Antigen processing. A new presentation pathway?

Review: analogies between trophoblastic and malignant cells

PROBLEM

The question of how trophoblastic and malignant cells evade immunologic recognition and rejection by their host was studied.

METHOD OF STUDY

A literature review was conducted.

RESULTS

Trophoblastic and malignant cells share a number of similarities. These include a lack of major histocompatibility complex antigen expression, resistance to lysis by natural killer cells, T-helper cell-2 (TH2)-biased response, prostaglandin E production, and response to transforming growth factor beta. In addition, the analogies between trophoblastic and malignant cells extend into immunotherapy in which anti-idiotype therapy has a viable role in the prevention of pregnancy loss and the treatment of cancer.

CONCLUSIONS

Trophoblastic and malignant cells use a number of similar mechanisms to resist rejection by their host. By using similar strategies these cells are able to successfully co-exist in an immunologically hostile environment.

Delivery of protein antigen to the major histocompatibility complex class I-restricted antigen presentation pathway

Major histocompatibility complex (MHC) class I-restricted antigen presentation normally requires a protein antigen to be synthesized in the cytosol of the antigen presenting cell (APC). Exogenous protein antigen could gain access to the class I presentation pathway if the protein is introduced into the cytosolic compartment of the APC. Approaches which release the protein antigen from endocytic vesicles have been employed to deliver protein antigen for the recognition by class I-restricted cytotoxic T lymphocytes (CTL). These include osmotic shock, electroporation, cationic and pH-sensitive liposomes. An alternative approach is to deliver a gene that encodes the protein antigen. In this case, the APC is transfected with a gene which synthesizes the "exogenous protein" in the cytosol. Delivery of protein antigen targeted for CTL induction in vivo follows a different strategy and generally requires an antigen carrier of lipidic/membranous nature, such as liposomes, immunostimulating complexes, and/or lipid conjugates. Macrophages that are responsible for scavenging the antigen play an important role in CTL induction. An optimal CTL inductive vaccine must contain other immuno-modulatory activities in addition to its activity in delivering antigen to the class I pathway. Attempts to attenuate viral infection and to improve anti-tumor immunity have been successful by delivering the exogenous antigen entrapped in liposomes. These animal model studies should be of great value in the development of potential vaccine formulation.

Lymphocyte-melanoma interaction: role of surface molecules

The coexistence of tumor-specific immunity with a progressing tumor is observed in most experimental systems and remains one of the major paradoxes of tumor immunology. Expression of several surface molecules on melanoma cells, e.g., intercellular adhesion molecule 1 (ICAM-1) or major histocompatibility complex (MHC) class II, has been associated with an aggressive tumor growth and an reduced host antitumor response. HLA class I expression is also frequently altered in melanoma compared to melanocytes. Given the central role of these molecules in the restriction of T cell recognition, regulation of tumor HLA class I expression might also be a strategy for the evasion of immune surveillance by the malignant cells. The fact that it is now possible to clone antigen-specific T cells from tumor patients, as well as the relevant autologous tumor cell lines, enabled us to establish a model system to investigate possible tumor escape mechanisms from immunosurveillance. Using this system, we were able to demonstrate that purified soluble ICAM-1 or 12-fold-concentrated cell-free melanoma supernatants, containing shed ICAM-1, were able to inhibit conjugate formation between T cell clones and the autologous melanoma cells as efficiently as monoclonal antibodies against CD11a, Soluble ICAM-1 also abrogated the MHC-restricted killing of the melanoma by T cell clones. We further observed that a number of CD4+ T cell clones and melanoma cell lines established from the same tumors form conjugates with each other, leading to an increase of [Ca2+]i in the T cell clone; however, this interaction failed to induce interleukin-2 production or proliferation of the T cell clone. Furthermore, this interaction rendered the T cell clone unresponsive to subsequent stimulation. All these effects were MHC class II restricted. Therefore, the melanoma was capable of delivering antigen-specific signals to the T cell clone, but did not deliver the costimulatory signals, e.g., a B7/CD28 interaction, necessary for full T cell activation. Transfection of the melanoma with an expression vector containing a B7 cDNA with subsequent B7 expression on its cell surface renders the melanoma a fully competent antigen-presenting cell which is able to induce a nuclear factor binding to the interleukin-2 promoter in the specific T cell clone, followed by enhanced interleukin-2 transcription, synthesis, and T cell proliferation.

T cell recognition of an HLA-A2-restricted epitope derived from a cleaved signal sequence

An alternative pathway for class I-restricted antigen presentation has been suggested on the basis of peptides bound to HLA-A2 molecules in cells lacking the transporter for antigen presentation (TAP). Most of these peptides were derived from signal sequences for translocation into the endoplasmic reticulum (ER). However, it is not known whether these peptides can be presented to T cells. The hydrophobic nature of an HLA-A2-restricted T cell epitope (M1 58-66) was exploited to test whether it could be presented to T cells when derived from a signal sequence. Replacing the signal sequence of the influenza virus hemagglutinin molecule H3 with an artificial sequence containing that HLA-A2-restricted T cell epitope resulted in efficient translocation of H3 molecules into the ER and transport to the cell surface. This signal sequence-derived epitope was presented to HLA-A2-restricted T cells. Involvement of cytosolic processing for this presentation is very unlikely, because (a) presentation occurred in cells lacking TAP; (b) expression of H3 molecules with the artificial signal sequence did not produce a detectable cytosolic form of H3; and (c) presentation of the same epitope expressed in cytosolic forms of antigen required TAP. Thus, a peptide derived from a signal sequence cleaved in the ER can provide an epitope for HLA-A2-restricted T cell recognition.

Peptide binding to the most frequent HLA-A class I alleles measured by quantitative molecular binding assays

Quantitative assays to measure the binding of defined synthetic antigenic peptides and purified MHC class I molecules are described for several common human HLA-A alleles (A1, A2.1, A3, A11 and A24). Under appropriate conditions, the binding of radiolabeled peptides to purified MHC class I molecules is very effective, highly specific, and appears to be dependent on the specific sequence motif of the peptide as defined by critical anchor residue positions. Establishment and optimization of the assay reveals that a relatively high fraction of the MHC class I molecules isolated from EBV transformed B cell line sources is capable of binding exogenously added peptide. Scatchard analysis for all alleles yields 5-10% occupancy values. There is a stringent peptide size requirement that is reflected by the direct influence of peptide length on the binding affinity. The peptide-MHC class I interactions demonstrate remarkable similarity to peptide-MHC class II interactions, both in overall affinity and kinetic behavior. The immunological relevance of the peptide-MHC class I binding assay is also demonstrated by measuring the affinity of a panel of previously described HLA restricted peptides for their HLA restriction element. In 91% (10/11) of the cases, the peptides bound with affinities of 50 nM or less, and in the remaining 9% (1/11) of the cases, in the 50 to 500 nM range. Thus, these data provide the first quantitative estimate of what level of HLA-A binding affinity is associated with a diverse panel of immunodominant CTL epitopes in man.

Interaction of MHC class I molecules with the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) delivers cytosolic peptides into the endoplasmic reticulum (ER) where they bind to nascent class 1 histocompatibility molecules. Class 1-peptide complexes are then displayed at the cell surface for recognition by cytotoxic T lymphocytes. Immunoprecipitation of either TAP or class 1 molecules revealed an association between the transporter and diverse class 1 products. TAP bound preferentially to heterodimers of the class 1 heavy chain and beta 2-microglobulin, and the complex subsequently dissociated in parallel with transport of class 1 molecules from the ER to the Golgi apparatus. The TAP-class 1 complexes could also be dissociated in vitro by the addition of class 1-binding peptides. The association of class 1 molecules with TAP likely promotes efficient capture of peptides before their exposure to the lumen of the ER.

Antigen processing mutant T2 cells present viral antigen restricted through H-2Kb

Cytotoxic T lymphocytes (CTL) recognize foreign antigens as short peptides presented by class I molecules of the major histocompatibility complex (MHC). T2 cells are profoundly defective in the presentation of endogenously synthesized antigens to CTL due to a deletion of MHC class II-encoded genes for transporters associated with antigen presentation (TAP1/TAP2). Surprisingly, we here demonstrate that T2 cells, after infection with Sendai virus, are readily killed by H-2Kb restricted CD8+ T cells. In contrast to classical class I-mediated antigen presentation, the presentation of Sendai virus antigen in T2Kb cells is brefeldin A (BFA) insensitive. The present findings may suggest the presence of an alternative pathway for MHC class I-mediated antigen presentation in T2 cells.