Probing Non-Covalent Interactions through Molecular Balances: A REG-IQA Study

The interaction energies of two series of molecular balances (1-X with X = H, Me, OMe, NMe2 and 2-Y with Y = H, CN, NO2, OMe, NMe2) designed to probe carbonyl…carbonyl interactions were analysed at the B3LYP/6-311++G(d,p)-D3 level of theory using the energy partitioning method of Interacting Quantum Atoms/Fragments (IQA/IQF). The partitioned energies are analysed by the Relative Energy Gradient (REG) method, which calculates the correlation between these energies and the total energy of a system, thereby explaining the role atoms have in the energetic behaviour of the total system. The traditional "back-of-the-envelope" open and closed conformations of molecular balances do not correspond to those of the lowest energy. Hence, more care needs to be taken when considering which geometries to use for comparison with the experiment. The REG-IQA method shows that the 1-H and 1-OMe balances behave differently to the 1-Me and 1-NMe2 balances because the latter show more prominent electrostatics between carbonyl groups and undergoes a larger dihedral rotation due to the bulkiness of the functional groups. For the 2-Y balance, REG-IQA shows the same behaviour across the series as the 1-H and 1-OMe balances. From an atomistic point of view, the formation of the closed conformer is favoured by polarisation and charge-transfer effects on the amide bond across all balances and is counterbalanced by a de-pyramidalisation of the amide nitrogen. Moreover, focusing on the oxygen of the amide carbonyl and the α-carbon of the remaining carbonyl group, electrostatics have a major role in the formation of the closed conformer, which goes against the well-known n-π* interaction orbital overlap concept. However, REG-IQF shows that exchange-correlation energies overtake electrostatics for all the 2-Y balances when working with fragments around the carbonyl groups, while they act on par with electrostatics for the 1-OMe and 1-NMe2. REG-IQF also shows that exchange-correlation energies in the 2-Y balance are correlated to the inductive electron-donating and -withdrawing trends on aromatic groups. We demonstrate that methods such as REG-IQA/IQF can help with the fine-tuning of molecular balances prior to the experiment and that the energies that govern the probed interactions are highly dependent on the atoms and functional groups involved.

Interacting Quantum Atoms and Multipolar Electrostatic Study of XH···π Interactions

The interaction energies of nine XH···π (X = C, N, and O) benzene-containing van der Waals complexes were analyzed, at the atomic and fragment levels, using QTAIM multipolar electrostatics and the energy partitioning method interacting quantum atoms/fragment (IQA/IQF). These descriptors were paired with the relative energy gradient method, which solidifies the connection between quantum mechanical properties and chemical interpretation. This combination provides a precise understanding, both qualitative and quantitative, of the nature of these interactions, which are ubiquitous in biochemical systems. The formation of the OH···π and NH···π systems is electrostatically driven, with the Qzz component of the quadrupole moment of the benzene carbons interacting with the charges of X and H in XH. There is the unexpectedly intramonomeric role of X-H (X = O, N) where its electrostatic energy helps the formation of the complex and its covalent energy thwarts it. However, the CH···π interaction is governed by exchange-correlation energies, thereby establishing a covalent character, as opposed to the literature's designation as a noncovalent interaction. Moreover, dispersion energy is relevant, statically and in absolute terms, but less relevant compared to other energy components in terms of the formation of the complex. Multipolar electrostatics are similar across all systems.

How to Compute Atomistic Insight in DFT Clusters: The REG-IQA Approach

The relative energy gradient (REG) method is paired with the topological energy partitioning method interacting quantum atoms (IQA), as REG-IQA, to provide detailed and unbiased knowledge on the intra- and interatomic interactions. REG operates on a sequence of geometries representing a dynamical change of a system. Its recent application to peptide hydrolysis of the human immunodeficiency virus-1 (HIV-1) protease (PDB code: 4HVP) has demonstrated its full potential in recovering reaction mechanisms and through-space electrostatic and exchange-correlation effects, making it a compelling tool for analyzing enzymatic reactions. In this study, the computational efficiency of the REG-IQA method for the 133-atom HIV-1 protease quantum mechanical system is analyzed in every detail and substantially improved by means of three different approaches. The first approach of smaller integration grids for IQA integrations reduces the computational overhead by about a factor of 3. The second approach uses the line-simplification Ramer-Douglas-Peucker (RDP) algorithm, which outputs the minimal number of geometries necessary for the REG-IQA analysis for a predetermined root mean squared error (RMSE) tolerance. This cuts the computational time of the whole REG analysis by a factor of 2 if an RMSE of 0.5 kJ/mol is considered. The third approach consists of a "biased" or "unbiased" selection of a specific subset of atoms of the whole initial quantum mechanical model wave-function, which results in more than a 10-fold speed-up per geometry for the IQA calculation, without deterioration of the outcome of the REG-IQA analysis. Finally, to show the capability of these approaches, the findings gathered from the HIV-1 protease system are also applied to a different system named haloalcohol dehalogenase (HheC). In summary, this study takes the REG-IQA method to a computationally feasible and highly accurate level, making it viable for the analysis of a multitude of enzymatic systems.

A Combined BET and IQA-REG Study of the Activation Energy of non-polar zw-type [3+2] Cycloaddition Reactions

A combined Bonding Evolution Theory (BET) and Interacting Quantum Atoms-Relative Energy Gradient (IQA-REG) study is carried out on a non-polar zw-type [3+2] cycloaddition (32CA) reaction. BET is the joint use...

Energy-entropy prediction of octanol-water logP of SAMPL7 N-acyl sulfonamide bioisosters

Partition coefficients quantify a molecule's distribution between two immiscible liquid phases. While there are many methods to compute them, there is not yet a method based on the free energy of each system in terms of energy and entropy, where entropy depends on the probability distribution of all quantum states of the system. Here we test a method in this class called Energy Entropy Multiscale Cell Correlation (EE-MCC) for the calculation of octanol-water logP values for 22 N-acyl sulfonamides in the SAMPL7 Physical Properties Challenge (Statistical Assessment of the Modelling of Proteins and Ligands). EE-MCC logP values have a mean error of 1.8 logP units versus experiment and a standard error of the mean of 1.0 logP units for three separate calculations. These errors are primarily due to getting sufficiently converged energies to give accurate differences of large numbers, particularly for the large-molecule solvent octanol. However, this is also an issue for entropy, and approximations in the force field and MCC theory also contribute to the error. Unique to MCC is that it explains the entropy contributions over all the degrees of freedom of all molecules in the system. A gain in orientational entropy of water is the main favourable entropic contribution, supported by small gains in solute vibrational and orientational entropy but offset by unfavourable changes in the orientational entropy of octanol, the vibrational entropy of both solvents, and the positional and conformational entropy of the solute.

Detection of Equid herpesvirus type 2 and 5 DNA in uterine flushings of mares with reproductive disorders

In recent years, there has been increasing evidence of the potential pathogenic significance of equine gammaherpesviruses in the horse. In humans, cattle and mice, gammaherpesviruses have already been associated with uterine infection. The aim of the present study was to investigate the presence of gammaherpesviruses in uterine flushings of mares with reproductive problems and to evaluate if there was a possible statistical association with clinical and laboratory findings in these cases. A total of 80 uterine flushings were collected from 61 mares with different reproductive problems and these were tested for equine herpesviruses (EHV) 1-5 by PCR. In the case of each mare in the study, the age, history of infertility, presence of anatomical defects in the reproductive tract, presence of systemic or local disease at time of sampling, phase in the oestrous cycle, post-partum interval, nature of uterine lavage performed (low versus large volume lavage), cytological and bacteriological examination results from the uterine flushing, and PCR herpesvirus results were recorded. Univariate analysis and multivariable logistic regression models were used to identify possible statistical associations and risk factors. Nine out of 61 mares (14.7%) had EHV-5 DNA in their uterine flushings. Co-infections with EHV-1 and EHV-2 were present in two cases. Of all the variables analyzed, only the cytological examination findings were associated with EHV-5 PCR positive results, both on univariate and multivariable analysis, especially in cases with an inflammation score of 3. It is postulated that presence of EHV-5 infection in the non-pregnant uterus may have a role to play in reproductive dysfunction and have a negative consequence on the pregnant uterus. Additional studies involving both healthy mares and mares with reproductive problems need to be performed, however, to elucidate whatever role equine gammaherpesviruses may play in the reproductive tract. This would be very worthwhile, since reproductive problems can have a significant impact on the equine breeding industry. Gaining a greater understanding of its causes could lead to new approaches for prevention and treatment.

Peptide and peptide mimetic inhibitors of antigen presentation by HLA-DR class II MHC molecules. Design, structure-activity relationships, and X-ray crystal structures

Molecular features of ligand binding to MHC class II HLA-DR molecules have been elucidated through a combination of peptide structure-activity studies and structure-based drug design, resulting in analogues with nanomolar affinity in binding assays. Stabilization of lead compounds against cathepsin B cleavage by N-methylation of noncritical backbone NH groups or by dipeptide mimetic substitutions has generated analogues that compete effectively against protein antigens in cellular assays, resulting in inhibition of T-cell proliferation. Crystal structures of four ternary complexes of different peptide mimetics with the rheumatoid arthritis-linked MHC DRB10401 and the bacterial superantigen SEB have been obtained. Peptide-sugar hybrids have also been identified using a structure-based design approach in which the sugar residue replaces a dipeptide. These studies illustrate the complementary roles played by phage display library methods, peptide analogue SAR, peptide mimetics substitutions, and structure-based drug design in the discovery of inhibitors of antigen presentation by MHC class II HLA-DR molecules.

Peptidomimetic compounds that inhibit antigen presentation by autoimmune disease-associated class II major histocompatibility molecules

We have identified a heptapeptide with high affinity to rheumatoid arthritis-associated class II major histocompatibility (MHC) molecules. Using a model of its interaction with the class II binding site, a variety of mimetic substitutions were introduced into the peptide. Several unnatural amino acids and dipeptide mimetics were found to be appropriate substituents and could be combined into compounds with binding affinities comparable to that of the original peptide. Compounds were designed that were several hundred-fold to more than a thousand-fold more potent than the original peptide in inhibiting T-cell responses to processed protein antigens presented by the target MHC molecules. Peptidomimetic compounds of this type could find therapeutic use as MHC-selective antagonists of antigen presentation in the treatment of autoimmune diseases.

Binding affinity independent contribution of peptide length to the stability of peptide-HLA-DR complexes in live antigen presenting cells

The effect of peptide length on the stability of peptide-HLR-DR1 (DR1) complexes was analyzed using two peptide series of increasing length, each containing a 7mer core with five DR1-binding anchors, extended stepwise with Ala residues at the N- and C-terminus, respectively. The Ala extensions, although did not affect binding affinity, significantly increased the half lives of peptide-DR1 complexes (from 1.5 h up to 10 h) in live antigen presenting cells (APC). Flanking residues from position -2 to 0 and 8 to 11 were involved in the affinity-independent increase of complex stability. The shortest (8mer and 9mer) peptides, with in vivo half lives of <2.5 h, were unable to form stable complexes with DR1 in presence of HLA-DM (DM) molecules, and were poor competitors of antigen presentation. Longer peptides were resistant to DM-mediated unloading, and were efficient competitors of antigen presentation. Thus, DM appears to limit short peptides in establishing biologically relevant DR occupancy, despite their high binding affinity. In APC, stable complexes can form only with high affinity peptides of >9 residues, and the longevity of complexes seems to depend on full of occupation of the binding site.

Thrombocytopenic purpura as adverse reaction to recombinant hepatitis B vaccine

Three cases of immune thrombocytopenic purpura after the first dose of recombinant hepatitis B vaccine occurred in infants under 6 months of age. Other possible causes of this condition were excluded. Antiplatelet antibodies were present. A defect in platelet production was excluded in two children. Corticosteroid treatment was effective. Subsequent administration of other vaccines (against polio, diphtheria, and tetanus) did not cause relapse of thrombocytopenia.

Influence of CD26 and integrins on the antigen sensitivity of human memory T cells

The antigen sensitivity of class II MHC restricted human CD4 T-cell clones is demonstrated to increase gradually with time after restimulation. This is manifested in a requirement of less antigen in culture, as well as decreased numbers of peptide-MHC complexes per APC for T-cell activation, and in an increased resistance to inhibition by class II MHC blockade. The increase in antigen sensitivity is accompanied by increased cell-surface expression of CD26, LFA-1, and VLA-1, whereas the expression of TCR and a series of other cell-surface molecules remains unchanged. Using appropriate monoclonal antibodies, we have shown that CD26 and LFA-1 contribute directly to the increased antigen sensitivity of "late-stage" T-cell clones. The late-memory T-cell phenotype established in this study is shown to occur also among T cells activated in vivo. We suggest that increasing the antigen sensitivity via antigen-nonspecific molecules is a physiologic mechanism for maintaining T-cell memory in face of decreasing antigen concentration, and for ensuring preferential activation of memory T cells upon repeated encounter with antigen.

Induction of class II major histocompatibility complex blockade as well as T cell tolerance by peptides administered in soluble form

Peptides binding to a particular class II major histocompatibility complex (MHC) molecule can inhibit the activation of T cells by other peptides binding to the same molecule, a phenomenon termed class II MHC blockade. All class II-binding peptides exert MHC blockade in vivo in depot form with adjuvant, and some also retain their blocking properties in soluble form. We demonstrate here that soluble peptides, when used at doses causing short-term MHC blockade, can also induce long-term antigen-specific T cell tolerance to themselves. The tolerogenicity of soluble peptides correlates with their antigenicity in adjuvant, but it is not necessarily related to their capacity to act as class II blockers in vivo. The tolerant state is manifested in a decreased production of both T helper cell 1 (Th1)-type and Th2-type lymphokines, and it cannot be reversed by interleukin-2. Once T cells are primed with a peptide in complete Freund's adjuvant, they are resistant to tolerization with the same peptide applied in soluble form. Tolerance induction is partially impaired in B cell-deficient mu MT-/- mice, suggesting a role for B cell antigen presentation in this process. The results suggest that the potential immunogenicity of class II MHC blockers could be circumvented by choosing a tolerogenic mode of application.

HLA-DR4-IE chimeric class II transgenic, murine class II-deficient mice are susceptible to experimental allergic encephalomyelitis

To investigate the development of HLA-DR-associated autoimmune diseases, we generated transgenic (Tg) mice with HLA-DRA-IE alpha and HLA-DRB1*0401-IE beta chimeric genes. The transgene-encoded proteins consisted of antigen-binding domains from HLA-DRA and HLA-DRB1*0401 molecules and the remaining domains from the IE(d)-alpha and IE(d)-beta chains. The chimeric molecules showed the same antigen-binding specificity as HLA-DRB1*0401 molecules, and were functional in presenting antigens to T cells. The Tg mice were backcrossed to MHC class II-deficient (IA beta-, IE alpha-) mice to eliminate any effect of endogenous MHC class II genes on the development of autoimmune diseases. As expected, IA alpha beta or IE alpha beta molecules were not expressed in Tg mice. Moreover, cell-surface expression of endogenous IE beta associated with HLA-DRA-IE alpha was not detectable in several Tg mouse lines by flow cytometric analysis. The HLA-DRA-IE alpha/HLA-DRB1*0401-IE beta molecules rescued the development of CD4+ T cells in MHC class II-deficient mice, but T cells expressing V beta 5, V beta 11, and V beta 12 were specifically deleted. Tg mice were immunized with peptides, myelin basic protein (MBP) 87-106 and proteolipid protein (PLP) 175-192, that are considered to be immunodominant epitopes in HLA-DR4 individuals. PLP175-192 provoked a strong proliferative response of lymph node T cells from Tg mice, and caused inflammatory lesions in white matter of the CNS and symptoms of experimental allergic encephalomyelitis (EAE). Immunization with MBP87-106 elicited a very weak proliferative T cell response and caused mild EAE. Non-Tg mice immunized with either PLP175-192 or MBP87-106 did not develop EAE. These results demonstrated that a human MHC class II binding site alone can confer susceptibility to an experimentally induced murine autoimmune disease.

Down-regulation of class II major histocompatibility complex molecules on antigen-presenting cells by antibody fragments

Certain HLA class II-specific monoclonal antibodies (mAb) cause up to 90% decrease in the cell surface expression of class II molecules. This down-regulation is isotype-specific, i.e. DR-specific mAb do not affect the expression of DP and DQ molecules. However, antibodies binding to one DR allotype down-regulate both allotypes in heterozygous antigen-presenting cells (APC), indicating that the phenomenon is not a direct consequence of ligation. All down-regulating mAb identified recognize the first (peptide binding) domains of class II heterodimers, and strongly inhibit the activation of class II-restricted human T cells in vitro. Conversely, non-down-regulating mAb fail to inhibit T cell activation, and most of them (four out of five) recognize class II second domains. Down-regulating antibodies are cytotoxic for B lymphoblastoid cell lines and for a small proportion of normal activated B cells. Their F(ab')2 fragments mediate both down-regulation and cytotoxicity, whereas the monovalent Fab fragments are not cytotoxic, but retain the down-regulatory and T cell inhibitory properties. These findings raise the possibility of a class II major histocompatibility complex-specific, antibody-based immunosuppressive therapy without cytotoxic side effects.

Self tolerance to T cell receptor V beta sequences

T cell tolerance to self is achieved by deletion or inactivation of clones recognizing peptides of self proteins presented by major histocompatibility complex molecules. A considerable fraction of self proteins accessible to the immune system is contributed by the system itself, for example, the receptors used for antigen recognition (antibodies and T cell receptors [TCRs]). Thus far, it has remained unclear, whether antigen receptors are subject to self tolerance, or on contrary, engage into network interactions implying immunity rather than tolerance. In this study, we demonstrate self tolerance to synthetic peptides corresponding to the first hypervariable region of the V beta 8.1 and V beta 8.2 TCR proteins. We also show that the tolerogenic synthetic peptide corresponds to a fragment produced by processing of the V beta protein, and conversely, that a V beta peptide not produced by processing is also not subject to self tolerance. Thus, the rules of tolerance seem to apply to antigen receptors, at least to their germline-encoded portions, in a similar fashion as to other self proteins. This finding has important implications for studies of natural and artificially induced immune networks.

Down-regulation of class II major histocompatibility complex molecules on antigen presenting cells after interaction with helper T cells

The recognition of antigenic peptides by CD4+ helper T cells is demonstrated here to result in a dramatic (up to 90%) decrease in expression of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APC). The reduction is selective to the class II isotype presenting the antigen, but if affects both allelic forms of the same isotype in heterozygous APC. The observed MHC down-regulation requires a specific T cell receptor-peptide-class II interaction, a direct contact between T cell and APC, and the involvement of CD2 molecules. These findings have important implications for the regulation of immune response, self tolerance, and autoimmunity.

Graft-versus-host resistance induced by class II major histocompatibility complex-specific T cell clones

Possible mechanisms of graft-vs.-host (GVH) resistance have been studied using a panel of seven class II major histocompatibility complex-specific T cell clones for elicitation and challenge. One clone recognized I-Ak,d,f, and expressed V beta 8.3 together with J beta 1.5. The remaining six clones were I-Ek specific and expressed V beta 15 rearranged to J beta 1.1 or J beta 1.3. The I-Ek-specific clones were also homologous to each other and different from the I-A-reactive one in the D and N regions. Four of the seven clones exhibited I-Ek-specific cytolytic activity. Each clone, when injected in sublethal numbers into appropriate recipients, could induce resistance to a subsequent lethal dose of any other clone in the panel. The resistance did not require sharing of either T cell receptor beta chains or antigen specificity, or MHC molecules by the eliciting and challenging clone. Cytolytic and noncytolytic clones were equally efficient in inducing GVH resistance. A prerequisite of resistance induction was the activation of eliciting clone subsequent to recognition of class II molecules in the host. Clones preactivated with high concentrations of recombinant interleukin 2, in vitro, could induce GVH resistance also in syngeneic hosts, suggesting that resistance induction was associated with the activated state of clone, rather than antigen recognition per se. In all instances of resistance, the challenging clones failed to induce vascular leakage, which was the cause of death in susceptible recipients (Lehmann, P. V., G. Schumm, D. Moon, U. Hurtenbach, F. Falcioni, S. Muller, and Z. A. Nagy. 1990. J. Exp. Med. 171:1485). Lipopolysaccharide (LPS) induced resistance to vascular leakage did not provide crossresistance to GVH and vice versa, suggesting that interleukin 1 alpha and tumor necrosis factor alpha implicated in LPS resistance are not involved in GVH resistance. Although the mechanism remains unclear, the most likely explanation for GVH resistance in this system is either the downregulation of permeability increasing effect in the challenging clone, or an induced refractoriness of blood vessels to this effect.

Flexibility of the T cell repertoire. Self tolerance causes a shift of T cell receptor gene usage in response to insulin

Bovine insulin(BI)-specific I-Ab-restricted T cell clones have been characterized for fine specificity and TCR gene usage. We have demonstrated that mouse strains carrying H-2b on three different genetic backgrounds (C57BL, BALB, and 129) rearrange and express the V beta 6 gene in a large proportion (36%) of insulin-specific clones. In these strains, the non-MHC background did not seem to influence TCR gene usage in response to BI. The V beta 6+ clones appeared to be selected by the antigen. In contrast, no V beta 6+ clones could be isolated from (B6 x DBA/2)F1 mice, where V beta 6+ (and V beta 8.1+) T cells are deleted by self tolerance to Mls-1a. Thus, although a small proportion of residual V beta 6+ cells had been demonstrated in Mls-1a mice, these cells could not be retrieved in a response that uses V beta 6 predominantly. In functional terms, therefore, the deletion of V beta 6 by self tolerance appears to be complete. Instead of V beta 6, the majority (up to 60%) of I-Ab- as well as I-Ad-restricted insulin-specific clones from the (B6 x DBA/2)F1 mice expressed V beta 8.2 and V beta 8.3. This shift of gene usage was not accompanied by any detectable change in the fine specificity pattern of response. Thus, in the insulin-specific response, the flexibility of T cell repertoire fully compensates for deletions caused by self tolerance.

Acute lethal graft-versus-host reaction induced by major histocompatibility complex class II-reactive T helper cell clones

T cell clones isolated from class II MHC-disparate MLR combinations, and specific for I-Ak and I-Ek molecules, respectively, are shown to induce acute lethal graft-vs-host disease in unirradiated recipients. Cytolytic and noncytolytic clones are equally efficient in this respect. The lethal disease is dependent on recognition of the stimulatory class II molecules in the host. The clones home to lungs and liver, and become activated in these organs as demonstrated by an in vivo thymidine incorporation assay. After activation, a severe vascular leak syndrome develops causing death of the recipients within 5 d after the injection of 5 x 10(6) to 10(7) cloned cells. The disease develops without the participation of secondary host-derived inflammatory mechanisms, such as mast cell degranulation, complement activation, and the release of prostaglandins, oxygen radicals, or proteolytic enzymes. The results raise the possibility that Th cells can directly influence vascular permeability, and control, thereby, the acute inflammatory reaction of blood vessels.

Inhibition of T cell response with peptides is influenced by both peptide-binding specificity of major histocompatibility complex molecules and susceptibility of T cells to blocking

Synthetic peptides corresponding to sequences 46-62 and 51-62 of mouse lysozyme and 46-61 of hen egg-white lysozyme (HEL) were used as competitors in a variety of T cell responses. The competitors, according to their binding specificity for major histocompatibility complex (MHC) were expected to inhibit T cell responses restricted to I-Ak, but not those restricted to I-Ad, I-Ek molecules. In competition experiments with T cell hybridomas, the poor binder I-Ed molecule required 10- to 15-fold higher competitor concentrations than the good binder I-Ak molecule to achieve 50% inhibition of antigen presentation. Similarly, the nonresponder state of H-2d mice to HEL peptide 46-61 could be overcome by increasing the immunizing dose, and proliferative T cell responses to different antigens in association with a variety of class II MHC molecules could be blocked by the mouse lysozyme and HEL peptides. Thus, the capability of some and failure of other MHC molecules to bind certain peptides appeared quantitative, rather than of an all or none nature, in these experimental systems. The susceptibility of uncloned T cell lines to peptide competitors was found to decrease with time. Lines maintained by repeated restimulation with antigen and APC, but without exogenous interleukin 2, acquired resistance within weeks. In contrast, T cell clones retained their susceptibility to peptide competitors over a long period of time. The latter data raise the possibility that a competition between ubiquitous (self) peptides and foreign antigen may result in the selection of T cells that have high avidity for the activating antigen-MHC complex, and are thus relatively resistant to competition at the level of antigen presentation.

Why peptides? Their possible role in the evolution of MHC-restricted T-cell recognition

The peptide-presenting function of major histocompatibility complex (MHC) molecules permits pathogenic microorganisms to evade the host's immune system in two different ways: first, by escape of pathogen-derived antigenic peptides from presentation, and second, by molecular mimicry, that is resemblance between MHC-bound self and foreign peptides. These two mechanisms could have served as selective pressures in the evolution of the MHC. In this article, Zoltan Nagy and colleagues propose that escape from presentation selects for one or a few MHC molecules with the capacity to bind a broad range of different peptides. In contrast, molecular mimicry is considered to be the driving force for MHC diversification, that is it increases the number (polymorphism) and selectivity of peptide-binding sites.

In vivo competition between self peptides and foreign antigens in T-cell activation

Cytotoxic and helper T lymphocytes recognize foreign antigen in the form of short peptides associated with class I and class II major histocompatibility complex (MHC) molecules, respectively. A recent study of the three-dimensional structure of a class I MHC molecule revealed a cleft formed by the amino-terminal half of the protein, which could serve as the binding site for these peptides. Because an individual possesses only a limited set of different MHC molecules, each molecule of this set must have the ability to bind a large number of different peptides in order to ensure full immunocompetence. Thus, it can be anticipated that peptides with unrelated sequences compete for binding to the same MHC molecule, and, indeed, this has been shown to occur in vitro. We therefore decided to see whether such competition could also regulate the cell responses in vivo. We have found that a synthetic peptide corresponding to residues 46-62 of mouse lysozyme, although not immunogenic itself, effectively inhibits the priming for T-cell responses when injected into mice together with foreign protein or peptide antigens. The inhibition observed strictly correlates with the capacity of the competitor to bind to the particular MHC molecule presenting the foreign antigen, and its extent depends on the molar ratio between antigen and competitor.

A simple and sensitive fluorometric immunoassay for the measurement of immunoglobulins in culture medium of mitogen-stimulated lymphocytes

A sensitive fluorescent immunoassay (FIA) for the quantification of immunoglobulins (IgG, IgM) in culture supernatants of human lymphocytes has been developed and compared with an ELISA method. FIA, like the ELISA assay, detects nanogram amounts of immunoglobulin (Ig), and has a high degree of specificity and reproducibility. The intra-assay coefficient of variation was about 5% in both test systems; the inter-assay coefficient of variation was approximately 9% in FIA and 13% in ELISA. Quantification of supernatants of lymphocyte cultures by both systems showed a high degree of concordance (correlation coefficient r = 0.93). The advantages of the FIA procedure are the short assay time and the more direct acquisition of signal.

Influence of cianidanol on specific and non-specific immune mechanisms

The influence of Cianidanol (Ci), a cytoprotective radical scavenger, on peripheral blood mononuclear cells (PBMC) was assessed with respect to its immunomodulatory function. In previous studies performed in our laboratory, a bidirectional influence of Ci on the immune response was observed, depending on its concentration. In order to elucidate this effect, the influence of Ci on macrophage (M phi) and B-cell function was investigated. A marked dose-dependent suppression of M phi phagocytosis by Ci could be detected. Furthermore, PGE2 synthesis of non-activated and PHA-activated PBMC was inhibited in the presence of Ci. This effect was shown to be due to an inhibition of M phi cyclooxygenase. It was also demonstrated that neither spontaneous nor Staphylococcus aureus Cowan I induced proliferation of highly purified B-cells was enhanced by Ci. Similar results were obtained by measuring the influence of Ci on immunoglobulin secretion of purified B-cells, exposed to Klebsiella membrane preparations. From these data it can be concluded that the previously described enhancing effect of Ci on immunoglobulin secretion is probably T-cell mediated.

Influence of TPA (12-O-tetradodecanoyl-phorbol-13-acetate) on human B lymphocyte function

The effect of the tumour-promoting agent TPA (12-0-tetra-dodecanoyl-phorbol-13-acetate) on the proliferation and Ig secretion response of blood and tonsil lymphocytes was investigated and compared to that of the T-cell-dependent polyclonal activators pokeweed mitogen (PWM) and group A streptococcal cell membranes (A-ScM) or the T-cell-independent B cell mitogen Staphylococcus aureus Cowan I (SAC) and a T-cell-independent B cell activator Klebsiella pneumoniae (Klebs M). In blood mononuclear cells (MNC), a rather weak, monocyte-dependent DNA synthetic response was observed after exposure to TPA, in comparison to PWM, A-ScM or SAC. Whereas highly purified B cells did not respond to TPA, purified T cells proliferated to a similar degree as unseparated MNC; moreover, the addition of T to B lymphocytes enhanced proliferation rates proportionally to the number of T cells added. This suggests that TPA acts as a polyclonal T cell activator (PTA) for human blood and tonsil cells. Similarly, TPA induced only small amounts of Ig secretion in blood and in tonsil MNC, as determined by an ELISA assay, and no significant Ig secretion in highly purified B cells. The rather weak B cell differentiation response was not due to a monocyte suppressor effect, since partially monocyte-depleted MNC or B cells responded similarly to the non-depleted cells. Thus, TPA cannot be considered as an alternative to other B cells stimulators, both with regard to DNA synthesis and Ig secretion.

Functionally distinct human T-lymphocyte clones sharing potent suppressive activity on immunoglobulin secretion

T-lymphocyte clones derived from populations sensitized to alloantigens in vitro were tested for their regulatory effects on pokeweed mitogen-stimulated immunoglobulin (Ig) secretion. Clones with natural killer (NK)-like cytotoxicity and/or suppressive activity for lymphoproliferative (LP) responses potently inhibited Ig secretion. Moreover, certain alloproliferative T4+ interleukin-2 (IL-2)-secreting 'helper' clones shared this strong suppressive activity on Ig secretion. The remaining clones enhanced, rather than suppressed, Ig production. Inhibition by all types of suppressive clones appeared not to be restricted by MHC products, since allogeneic HLA-mismatched donors were suppressed as efficiently as the autologous donor. Suppression was radioresistant, and was apparently not caused by absorption of IL-2, or cytotoxicity of the clones. Suppression was still detectable at plateau levels when cloned cells were added as late as 96 hr after the initiation of the cultures, suggesting an inhibitory mechanism divorced from early B-cell activation events. Thus, T-lymphocyte clones with distinct different functional activities share similar profound suppressive effects on Ig secretion in vitro.

[Investigation of drug users reported to the juvenile court of Rome]

After illustrating the new set of rules in matter of stupefacient and psychotropic drugs, introduced in Italy on December 1975, and the duties assigned by the law to the judicial power, the findings are set forth here of an investigation carried out on 40 youths whose addiction to the use of drugs had been reported to the juvenile court of Rome up to April 30, 1977. Anamnestic, social, judicial and clinical data on such subjects were obtained. Even with the obvious reservations resulting from the extremely small sample, some findings are observed that appear to be significant: a considerable portion of the minors addicted to the use of drugs was the recipient of other interventions by the juvenile court in the civil, administrative and criminal sphere; for the males, the prevailing anamnestetic and social data correspond to the characteristics of the other subjects, with whom the court usually deals at the "re-educational" or criminal level, while, with regard to females, the picture is remarkably different. The use of heroin seems to be wide-spread among the males, where also prevail previous hospitalizations caused by the use of drugs. The Authors advocate a new and more extensive verification, to be carried out after a period of time.