Biochemical analysis of a novel H-2 class I-like glycoprotein expressed in five AKR-(Gross virus) derived spontaneous T cell leukemias

The H-2 class I Ag profiles of five spontaneous AKR (H-2K) Gross virus leukemic cell lines were analyzed. A novel H-2 class I, "alloantigen"-like glycoprotein was immunoprecipitated and isolated from all the tumor cell lines using an H-2Dd-specific mAb 35-5-8. The novel Ag was also recognized in vitro by anti-H-2Dd-specific CTL. In addition, DNA from all the thymomas, but not the DNA from normal adult AKR thymic cells showed a transcribed gene detectable with an H-2Dd-specific oligonucleotide probe. The molecular profile of the novel antigen was further studied by two-dimensional gel electrophoresis and analyzed by a computer based image analyzer system and reverse-phase HPLC tryptic peptide mapping. Its molecular pattern was different from the syngeneic H-2Kk, H-2Dk, and the allogeneic H-2Dd gene products. The two-dimensional gel pattern of the novel H-2 class I molecule had a different overall structure reflected in isoelectric point, number, and distribution of polypeptide spots. The tryptic peptide map analysis showed six peaks exclusively identified with the novel Ag. The calculated degree of homology with the corresponding H-2Dd, H-Dk, and H-Kk peptides was 41, 56, and 51%, respectively. In addition, an unusual cell surface distribution of the novel Ag was observed in most of the leukemic lines. The removal of sialic acid residues by neuraminidase treatment facilitated the detection of the allodeterminants by anti-H-2Dd-specific mAb and CTL. Furthermore, we showed that in one AKR tumor line, 424, there is a close association of the novel Ag with the syngeneic class I molecules. Prior preclearance of the syngeneic class I molecules revealed the presence of the H-2Dd-like allospecificity. The genetic and molecular relationship between the expression of this novel class I-like glycoprotein and the recently sequenced Q5 gene is under current investigation.     

Analysis of hybrid H-2D and L antigens with reciprocally mismatched aminoterminal domains: functional T cell recognition requires preservation of fine structural determinants

Studies of immune recognition of hybrid class I antigens expressed on transfected cells have revealed an apparent general requirement that the N(alpha 1) and C1(alpha 2) domains be derived from the same gene in order to preserve recognition by virus-specific H-2-restricted and allospecific T cells. One exception has been the hybrid DL antigen in which the N domain of H-2Ld has been replaced by that of H-2Dd. Cells bearing this molecule serve as targets for some virus and allospecific CTL. Because cells expressing the reciprocal hybrid LD (N domain of H-2Dd replaced by that of H-2Ld) antigen have not been available, it has not been possible to evaluate whether this exception stemmed from the relatedness of H-2Ld and H-2Dd or whether the DL antigen fortuitously preserved some function of the parent molecule as a rare exception. To assess this question, and to evaluate the contribution of the N and C1 domains of H-2Ld and H-2Dd to serologic and T cell recognition, we have constructed the reciprocal chimeric gene pLD (the N exon of H-2Ld substituted for that of H-2Dd), introduced this into mouse L cells by DNA-mediated gene transfer, and analyzed the expressed product biochemically, serologically, and functionally. Transformant L cells expressing either LD or DL antigens were both reactive with a number of anti-H-2Ld or anti-H-2Dd N/C1-specific monoclonal antibodies, indicating the preservation in the hybrid molecules of determinants controlled by discrete domains. Mab binding was generally greater with cells expressing hybrid DL antigen than with those transformants expressing LD molecules. Moreover, the amount of beta 2M associated with DL antigens was more than that associated with LD. Cells expressing hybrid DL antigens were recognized as targets by bulk and cloned allospecific anti-H-2Dd and anti-H-2Ld CTL, whereas cells expressing LD molecules were not recognized by any of the T cells tested. VSV-specific H-2Ld-restricted CTL failed to lyse VSV-infected targets expressing either DL or LD. These results indicate that T cell reactivity of cells expressing the DL hybrid antigen is an exception to the observed general requirement for class I antigens to possess matched N and C1 domains for functional T cell recognition by T cells restricted to parental antigens.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cytotoxic T lymphocyte responses to minor H-43 alloantigens in H-43a and H-43b mice. Both anti-H-43b and anti-H-43a CTL activities are generated exclusively in the context of the same H-2Kb restriction element

We have previously demonstrated that anti-H-43a CTL response of H-43b responder mice was exclusively restricted by self H-2Kb (Kb) but not by the other nine self MHC class I alleles from independent origins, i.e., Kbml,d,k,s and Db,d,k,q,s. In the present study, we verified that Kf,q,r and Df,r alleles could also not serve as restricting class I elements in the CTL response to H-43a alloantigen. Another notable observation made in the earlier study was the fact that, in H-43 incompatibility of the alternative combination, H-43a mice were incapable of generating CTL activity against H-43b alloantigen. However, by means of employing new in vivo immunization procedures, we discovered that some but not all genetically identical H-43a responder mice could mount anti-H-43b CTL response restricted by self Kb. Again, no anti-H-43b CTL activity could be generated in the context of self Kk, Kj, Db or Dk molecules. Although the number of class I alleles we examined is still limited, these results indicate that antigenic fragments derived from the processed H-43a and H-43b alloantigens possess an indistinguishable epitope (agretope), and that such agretope either interacts only with the privileged Kb molecules or allows to bestow the immunogenic conformation of allodeterminants on the fragments solely in the context of the restricting Kb element.     

Broad recognition of cytotoxic T cell epitopes from the HIV-1 envelope protein with multiple class I histocompatibility molecules.

A few cases have been described of antigenic determinants that are broadly presented by multiple class II MHC molecules, especially murine I-E or human DR, in which polymorphism is limited to the beta chain, and the alpha chain is conserved. However, no similar cases have been studied for presentation by class I MHC molecules. Because both domains of the MHC peptide binding site are polymorphic in class I molecules, exploring permissiveness in class I presentation would be of interest, and also such broadly presented antigenic determinants would clearly be useful for vaccine development. We had defined an immunodominant determinant, P18, of the HIV-1 gp160 envelope protein recognized by human and murine CTL. To determine the range of class I MHC molecules that could present this peptide and to determine whether two HIV-1 gp160 Th cell determinants, T1 and HP53, could also be presented by class I MHC molecules, we attempted to generate CTL specific for these three peptides in 10 strains of B10 congenic mice, representing 10 MHC types, and BALB/c mice. P18 was presented by at least four different class I MHC molecules from independent haplotypes (H-2d, p, u, and q to CD8+ CTL. In H-2d and H-2q the presentation was mapped to the D-end class I molecule, and for Dd, a requirement for both the alpha 1 and alpha 2 domains of Dd, not Ld, was found. HP53 was also presented by the same four different class I MHC molecules to CD8+ CTL although at higher concentrations. T1 was presented by class I molecules in three different strains of distinct MHC types (B10.M, H-2f; B10.A, H-2a; and B10, H-2b) to CTL. The CTL specific for P18 and HP53 were shown to be CD8+ and CD4- and to kill targets expressing endogenously synthesized whole gp160 as well as targets pulsed with the corresponding peptide. To compare the site within each peptide presented by the different class I molecules, we used overlapping and substituted peptides and found that the critical regions of each peptide are the similar for all four MHC molecules. Thus, antigenic sites are broadly or permissively presented by class I MHC molecules even without a nonpolymorphic domain as found in DR and I-E, and these sequences may be of broad usefulness in a synthetic vaccine.     

Endocytosis and de novo expression of major histocompatibility complex encoded class I molecules: kinetic and ultrastructural studies

The endocytic pathway and expression of the major histocompatibility complex encoded class I molecule H-2Kk was investigated in murine fibroblasts. Internalization of H-2K molecules did not occur constitutively. Endocytosis of the molecules was induced by addition of multivalent ligands such as rabbit anti-mouse immunoglobulin serum or protein A-bearing liposomes to cells pretreated with anti-H-2Kk antibodies. The complete removal of H-2K molecules took about 5 h at 37 degrees C and was not inhibited by the lysosomotropic agent NH4Cl or the protein synthesis inhibitor cycloheximide. When targeted liposomes that contained carboxyfluorescein at a self-quenched concentration were directed against H-2K molecules, the cells became highly fluorescent after 30 min: a consequence of carboxyfluorescein release from the liposomes. This process was inhibited by NH4Cl but not by cycloheximide, suggesting internalization of H-2K molecules into acidic intracellular compartments. The endocytic pathway of liposomes directed against H-2K molecules and the subcellular compartments involved in this process were investigated with targeted liposomes containing horseradish peroxidase. By electron microscopy, the endocytic process was shown to start very rapidly (1-2 min) and involved uncoated cell surface invaginations. The cytoplasmic uncoated vesicles fused together into larger vacuoles containing concentrated liposomes and by 1 h, liposomes began to be destroyed in lysosomal compartments. Within 4 h, 90% of liposomes were lysed inside the cell. The fate of radiolabeled anti-H-2K antibody was also investigated. Degradation of the antibody occurred only when cross-linked with a second layer of antibody, beginning after 2 h and becoming more pronounced after 20 h of incubation. The original cell surface abundance of H-2K molecules was reestablished after 5 to 7 h. During this time neither NH4Cl nor cycloheximide had any effect on the cell surface expression of the molecule. However, after a second cycle of internalization, cells incubated with cycloheximide no longer expressed these molecules. These results suggested that H-2K molecules were not recycled back to the surface after internalization but were degraded in lysosomal compartments together with their ligand. Preexisting molecules, already present in intracellular pools, were expressed to replace them. By immunoprecipitation of metabolically labeled intracellular and surface H-2K molecules, we observed an intracellular pool of H-2K of about 70 to 80% of the total cellular H-2K.     

H-2K molecules positively select V beta 17a+ CD4(-)8+ T cells in bone marrow and thymic chimeras

Population size of V beta 17a brightly positive cells among CD4(-)8+ thymocytes was analyzed in thymic chimeras as well as bone marrow (BM) chimeras in which SWR/J mice were used as BM donors and various strains of mice including H-2Kb mutant (bm) mice as recipients. It was shown that the proportion of V beta 17a+ CD4(-)8+ thymocytes was determined by H-2K molecules expressed on thymic epithelial cells. The highest proportion was observed in Ks and Kb thymuses, the intermediate proportion in Ks/q and Kk, and the lowest in Kq thymuses. Fine analysis of the H-2Kbm molecules involved in the positive selection revealed that the region important to the selection was located on the beta-pleated floor of antigen recognition site. According to the three-dimensional class I structure, this site appears not to be directly accessible to the T cell antigen receptor. Thus, the present finding suggests that the substitutions of amino acids at this site alter the shape and charge of the peptide binding site and eventually influence the positive selection of the V beta 17a+ T cell repertoire during differentiation.     

Structural basis of Kbm8 alloreactivity. Amino acid substitutions on the beta-pleated floor of the antigen recognition site

We have analyzed the functional significance of the four amino acid differences between the parental H-2Kb and mutant H-2Kbm8 glycoproteins. Six bm8 variants including single substitutions at residues 22, 23, 24, and 30 as well as paired substitutions at residues 23, 30 and 22, 24 were generated and transfected into L cells. Surface expression of these H-2Kb variants was analyzed using monoclonal antibodies which bind to well-defined H-2Kb epitopes. No alterations introduced into the conformational structure of H-2Kb by the amino acid substitutions were detected. The effect of these substitutions on CTL recognition was initially analyzed using the following bulk CTL: either H-2Kb anti-H-2Kbm8, H-2Kbm8 anti-H-2Kb, or third party anti-H-2Kb. The alloreactivity between H-2Kb and H-2Kbm8 is dominated by the amino acid substitution at residue 24 (Glu----Ser). The complete bm8 phenotype, however, also requires the additional substitution at residue 22 (Tyr----Phe). The H-2Kbm8 anti-Kb bulk CTL reacted with both variant H-2Kbm8 molecules containing single substitutions at amino acid positions 22 or 24 but not the variant molecule containing both substitutions. Further analysis using three individual H-2Kbm8 anti-Kb CTL clones indicated the complexity of the self Kbm8 phenotype. Clone 8B1.20 did not react to changes in residues 22 or 24. The 8B1.32 clone reacted with the change at residue 22 but not with the change at residue 24, although the 8B1.54 clone reacted with the change at residue 24 but not with the change at residue 22. The changes in residues 23 (Met----Ile) and/or 30 (Asp----Asn) did not impact significantly on the alloantigenic properties of Kbm8 as determined by both the bulk and cloned CTL populations. According to the three-dimensional class I structure the substitution at amino acid 24 is inaccessible to the TCR. The location of this substitution within the Ag recognition site implies that altered peptide binding, and not a disruption of MHC residues that interact with the TCR, is responsible for the alloreactivity between H-2Kb and H-2Kbm8.     

In vivo priming of mouse CTL precursors directed to product of a newly defined minor H-42 locus is under a novel control of class II MHC gene

In a previous study, we discovered a new mouse minor histocompatibility antigen encoded by a locus at 8.5 cM apart from the H-2 complex, and we have since named the locus H-42. One allele of H-42, which is named H-42a, had been elucidated, but the other alleles, which we tentatively named H-42b, have not been elucidated. In the present study, we explored MHC control on the anti-H-42a cytotoxic T lymphocyte (CTL) responsiveness in H-42b mice. In vivo immunization (i.v. injection) of H-42b mice with 5 to 30 X 10(6) spleen cells (SC) bearing allogeneic H-42a antigen but carrying H-2 complex (mouse MHC) matched with the H-42b mice failed to prime anti-H-42a CTL but induced stable and specific anti-H-42a CTL unresponsiveness, i.e., tolerance, in the H-42b recipient mice. In contrast, H-2 heterozygous H-42b F1 mice injected with SC bearing H-42a alloantigen on either of the parental H-2 haplotypes were effectively primed to generate anti-H-42a CTL. Exploration of the region or subregion in the H-2 complex of H-42a donor SC that should be compatible with H-42b recipient mice for the induction of their anti-H-42a CTL tolerance demonstrated that the compatibility at I region, most probably I-A subregion, but not at K, S, or D region, determined the induction of the tolerance. MHC class II compatible H-42a skin graft (SG) to H-42b mice, however, consistently primed the anti-H-42a CTL in the H-42b recipients. These results were discussed in several aspects, including uniqueness of MHC class II control on the CTL response to minor H-42a antigen, possibility of inactivation of responding anti-H-42a precursor CTL or helper T cells in H-42b mice by encountering the veto cells present in MHC class II-matched H-42a SC population, and significance of the present observations as a mechanism of CTL tolerance to self-components.     

Multiple CTL subsets generated by H-2.L locus products stimulation: evidence for an antigenic determinant private to H-2.Ld

Analysis of the fine specificity of CTL subpopulations raised by an H-2.L locus products stimulation (H-2dm2 anti-H-2d) was performed by absorption experiments by using monolayers of macrophages of H-2m, H-2q, H-2b, and H-2k haplotypes. The results show the existence of four CTL subsets. The pattern of reactivity of three of them could be correlated with that of antibodies present in H-2dm2 anti-H-2d antisera (anti-H-2.64, anti-H-2.65, and anti-H-2.Kk). The fourth CTL subset reacted with a specificity unique to H-2.Ld molecules (a private specificity?), absent on cells from H-2m, H-2q, H-2b, and H-2k haplotypes, and undescribed as yet by serologic methods. These data support the hypothesis that the H-2.L locus products are comparable in their antigenic properties to those of the H-2.K and H-2.D loci.     

Determination of structural principles underlying three different modes of lymphocytic choriomeningitis virus escape from CTL recognition

Lymphocytic choriomeningitis virus infection of H-2(b) mice generates a strong CD8(+) CTL response mainly directed toward three immunodominant epitopes, one of which, gp33, is presented by both H-2D(b) and H-2K(b) MHC class I molecules. This CTL response acts as a selective agent for the emergence of viral escape variants. These variants generate altered peptide ligands (APLs) that, when presented by class I MHC molecules, antagonize CTL recognition and ultimately allow the virus to evade the cellular immune response. The emergence of APLs of the gp33 epitope is particularly advantageous for LCMV, as it allows viral escape in the context of both H-2D(b) and H-2K(b) MHC class I molecules. We have determined crystal structures of three different APLs of gp33 in complex with both H-2D(b) and H-2K(b). Comparison between these APL/MHC structures and those of the index gp33 peptide/MHC reveals the structural basis for three different strategies used by LCMV viral escape mutations: 1) conformational changes in peptide and MHC residues that are potential TCR contacts, 2) impairment of APL binding to the MHC peptide binding cleft, and 3) introduction of subtle changes at the TCR/pMHC interface, such as the removal of a single hydroxyl group.     

Cytotoxic T lymphocyte response to minor H-43a alloantigen in H-43b mice. Privileged H-2Kb restriction to the response is not due to immunodominance or epistatic effect but due to Ir gene function of H-2Kb itself

Previous study demonstrated that anti-H-43a cytotoxic T lymphocyte (CTL) response of H-43b CWB (H-2b) stain carrying non-major histocompatability complex (MHC) genes of C3H and F1 strains raised by crossing CWB with various H-43b strains was restricted exclusively by self H-2Kb (Kb). In the present study, newly produced C3W strain (H-2k, H-43b), which is H-43-congenic to C3H/HeN (H-2k, H-43a), was used as H-43b mice, and possibility of immunodominance of Kb was examined. No anti-H-43a CTL response could be induced in C3W strain and F1 strains raised by crossing C3W with other H-43b strains not carrying Kb. Thus, the possibility of immunodominance of Kb over the other MHC class I alleles could not be supported. We also examined possibility of epistatic effect of I region genes and non-MHC genes on the Kb restriction. (C3W x C57BL/6)F1(I-Ak/b) and (C3W x B6.CH-2bm12)F1(I-Ak/bm12)mice showed equally anti-H-43a CTL response restricted exclusively by self Kb, and (C3W x B10.MBR)F1(Ik/k) mice also showed anti-H-43a CTL response restricted solely by self Kb. Cold target competition experiments demonstrated that H-43b C57BL/10 or A.BY mice, which do not have non-MHC genes of C3H mounted anti-H-43a CTL response restricted solely by self Kb. Thus, no relation of I region genes or non-MHC genes to the Kb restriction was shown. All the results indicate that H-43b mouse strains, including F1, can not achieve anti-H-43a CTL response unless they carry Kb allele. Notably, (C3W x C57BL/6)F1 mice mounted self Kb-restricted anti-H-43a CTL response, whereas (C3W x B6.CH-2bm1)F1 mice carrying mutated Kb could not mount anti-H-43a CTL response at all. These findings indicate strongly that Kb itself is classical Ir gene of anti-H-43a CTL response and directs self Kb restriction of the response.     

Cytotoxic T-lymphocyte tolerance to minor H-43a alloantigen is induced exclusively in the context of the self major histocompatibility complex class I H-2Kb molecules

We elucidated previously that cytotoxic T lymphocyte precursors (CTLp) against H-43a allo-antigen, which we had discovered as a new mouse minor H antigen, were primed in H-43b mice only in the context of self H-2Kb restriction element, and that anti-H-43a CTLp tolerance was induced in H-43b mice by injection with H-43a spleen cells (SC) from H-43 congenic mice, i.e., under the condition of disparity at only the H-43 locus. The present study attempted to determine whether the H-2Kb restriction element for anti-H-43a CTLp priming is also implicated in the induction of anti-H-43a CTLp tolerance. For this purpose, we used a newly established H-43b C3W (H-2k) strain which is H-43 congenic to H-43a C3H/HeN. When (C3W X B10.MBR)F1 (H-43b, H-2Kk/b, Ik/k, Dk/q) mice were injected with H-43a-bearing (C3H/HeN X B10.AKM)F1 (H-43a/b;H-2Kk/k,Ik/k,Dk/q)SC, their selfH-2Kb-restricted anti-H-43a CTLp were were primed (cross-priming). By contrast, injection of H-43a-bearing (C3H/HeN X B10.MBR)F1 (H-43a/b; H-2Kk/b,Ik/k, Dk/q)SC, which differ from (C3H/HeN x B10.AKM) F1 SC solely at H-2K and possess H-2Kb molecules, did not prime but specifically inactivated the anti-H-43a CTLp of (C3W x B10.MBR)F1 mice. These results indicate clearly that anti-H-43a CTLp tolerance is induced exclusively in the context of the H-2Kb element expressed on the antigenic H-43a SC.     

Inhibitory effect of herpes simplex virus infection to target cells on recognition of minor histocompatibility antigens by cytotoxic T lymphocytes

Target cell lysis by CTL specific for minor histocompatibility Ag (minor HA), which were generated in (C3H/He x BALB/c)F1 mice immunized with A/J mouse spleen cells, was dramatically reduced by infection of HSV to Neuro-2a (A/J mouse origin) cells as target. The reduction was apparent at 5 h after infection of HSV to target cells, when many viral proteins were produced in the cells. Conversely, MHC-restricted HSV-specific CTL-mediated cell lysis increased time dependently. Using an RNA virus, vesicular stomatitis virus, significant reduction of minor specific CTL-mediated target cell lysis was also found. During the time when this reduction of target cell lysis by HSV occurred, the surface expression of class I H-2Dd molecules was maintained, and anti-H-2a allo-MHC-specific CTL lysed HSV-infected Neuro-2a cells as strongly as uninfected Neuro-2a cells. When HSV-infected or uninfected Neuro-2a cells were treated with Brefeldin A that selectively blocks transportation of newly synthesized proteins out of endoplasmic reticulum, both HSV- and minor HA-specific CTL-mediated cell lyses were blocked. These observations demonstrated that minor HA are continuously synthesized and associated with class I molecules at pre-Golgi and transported via trans Golgi system with quick turnover, and that newly synthesized HSV Ag, which are also associated with class I molecules and transported via the same system, should take the place of intrinsic minor HA and be presented on the surface of the cells to be recognized by MHC-restricted CTL.     

Direct binding of peptides to MHC class I molecules on living cells. Analysis at the single cell level.

To directly assess the binding of exogenous peptides to cell surface-associated MHC class I molecules at the single cell level, we examined the possibility of combining the use of biotinylated peptide derivatives with an immunofluorescence detection system based on flow cytometry. Various biotinylated derivatives of the adenovirus 5 early region 1A peptide 234-243, an antigenic peptide recognized by CTL in the context of H-2Db, were first screened in functional assays for their ability to bind efficiently to Db molecules on living cells. Suitable peptide derivatives were then tested for their ability to generate positive fluorescence signals upon addition of phycoerythrin-labeled streptavidin to peptide derivative-bearing cells. Strong fluorescent staining of Db-expressing cells was achieved after incubation with a peptide derivative containing a biotin group at the C-terminus. Competition experiments using the unmodified parental peptide as well as unrelated peptides known to bind to Kd, Kb, or Db, respectively, established that binding of the biotinylated peptide to living cells was Db-specific. By using Con A blasts derived from different H-2 congenic mouse strains, it could be shown that the biotinylated peptide bound only to Db among > 20 class I alleles tested. Moreover, binding of the biotinylated peptide to cells expressing the Dbm13 and Dbm14 mutant molecules was drastically reduced compared to Db. Binding of the biotinylated peptide to freshly isolated Db+ cells was readily detectable, allowing direct assessment of the relative amount of peptide bound to distinct lymphocyte subpopulations by three-color flow cytometry. While minor differences between peripheral T and B cells could be documented, thymocytes were found to differ widely in their peptide binding activity. In all cases, these differences correlated positively with the differential expression of Db at the cell surface. Finally, kinetic studies at different temperatures strongly suggested that the biotinylated peptide first associated with Db molecules available constitutively at the cell surface and then with newly arrived Db molecules.     

Association between H-2 and vaccinia virus-induced antigens on the surface of infected cells.

The relationship between H-2 molecules and vaccinia virus-induced antigens on the surface of H-2d infected cells was investigated by the differential redistribution method and by the blocking capacity of monospecific anti-H-2 sera on an anti-vaccinia cell-mediated cytotoxicity (CMC). Capping of either H-2K or H-2D molecules upon addition of monospecific and anti-H-2 sera was followed by the complete redistribution of viral antigens, suggesting the formation, on the cel membrane, of complexes of H-2K, H-2D molecules and vaccinia virus-induced antigens. However, not all H-2 molecules were involved in this association since i) free H-2K and H-2D molecules still moved independently on the cell surface, and ii) capping of vaccinia virus-induced antigens failed to induce the redistribution of all the H-2K and H-2D molecules. In addition, either monospecific anti-H-2K or anti-H-2D antiserum was found to exert potent blocking activity on anti-vaccinia CMC, indicating also a close topographical relationship between H-2K, H-2D molecules and vaccinia virus-induced antigens.     

Direct binding of peptide to empty MHC class I molecules on intact cells and in vitro

MHC class I molecules devoid of peptide are expressed on the cell surface of the mouse mutant lymphoma cell line RMA-S upon culture at reduced temperature. Empty class I molecules are thermolabile at the cell surface and in detergent lysates, but can be stabilized by the addition of presentable peptide; peptide binding appears to be a rapid process. Furthermore, class I molecules on the surface of RMA-S (H-2b haplotype) cells cultured at 26 degrees C can efficiently and specifically bind iodinated peptide presented by H-2Kb. Binding of iodinated peptide is also observed at a lower level for nonmutant cells (RMA) cultured at 26 degrees C. These experiments underscore the role for peptide in maintenance of the structure of class I molecules and, more importantly, provide two assay systems to study the interactions of peptides with MHC class I molecules independent of the availability of T cells that recognize a particular peptide-MHC class I complex.     

Recognition of a sequestered self peptide by influenza virus-specific CD8+ cytolytic T lymphocytes

The Ag receptors on CD8+ CTL recognize foreign antigenic peptides associated with cell surface MHC class I molecules. Peptides derived from self proteins are also normally presented by MHC class I molecules. Here we report that an H-2Kd-restricted murine CD8+ CTL clone directed to an influenza hemagglutinin epitope can recognize a peptide derived from the murine mitochondrial aconitase enzyme in association with H-2Kd molecules. Surprisingly, this self peptide is not normally displayed on the cell surface associated with the restricting MHC class I molecule. Several lines of evidence suggest that this self peptide, although requiring association with the Kd molecule for CTL recognition, is not associated with this or other MHC class I allele under physiologic conditions in intact cells. Rather, it is sequestered in the cytoplasm associated with a carrier protein and is released only upon cell disruption. These results suggest a means of restricting the entry of self peptide into the class I pathway. In addition, this finding raises the possibility that self peptides sequestered within the cell can, after release from damaged cells, interact with MHC class I molecules on bystander cells and trigger autoimmune injury by virus-specific CTLs during viral infection.     

Molecular signals for phosphatidylinositol modification of the Qa-2 antigen

Most cell surface proteins are anchored to the cell bilayer by hydrophobic membrane-spanning domains. Recently it has been shown that a small class of molecules are attached to cell surfaces via a phosphatidylinositol moiety covalently linked to the C-terminus of the mature processed polypeptide. The molecular signals that identify a polypeptide for phosphatidylinositol (PI) attachment have not been well defined in any system, but are thought to reside in the C-terminus of the primary translation product. We report that all the signals responsible for PI anchoring of Qa-2 Ag are confined to the 36 C-terminal residues of the precursor proteins. To investigate further the features that signal cleavage and PI addition, we have studied mutants of two closely related murine class I MHC molecules: the PI-linked Ag, Q9b, from the Qa-2 Ag family, and the integral membrane transplantation antigen, H-2Ld. The addition of 15 amino acids to the three residue long cytoplasmic domain of Q9b or the mutation of Asp295 found in its C-terminal hydrophobic domain to Val converts this molecule into an integral membrane protein. However, the introduction of a short three residue cytoplasmic tail and Asp295 into the transmembrane domain of H-2Ld does not convert this molecule to a PI-linked one. The results of these analyses suggest that the PI-processing signals may depend on overall conformation, hydrophobicity, and length of the C-terminal domain of the precursor protein. In addition these data indicate that PI anchoring of class I Ag requires more than two mutational steps and may have been selected during the evolution.     

Isolation and analysis of H-2 and Ia alloantigens from wild mouse strains.

Immunoprecipitation and SDS-PAGE analysis were used to examine H-2 and Ia antigens from mouse strains with wild-derived MHC haplotypes. Antisera raised against the wild-derived strains contained anti-H-2 and anti-Ia antibodies which precipitated antigen molecules readily distinguishable by a single assay procedure. The antibodies to wild strains were cross-reactive with standard laboratory haplotypes. Evidence supporting the similar genetic organization of wild and standard haplotypes was found, including isolation of separate H-2K and H-2D molecules from a wild-derived strain, and isolation of two separate Ia molecules from a wild-derived strain.