RMA/S cells present endogenously synthesized cytosolic proteins to class I-restricted cytotoxic T lymphocytes

Immunization against tumor and minor histocompatibility antigens by eluted cellular peptides loaded on antigen processing defective cells

Material eluted from RMA lymphoma or B6 spleen cells under acid conditions was fractionated by reverse phase high-performance liquid chromatography, and tested for ability to restore the sensitivity to cytotoxic T lymphocytes of the processing/presentation defective mutant line RMA-S. This allowed identification of three fractions (termed M1, M2 and M3) carrying B6 antigens recognized by cytotoxic T lymphocytes (CTL) elicited across the minor histocompatibility barrier A.BY anti-B6 (both H-2b) and one fraction (termed T1) carrying a tumor antigen recognized by B6 anti-RMA CTL. By parallel runs of material from cell lysates over major histocompatibility complex class I affinity columns, the M2 and M3 antigens were defined as Kb restricted, and M1 and T1 as Db restricted. Isolated fractions loaded onto RMA-S cells could be used to prime anti-minor histocompatibility antigen and tumor CTL in vivo. They could also be used for in vitro restimulation of spleen cells from mice that had been primed either by antigen-loaded RMA-S, or by wild-type RMA tumor cells and B6 splenocytes. The CTL generated by these methods were specific for the loading antigen, and they also recognized the antigen on the "physiological" target, i.e. RMA or B6 lymphoblasts. This system based on RMA-S as an immunization and target antigen reporter cell may be used for dissection of complex CTL responses, e.g. in studies of clonal composition and epitope dominance, or for studies of tumors that are poor stimulators of immunity.

Evidence that transporters associated with antigen processing translocate a major histocompatibility complex class I-binding peptide into the endoplasmic reticulum in an ATP-dependent manner

We have investigated the role of the putative peptide transporters associated with antigen processing (TAP) by using a permeabilized-cell system. The main objective was to determine whether these molecules, which bear homology to the ATP-binding cassette family of transporters, translocate antigenic peptides across the endoplasmic reticulum membrane for assembly with major histocompatibility complex (MHC) class I molecules and beta 2-microglobulin light chain. The pore-forming toxin streptolysin O was used to generate permeabilized cells, and peptide translocation was determined by measuring the amount of added radiolabeled peptide bound to endogenous class I molecules. No radiolabeled peptide was associated with MHC class I glycoproteins from unpermeabilized cells. We found that efficient peptide binding to MHC class I molecules in permeabilized cells is both transporter dependent and ATP dependent. In antigen-processing mutant cells lacking a functional transporter, uptake occurs only through a less-efficient transporter and ATP-independent pathway. In addition, short peptides (8-10 amino acids) known to bind MHC class I molecules compete efficiently with a radiolabeled peptide for TAP-dependent translocation, whereas longer peptides and a peptide derived from an endoplasmic reticulum signal sequence do not compete efficiently. This result indicates that the optimal substrates for TAP possess the characteristics of MHC-binding peptides.

Rejuvenated expression of H-2Kb in RMA-S cells does not affect alloreactive T cell- and natural killer cell-mediated lysis

Reduced sensitivity to alloreactive cytotoxic lymphocytes (CTLs) and increased susceptibility to natural killer (NK) cells in vivo and in vitro are two phenotypic characteristics that have been attributed to the reduced major histocompatibility complex (MHC) ligand density on the surface of an antigen-presentation-defective cell line, RMA-S. As RMA-S exhibits both defective processing of antigenic peptides and very low class I expression, it is uncertain which is responsible for the above characteristics. In this report, we show that the phenotype cannot be reversed by increasing the number of Kb + beta 2-M complexes expressed on the cell surface. These findings emphasize the importance of association of MHC class I molecules with endogenously processed peptides for biological interaction with alloreactive CTLs and NK cells.

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.

TAP2-defective RMA-S cells present Sendai virus antigen to cytotoxic T lymphocytes

The murine antigen-processing-defective mutant cell line RMA-S is leaky in the presentation of certain endogenously synthesized minor histocompatibility and viral antigens to major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL). The viral antigens include influenza virus nucleoprotein, vesicular stomatitis virus (VSV) nucleocapsid and Rauscher murine leukemia virus (MuLV) antigen. Here we demonstrate Sendai virus antigen presentation by the HAM2 (murine TAP2, transporter associated with antigen presentation type 2)-defective RMA-S cell line and compare antigen presentation after restoration of the defect by murine TAP1/2 gene transfection. Kinetic studies revealed that RMA-S cells required 2-3 h longer incubation and approximately 10 times higher doses of Sendai virus to reach the same level of killing as the RMA parental line. After transfection of RMA-S cells with the murine TAP1/2 gene, Sendai virus antigen presentation was restored to levels of the RMA wild-type line with regard to time of virus infection and dose of virus needed for sensitizing target cells. The presentation of Sendai virus antigen in RMA-S cells was sensitive to brefeldin A (BFA), suggesting that the presentation was mediated via the endogenous pathway. Our findings confirmed leakiness of antigen presentation in RMA-S cells and extended it to Sendai virus. The results underscored the role for intact expression of the TAP 1/2 molecules for efficient MHC class I-mediated antigen presentation.

Presentation of numerous viral peptides to mouse major histocompatibility complex (MHC) class I-restricted T lymphocytes is mediated by the human MHC-encoded transporter or by a hybrid mouse-human transporter

The major histocompatibility complex-encoded transporter associated with antigen processing (TAP) is required for the efficient presentation of cytosolic antigens to class I-restricted T cells. TAP is thought to be formed by the interaction of two gene products, termed TAP1 and TAP2. We find that TAPs consisting either of human subunits, or mouse TAP1 and human TAP2, facilitate the presentation of numerous defined viral peptides to mouse class I-restricted T cells. As human and mouse TAP2 and TAP1 differ in 23 and 28% of their residues, respectively, this indicates that TAP1 and TAP2 can form a functional complex with partners considerably different from those they coevolved with. Moreover, these findings indicate that widely disparate TAPs facilitate delivery of the same peptides to class I molecules. These findings suggest that TAP polymorphism does not greatly influence the types of peptides presented to the immune system.

Major histocompatibility complex class I-specific and -restricted killing of beta 2-microglobulin-deficient cells by CD8+ cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) recognize major histocompatibility complex (MHC) class I molecules, normally composed of a heavy chain, a beta 2-microglobulin (beta 2m), and peptide antigens. beta 2m is considered essential for the assembly and intracellular transport of MHC class I molecules as well as their peptide presentation to CTLs. Contrary to this dogma, we now report the generation of allospecific and restricted CD8+ and TCR alpha beta+ CTLs (where TCR is T-cell receptor) capable of killing beta 2m-deficient cells. Such CTLs were obtained by priming mice with live allogeneic beta 2m- spleen cells or mutant lymphoma cells producing MHC class I protein but no detectable beta 2m. Although both beta 2m- and beta 2m-expressing lymphoma cells were rejected in allogeneic mice, only the former were efficient inducers of CTLs recognizing beta 2m- cells. These CTLs were MHC class I (H-2Kb or Db)-specific and CD8-dependent and did not require serum as a source of external beta 2m in the culture. They could be induced across major and minor histocompatibility barriers. The H-2-restricted CTLs generated in the latter case failed to kill the antigen-processing-deficient target RMA-S cells. The results show that MHC class I heavy chains in beta 2m- cells can be transported to the cell surface and act as antigens or antigen-presenting molecules to allospecific and MHC-restricted CTLs.

Cytotoxic T lymphocytes against the antigen-processing-defective RMA-S tumor cell line

RMA-S is an antigen processing-defective cell line, obtained from a Rauscher virus-induced tumor. The cells express only a low level of cell surface major histocompatibility complex (MHC) class I molecules, which are supposed to be devoid of internally derived antigenic peptides. We investigated Rauscher virus expression and Rauscher peptide presentation to virus-specific cytotoxic T lymphocytes (CTL) by this cell line. Viral proteins are expressed properly, both intracellularly and at the cell surface of RMA-S. Rauscher peptides are presented to virus-specific CTL in the groove of both the class I H-2Kb and Db molecules, but at a low level. Culture of RMA-S cells at room temperature increases their susceptibility to CTL. The RMA-S defect thus affects, but not totally abrogates, Rauscher peptide presentation by MHC class I molecules via the endogenous pathway. This indicates that the RMA-S antigen processing deficit is not absolute.

The MHC: relationship between linkage and function

It is intriguing that several genes with associated functions, including all of class I and class II genes, as well as some genes affecting antigen presentation of both class I and class II pathways, are linked in the MHC. Recent observations have led to speculation that there may be a functional explanation for keeping these related genes together.

Evidence for peptide transport across microsomal membranes

Antigenic peptides bound to class I molecules of the major histocompatibility complex (MHC) are recognized by T-cell receptors during development of an antiviral immune response. T cells respond to peptides derived from cytoplasmic viral proteins as well as viral membrane proteins, indicating that a pathway exists for the transport of proteins or peptides from the cytosol into the compartment(s) where the MHC class I molecules assemble. To investigate this pathway, we have developed an in vitro assay for the transport of peptides into microsomal vesicles. This assay provides evidence for the transport of chemically synthesized peptides (13-21 amino acids) containing N-linked glycosylation acceptor sequences, which serve as glycosylation substrates. Their transport results in depletion of the pool of available dolichol high-mannose oligosaccharides in the lumen of the microsomal vesicles. We have observed transport of peptides derived from antigenic human immunodeficiency virus gag and influenza B nucleoprotein sequences, but transport of a third randomly selected peptide was not detected, suggesting specificity of the transport process. We were not able to demonstrate ATP dependence of this peptide transport process by using apyrase and an ATPase inhibitor. This result was unexpected in light of the recent identification of MHC-linked genes with homology to ATP-binding cassette transporters, which have been proposed to mediate peptide transport.