Identification of functional regions on the I-Ab molecule by site-directed mutagenesis

Role of endocytosis and trans-endocytosis in ICOS costimulator-induced downmodulation of the ICOS Ligand

The interaction between the T-lymphocyte costimulatory molecule ICOS and its ligand (ICOS-L) is needed for efficient immune responses, but expression levels are tightly controlled, as altered expression of ICOS or ICOS-L may lead to immunodeficiency, or favor autoimmune diseases and tumor growth. Using cells of mouse B cell lymphoma (M12.C3) and melanoma (B16), or hamster CHO cells transfected with various forms of mouse ICOS-L, and ICOS⁺ T cell lines, we show that, within minutes, ICOS induces significant downmodulation of surface ICOS-L that is largely mediated by endocytosis and trans-endocytosis. So, after interaction with ICOS⁺ cells, ICOS-L was found inside permeabilized cells, or in cell lysates, with significant transfer of ICOS from ICOS⁺ T cells to ICOS-L-expressing cells, and simultaneous loss of surface ICOS by the T cells. Data from cells expressing ICOS-L mutants show that conserved, functionally important residues in the cytoplasmic domain of mouse ICOS-L (Arg₃₀₀ , Ser₃₀₇ and Tyr₃₀₈ ), or removal of ICOS-L cytoplasmic tail have minor effect on its internalization. Internalization was dependent on temperature, and was partially dependent on actin polymerization, the GTPase dynamin, protein kinase C, or the integrity of lipid rafts. In fact, a fraction of ICOS-L was detected in lipid rafts. On the other hand, proteinase inhibitors had negligible effects on early modulation of ICOS-L from the cell surface. Our data add a new mechanism of control of ICOS-L expression to the regulation of ICOS-dependent responses.

Class II major histocompatibility complex mutant mice to study the germ-line bias of T-cell antigen receptors

The interaction of αβ T-cell antigen receptors (TCRs) with peptides bound to MHC molecules lies at the center of adaptive immunity. Whether TCRs have evolved to react with MHC or, instead, processes in the thymus involving coreceptors and other molecules select MHC-specific TCRs de novo from a random repertoire is a longstanding immunological question. Here, using nuclease-targeted mutagenesis, we address this question in vivo by generating three independent lines of knockin mice with single-amino acid mutations of conserved class II MHC amino acids that often are involved in interactions with the germ-line-encoded portions of TCRs. Although the TCR repertoire generated in these mutants is similar in size and diversity to that in WT mice, the evolutionary bias of TCRs for MHC is suggested by a shift and preferential use of some TCR subfamilies over others in mice expressing the mutant class II MHCs. Furthermore, T cells educated on these mutant MHC molecules are alloreactive to each other and to WT cells, and vice versa, suggesting strong functional differences among these repertoires. Taken together, these results highlight both the flexibility of thymic selection and the evolutionary bias of TCRs for MHC.

Overexpression of Peroxisome Proliferator-activated Receptor-α (PPARα)-regulated Genes in Liver in the Absence of Peroxisome Proliferation in Mice Deficient in both l- and d-Forms of Enoyl-CoA Hydratase/Dehydrogenase Enzymes of Peroxisomal β-Oxidation System

Peroxisomal beta-oxidation system consists of peroxisome proliferator-activated receptor alpha (PPARalpha)-inducible pathway capable of catalyzing straight-chain acyl-CoAs and a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs. Disruption of the inducible beta-oxidation pathway in mice at the level of fatty acyl-CoA oxidase (AOX), the first and rate-limiting enzyme, results in spontaneous peroxisome proliferation and sustained activation of PPARalpha, leading to the development of liver tumors, whereas disruptions at the level of the second enzyme of this classical pathway or of the noninducible system had no such discernible effects. We now show that mice with complete inactivation of peroxisomal beta-oxidation at the level of the second enzyme, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase (L-PBE) of the inducible pathway and D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase (D-PBE) of the noninducible pathway (L-PBE-/-D-PBE-/-), exhibit severe growth retardation and postnatal mortality with none surviving beyond weaning. L-PBE-/-D-PBE-/- mice that survived exceptionally beyond the age of 3 weeks exhibited overexpression of PPARalpha-regulated genes in liver, despite the absence of morphological evidence of hepatic peroxisome proliferation. These studies establish that peroxisome proliferation in rodent liver is highly correlatable with the induction mostly of the L- and D-PBE genes. We conclude that disruption of peroxisomal fatty acid beta-oxidation at the level of second enzyme in mice leads to the induction of many of the PPARalpha target genes independently of peroxisome proliferation in hepatocytes, raising the possibility that intermediate metabolites of very long-chain fatty acids and peroxisomal beta-oxidation act as ligands for PPARalpha.

Hidden variables: unstable Abeta chain genes encoding antigen recognition structures in tumor survivors

Novel single exon genes Abeta4-7 comprising the Abeta6 gene family have been cloned from mouse mutants surviving transplantable metastatic tumors. Their protein coding sequences are similar to H2-Ab cDNA which encodes antigen-binding molecules of antigen presenting cells (APC); their promoters and/or signal sequences are unrelated to Ab sequences but found in other eukaryotic genes. Abeta4(b) protein was demonstrated on macrophages and B cells that are APC. The Abeta6(w302) appears to be an ancient gene ancestral to major histocompatibility complex (MHC) class II beta genes. However, unlike the MHC class II, the Abeta4-7 genes are not involved in skin graft rejection. Despite inbreeding, the Abeta6(w302) locus remains unfixed in several strains of mice. The number of Abeta genes and their alleles varied between individual mice; they do not map into the H2 region but appear to be scattered over the genome. The Abeta6 gene family is molecularly unstable in Abeta6(w302)-positive (but not in Abeta6(w302)-negative) mice which are somatic mosaics for these genes. Biological features of Abeta4-7 genes make them remarkably different from the classical MHC gene system. All available evidence strongly suggests that these genes control susceptibility/resistance to the spread of metastatic tumors.

Effect of the bm12 class II mutation on proliferative and cytokine responses of encephalitogenic T cells in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis

The bm12 mutation in the class II I-A(b)molecule can profoundly influence experimental autoimmune disease, enhancing the development of systemic lupus erythematosus-like syndromes in NZB.H-2(bm12)mice or, conversely, abolishing the susceptibility of C57BL/6J (H-2(b)) mice to the induction of experimental autoimmune myasthenia gravis. We have studied the effect of this mutation on experimental autoimmune encephalomyelitis (EAE), induced in H-2(b)mice by myelin oligodendrocyte glycoprotein (MOG), and recently showed that MOG 35-55 peptide (pMOG 35-55), which represents the immunodominant encephalitogenic region for H-2(b)mice, is also a strong encephalitogen for H-2(bm12)mice. Nevertheless, although the differences in fine epitope specificity and TCR-Vbeta gene usage between encephalitogenic pMOG 35-55-specific T cells from H-2(b)and H-2(bm12)mice were subtle, H-2(bm12)and H-2(b)antigen presenting cells failed to effectively cross-present pMOG 35-55 non-syngeneically to I-A(b)/pMOG 33-55- and I-A(bm12)/pMOG 35-55-specific T cells, respectively. In the present study, we show that the abrogation of the response to pMOG 35-55 by the Th1 encephalitogenic pMOG 35-55-specific T cells upon non-syngeneic cross-presentation is neither due to a cytokine shift to a Th2 pattern, nor a result of anergy induction. Therefore, we suggest that presentation of pMOG 35-55 to I-A(b)/pMOG 35-55-specific T cells via the bm12 class II MHC molecule resulted in ineffective stimulation, similar to a weak agonistic effect.

T cell cross-reactivity between coxsackievirus and glutamate decarboxylase is associated with a murine diabetes susceptibility allele

Limited regions of amino acid sequence similarity frequently occur between microbial antigens and host proteins. It has been widely anticipated that during infection such sequence similarities could induce cross-reactive T cell responses, thereby initiating T cell-mediated autoimmune disease. However, the nature of major histocompatibility complex (MHC)-restricted antigen presentation confers a number of constraints that should make this type of T cell cross-reactivity a rare, MHC allele-dependent event. We tested this prediction using two insulin-dependent diabetes mellitus (IDDM)-associated antigens, coxsackievirus P2-C (Cox P2-C) protein and glutamate decarboxylase (GAD65), which share a prototypic sequence similarity of six consecutive amino acids within otherwise unrelated proteins. We surveyed a panel of 10 murine MHC class II alleles that encompass the spectrum of standard alleles for the ability to cross-reactively present Cox P2-C and GAD65. Out of the 10 restriction elements tested, the sequence similarity regions were both dominant determinants and were cross-reactively displayed after the natural processing of whole antigens, only in the context of I-Anod. These data show that cross-reactive T cell recognition of sequence similarity regions in unrelated proteins is confined to certain MHC alleles, which may explain MHC association with autoimmune disease. It is striking that these two diabetes-associated antigens were cross-reactively recognized only in the context of a diabetes susceptibility allele. Since the human and the murine class II alleles associated with IDDM share conserved features, cross-reactive T cell recognition of GAD65 and Cox P2-C may contribute to the pathogenesis of human IDDM and account for the epidemiological association of coxsackievirus with IDDM.

Direct cleavage of a DNA fragment by a bleomycin-oligonucleotide derivative

Bleomycin A5 oligonucleotide derivative was used for direct cleavage of a DNA target. In the presence of Fe2+ ions and 2-mercaptoethanol, Blm-R-pd(CCAAACA) (I) damaged the target. pd(TGTTTGGCGAAGGA), with the yield of 80% , without affecting its own oligonucleotide tail. The sites of cleavage were T3-T5 and G6-G7. Unbound bleomycin A5 damaged the G6-G7-C8 site. Reagent I formed more stable complementary complexes with the target than parent oligonucleotide (ATm = 11 C).

Point mutations define positions in HLA-DR3 molecules that affect antigen presentation

Allelic differences in major histocompatibility complex (MHC)-encoded class II molecules affect both the binding of immunogenic peptides to class II molecules and the recognition of MHC molecule-peptide complexes by T cells. As yet, there has been no extensive mapping of these functions to the fine structure of human class II molecules. To determine sites on the HLA-DR3 molecule involved in antigen presentation to T cells, we used monoclonal antibodies specific for HLA-DR3 to immunoselect mutants of a B-lymphoblastoid line. We located the sites of single amino acid substitutions in the HLA-DR3 molecule and correlated these structural changes with patterns of recognition by HLA-DR3-restricted, antigen-specific T cells, allospecific T cells, and allospecific anti-DR3 monoclonal antibodies. We analyzed seven mutations. One mutation, at position 74 in domain 1 of the DR beta chain, affected recognition by all T cells tested, whereas others, at positions 9, 45, 73, 151, and 204 of the DR beta chain and position 115 of the DR alpha chain, altered recognition by some T cells, but not others. Each of the substitutions resulted in a unique pattern of T-cell stimulation. In addition, each T-cell clone recognized a different subset of the mutants. These results indicate that different residues of the DR3 molecule are involved in presentation of antigen to different DR3-restricted T cells. These studies further show that substitutions which most likely affect peptide binding alter recognition of DR3 molecules by an alloreactive T-cell clone and some allospecific antibodies.

Mapping of the full profile of T cell allorecognition regions on HLA-DR2 beta subunit

Alloreactive human T cell lines were developed by repeated in vitro restimulation with alternate allogeneic cells of similar DR2 serotype, but differing at other HLA loci. These polyclonal T cells recognized DR specificities in common with all DR2 serotypes. The allorecognition profiles of DR2 beta by the T cell lines were mapped by consecutive uniform-sized overlapping peptides encompassing the entire extracellular and intracellular regions of the DR beta subunit. It was found that the T cell allorecognition sites are focused around the first and third polymorphic regions of the N-terminal domain, and interestingly, include conserved areas in the DR beta second domain as well as the intracellular segment beta 222-237. These findings have important implications for allorecognition. Comparison with the previous analysis of antibody-binding regions of DR2 beta revealed some similarities and differences in the antibody and T cell alloresponses to this histocompatibility antigen.

The contribution of mutant amino acids to alloantigenicity

The I-Abm12 mutation has been used extensively to study the relationship between structure and function of murine class II major histocompatibility molecules. I-Abm12 differs from I-Ab by three amino acid replacements in the A beta chain, and the proposed structural model of the I-Abm12 molecule places these three amino acid substitutions along one of the alpha-helices where they may affect both antigen and TCR binding. Two of the substitutions, Ile----Phe67 and Thr----Lys71, are thought to point into the binding site, whereas the third substitution, Arg----Gln70, is thought to point up and hence, be available for binding to the TCR. These predicted orientations are consistent with serologic analysis of the bm12 molecule, which demonstrates that residue 70 is uniquely accessible to mAbs distinguishing I-Ab from I-Abm12. In this study we have determined the influence of each of these amino acid substitutions on the ability of the resulting molecules to stimulate a panel of I-Abm12 (allo) reactive T cell hybridomas. Our experiments indicate that reversion of the amino acid at position 70 from Gln (I-Abm12) to Arg (I-Ab) interferes with allorecognition by 33 of 35 I-Abm12-reactive hybridomas. On the other hand, many hybrids can tolerate amino acid substitutions at positions 67 or 71. Single amino acid substitutions at position 67, 70, or 71 are recognized by only a minority of I-Abm12-specific hybrids and usually the reactivity is greatly diminished. These data are most consistent with the idea that the amino acid at position 70 directly interacts with the TCR during allorecognition. The additional effects of residues 67 and 71 are consistent with a contribution by bound peptide to the allorecognition process.

The role of polymorphic I-Ak beta chain residues in presentation of a peptide from myelin basic protein

Proteins encoded by genes in the MHC are highly polymorphic. For class II proteins the highest level of polymorphism is found in distinct regions of variability, notably in the membrane-distal domains. To investigate the role of such residues in antigen presentation, we have tested cells transfected with wild-type or mutant I-Ak beta chains for their ability to present the NH2-terminal peptide of myelin basic protein to a panel of T cell clones. We were unable to detect a gross effect on peptide binding, in that all of the mutant cell lines presented antigen to at least one of the cloned T cells. However, the results imply that the more NH2-terminal residues, particularly 12 and 14, are involved in peptide interactions. Mutations at these residues presented antigen only at high antigen concentrations. Furthermore, residues of the more COOH-terminal regions appear to determine TCR interactions. Mutations in the predicted alpha-helical regions of the beta chain affected antigen presentation without abolishing peptide binding.

A hypothetical model of the foreign antigen binding site of class II histocompatibility molecules

Class II and class I histocompatibility molecules allow T cells to recognize 'processed' polypeptide antigens. The two polypeptide chains of class II molecules, alpha and beta, are each composed of two domains (for review see ref. 6); the N-terminal domains of each, alpha 1 and beta 1, are highly polymorphic and appear responsible for binding peptides at what appears to be a single site and for being recognized by MHC-restricted antigen-specific T cells. Recently, the three-dimensional structure of the foreign antigen binding site of a class I histocompatibility antigen has been described. Because a crystal structure of a class II molecule is not available, we have sought evidence in class II molecules for the structural features observed in the class I binding site by comparing the patterns of conserved and polymorphic residues of twenty-six class I and fifty-four class II amino acid sequences. The hypothetical class II foreign-antigen binding site we present is consistent with mutation experiments and provides a structural framework for proposing peptide binding models to help understand recent peptide binding data.

Functional polymorphism of each of the two HLA-DR beta chain loci demonstrated with antigen-specific DR3- and DRw52-restricted T cell clones

HLA-DR3- and HLA-DRw52-associated functional polymorphism was investigated with selected tetanus toxoid (TT)-specific T cell clones. We have shown earlier that HLA-DR antigens are encoded by two distinct loci, DR beta I and DR beta III. The alloantigenic determinant(s) defined by the serological HLA-DR3 specificity map to the former, while the supratypic HLA-DRw52 determinants map to DR beta III. Furthermore, we have recently recognized by DNA sequencing three alleles of HLA-DRw52 at locus DR beta III, referred to as 52 a, b, and c. Our objective was to correlate the pattern of T cell restriction with the gene products of individual DR beta chain loci and with the three newly described alleles of locus DR beta III. Among the selected T cell clones, 5 reacted exclusively when TT was presented by HLA-DR3+ APCs (TT-DR3-APC). In contrast, two T cell clones were stimulated by TT-DRw52-APC. More specifically, these two T cell clones (Clones 10 and 16) were stimulated by different subsets of TT-DRw52-APC. Clone 16 responded to some DR3 and TT-DRw6-APC, while clone 10 was stimulated by other TT-DR3 and TT-DRw6, and all TT-DR5-APC. This same pattern of DRw52 restriction was found in panel, as well as in family studies. Because this suggested a correlation with the pattern of DRw52 polymorphism observed earlier by DNA sequencing and oligonucleotide hybridization, the APC used in these experiments were typed for the 52 a, b, and c alleles of locus DR beta III by allele-specific oligonucleotide probes. This distribution overlapped exactly with the stimulation pattern defined by the T cell clones. Clone 16 responded to TT-52a-APC, clone 10 to TT-52b-APC, and both clones to a TT-52c-APC. The response of the T cell clones was inhibited differentially by mAbs to DR. Raising TT concentration, or increasing HLA-class II expression with INF-gamma both affected the magnitude of response of the TT-specific clones but did not modify their specificities. These results demonstrate that a restriction specificity can be attributed to the DR beta III locus and illustrate the functional relevance of the polymorphism observed at this locus. This is of special interest in view of the striking difference in the pattern of structural diversity among alleles of DR beta I and DR beta III.

Identification of an immunodominant region on the I-A beta chain using site-directed mutagenesis and DNA-mediated gene transfer

To identify which polymorphic residues determine the allospecific antibody binding sites on A beta polypeptides, mutant Ak beta genes were constructed encoding single or multiple amino acids of the d allele at 14 polymorphic positions in the beta 1 domain. Cell lines expressing these genes were analyzed by quantitative immunofluorescence using 16 mAbs reactive to Ak beta or Ad beta. Substitution of d allele residues at positions 63 and 65-67 in the Ak beta polypeptide resulted in the loss of binding of all Ak beta-reactive antibodies and the gain of binding of most Ad beta-reactive antibodies. Two Ad beta-reactive mAbs bound to the mutant Ak beta polypeptide containing d allele-characteristic residue at position 40. In contrast, substitution of the other polymorphic residues in the NH2-terminal and COOH-terminal regions of the beta 1 domain did not alter antibody binding.

Structural mutation affecting intracellular transport and cell surface expression of murine class II molecules

We have selected Ia variants from the Ia+ (H-2d) M12.4.1 B cell lymphoma that are negative on the cell surface for one or both Ia isotypes. The molecular analysis of two such independently selected cell lines, M12.A2 and M12.C3, is reported here. This analysis revealed that the genes encoding Ad beta (M12.A2) and Ed beta (M12.C3) contained identical single-nucleotide transitions that resulted in the substitution of Ser (mutant) for Asn (wild-type) at residue 82/83 of the extracellular NH2-terminal (membrane distal) beta 1 domain. This conservative substitution caused a cytoplasmic accumulation of I-A or I-E molecules in the respective cell line although predicted secondary-structure analysis suggests a minimal effect on protein conformation. Thus, the mutation appears to have either created a negative signal that stops transport or eliminated a positive signal that is required for transport and targeting to the cell surface.

DNA sequence analysis of I-Ak beta mutants reveals serologically immunodominant region

We have produced a series of in vitro serologically selected cell lines that express mutant I-Ak molecules. In this report we describe the DNA sequence analysis of the Ak beta gene of four cell lines that express serologically altered Ak beta polypeptides in association with wild-type Ak alpha polypeptides. Each of the major serologic epitopes on the Ak beta polypeptide has been altered in one or more of the four mutants. In addition, the four mutants exhibit a broad spectrum of functional defects when used to stimulate a panel of T hybridomas of various specificities. The DNA sequence analysis revealed that each mutant had sustained a single nucleotide substitution resulting in a single amino acid substitution. All four independent substitutions occurred within or near the third of the four variable regions defined in the beta 1 domain of the A beta polypeptide by allelic comparisons. These data strongly suggest that the third variable region is the major determinant of alloantigenicity on the Ak beta polypeptide.

Allele-specific control of Ia molecule surface expression and conformation: implications for a general model of Ia structure-function relationships

Sequence polymorphism of class II major histocompatibility complex-encoded molecules (Ia) not only accounts for the allelic variability in Ia structure relevant to T-lymphocyte responses but also seems to result in differential quantitative expression of particular Ia heterodimers. The contributions of different allelically variable regions of Ia molecules to both of these processes were analyzed by transfection of L cells with various A beta and A alpha gene pairs. The results show that, with regard to quantitative and qualitative aspects of Ia expression, the polymorphisms in the A beta chain segregate into two groups. Those in the NH2-terminal half of A beta 1 have a consistent role in controlling beta-alpha chain interactions, efficiency of dimer expression, and Ia conformation and probably are in the interior of the Ia molecule at the site of beta-alpha domain interaction. Polymorphisms in the COOH-terminal half of A beta 1 contribute to those structures that directly interact with antibodies, antigen, and/or T-cell receptors, consistent with their presence on the surface of the Ia heterodimer. This analysis provides a model for understanding both overall class II molecular structure and the relationship between this structure and immune recognition. It also suggests an explanation for the evolution of certain features of class II genes.

Expression of human class II major histocompatibility complex antigens using retrovirus vectors

Retrovirus vectors [direct orientation (DO) vectors] that permit the simultaneous expression of an inserted protein-coding sequence and a dominant-acting selectable marker have been constructed. In these vectors, an internal simian virus 40 or human metallothionein promoter sequence serves to drive the expression of the bacterial neomycin phosphotransferase or guanine-xanthine phosphoribosyltransferase genes, whereas the viral long terminal repeat sequences are utilized to promote expression of inserted sequences. In some of the vectors, the viral 5' splice site, normally used in the biogenesis of the subgenomic env-encoding mRNA, has been eliminated. These vectors yield high transient and stable titers of virus after transfection of viral packaging cell lines, show little or no depression of virus titer with a variety of inserts, and faithfully transmit recombinant proviral sequences to recipient cells. To characterize the expression potential of these vectors, a variety of inserts encoding the alpha and beta subunits of the human major histocompatibility complex class II antigen HLA-DR have been introduced into these vectors. NIH 3T3 cells infected by viruses containing HLA-DR alpha or beta cDNAs express these proteins as shown by immunoprecipitation of metabolically labeled extracts. In addition, through the sequential infection of cells with retrovirus constructions expressing two different selectable markers, both subunits of the class II antigen have been introduced into NIH 3T3 cells. Such infected cells express HLA-DR molecules at the cell surface.

A single base mutation in an I-A alpha-chain gene alters T-cell recognition

The interaction between the clonally selected T-cell antigen receptor, antigen, and Ia molecule is poorly understood at the molecular level. A cell line bearing an altered I-Ak alpha-chain (Ak alpha) molecule has been examined in order to provide more information about the relationship between Ia structure and function. The cell line, 3J9, was derived from the TA3 B-cell hybridoma through a series of negative and positive immunoselection steps. The 3J9 mutant lacked the binding site recognized by the Ak alpha-specific monoclonal antibody 39J and failed to present antigen to two T-cell hybridomas out of a large panel of I-Ak-restricted T-cell hybridomas examined. Sequence analysis of the mutant Ak alpha gene showed a single base transition (G----A) that resulted in a glutamic acid to lysine substitution at amino acid 75 of the alpha 1 domain. This mutation confirms the importance of amino acid 75 in the expression of the Ia.19 epitope, demonstrates the involvement of this region in the presentation of antigen to specific T cells, and provides a further example of the multiple functional domains on the Ia molecule that are involved in antigen presentation.

T cell clones specific for an amphipathic alpha-helical region of sperm whale myoglobin show differing fine specificities for synthetic peptides. A multiview/single structure interpretation of immunodominance

The T cell response to sperm whale myoglobin in the H-2d haplotype has been shown to be largely focused on a limited region around glutamic acid 109 recognized in association with I-Ad. T cell clones 9.27 and 1.2 have been previously (4, 5) shown to reflect this specificity and MHC restriction. In this study we have used a panel of synthetic peptides from the region 102-118 of myoglobin to characterize the specificities of these representative clones. The segment from 106-118 was found to represent a consensus region for recognition by both clones. However, we saw significant differences between clones in the hierarchy of responsiveness to peptides within the panel. In as much as the peptide and the I-Ad molecule remain constant, these differences derive from differences in how each T cell receptor interacts with the antigen. This peptide segment is an amphipathic alpha helix in native myoglobin, meaning that one side is hydrophobic and the other hydrophilic. It is one of the prototype cases that led us to find that amphipathic helices constitute the majority of immunodominant sites recognized by helper T cells (1). It is likely that the peptide will refold into an amphipathic helix stabilized by the interface at the surface of the presenting cell. When such secondary conformation is considered, these data are consistent with a model of multiple T cell specificities arising from multiple views of a single antigen conformation at a single Ia-binding site and do not require postulation of multiple conformations or binding sites. Additionally, the finding of distinct specificities suggests that the immunodominance of this site depends not on the dominance of a single clone, but on the focusing of a polyclonal response on a single region of the molecule in association with I-Ad. The immunodominance of this particular region of the protein may thus depend on intrinsic features of the site, such as potential to form an amphipathic helix, as well as extrinsic factors such as binding properties of the I-A molecule.

Cell surface and cell cycle analysis of metal-induced murine T cell proliferation

The heavy metal cations Pb2+, Ni2+ and Zn2+ have previously been shown to induce T cell proliferation which required the presence of both T cells and Ia+ cells at the initiation of culture. This work has examined the ability of these metals to induce cell cycle entry as determined by acridine orange cell cycle analysis. Cell surface phenotype analysis, performed on splenocytes stimulated with optimum metal concentrations (100 microM), indicated that in vitro T cell recovery (growth and/or longevity) was enhanced by Pb2+, Ni2+, and Zn2+. Furthermore, simultaneous examination of cell surface phenotype and cell cycle progression (propidium iodide) indicated that the predominant cell type proliferating in response to these metals was Thy-1.2+. The metals differentially induced L3T4+ and Lyt-2+ cells to enter the cell cycle. The ability of various monoclonal antibodies to modulate metal-induced proliferation was examined. Anti-L3T4a, anti-I-A and anti-I-E blocked metal-induced proliferation. Anti-Lyt-2 only partially inhibited whereas anti-Lyt-1 was stimulatory. These results suggest that recognition of major histocompatibility complex-encoded class II molecules is required for the induction of proliferation by these metals (similar to the autologous mixed lymphocyte response).