans cells into the central regions of the cornea. In the present study, Langerhans cell migration was induced by the instillation of either sterile latex beads or formalin-killed Staphylococcus aureus into shallow incisions in the central corneal epithelium. Langerhans cells could be detected in the central cornea as early as 24 hours following instillation of either latex beads or S. aureus and remained for at least 6 weeks. Phagocytosis of latex beads by corneal epithelial cells was demonstrated in vivo and in vitro. Moreover, phagocytosis of latex beads stimulated corneal epithelial cells to secrete increased amounts of interleukin-1 (21-83% increase). The centripetal migration of peripheral corneal Langerhans cells in response to phagocytosis of latex beads could be mimicked by injecting as little as 0.001 units of human purified interleukin-1 (IL-1) into the central corneal epithelium. The IL-1 induced chemotaxis could be blocked by coinjection of anti-IL-1 antibody but not irrelevant antibody. The findings indicate that the exclusion of Langerhans cells from the central corneal epithelium is a dynamic process that can be regulated by the resident corneal epithelial cells themselves and raises the possibility that corneal epithelial cells and Langerhans cells collaborate in antigen processing within this organ.

The immunogenic privilege of corneal allografts.

Corneal grafts enjoy the highest success rate of any form of organ transplantation. The low incidence of graft rejection is particularly impressive considering that human leukocyte antigen matching of donor and recipient is not normally performed. Although corticosteroids are applied topically, systemic immunosuppressive drugs are not routinely implemented for keratoplasty. The most widely accepted explanation to account for corneal allograft success suggests that the avascularity of the graft bed prevents corneal alloantigens from reaching the regional lymphoid tissues and therefore results in an "afferent blockade" of the immune response. However, recent findings suggest that the unique immunologic characteristics of the corneal graft itself may play a crucial role in determining the fate of the transplant. In particular, the presence and distribution of donor-derived Ia+ Langerhans cells can have a profound impact on graft immunogenicity and thus, graft survival even if the graft bed is initially free of lymphatic and blood vascular drainage channels. Thus, the immunologically unique characteristics of the corneal graft conspire with the avascular graft bed to produce an "immunologically privileged" environment that promotes graft survival.

Histopathology of rejected orthotopic corneal grafts in the rat.

We have used an orthotopic graft model in the rat to study the histologic characteristics of corneal allograft rejection. Unrejected allogeneic grafts could not be distinguished from clear syngeneic grafts. Although donor Langerhans cells are necessary for the development of delayed-type hypersensitivity (DTH), the histopathological characteristics of rejecting corneal allografts in immunologically naive hosts were identical regardless of the presence or absence of donor Langerhans cells. By contrast, preimmunization had a dramatic effect on the histology of graft rejection. Untreated allografts placed onto pre-immunized recipients underwent a marked cellular necrosis accompanied by minimal inflammation that easily distinguished these grafts from the previous groups. These results suggest that neither the presence nor absence of DTH responsiveness correlates with the histopathological events that accompany corneal graft rejection. However, preimmunization leads to a different histologic pattern of rejection that is characterized by an intense cellular necrosis.

Presentation of the H-Y antigen on Langerhans' cell-negative corneal grafts downregulates the cytotoxic T cell response and converts responder strain mice into phenotypic nonresponders.

We have used the murine cornea is an allograft model to investigate the relative roles of graft-derived IA+ APC (Langerhans' cells) and host-derived APC during the induction of CTL responses to H-Y. The natural exclusion of LC from the immunizing corneal graft led to a specific state of unresponsiveness to H-Y in responder strain mice, while inclusion of LC resulted in responsiveness. Failure to respond to H-Y could not be attributed to the absence of H-Y or IA antigen expression on the surface of LC-deficient grafts but instead, appeared to be due to active suppression of the T helper cell response during in vivo priming. Reprocessing of the H-Y antigen by host APC did not occur after immunization with H-Y presented on H-2-incompatible grafts unless presented initially by graft-derived LC. H-2 as well as some non-H-2 alloantigens were presented to the host without a requirement for donor-derived LC. Thus there appear to be differential requirements for the processing and presentation of alloantigens.

Expression of HLA class I and II antigens on cells of the human trabecular meshwork.

Human corneoscleral tissue containing trabecular meshwork and cultured human trabecular cells were examined for HLA-ABC (class I) and HLA-DR (class II) antigens of the major histocompatibility complex using an indirect immunofluorescence assay. Class I antigens were detected in the trabecular meshwork on frozen sections and on cultured trabecular cells. Class II antigens were constitutively expressed on some, but not all, cells within the trabecular meshwork. Many more cells could be induced to express class II antigens by pre-incubation in human gamma interferon. Cultured trabecular cells did not express class II antigens constitutively, but expression could be induced by gamma interferon. This study suggests that, in addition to Langerhans' cells at the limbus, other cell types within the anterior segment express major histocompatibility complex-encoded class II antigens either constitutively or inducibly. These cells may be important for the initiation and regulation of ocular immunity.

T cell subsets in the immune rejection of murine heterotopic corneal allografts.

Using a previously described murine heterotopic corneal allograft model, we examined the roles of delayed-type hypersensitivity (DTH) and cytotoxic T lymphocytes (CTL) in corneal allograft rejection. We have previously shown that normal C57BL/6 mice consistently reject heterotopic corneal allografts within 14 days of grafting. These hosts develop antigen-specific CTL responses but no evidence of DTH reactivity. The absence of DTH suggested that this T cell subset was unnecessary for corneal allograft rejection. The present studies using T cell-deficient mice selectively reconstituted with specific T cell subsets confirmed this suspicion. T cell-deficient (i.e., adult thymectomized, lethally irradiated, bone marrow-reconstituted = ATXBM) C57BL/6 mice were selectively reconstituted with the following categories of syngeneic lymph node cells (LNC): BALB/c skin-immune LNC treated with anti-Lyt 1 antibody + complement; BALB/c skin- or cornea-immune LNC treated with anti-Lyt 2 antibody + complement; or BALB/c cornea- or skin-immune LNC not treated with antibody. ATXBM mice reconstituted with syngeneic Lyt 1-depleted, BALB/c skin-immunized LNC failed to develop DTH, yet rapidly rejected BALB/c corneal allografts. Similarly, ATXBM mice reconstituted with Lyt 1-depleted cornea-immune LNC did not demonstrate DTH responses but were able to reject 100% of the corneal allografts in an accelerated fashion. By contrast, corneal allograft rejection was significantly delayed in ATXBM mice reconstituted with cornea-immune LNC partially depleted of Lyt 2+ T cells. Collectively, the results indicate that: heterotopic corneal allografts can be rejected in the absence of DTH; heterotopic corneal allografts fail to induce allospecific DTH; and partial depletion of Lyt 2+ CTL leads to a prolongation of heterotopic corneal allograft survival. Thus, the primary T cell-dependent immune effector elements responsible for heterotopic corneal allograft rejection appears to reside in the cytolytic T lymphocyte population.

Antigen presentation by Langerhans cells in vivo: donor-derived Ia+ Langerhans cells are required for induction of delayed-type hypersensitivity but not for cytotoxic T lymphocyte responses to alloantigens.

T cell activation in response to allogeneic stimulation and hapten-specific delayed-contact hypersensitivity responses in vivo can be initiated by Ia-bearing epidermal Langerhans cells (LC). By using a murine heterotopic corneal allograft model, we have investigated the requirement for allogeneic LC as antigen-presenting cells (APC) in the in vivo induction of delayed-type hypersensitivity (DTH) and cytolytic T lymphocyte (CTL) responses to alloantigens in fully allogeneic and H-2 I region-disparate strain combinations. LC-deficient, avascular central corneal allografts from BALB/c donors failed to induce DTH responsiveness when grafted to a subdermal bed on C57BL/6 recipients (p greater than 0.05), yet antigen-specific primary CTL reactivity developed within 7 days after grafting. LC-containing corneal-limbus allografts or central corneal allografts containing a latex bead-induced infiltrate of LC resulted in intense DTH as well as CTL responsiveness when grafted in this same strain combination. Similarly, LC-containing but not LC-deficient corneal allografts from A.TL donors induced DTH responsiveness in I region-disparate A.TH hosts despite the fact that these grafts survived for prolonged duration (less than 28 days). By contrast, CTL induction in I region-disparate hosts was independent of the presence of allogeneic LC. Corneal epithelial cells of grafts removed from I region-disparate hosts 7 days posttransplantation were shown by immunohistology to express the Iak antigens of donor origin. The possibility that bone marrow-derived allogeneic LC were a sufficient requirement for DTH induction was confirmed in experiments performed with CB6F1----B6 bone marrow chimeras used as corneal allograft donors. Corneal-limbus grafts obtained from mice 90 days after chimerization were shown by immunohistology to contain Iad-bearing CB6F1 LC as a sole source of class II alloantigens. When grafted to C57BL/6 recipients, LC-containing chimeric corneas induced DTH responsiveness that was similar in magnitude to that observed in C57BL/6 mice grafted with chimeric skin, yet no DTH response to LC-deficient chimeric central corneal grafts was observed. Moreover, in all cases, the chimeric corneal and skin allografts survived for prolonged duration (greater than 28 days). These results demonstrate that donor-derived LC act as APC in the induction of DTH responsiveness to allogeneic tissue; however, there was no apparent requirement for allogeneic LC in the induction of CTL responses to class I or class II MHC alloantigens.(ABSTRACT TRUNCATED AT 400 WORDS)