Immune response gene control of determinant selection. I. Intramolecular mapping of the immunogenic sites on insulin recognized by guinea pig T and B cells

Insights in regulated bioanalysis of human insulin and insulin analogs by immunoanalytical methods

Despite the long and illustrious history of insulin and insulin analogs as important biotherapeutics, the regulated bioanalysis (in this article, regulated bioanalysis refers to the formalized process for generating bioanalytical data to support pharmacokinetic and toxicokinetic assessments intended for development of insulin and insulin analogs as biotherapeutics, as opposed to the analytical process used for measuring insulin as a biomarker) of these peptides remains a challenging endeavor for a number of reasons. Paramount is the fact that the therapeutic concentrations are often low in serum/plasma and not too dissimilar from the endogenous level, particularly in patients with insulin resistance, such as Type 2 diabetes mellitus. Accordingly, this perspective was written to provide helpful background information for the design and conduct of immunoassays to support regulated bioanalysis of insulin and insulin analogs. Specifically, it highlights the technical challenges for determination of insulin and insulin analogs by immunoanalytical methods that are intended to support evaluations of pharmacokinetics and toxicokinetics. In a broader sense, this perspective describes the general bioanalytical issues that are common to regulated bioanalysis of peptides and articulates some of the bioanalytical differences between conventional monoclonal antibodies and peptide therapeutics.

Hindering auxiliary anchors are potent modulators of peptide binding and selection by I-Ak class II molecules

Selection of particular antigen-derived peptides by class II MHC molecules determines the population of complexes represented on the antigen-presenting cell surface and available for T cell receptor engagement. This discriminating selection process results from unique interactions between the spectrum of peptides generated during antigen processing and the MHC molecules. Here, we examined the selection of peptides by the class II MHC, I-A(k). Our results indicate that although peptide primary anchors are key in MHC binding, auxiliary anchors are a powerful regulatory component in the selection of peptides by I-A(k). Study of the segments surrounding the dominant hen egg white lysozome(48-61) epitope demonstrates that auxiliary anchors also are involved in determining the binding register of I-A(k) along an extended peptide. In addition, we found that unique combinations of auxiliary anchors can act in concert to modulate the binding of peptides to I-A(k).

Alanine scanning mutants of rat proinsulin I show functional diversity of anti-insulin monoclonal antibodies

In contrast to autoantibodies that are functionally silenced or deleted, IgG Abs that react with autologous insulin routinely follow hormone administration and arise spontaneously in autoimmune (type I) diabetes mellitus. To understand Ab interactions with autologous insulin, rat proinsulin I and 32 alanine substituted analogues were expressed as fusion proteins and used to examine 16 anti-insulin mAb in ELISA. The results identify several amino acid residues that contribute to binding by a large majority (>75%) of mAb, although no single residue is uniformly required for binding by all mAb. Replacements at charged or polar residues on the insulin surface including A4 (Asp), A5 (Gln), A9 (Ser) A12 (Ser), A17 (Gln), A18 (Asn), B13 (Glu), and B21 (Glu) consistently decreased mAb binding. Single alanine substitutions at positions A16 (Leu), A11 (Cys), B8 (Gly), and B15 (Leu) that are predicted to alter the core structure or chain folding vary widely in their impact on Ab binding. mAb that bind insulin preferentially on solid phase (i.e., ELISA) are highly sensitive to replacement of single residues, and substitutions that alter conformation abolish binding. In contrast, high affinity mAb that bind insulin in solution are relatively insensitive to substitutions at single residues, and they maintain binding to all mutants, including those with disrupted conformation. For such high affinity mAb, replacement of long hydrophobic side chains can augment binding, suggesting mAb interactions with insulin include an induced fit. Thus, the ability of insulin to function as a "molten globule" may contribute to the diversity and autoreactivity of the anti-insulin repertoire.

Mechanisms and consequences of peptide selection by the I-Ak class II molecule

Important quantitative parameters can be utilized to define the selection and the immunogenicity of protein antigens precisely at a biochemical and a cellular level. Here we describe a naturally processed family of peptides comprising the dominant hen egg white lysozyme epitope, its major contribution to surface I-Ak molecules, the primary and auxiliary peptide anchors involved in its selection, and its display of T-cell receptor contacts. In addition, we explore the importance of the processing events that lead to the generation of residues flanking the minimal core epitope, the quantification of T-cell responses directed toward the epitope, and the ability of the dominant epitope to form two unique conformations within the binding groove. Lastly, we address the relationship between this dominant and a minor lysozyme epitope.

Identification of an autologous insulin B chain peptide as a target antigen for H-2Kb-restricted cytotoxic T lymphocytes

We have examined the CD8+ peripheral T cell repertoire of C57BL/6 (H- 2b) mice for cytotoxic T lymphocyte (CTL) reactivities to insulin, using in vitro immunization with a chymotryptic digest of reduced bovine insulin. The results presented in this study demonstrate that potentially autoreactive H-2Kb-restricted cytotoxic T cells specific for an autologous insulin B chain peptide are present in the preimmune splenic T cell repertoire. The immunogenic peptide comprises residues 7- 15 from the insulin B chain and has features in common with naturally processed Kb-restricted peptides identified by others. The minimal peptide sequence recognized by these cytotoxic T cells is 10-15, which is highly conserved in mammalian species and constitutes a self-peptide in mice. The presence of class I major histocompatibility complex- restricted CTLs with potentially autoreactive specificities in preimmune animals raises the possibility of a role for such cells in autoimmune disease states. Possible mechanisms for the in vivo expansion of insulin peptide-specific CTLs are discussed.

Definition of immunogenic determinants of the human papillomavirus type 16 nucleoprotein E7

Specific T lymphocyte lines and T cell clones were established from peripheral blood mononuclear cells of asymptomatic seropositive individuals employing synthetic peptides which correspond to the sequence of the human papillomavirus (HPV) type 16 transforming protein E7. Specificity analysis of T cells as determined by means of [3H] thymidine incorporation after stimulation with individual peptides revealed three immunogenic determinants of E7 that are recognised in association with at least two different HLA haplotypes. One N-terminal region (aminoacids 5-18) was recognised by one T cell line. T cell clones and the corresponding T cell line established from another donor responded to a different N-terminal (17-38) and to a C-terminal region (69-86). The N-terminal sequence 5-18 and the C-terminal determinant contain a periodicity of hydrophilic and hydrophobic residues that have been found in many T cell epitopes. Phenotypic characterisation of T cell clones by indirect immunofluorescence revealed that the T cell clones expressed the CD4 surface glycoprotein suggesting that the specific E7 determinants were recognised in association with major histocompatibility complex (MHC) class II molecules. With regard to functional properties, at least three T cell clones exhibited specific cytotoxic activity towards autologous B lymphocytes transformed by Epstein-Barr virus in the presence of the relevant HPV16 E7 peptides. The implications of these results regarding the development of vaccination strategies and host-virus interaction are discussed.

Comoviruses and enteroviruses share a T cell epitope

An in vitro murine T cell proliferation assay was used to determine whether an antigenic epitope(s) recognized by enterovirus-immune T cells is held in common between plant comoviruses and human enteroviruses. Splenocytes isolated from C3H/HeJ mice infected with coxsackievirus B3 (CVB3) proliferated in vitro not only against a variety of enterovirus (CVB2, CVB3, CVB6, CVA16, PV1) antigens, but against comovirus (CPMV, BPMV) antigens as well. Splenocytes from mice inoculated with bean pod mottle virus (BPMV) also proliferated in response to comoviral and enteroviral antigens in vitro. However, if the viral inocula were highly purified prior to inoculation, then the splenocyte response was generated only against the group used to inoculate, suggesting that the epitope shared between the comoviruses and the enteroviruses resided in the nonstructural region. B (nonstructural) and M (structural) genomic segments of CPMV were translated in rabbit reticulocyte lysates and used as in vitro antigens. Splenocytes from mice inoculated with live CVB3 proliferated in response to the B-RNA-encoded but not the M-RNA-encoded polypeptides, confirming the nonstructural coding region location of the common epitope. Comparison of predicted amino acid sequences in the nonstructural coding regions of the comoviruses and picornaviruses suggested a potentially immunogenic linear epitope in protein 2C. The consensus peptide LEEKGI was synthezized and shown to be immunogenic for both BPMV- and CVB3-immune splenocytes.

Chemical and functional analysis of MHC class II-restricted T cell epitopes

Various aspects of antigen degradation and presentation are reviewed, in particular with respect to fragmentation of native vs. denatured proteins, different enzymatic machinery present in different cells and individuals, characterization of epitopes and their persistence on antigen-presenting cells as well as their capacity to interact with different MHC class II molecules. Finally, the structure of antigenic peptides is discussed.

Immunogenicity of B chain in insulin responder and nonresponder mice

The potential immunogenicity of insulin B chain in beef insulin low-responder H-2k,a and high-responder H-2b,d mice was examined using lymph node proliferation assays. Oxidized B chain was immunogenic in H-2k,a, but not H-2b,d, mice. The T cell population recognized a determinant in OX-B chain associated with I-Ak. These cells did not respond to intact insulin, suggesting that the B chain determinant was not available to I-Ak during immunologic processing of insulin. Responses were observed in H-2k and H-2d, but not H-2b, after immunization with reduced and carboxyamidomethylated-insulin which contains equimolar A chain and B chain. These responses were I-A-restricted and heterogeneous, with reactivity to A chain and B chain determinants. In each case, little or no cross-reactivity was observed between RCAM-insulin and intact insulin. Furthermore, T cell populations induced in H-2k mice selectively recognized OX-B chain or RCAM-B chain, which differ in chemical modification of the thiols of Cys B7 and Cys B19. Similarly, RCAM-BINS-immune T cells from H-2d did not react to OX-B chain. These results indicate that derivatization of the cysteine thiols, through disulfide bonds, oxidation, or carboxyamidomethylation, radically affects T cell recognition of insulin B chain.

Characterization of agretopes and epitopes involved in the presentation of beef insulin to T cells

Beef insulin-specific I-Ad-restricted T cell hybridomas were derived from the fusion of antigen-primed (BALB/c X B6)F1 T cells with BW5147 thymoma. Specificity analysis revealed that the A-chain loop region is involved in antigen recognition. Hybridoma A20.2.15 is specific for beef insulin and cross-reacted with sheep insulin, but not with pork insulin. Using synthetic peptides we showed that the A-chain loop containing peptide A1-A14 jointed to the B7-B15 peptide by a disulfide bond can activate this hybridoma. Fragments generated by enzyme digest further suggest that the peptide recognized on beef insulin appears to involve A-chain loop residues A5-A12 and B-chain residues B7-B13 that are linked by the A7-B7 disulfide bridge. We found that beef insulin needs to be processed prior to T cell activation. Glutaraldehyde fixation and chloroquine treatment of presenting cells abolished their capacity to present insulin. Beef insulin denatured by pH changes cannot activate, thus suggesting that simple denaturation is not sufficient for presentation by antigen presenting cells. Finally, the agretope on beef insulin is comprised of two functional regions B7-B13 on the B chain and the A-chain loop in the A-chain, while residues A8 and A10 are probably involved in interaction with the T cell receptor.

Competitor analogs for defined T cell antigens: peptides incorporating a putative binding motif and polyproline or polyglycine spacers

We describe a new approach for modeling antigenic peptides recognized by T cells. Peptide A24 170-182 can compete with other antigenic peptides that are recognized by H-2kd-restricted cytolytic T cells, presumably by binding to the Kd molecule. By comparing substituted A24 peptides as competitors in a functional competition assay, the A24 residues Tyr-171, Thr-178, and Leu-179 were identified as possible contact residues for Kd. A highly active competitor peptide analog was synthesized in which Tyr was separated from the Thr-Leu pair by a pentaproline spacer. The choice of proline allowed the prediction of a probable conformation for the analog when bound to the Kd molecule. The simplest conformation of the A24 peptide that allows the same spacing and orientation of the motif as in the analog would be a nearly extended polypeptide chain incorporating a single 3(10) helical turn or similar structural kink.

Analysis of anti-insulin antibodies using time-resolved fluoroimmunoassay

This paper describes the analysis of both monoclonal and polyclonal murine insulin antibodies with time-resolved fluoroimmunoassay (TRFIA). This assay is based on the binding of antiserum by coated isotype-specific capture antibodies and the detection of insulin binding using europium-labeled human insulin (HI-Eu). This reagent was shown to have a detection limit of 10(-16) mol. Among various applications this technique permitted an isotype-specific determination of the affinity distribution of polyclonal antibodies from individual mice.

Intradomain H-2Kd/Dd recombinants define the same regions as crucial for recognition by alloreactive or major histocompatibility complex-restricted cytolytic T cells

To identify residues on class I major histocompatibility complex (MHC) antigens that are important in T cell recognition, we analyzed a series of 11 intradomain recombinant mouse MHC (H-2) molecules in which N-terminal H-2Kd segments of varying lengths are followed by H-2Dd segments. Lysis of L cell transfectant target cells by a series of alloreactive cytolytic T cell (CTL) clones specific for Kd or for Dd revealed several regions that contain residues critical for specific recognition. These residues map within the presumed antigen-binding site of the MHC molecule. Of particular interest was the finding that the two regions identified as important for Kd allorecognition match those that influence the recognition of synthetic peptide antigens by Kd-restricted CTL.

Antigenicity of the carboxyl terminus of insulin: isolation of human insulin-specific monoclonal antibodies

Monoclonal technology was used to isolate antibodies binding the B30 (carboxy) terminal residue in the polyclonal-provoked immune response to human insulin. Although both spleen and lymph node cell fusions were carried out, only the latter were successful in isolating monoclonal antibodies that bound the carboxy terminal of human insulin. The binding of such antibodies was abolished or diminished by substitutions of the B30 residue. Studies with insulin species variants showed that the molecular binding between antibody and insulin may be critically determined by a subresidue feature, e.g. presence or absence of a single methyl group, as shown by the binding of the monoclonal antibody D10 to human insulin (threonine at B30) but not to rabbit insulin (serine at B30). Such studies are of interest in the study of the molecular basis of antibody-antigen interaction.

Antibody response of three different strains of mice to alpha s1-casein analyzed by using proteolytic and synthetic peptides

The location of immunodominant regions on bovine alpha s1-casein (alpha s1-CN) was compared among three strains of mice (BALB/c, C3H/He and C57BL/6). Anti-alpha s1-CN antisera were separated 5 weeks after the first immunization. Thirty-seven kinds of peptides were obtained from alpha s1-CN by proteolysis, and the segmental 19- to 20-residue peptides overlapping with their adjacent neighbors by 5 residues were synthesized over the entire polypeptide chain of alpha s1-CN. The ability of the anti-alpha s1-CN antibody to bind peptides was tested by an enzyme-linked immunosorbent assay, in which the peptides were adsorbed to the solid phase. All the strains responded highly to alpha s1-CN. The immunodominant antigenic regions of alpha s1-CN were not common to the three strains of mice, the H-2 haplotypes being different from each other (BALB/c, regions 1, 3, 5, 6 and 7; C3H/He, regions 2, 3, 5, 6, 6', 7 and 7'; and C57BL/6, regions 2, 4, 5, 6 and 7).

Analysis of host-virus interactions in AIDS with anti-gp120 T cell clones: effect of HIV sequence variation and a mechanism for CD4+ cell depletion

The primary human T cell response to HIV was analyzed by isolating from seronegative donors T cell clones specific for HIV gp120. T cell epitopes restricted by different MHC elements were identified within gp120, and synthetic peptides were used to address the fundamental problem of how HIV sequence variability affects T cell recognition. Even one conservative substitution can drastically reduce recognition; thus the interaction of gp120 epitopes with T cell receptors and MHC is precise and poorly crossreactive. Importantly, a subset of CD4+ gp120-specific clones manifest cytolytic activity and lyse uninfected autologous CD4+Ia+ T cells in the presence of gp120 in a process that is strictly dependent upon CD4-mediated uptake of gp120 by T cells. Assuming gp120 is shed from HIV-infected cells in vivo, this novel CD4-dependent autocytolytic mechanism may contribute to the profound depletion of CD4+ cells in AIDS.

Synthetic peptides as antigens and competitors in recognition by H-2-restricted cytolytic T cells specific for HLA

The specificity of peptide recognition by a number of Kd-restricted CTL clones specific for HLA-CW3 or HLA-A24 was investigated. The CTL clones were derived from DBA/2 (H-2d) mice immunized with syngeneic P815 mouse cells transfected with genes encoding HLA-CW3 or HLA-A24 class I molecules. We had previously shown that CTL clones that lysed P815-CW3 transfectant target cells could lyse P815 (HLA-) target cells incubated with synthetic CW3 peptides corresponding to the COOH-terminal end of the alpha 2 domain. In the present study, we found that Kd-restricted CTL clones that lysed P815-A24 transfectant target cells recognized a synthetic peptide from the same region (residues 170-182) of the A24 molecule. CW3 and A24 differ by only one amino acid within this region. Recognition of CW3 or A24 peptides corresponded exactly with lysis of P815-HLA transfectants both for clones that mutually exclusively lysed CW3 or A24 transfectant target cells and for CW3/A24 crossreactive CTL clones. The latter CTL clones that lysed both CW3 and A24 transfectant target cells showed a clear preference for the peptide corresponding to the immunizing HLA allele. The homologous CW3 and A24 peptides could compete with each other for recognition, in contrast to a peptide from the same region of HLA-B7. Peptides from the corresponding region of the endogenous Kd and Dd/Ld molecules could also inhibit recognition of CW3 and A24 peptides. Competition with peptides apparently occurred at the level of the target cell. These results are consistent with a model whereby MHC class I molecules position protein fragments or peptides for specific recognition by T cells.

Characteristics of blood cells responding to insulin in children with insulin-dependent diabetes

Cells responding to insulin in peripheral blood of children with insulin-dependent diabetes mellitus were characterized using monoclonal antibodies and autoradiography. The results indicate that the cells responding to insulin were T cells mainly of the helper (CD4+) phenotype and that they required adherent cells of the monocyte/macrophage lineage for proliferation to occur.

Limiting dilution comparison of the repertoires of high and low responder MHC-restricted T cells

To approach the mechanism that determines Ir gene-controlled high or low responsiveness to whole proteins, such as sperm whale myoglobin (SWMb), we compared the repertoires of high and low responder haplotype-restricted T cells for different myoglobin epitopes by limiting dilution frequency analysis. Poisson analysis was performed using long-term limiting dilution cell lines of (B10.BR [low] X B10.D2[high])F1 T cells maintained on high or low responder APCs. The cell lines were tested with SWMb peptides and fragments for T cell repertoire fine specificities and Ia restrictions. The frequency of SWMb-specific F1 T cells responsive on B10.BR (H-2k) APCs was 2.5-3.6-fold lower than on B10.D2 (H-2d) APCs. Strikingly, all of the H-2k-restricted T cells used I-Ek as a restriction element, whereas both I-Ad- and I-Ed-restricted T cells were found among the H-2d-restricted lines. The I-Ad-restricted T cells were dominant, and the majority was specific for the synthetic peptide 102-118. T cells specific for peptide 132-146, dominant in association with I-Ed, were less frequent. However, no detectable H-2k-restricted T cells were specific for either of these peptides, but instead they were specific for fragment 1-55 or peptide 59-80. Fragment 1-55 also stimulated a similar number of H-2d-restricted T cells. Therefore, the low response of F1 T cells on H-2k-presenting cells may be due to the failure to see myoglobin plus I-Ak, in particular the immunodominant site around Glu 109, in contrast to the dominant response of high responder mice (both H-2d and H-2s) focused on the I-A molecule and the site around residue Glu 109. The I-E- low responder B10 strain also failed to respond to peptide 102-118, supporting the idea that the low responder status results from a limited repertoire lacking response to 102-118 plus I-A. In those strains that respond to the immunodominant site 102-118, the frequency of T cells in the repertoire specific for this site was always considerably greater than that for other sites. These results suggest that there is an important difference between immunodominant epitopes and minor epitopes and that Ir gene-controlled low responsiveness to a natural whole protein may be due primarily to the failure to respond to a single immunodominant site, even though a number of other epitopes can be recognized.

Spectrotypic analysis of antibodies to insulin A and B chains

Antibodies produced by immunization with native insulin were analyzed by isoelectric focusing for binding to isolated A and B chains. Antibodies to isolated A chain of beef insulin were found to have restricted spectrotypes and were seen after immunization with either beef or human insulin. Hyperimmunization with beef insulin, but not human, increased the heterogeneity of anti-A chain antibodies. Antibodies to isolated B chains were also electrophoretically restricted but showed less heterogeneity after hyperimmunization than anti-A chain responses. When binding to chains was carried out in the presence of excess cold insulin, anti-B chain spectrotypes were inhibited by the native molecule. In contrast, only a portion of anti-A chain spectrotypes were inhibited by native insulin, suggesting that these clonotypes are directed at epitopes not present on the surface of the molecule. These data indicate that the anti-insulin repertoire includes antibodies that can bind isolated chains as well as the native molecule. Some of the determinants on isolated A chain are not available on intact insulin and may arise from antigen catabolism.

Human T-helper lymphocytes in myasthenia gravis recognize the nicotinic receptor alpha subunit

Myasthenia gravis is a human disease caused by an autoimmune response against the nicotinic acetylcholine receptor (AcChoR). Since the molecular structure of AcChoR is well known, myasthenia gravis is an excellent system for studying the recognition of a complex membrane antigen in the human immune system. Human T-helper (TH) cell lines reactive to the AcChoR were isolated from four myasthenic patients by selection with native AcChoR from Torpedo californica. The selected TH cells could efficiently recognize native and fully denatured AcChoR. The vast majority of the TH-stimulating AcChoR epitopes were located on the denatured alpha subunit of AcChoR. Antibody competition experiments using a panel of rat anti-AcChoR monoclonal antibodies showed that 39-45% of the autoantibodies present in the sera of these same patients bound to the conformation-sensitive "main immunogenic region" (MIR), also located on the alpha subunit. However, AcChoR-induced stimulation of the T cells could not be inhibited with up to 20-fold molar excess of different rat anti-MIR monoclonal antibodies. These results suggest that the Torpedo AcChoR alpha subunit contains conformation-insensitive epitopes that play a role in the autosensitization of TH cells and that seem to be physically separated from the MIR. The specificity of the TH cell response may contribute to directing the B-cell response to other alpha-subunit determinants, such as the MIR itself.

The basis for the immunoregulatory role of macrophages and other accessory cells

Macrophages handle extracellular proteins and secrete diverse bioactive molecules and, therefore, influence the physiology of many tissues. They also have an important immunoregulatory role. The immune response to proteins involves the activation of the T helper subset of lymphocytes. The T helper cell is activated only when it interacts with the protein displayed on the surface of a macrophage or other accessory cell. This interaction involves restrictive proteins encoded in the major histocompatibility gene complex as well as growth-differentiating proteins.

Monoclonal antibody characterization of a unique immune response control locus between H-2S and D

The primary in vitro antibody response to the type 2 antigen, trinitrophenyl (TNP)-Ficoll, is controlled by two complementing loci in the H-2 region of the mouse major histocompatibility complex (MHC). High responder alleles at both loci are necessary for a high responder phenotype. Previous studies mapped one locus of control to the I-A subregion. In this report we demonstrate by recombinant analysis that the second locus of control is located between the H-2S and D regions. A comparison of responses in the B10.BAR6, B10.BAR10, and B10.BAR11 strains defined a locus controlling the response to TNP-Ficoll in a single haplotype, bounded on the left by the crossover event in the B10.BAR10 and on the right by the crossover event in the B10.BAR6 strain. A monoclonal antibody directed against this right-hand region of control has been produced (48.21.7) that blocks the response to TNP-Ficoll at the level of the antigen-presenting cell. The monoclonal antibody 48-21.7 is specific for the high responder b allele at the right-hand locus and did not inhibit responses to other protein antigens tested. The immune response to TNP-Ficoll was not inhibited by monoclonal antibodies that react with H-2Db or Qa-2 specificities, suggesting that the TNP-Ficoll response is controlled by a unique locus located between H-2S and D. Finally, 48-21.7 recognizes and precipitates a unique product of approximately 40,000 mol wt that is distinct from the H-2D region product recognized by the monoclonal antibody B22/249.

Insulin-specific T cell hybridomas derived from (H-2b x H-2k)F1 mice preferably employ F1-unique restriction elements for antigen recognition

T cell hybridomas of (B10 X B10.BR)F1 genotype with reactivity to bovine insulin (BI) were established to analyze the restriction and antigen fine specificity of (H-2b X H-2k)F1 T cells towards BI. Our data indicate a focusing of the response on two epitopes on the insulin molecule, the A chain loop determinant comprising amino acids A8 and A10, as well as the glutamic acid residue in position 4 of the A chain. Both were recognized either separately or in conjunction. Unexpectedly, the T cell hybridomas exhibited a marked preference for recognizing insulin in the context of F1-unique restriction elements of Ab alpha Ak beta type rather than parental high-responder I-Ab molecules. Analysis of the response of primed lymph node T cells of (B10 X B10.BR)F1 mice towards BI corroborated the finding of a preponderant corecognition of F1-unique I-A molecules.

A unidirectional carrier effect

A surprising unidirectional carrier effect has been observed in the antibody response to myoglobin-ferritin conjugate. This conjugate serves as a hapten-carrier complex for myoglobin-specific T cells to help ferritin-specific B cells make anti-ferritin antibodies, but it does not function for ferritin-specific T cells to help myoglobin-specific B cells to make anti-myoglobin. Therefore, myoglobin-ferritin does not bypass the Ir gene defect of low responders to myoglobin. In contrast, myoglobin-fowl gamma-globulin does induce anti-myoglobin antibodies in low responder mice and thus bypasses the Ir gene defect. Both complexes are covalently coupled. Since the myoglobin-ferritin conjugate serves for myoglobin-specific T cells to help myoglobin-specific B cells, the myoglobin in the conjugate is not altered in a way that would prevent recognition by myoglobin-specific B cells. Similarly, the conjugate serves for ferritin-specific helper T cells to help ferritin-specific B cells, so it can be recognized functionally by ferritin-specific T helper cells. Explanations such as unidirectional induction of or sensitivity to bystander help, or T-cell suppression, have been excluded. While the explanation for this unexpected observation is not yet certain, several possibilities are discussed to explain this novel phenomenon, which is believed to be the first example of such a unidirectional carrier effect between two proteins.

Hypothesis: the MHC-restricted T-cell receptor as a structure with two multistate allosteric combining sites

This paper presents a dual-recognition model of the T-cell receptor that has been constructed to account for the phenomenon of MHC restriction as well as the paradoxical ability of T-cells to be both multispecific and precisely specific at the same time. In our model the combining sites for antigen and MHC are not independent as in classical dual-recognition models, but interact with each other by an allosteric mechanism. We envision a flexible receptor with combining sites for antigen and MHC that are capable of existing in a multitude of distinct complementarity states. MHC and antigen molecules act as allosteric effectors such that one ligand perturbs the conformation and therefore the specificity of the site for the other ligand. An essential feature of the model is that different MHC determinants induce different conformations at the anti-antigen site. In this way the receptor acquires multiple specificities. Within a particular complementarity state, precise recognition results from the requirement that antigen and MHC exhibit positive cooperativity in their binding to the T-cell receptor. Positive cooperativity is also the basis for MHC restriction. Reaction mechanisms are presented which describe the requirement that antigen and MHC both induce conformational changes in order to generate high-affinity binding to either ligand. As a precedent for the multistate allosteric receptor model, we discuss the properties of allosteric enzymes, especially ribonucleotide reductase, whose properties are analogous to those we have postulated for the T-cell receptor. Also discussed is the possibility that molecules such as Ly2, L3T4 and the Mls antigen, which have been found to play a role in antigen recognition, function as affinity-enhancing allosteric effectors that interact with the constant portion of the T-cell receptor.

HLA phenotype and insulin antibody production

HLA phenotypes have been determined in 79 patients as part of a prospective study of factors governing the immune response to injected insulin. IgG insulin antibody levels 6 months after starting treatment with bovine insulin were significantly higher in patients bearing HLA-DR7 and this in conjunction with the lack of a similar pattern in the IgG response to Helix pomatia haemocyanin, suggests the presence of an immune response gene for insulin. The hyporesponsiveness of HLA-B8/DR3/C4AQ0 positive individuals is more likely to reflect a non-specific abnormality of immunity.

Analysis of the (H-2b X H-2k)F1-restricted response to insulin. Ab alpha Ak beta hybrid Ia molecules restrict the response towards the glutamic acid A4 epitope

The aim of these studies was to characterize the (H-2b X H-2k)F1-unique restriction element(s) responsible for presentation of bovine insulin (BI) to a long-term cultured T-cell line (BK-BI-1.2). (B10.BR X bm12)F1 spleen cells, which express a normal Ab alpha Ak beta molecule but a mutated Ak alpha Abm12 beta product on their cell surface, were perfectly able to act as BI-presenting cells. Antibody inhibition experiments with antibodies directed at I-Ak products revealed that monoclonal antibody 10-2.16, which reacts with the Ak beta polypeptide chain, abrogated BI-directed T-cell proliferation, whereas antibody H116-32.R5 with specificity for the Ak alpha chain was not inhibitory. These results identified the Ab alpha Ak beta complex as restriction structure. Recognition of BI in the context of the Ab alpha Ak beta molecule depended on the glutamic acid residue in position 4 of the A chain of bovine insulin. Twenty to twenty-five percent of the secondary proliferative response of (B10 X B10.BR)F1 lymph node T cells primed with BI in vivo was directed at the A4 determinant, suggesting that BK-BI-1.2 T blasts are representative of T-cell clones with measurable frequency. In (B10.BR X bm12)F1 mice, which lack a functional Ab alpha Ab beta restriction element, up to 80% of the proliferative response was dependent on the A4 epitope.

Polyglandular autoimmune syndromes

One of the basic caveats in endocrinology is that glandular abnormalities tend to occur together. Continued suspicion of other glandular hypofunction should be maintained in following patients with any type of endocrine gland hypofunction, since the risk of multiple glandular involvement is significant. Family members should be alerted to the high prevalence of endocrinopathies especially among first-degree relatives of patients with polyglandular autoimmune disease. Parameters such as antiorgan antibodies, although occasionally helpful, have not been shown to be consistently useful in predicting the future development of clinical organ-specific autoimmune disease. HLA typing remains a research tool at this time, as does evaluation of humoral and cell-mediated immunity.

Factors governing the human immune response to injected insulin

Seventy-nine patients were observed prospectively during their initial period of treatment with conventional bovine insulins. Insulin antibody levels 6 months after starting insulin therapy did not correlate with age, gender or beta cell function at onset of treatment. Patients who required soluble insulin in addition to isophane insulin developed higher levels of insulin antibody. Patients bearing the HLA-B8, DR3 and C4AQO alleles had lower levels of insulin antibody, whereas those bearing DR7 produced significantly higher levels. Other alleles at the C4A, C4B, C2, factor B or Gm loci did not appear to have a significant effect on insulin antibody production. The hyporesponsiveness of B8/DR3/ C4AQO -positive individuals probably reflects a non-specific abnormality of immunity whereas the enhanced responsiveness of those positive for DR7 suggests the presence of a specific immune response gene for insulin.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Serologic and functional characterization of a panel of antigen-presenting cell lines expressing mutant I-A class II molecules

An improved method is described for selecting mutant cells with an altered pattern of Ia antigenic determinants and antigen-presenting properties from an homogeneous population of functional antigen-presenting cells (APC). The APC line used, TA3, was a somatic cell hybrid obtained by fusing normal heterozygous H-2a/d B cells with a drug-marked variant of a BALB/c B lymphoma line. Two phenotypic groups of mutants were obtained by this method. Serologic analysis with a panel of anti-I-Ak monoclonal antibodies suggested that the change in the first group of mutants (type A mutants) involved the alteration of a portion of one epitope of the I-Ak molecule while in the second group of mutants (type B), an alteration of a different Ia epitope group had occurred. Functional studies using a panel of cloned antigen-specific and autoreactive T cell hybridomas demonstrated that the loss of a limited number of I-Ak determinants in the type A mutants correlated with the loss of some but not all I-Ak-encoded restriction elements, while the type B mutation(s) resulted in the ablation of all I-Ak-restricted APC functions tested. These mutations may occur in the region of the Ia molecule that interacts with the T cell receptor (the histope) or in a postulated region that interacts with antigen (the desetope). The finding that both type A and B mutations lead to loss in the capacity to be corecognized with many different antigens by I-Ak-restricted T cell hybridomas suggests that the Ia molecule may possess very few distinct histotopes and/or desetopes or that the tertiary structure of the Ia molecule is crucial in the formation of these sites. Alternatively, the mutations, particularly the type B mutations, may have led to the failure of expression of an entire alpha or beta chain.

In vitro lymphocyte proliferation response to therapeutic insulin components. Evidence for genetic control by the human major histocompatibility complex

Genes in the major histocompatibility complex of mice and guinea pigs control immunologic responsiveness to insulins from other animal species. In order to determine if similar genetic control exists in man, we have examined lymphocyte proliferation responses to components of therapeutic insulins by employing lymphocytes from diabetic patients that receive insulin. Distinct groups of individuals demonstrated positive lymphocyte proliferative responses to beef insulin, beef and pork insulin, beef proinsulin, pork proinsulin, and protamine. Lymphocytes from the patient population were typed for the HLA-A, B, C, and DR antigens. An increased frequency of certain HLA antigens was found in those individuals that responded to the following therapeutic insulin components: beef, HLA-DR4; beef and pork, HLA-DR3; beef proinsulin, HLA-BW4, CW2, CW5, DR2, and DR5; protamine, HLA-CW3, CW5, and DR7. The results demonstrate that the human immune system recognized the structural differences between human and beef and/or pork insulin. These differences are two amino acids in the A chain, alpha loop, of beef insulin and the single terminal amino acid, alanine, which is common to pork and beef insulins. Positive responses to both beef proinsulin and pork proinsulin demonstrated the capability of restricted recognition of more complex proteins represented by the C-peptide in these insulin preparations. Lymphocyte proliferative responses to protamine were also restricted, which suggests a genetic control to this antigen. The association of these responses with HLA alloantigens strongly suggests that genes within the human major histocompatibility complex control recognition and lymphocyte response to therapeutic insulin components.

Mapping epitopes on the insulin molecule using monoclonal antibodies

A panel of 18 monoclonal antibodies (mAb) delta to insulin have been prepared and used to begin to map antigenic determinants on the insulin molecule. All 18 mAb were of the IgG class, with 14 IgG1, 2 IgG2a and 2 IgG2b. The affinities of these mAb for their immunizing insulin ranged from 1 X 10(6) to 3 X 10(8) 1/M. The epitope recognized by three of the mAb, 1, 7 and 16 involves the three residues of the A chain, A 8-10, the so called A chain-loop determinant. This A chain loop is one of the most evolutionarily diverse regions of insulins from different species. Another mAb, 10, has been hypothesized to recognize a nearby epitope composed of the A chain residues, A4 and A8 and a B chain residue, B29, that are adjacent on the surface of the insulin molecule. Four of the mAb bind to synthetic B chain. The epitopes recognized by these 4 mAb and the last 10 mAb are unknown but the mAb are grouped according to their ability to bind to different species of insulin or proinsulin. The results of an 18 X 18 matrix analysis of pairs of mAb binding simultaneously to insulin indicate that, despite the finding that some mAb see similar antigenic sites on the insulin molecule, each of the mAb recognizes a unique site on the insulin molecule. Finally, a lower estimate of the number of possible antibodies made to insulin has been calculated to be greater than or equal to 115, a number only 10-fold lower than the lower limit of antibodies made to dinitrophenyl (DNP) or (4-hydroxy-5-iodo-3-nitrophenyl)acetyl (NIP), following hapten protein immunization.

Different repertoires of mouse T cells for bovine insulin presented by syngeneic and allogeneic cells

Splenic T cells were primed, after removal of alloreactive cells, to beef insulin on allogeneic antigen-presenting cells (APC). The fine specificity of in vitro secondary response was tested in combinations H-2b (responder) T cell-H-2k (nonresponder) APC, and vice versa, using separated chains of beef and pork insulin. The response in both combinations exhibited identical specificity patterns demonstrating that both responder and nonresponder APC could present the same array of insulin epitopes to allogeneic T cells. The determinants presented to allogeneic T cells include the A-chain loop epitope and the B-chain determinant(s) that were found to be immunogenic for H-2b and H-2d T cells, respectively, in the context of syngeneic major histocompatibility complex (HC) molecules. In addition, minor determinants were detected in the A chain outside the loop that are not immunogenic in syngeneic T cell-APC combinations. Inhibition of T cell proliferation with monoclonal antibodies has shown that class II MHC molecules of the nonresponder (Ak alpha Ak beta, Ek alpha Ek beta) as well as those of the responder APC (Ab alpha Ab beta) are equally capable of presenting virtually all insulin epitopes recognizable by T cells. The data, therefore, demonstrate that the selective recognition of different insulin epitopes observed in syngeneic or semisyngeneic T cell-APC combinations does not result from determinant selection at the level of APC.

IR gene regulation of the response to trinitrophenyl-polysaccharides. Two independent genes are required for antibody production

The primary in vitro antibody response to TNP-Ficoll was found to be under H-2-restricted Ir gene control. Strains B10(H-2b), B10.A(H-2a), and B10.S(9R) (H-2t4) were consistently low responders while strains D2.GD(H-2g2), B10.GD(H-2g2), and B10.S(H-2s) were high responders. The in vitro TNP-Ficoll response in congenic recombinant and F1 hybrid mice demonstrated the requirement for complementation of two independent Ir genes. One Ir gene mapped in or to the left of the I-A subregion with high responder alleles being s or d. The second Ir gene mapped to the right of the I-E subregion and required b or s alleles for complementation. These results were further supported by the ability to block the TNP-Ficoll response by appropriate anti-Ia serum pretreatment of the antigen-presenting macrophages. When a structurally different polysaccharide antigen TNP-dextran was used, an identical pattern of restriction was observed.

Antigen-presenting cells from nonresponder strain 2 guinea pigs are fully competent to present bovine insulin B chain to responder strain 13 T cells. Evidence against a determinant selection model and in favor of a clonal deletion model of immune response gene function

To test directly the determinant selection hypothesis of immune response gene function, we primed strain 13 T lymphocytes in vitro with allogeneic bovine insulin pulsed strain 2 macrophages. Strain 2 macrophages were found to be fully competent to present bovine insulin B chain to strain 13 T cells despite the fact that strain 2 guinea pigs are normally totally unresponsive to this antigen. These results are incompatible with a strict interpretation of the determinant selection hypothesis, which would have predicted that strain 2 macrophages would have been restricted to the presentation of A chain loop determinants. In addition, a comparison of the reactivity profiles of self-Ia- and allo-Ia-restricted strain 13 T cells to a series of synthetic B chain peptide fragments revealed that the allo-Ia-restricted populations could be activated by autologous guinea pig insulin. Taken together, these observations strongly suggest that the clonal deletion of self-reactive cells is likely to be I region restricted and that nonresponsiveness to any protein antigen may result from a restriction in the T cell repertoire that is generated during ontogeny by a clonal deletion mechanism of tolerance to self.

Corneal allograft rejection. The role of the major histocompatibility complex

The greater success of corneal transplantation compared to other organ transplants has led to the concept that the cornea is a site of "immunological privilege." Corneal cells possess the antigens of the major histocompatibility complex responsible for allograft rejection in other tissues (i.e., HLA antigens). The avascularity of the cornea accounts for the relative protection of the donor cornea from the immunological surveillance of the recipient. As the roles and functions of the major histocompatibility complex are unravelled, the mechanisms responsible for host sensitization, lymphocyte activation and allograft rejection are becoming better understood. In particular, the HLA-DR antigen in humans is believed to play an integral part in allograft rejection. Langerhans cells in human corneal epithelium have been shown to bear this antigen. Evidence suggests that these cells or similar HLA-DR-bearing cells in the cornea play a major role in corneal allograft rejection. In light of these advances in transplantation immunobiology, new methods of suppressing and possibly preventing allograft rejection in corneal transplantation are presented.