The nature of the colicin K receptor of Escherichia coli Cullen.

Cell walls of E. coli strains B and Cullen contain specific receptors for colicin K and for the T2, T6, and C16 phages. The receptors for the bacteriocin and the T6 virus are located in the outer layers of the cell wall of these microorganisms and are absent in their cytoplasmic membrane. The receptors for colicin K, phage T2, and the T6 and C16 viruses differ in their stability toward enzymes and chemical reagents. Their specificity must therefore be determined by different chemical groupings. The colicin K receptor is inactivated by certain proteolytic enzymes and by reagents which combine with tryptophan. It is concluded therefore that proteins or peptides containing this amino acid are essential for biological activity of the receptor.

Altered hapten ligands antagonize trinitrophenyl-specific cytotoxic T cells and block internalization of hapten-specific receptors.

Low molecular chemicals (haptens) frequently cause T cell-mediated adverse immune reactions. Our previous work provided evidence that hapten-specific T cells, in analogy to those specific for nominal peptide antigens, direct their TCR towards hapten-modified, MHC-associated peptides. We now demonstrate that trinitrophenyl (TNP)-specific, class I MHC-restricted CTL from mice may exhibit exquisite specificity for subtle structural details of these hapten determinants, surpassing even the specificity of immunoglobulins. More importantly, these CTL could be antagonized by ligands altered either in their peptide sequence or in their hapten structure. The system was employed to examine the molecular basis of T cell antagonism. Whereas agonists resulted in a dose-dependent downregulation of TCR in different mouse T cell clones, antagonistic peptides totally failed to do so despite engaging the specific TCR. Moreover, simultaneous presentation of antagonist and agonist on the same antigen presenting cell prevented TCR internalization. No signs of anergy or functional receptor inactivation were observed in CTL treated with antagonist-loaded target cells. Based on a serial triggering model of T cell activation, our data favor a model in which antagonists block T cell functions by competitively engaging the specific TCR in unproductive interactions.

Involvement of the T cell antigen receptor and of Lyt-2 in the cytotoxic function of aged killer (AK) T cells.

Aged killer (AK) T cells are antigen-independent, IL-2-requiring variants of antigen-dependent CTL clones that have lost their original antigen specificity and have acquired, instead, specific cytotoxicity for P815 target cells. In this report we study whether AK cells use a similar or a different target cell recognition system than that of bona fide CTL. To this end, we selected from a cloned AK line variants that are partially or completely deficient in specific target recognition and/or in cytotoxic function, and analyzed these variants for expression of the T cell antigen receptor and of Lyt-2. Variants were selected from the prototype AK line (Cl 96) with specific, as well as lectin-facilitated, cytotoxicity for P815 tumor cells. Variants could be grouped into four types with increasing degrees of functional deficiency, which correlated with loss of T cell receptor and/or loss of Lyt-2. In short, loss of Lyt-2 was reflected in loss of specific target recognition, and loss of the T cell antigen receptor was reflected in loss of all cytotoxic activity. We conclude from these results that both Lyt-2 and the T cell antigen receptor are required for specific target cell recognition and the T cell antigen receptor is, in addition, required for cytotoxic function. Moreover, since AK cells express a somatically acquired specificity that differs from that of their clonal precursors, it appears that cytotoxic T cells may change their antigen receptor from one specificity to another during tissue culture.

Reactivity of V beta 17a+ CD8+ T cell hybrids. Analysis using a new CD8+ T cell fusion partner.

Tolerance to IE molecules leads to deletion of V beta 17a-bearing T cells. Both, the CD4+ as well as the CD8+ T cell subsets are affected. A large percentage of CD4+ V beta 17a+ T cell hybrids recognize IE molecules. We now have investigated the reactivity for IE antigens of CD8+ V beta 17a+ T cell hybrids. Using a transfection approach, we have introduced the murine CD8 molecule into different V beta 17a+ T cell hybrids. Furthermore, the CD8 cDNA was transfected into the BW5147 alpha-beta- fusion partner. This allowed us to generate a large number of V beta 17a+ T cell hybrids by fusion with the appropriate T cells. Only 6% of T cell hybrids were stimulated to produce IL-2 upon incubation with IE+ cells. However, in those, the CD8 molecule seemed not to contribute to the IE reactivity of the hybrid, since mAbs against the CD8 molecule failed to inhibit their reactivity. This low percentage of V beta 17a+ CD8+ IE-reactive T cell hybrids contrasts with the strong reduction of CD8+ V beta 17a+ T cells in IE+ mice, strongly suggesting that elimination of such cells in the thymus occurs when they are coexpressing CD4 and CD8. This view was confirmed by the occasional expression of CD4 in some hybrids in which case IE reactivity was detected. Furthermore, we demonstrated the functional integrity of the introduced CD8 molecule by: (a) reconstitution of the IL-2 response in a class I-restricted TNP-specific T cell hybrid; and (b) by generation of alloreactive class I-restricted T cell hybrids using the new CD8+ fusion cell line. This CD8+ fusion partner, BWLyt2-4, should prove useful to study antigen processing and antigen presentation requirements of class I-restricted T cells.

Distribution and metabolism of synthetic alkyl analogs of lysophosphatidylcholine in mice.

Stability, distribution and metabolic fate of alkyl analogs of lysophosphatidylcholine have been studied in mice. Analogs employed were 1-hexadecyl- and 1-dodecyl-propanediol-3-phosphorylcholine (2-deoxy-lysophosphatidylcholine) and rac-1-octadecyl-2-methyl-glycero-3-phosphorylcholine (2-methoxy-lysophosphatidylcholine). It is demonstrated that (1) the half-life of these compounds in the organism was increased to the order of days as compared to minutes or hours for natural lysophosphatidylcholine, (2) the analogs were slowly degraded in vivo with partial incorporation of the hydrophobic part into neutral lipids and phosphatidylcholine. Deoxy analogs were apparently degraded faster than methoxy derivatives, and (3) due to this metabolism, accumulation of these compounds, even after repeated injections, did not occur in mice. Intravenous and intraperitoneal injections led to rapid and complete distribution of the lysolipids with slight enrichment in liver, kidneys and intestine. Subcutaneous or oral application resulted in a similar distribution, however, remarkable depot effects at the sites of administration were observed. Experiments with tumor-bearing mice indicated a reduced metabolic turnover and thus a slight enrichment of ether-lysophosphatidylcholine analogs in tumor tissue.

Quantitative studies on lysolecithin-mediated hemolysis. Use of ether-deoxy lysolecithin analogs with varying aliphatic chain-lengths.

The process of lysolecithin-mediated hemolysis has been investigated by use of various ether-deoxy lysolecithin analogs (1-alkyl-propanediol-3-phosphorylcholine) with alkyl residues of 10-22 carbon atoms. Hemolytic activities were defined either as molar amounts to be added for 50% lysis (L50) or as cell-bound amounts at 50% lysis (A50). It was found, that in contrast to L50, A50 values are independent of experimental conditions. Moreover, L50 values primarily reflect the binding affinities, while A50 values give more accurate information on the actual membrane-disturbing potential. The strongest hemolytic C16-lysolecithin analog required 2 - 10(7) or 5 - 10(7) molecules bound per cell for 50% lysis at 0 or 37degrees C, respectively, corresponding to about 10 or 25% of the total membrane phospholipids. Evidence is presented, indicating that (a) lysophosphatides bind to cells below their critical micelle concentration, (b) micelles themselves are not generally necessary for cell lysis. Red cells of different species (man and cattle) as well as at varying temperatures exhibit significantly different sensitivities in terms of L50 and A50 values. These differences, however, depend on the degree of hydrophobicity of the lysolecithins and disappear in the case of lysolipids having C10 or C12 aliphatic residues. The data are in agreement with our hypothesis that cellular sensitivity to lysolecithin lysis may be determined by the degree of segregation of lysolecithin-rich areas within the membrane lipid phase.

Quantitative studies on lysolecithin mediated hemolysis. Benzylated lysolecithin as a probe to study effects of temperature and red cell species on the hemolytic reaction.

The slow reacting hemolytic lysolecithin analog 1-octadecyl-2-benzyl-glycero-3-phosphorylcholine has been employed for a detailed study of the process of lysolecithin induced hemolysis. Using a radiolabelled analog we found that the different sensitivities of red cells from different species (chicken, man, cattle) are not paralleled by the binding affinities of lysolecithin. Moreover, lysophosphatide binding to the cells is reduced at low temperatures while the hemolytic activity increases. In contrast to continuous changes of lytic activity and binding between 0 and 37 degrees C, the velocity of the hemolytic reaction with human erythrocytes is extraordinarily fast at 10 degrees C. Experiments in sucrose containing buffer indicated principally different lysis mechanisms below and above 15 degrees C. We have further shown that cells loaded sublytically with the lysolecithin at 37 degrees C undergo spontaneous lysis upon cooling to 0 degrees C. The degree of lysis under these conditions, however, is diminished with increasing amounts of cell-bound lysolipid. Determinations of membrane microviscosities by means of fluorescence polarization revealed some qualitative relations between membrane fluidity and sensitivity to lysolecithin. The data are discussed on the basis of recent reports indicating that lysolecithin-distribution in mixed lipid phases may be heterogeneous depending on lipid composition and temperature.

Relation of cytolytic activity of an ether-deoxy lysophosphatidylcholine analog to available membrane surface. Comparison of normal and tumor cells from mice.

1-Hexadecylpropanediol-3-phosphorylcholine, an ether-deoxy analog of lysophosphatidylcholine, has been employed to study the sensitivity of various types of mouse cells with respect to changes in membrane permeability induced by lysophosphatidylcholine. Cells used included erythrocytes, thymocytes, spleen cells and macrophage, as well as 4 different tumors (2 lymphomas, 1 Ehrlich acites and 1 methylcholanthren-induced fibrosarcoma). The sensitivity to the lysophosphatide (on a per-cell basis) of the above cell types varied by a factor of 65. When lytic concentrations were related to available membrane surface, this variation was reduced to a factor of 2.5. No principal difference was observed between the sensitivity of normal versus tumor cell membranes with respect to lysophosphatidylcholine lysis. Membrane surface, available for lysophosphatidylcholine, has been estimated from binding equilibria of 14C-labelled deoxy-lysophosphatidylcholine to the cells under standardized conditions. This method is based on the finding that binding equilibria of lysophospholipids to cells are predominantly determined by the available membrane surface.

Spontaneous and detergent-induced vesiculation of thymocyte plasma membranes.

An analog of lysophosphatidylcholine (1-dodecyl-propanediol-3-phosphocholine) which does not impair membrane-bound enzymes was used for the induction of shedding of membrane vesicles from intact calf thymocytes. Without liberation of intracellular enzymes such as lactate dehydrogenase (EC 1.1.1.27) the shedded membranes contained 15--25% of the total activity of the plasma membrane enzymes alkaline phosphatase (EC 3.1.3.1), nucleotide pyrophosphatase (EC 3.1.4.1) and gamma-glutamyl transferase (EC 2.3.2.2). Membrane-free supernatants only exhibited trace activities of these enzymes. Without further purification, the specific enzyme activities in shedded membranes were of the same order of magnitude as in purified plasma membranes prepared after nitrogen cavitation of thymocytes. Small amounts of membrane vesicles which showed a different composition could be removed without detergent. These membranes exhibited a 3-fold lower specific activity of the gamma-glutamyl transferase while that of the alkaline phosphatase and nucleotide pyrophosphatase was similar as in detergent induced membrane vesicles. Distinct differences also were found in the protein pattern. The content of total cholesterol and phospholipid in vesicles shed spontaneously or after detergent treatment was nearly identical, however, significant differences were found in the fatty acid composition of the main phospholipids. The content of polyunsaturated fatty acids (linoleic and arachidonic acid) increased in the order: spontaneously shedded membranes, detergent induced vesicles, conventional purified plasma membranes. These results are discussed in terms of the heterogeneous composition of areas of the thymocyte plasma membrane.

Stabilization of detergent-solubilized Ca2+-ATPase by poly(ethylene glycol).

The (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum (SR) has been solubilized with 1-alkanoyl propanediol-3-phosphorylcholines with chainlengths ranging between 8 and 12 C atoms. A marked dependence of the ATPase activity upon the chainlength was found, indicating that alkyl chainlengths with 12 C atoms are necessary for retention of activity. Addition of poly(ethylene glycol) to the eluting buffers used for gel filtration of the ATPase-detergent micelles was found to increase the activity and the long-term stability significantly. In the presence of Ca2+, the elution volume indicated an ATPase dimer, whereas in the absence of Ca2+ the elution volume indicated a monomeric solution. The purity of the preparations after gel filtration was improved by subsequent chromatography with a hydroxyapatite column.

Acidic "peptidophospholipids", a new class of hapten-bearing cell surface modifying reagents.

Covalent coupling of glutamyl-glutamic acid to the amino group of ether-phosphatidylethanolamine (EPE) yields an acidic "peptidophospholipid" (Glu2-EPE) which is water-soluble above pH 7.0 and stable to phospholipase A. The terminal amino group of Glu2-EPE is free for coupling with amino-reactive determinants. We describe the synthesis of various hapten-substituted peptidophospholipids as well as of an intermediate compound, coupled with the heterobifunctional reagent 3-(2'-pyridyl)-dithiopropionic acid N-hydroxysuccinimide ester. The latter derivative allows binding to sulfhydryl-containing molecules, e.g. peptides or proteins. So far, beef and pig insulin as well as trinitrophenyl-substituted ribonuclease A have thus been linked to Glu2-EPE. All derivatives of Glu2-EPE are water-soluble at physiological pH and readily adsorb to cell surfaces from aq. solution. Binding to cells is fast, stable and "non-toxic" over a wide range of concns. The adsorbed determinants are accessible to specific antibodies and facilitate complement-mediated cell lysis. Glu2-EPE thus appears to be a universal carrier molecule for fast, simple and mild modification of cells with foreign determinants, e.g. for Jerne plaque assays.

Molecular features of penicillin allergy.

Haptens, such as drugs and other low molecular weight chemicals, become immunogenic only upon binding to proteins. Among antibiotics, penicillins are most commonly used for the treatment of bacterial infections and constitute a typical example of allergy inducing drugs in humans. Previous work on their immunologic properties focused mainly on the examination of IgE-mediated hypersensitivity reactions; however, drug-specific T cell reactions are also involved in causing a serious allergic inflammatory response. This review will focus on the interaction between antibiotic molecules and penicillin-specific T lymphocytes in humans. Experimental data accumulated so far on the reactivity of T cells with penicillin G point to penicilloyl-modified, major histocompatibility complex-associated peptides as T cell epitopes. The recognition specificity of the respective T cell receptors appears to be directed at both the backbone and the specific side chain of penicillin. In contrast, the sequence of the carrier peptides appears to contribute little to the antigenic specificity, mainly as a holder for the haptenic determinant. Finally, recent results demonstrating the capacity of penicillins to modulate, in vitro, the Th0/Th2 phenotype of established T cell clones will be presented and discussed in relation to possible therapeutic applications.

MHC-dependent and -independent activation of human nickel-specific CD8+ cytotoxic T cells from allergic donors.

T lymphocytes are critical effectors in the pathogenesis of contact hypersensitivity. Nickel is the most common contact sensitizer in humans and nickel-specific CD4+ T helper cells have been extensively characterized. Because recent observations have suggested the activation of CD8+ T cells in murine models of contact hypersensitivity, we investigated the existence of CD8+ hapten-specific T lymphocytes in patients with allergy to nickel. Nickel-specific T cell lines were generated from the peripheral blood of three allergic donors. The T cell lines were composed of a majority of CD4+ T cells, but CD8+ T cells were also present and their percentage increased with repeated in vitro stimulations. In addition to nickel-reactive helper T cell-0-type or helper T cell-2-type CD4+ T cell clones, CD8+ T cell clones could be derived from these cell lines and a total of 15 clones were further studied. Cytokine production was evaluated for 11 CD8+ T cell clones that were either cytotoxic T cell-0- or cytotoxic T cell-1-type clones. Additional effector functions were investigated on the complete panel of T cell clones. These CD8+ T cells did not only display hapten-specific proliferation, but also specific cytotoxic activities towards autologous EBV-B cells in the presence of nickel. Two different types of CD8+ T cells were characterized. Most of the clones lysed only autologous targets in the constant presence of nickel; however, one clone was able to lyse numerous targets in the presence of NiSO4, irrespective of the expression of either major histocompatibility complex class I or class II molecules. The characterization of nickel-specific cytotoxic CD8+ T cells with different requirements for nickel-specific target lysis, may have important implications in the development or in the control of human contact hypersensitivity reactions to nickel in vivo.

Functional expression and analysis of a human HLA-DQ restricted, nickel-reactive T cell receptor in mouse hybridoma cells.

Nickel-induced contact dermatitis represents a T cell mediated delayed type hyperreactivity. The elucidation of the molecular basis of T cell activation by Ni2+ ions may serve as a model for the understanding of other metal allergies. We describe here the expression of hybrid T cell antigen receptor (TCR) alpha- and beta-genes, containing rearranged human Ni-reactive variable and mouse constant regions, together with human CD4 in a mouse T cell hybridoma. The resulting hybridoma specifically responds to IL-2 secretion to Ni, but not to other metal ions in the presence of HLA-matched antigen-presenting cells. Loss of CD4 decreases, but does not completely abrogate this reactivity. The restricting HLA-DQ element is identified as consisting of DQA1*0101 and DQB1*0501; however, only some of the B cell lines homozygous for these molecules effectively present Ni to the hybridoma. We interpret these data to show that (i) Ni-reactivity is definitely mediated by alpha beta TCR variable regions; (ii) as for peptide-specific TCR, the CD4 co-receptor enhances Ni-reactivity, but is not absolutely essential; (iii) Ni2+ ions like nominal peptide antigens require HLA (here class II) molecules of the APC for presentation; (iv) the restricting molecule may require a special conformation or the association with a particular type of peptide or an as yet unidentified other surface structure on the antigen-presenting cell for effective Ni-presentation.

Peptide immunization indicates that CD8+ T cells are the dominant effector cells in trinitrophenyl-specific contact hypersensitivity.

The identity of the effector T cell population involved in contact hypersensitivity is still questionable with evidence promoting both CD4+ or CD8+ T cells. Previous experimental studies have relied on the in vivo depletion of T cell subsets using antibody, or the use of knock-out mice with deficiencies in either CD4+ or CD8+ T cell-mediated immunity. To address the role of the class I- and class II-mediated pathways of T cell activation in contact hypersensitivity responses in mice with an intact immune system, we utilized various trinitrophenyl-derivatized peptides, which bind specifically with H-2Kb (major histocompatibility complex class I) or H-2I-Ab (major histocompatibility complex class II). The subcutaneous injection of major histocompatibility complex class II-specific, but not of class I-binding, hapten-derivatized peptides in incomplete Freund's adjuvant induced specific, albeit low, contact hypersensitivity responsiveness to trinitrochlorobenzene. When bone-marrow-derived dendritic cells, however, were pulsed with the same peptides and administered intradermally, the opposite result was observed, namely that the class I binding peptides induced contact hypersensitivity responses similar to that observed after epicutaneous trinitrochlorobenzene application. In contrast, dendritic cells pulsed with major histocompatibility complex class II binding peptides did not reproducibly sensitize for contact hypersensitivity responses. Surprisingly, both immunization protocols efficiently induced CD8+ effector T cells. These results support the notion that CD8+ T cells are the dominant effector population mediating contact hypersensitivity responsiveness and that the CD4+ T cell subset only contributes little if at all.

Intracellular processing of hapten-modified protein for MHC class I presentation: cytoplasmic delivery by pH-sensitive liposomes.

Phagocytic or endocytic uptake of pH-sensitive liposomes has been shown to result in the release of entrapped material into the cytosol. This system can therefore be applied to the targeted delivery of protein antigens into the MHC class I presentation pathway of antigen-processing cells. We have used trinitrophenyl (TNP)-modified chicken ovalbumin encapsulated in liposomes to examine the intracellular processing of haptenated proteins and the presentation of TNP-modified peptides to MHC class I-restricted hapten-specific CTL. Here, we demonstrate for the first time that hapten-modified proteins can undergo intracellular processing by macrophages, that similar peptides are produced in the form of unmodified or haptenated derivatives, and that TNP-peptides are transported to the cell surface and presented to class I-restricted CTL via the ER/Golgi pathway. This system can now be used to study T-cell responses to naturally processed hapten-conjugated peptides in vitro and in vivo.

Purified MHC class I molecules present hapten-conjugated peptides to TNP/H-2Kb-specific T cell hybridomas.

Several trinitrophenyl (TNP)-specific mouse cytotoxic T cell (CTL) clones recognize TNP-conjugated peptides in association with class I MHC Kb-molecules. Here we show that CD8+ T cell hybridomas derived from these CTL exhibit the same pattern of antigen-specificity as their parent CTL-clones. These T cell hybridomas reacted with TNBS- or TNP-peptide modified syngeneic target cells, and also with affinity purified, immobilized Kb-molecules preloaded with TNP-peptides. These findings demonstrate most directly that MHC-associated, haptenated peptides create functional antigenic epitopes for TNP-specific CTL. Furthermore, using purified Kb-molecules and a panel of Kb-binding TNP-conjugated peptides, we demonstrated that the epitope density is a critical factor in triggering these T cell hybridomas. Chemical modification of immobilized Kb-layers resulted in poor antigenicity, implying low epitope density and therefore arguing against covalent MHC-haptenization as a major source of T cell antigenic determinants.

Alkyl-lysophospholipid induced suppression of human lymphocyte response to mitogens and selective killing of lymphoblasts.

Alkyl-analogs of 2-lysophosphatidylcholine have been found to inhibit the response of human peripheral blood lymphocytes to mitogens and allogeneic cells. Furthermore, these compounds kill selectively transformed lymphocytes in vitro while resting lymphocytes are not affected in their viability. The increased incorporation of fatty acids into cellular phospholipids during lymphocyte stimulation has been shown to be inhibited by these alkyl-lysophospholipids. Both resting and transformed lymphocytes could be shown to have an 1-0-alkyl-cleavage enzyme. Thus, selective cytotoxicity for lymphoblasts is not due to principal differences in the metabolism of alkyl-lysophospholipids as we have demonstrated to be the case between normal and leukemic cells, but is most likely due to the interference of these substances with the enhanced turnover of cellular phospholipids in stimulated lymphocytes.

Further characterization of UVB radiation effects on Langerhans cells: altered expression of the costimulatory molecules B7-1 and B7-2.

We have reported previously that low-dose UVB radiation (UVBR, 50-200 J/m2) perturbs the antigen-presenting cell (APC) function of murine Langerhans cells (LC) by interfering with yet undefined costimulatory signals. In this study, we investigated (1) the effects of UVBR on the expression of the costimulatory molecules B7-1 and B7-2 on murine LC, (2) the functional consequences of defective B7-1 and B7-2 signalling on primary and secondary T-cell responses induced by LC and (3) the mechanism by which UVBR interferes with B7-1 and B7-2 expression. Ultraviolet-B radiation dose-dependently inhibited the culture-induced upregulation of B7-1 and B7-2 on LC from both UVB-susceptible (UVBs, C57BL/6) and UVB-resistant (UVBR, Balb/c) mice and abrogated their capacity to stimulate proliferation of naive alloreactive T cells and of the KLH (keyhole limpet hemocyanin)-specific T helper (Th)1 clone HDK-1. The UVBR-induced suppression of B7-1 and B7-2 on LC and their perturbed APC function were related, because exogenous triggering of the B7/CD28 pathway with a stimulatory monoclonal antibody (mAb) for CD28 to UVB-irradiated LC partially restored T-cell proliferation. Such reconstitution was not observed when the mAb was added to killed LC, indicating that the UVBR-induced suppression of APC function was not due to lethal effects on LC. Conditioned supernatants from UVB-irradiated epidermal cells did not inhibit the functional upregulation of B7-1 and B7-2, suggesting that UVBR inhibits B7-1 and B7-2 upregulation by acting directly on LC and not by altering LC costimulatory function via release of soluble immunosuppressive factors. In conclusion, UVBR distorts the functional expression of B7-1 and B7-2 on LC from both UVBS and UVBR mice, thereby contributing to the failure of UVB-irradiated LC to stimulate resting alloreactive T cells or KLH-specific Th1 cells.

T cell immune responses to haptens. Structural models for allergic and autoimmune reactions.

Protein-reactive chemicals, metal salts and drugs, commonly classified as immunological haptens, are major environmental noxes targeted at the immune system of vertebrates. They may not only interfere with this defense system by toxicity alone, but more often by evoking hapten-specific immune responses resulting in allergic and eventually autoimmune responses. Here, we review recent developments in the analysis of the structural basis of hapten recognition, particularly by T lymphocytes, which represent central elements in cell-mediated, as well as in IgE dependent, allergies. A break-through in this field was the finding that T cells detect haptens as structural entities, attached covalently or by complexation to self-peptides anchored in binding grooves of major histocompatibility antigens (MHC-proteins). Synthetic hapten-peptide conjugates were shown to induce hapten-specific contact sensitivity in mice, opening new routes for studying hapten-induced immune disorders.