Analysis of lysozyme-specific immune responses by synthetic peptides. I. Characterization of antibody and T cell-mediated responses to the N-terminal peptide of hen egg-white lysozyme

Pharmacological Modulation of Host Immunity with Hen Egg White Lysozyme (HEWL)-A Review

In the 100 years since its discovery, lysozyme has become an important molecule, both as model for studies in different fields and as a candidate for the therapy of various pathological conditions. Of the dozens of known lysozymes, in this review we focus on one in particular, lysozyme extracted from hen egg white (HEWL), and its interaction with the immune system when it is administered orally. Experimental data show that there is an axis that directs immune system activation from GALT (gut-associated lymphoid tissue) and the intestinal lymphocyte clusters. Although a contribution of peptidoglycans from digestion of the bacterial cell wall in the intestinal lumen cannot be excluded, immune stimulation is not dependent on the enzymatic activity of HEWL. The immune responses suggest that HEWL is able to recover from immunodepression caused by tumor growth or immunosuppressants, and that it also improves the success of chemotherapy. The positive results obtained in a small Phase 2 study in patients, the ease of oral administration of this protein, and the absence of adverse effects suggest that HEWL may play an important role in all diseases where the immune system is weakened or where its enhancement plays a critical role in the resolution of the pathology.

The majority of immunogenic epitopes generate CD4+ T cells that are dependent on MHC class II-bound peptide-flanking residues

Peptides bind to MHC class II molecules with a defined periodicity such that the peptide-flanking residues (PFRs) P-1 and P11, which lie outside the core binding sequence (P1-P9), are solvent exposed and accessible to the TCR. Using a novel MHC class II:peptide binding assay, we defined the binding register for nine immunogenic epitopes to formally identify the flanking residues. Seven of the nine epitopes, restricted by H-2A(k), H-2A(g7), or H-2E(k), were found to generate T cells that were completely dependent on either P-1 or P11, with dependency on P-1 favored over P11. Such PFR dependency appears to be influenced by the type of amino acid exposed, in that residues that can form salt bridges or hydrogen bonds are favored over small or hydrophobic residues. Peptides containing alanine substitutions at P-1 or P11 in place of PFRs that mediate dependency were considerably less immunogenic and mediated a substantially reduced in vitro recall response to the native protein, inferring that PFR recognition increases immunogenicity. Our data suggest that PFR recognition is a common event characteristic of all MHC class II-restricted T cell responses. This key feature, which is not shared by MHC class I-restricted responses, may underlie the broad functional diversity displayed by MHC class II-restricted T cells.

Impaired antigen presentation by splenocytes of ethanol-consuming C57BL/6 mice

Excessive alcohol consumption impairs T-cell-dependent immune function. Whether this impairment results from the direct inhibition of helper T (Th) cells or from inhibition of the cells that process and present antigen to Th cells is unclear. The present study examines the effect of dietary alcohol on the ability of spleen cells from C57BL/6 mice to present antigen to T-cell hybridomas. We find that ethanol consumption impairs the ability of spleen cells to present hen egg lysozyme (HEL) in vitro. This impairment was seen for native HEL protein, a hapten-modified HEL, and a peptide bearing a minimal T-cell epitope (HEL 51-60) that requires no additional enzymatic processing. These results suggest that deficiencies in immune responsiveness in alcohol-consuming individuals may include antigen presentation.

T cell epitope recognition involved in the low-responsiveness to a region of hen egg lysozyme (46-61) in C57BL/6 mice

The predominant T cell epitope of hen egg lysozyme (HEL) in high-responder C3H mice has been previously identified as the HEL 46-61 region. In contrast, this region is poorly recognized by T cells from low-responder C57BL/6 mice upon immunization with HEL. In previous studies, we have demonstrated that several C57BL/6 derived T cell hybridomas reactive to this epitope and other HEL epitopes preferentially recognize phosphorylcholine (PC)-conjugated HEL over unconjugated HEL. To understand the mechanisms involved in this difference of T cell recognition, we have further analysed the reactivity of T cells and T cell hybridomas from low-responder C57BL/6 mice. T cells from HEL-immunized mice were preferentially reactive to HEL 47-60. These results suggest a potential deficiency in generating an appropriate T cell epitope from the 46-61 region of native HEL in low-responder C57BL/6 mice. The minimal T cell epitope of this region was defined as HEL 51-60 using the PCH4.1 T hybridoma clone. This minimal epitope represents a single amino acid shift from the minimal epitope of HEL high-responder C3H mice (HEL 52-61). Various peptides representing this region were synthesized with single alanine substitutions at each position. The residues at positions 51, 52, 53 and 57 of HEL appear to be involved in Ia binding and the residues at 55 and 56 in contracting the TCR. T cell reactivity to HEL 51-61 peptides with various substitutions at position 61 strongly suggest that primarily the size of the C-terminal residue interferes with binding to the Ia molecules of low-responder mice. In addition, substitutions of the TCR contacting residues at positions 55 and 56 with similar residues (isoleucine-->leucine or leucine-->isoleucine) significantly increased the T cell reactivity, suggesting a low reactivity with the native residues. Therefore, the requirement of many residues in the T cell epitope for interaction with Ia, the necessity for additional Ag processing to facilitate Ia binding, and the low affinity of the TCR contacting residues may together render C57BL/6 mice unresponsive to the HE 46-61 region.

Sequence features that correlate with MHC restriction

Identification of common sequence motifs in antigenic peptides restricted to a specific class II molecule has not been easy due to the large variation in length and sequence that is observed in these peptides. The goal of this study is to develop an automated computerized method for the identification of sequence features and structural determinants that play a role in the MHC restriction of helper T-cell antigenic peptides. For this, we compiled an extended database of helper T-cell sites, including the information on MHC restriction, when available. Two groups of peptides are assigned to each MHC type: (1) peptides that bind to that MHC molecule to elicit a T-cell response, and (2) peptides that were shown experimentally either not to bind to or not to elicit a T-cell proliferative response in association with that MHC molecule. We search for common motifs in the group of binding peptides, and identify significant motifs that are frequent among these peptides but almost absent in the group of non-binding peptides. A motif consists of physical-chemical and structural properties that may be responsible for binding specificity and can be extracted from sequence data, such as, hydrophobicity, charge, hydrogen bonding capability, etc. The first search is performed on the non-aligned binding peptides. Next, the sequences are aligned according to an identified motif and a search for additional, conserved, properties is performed. The statistical significance of the motifs is evaluated as well as their compatibility with published experimental results on substitution effects. Here we demonstrate the general scheme of the analysis and results for I-Ek and I-Ak associated peptides.

Constraints in antigen processing result in unresponsiveness to a T cell epitope of hen egg lysozyme in C57BL/6 mice

T cell hybridoma clones were derived after fusion of BW-5147 parent cells with lymphocytes from C57BL/6 mice injected with phosphorylcholine (PC)-hen egg lysozyme (HEL) conjugates. Several T cell hybridomas were preferentially reactive with PC-HEL over unconjugated HEL, and a particular clone (PC-H4.1) was further analyzed. This T cell hybridoma clone could respond to its maximal level toward unconjugated HEL only when the dose of HEL was increased to 5-10-fold of the PC-HEL concentration. Interestingly, this clone was not stimulated by unfolded HEL (by S-carboxymethylation) to the level of PC-HEL. A synthetic peptide representing the amino acid position 47-61 of HEL, which is known to be non-immunogenic upon HEL injection in C57BL/6 mice, was able to stimulate the hybridoma only to a level comparable to that induced by unconjugated HEL. The T cell response to this synthetic peptide required an additional antigen-processing step, based on its inability to stimulate T cells after treatment of antigen-presenting cells with leupeptin, chloroquine or paraformaldehyde. Deletion of a single C-terminal amino acid residue of HEL 47-61 (arginine) significantly enhanced (10-100-fold of HEL 47-61) the T cell reactivity and abrogated the necessity of further antigen processing. These results suggest that the lack of a T cell response to a certain epitope may not be due to the lack of a T cell repertoire reactive to the epitope. In some cases, the unresponsiveness may be due to the difficulty in generating the particular epitopes. Taken together, modification of the lysozyme molecule with PC conjugation may facilitate further antigen processing of HEL to generate an optimal epitope for the nonresponder mice.

Immunogenicity of peptides having pre-determined alpha-helical and alpha-alpha fold topologies

A panel of three synthetic peptides based on the 310-327 region of mouse LDH-C4 was used to examine the effect of peptide conformation on immunogenicity. The peptides, without prior conjugation to carrier molecules, were injected into outbred mice and the antisera were assayed for peptide- and LDH-C4-reactive antibodies by ELISA. An 18-residue random coil peptide (alpha N) and an 18-residue amphipathic alpha-helix peptide (alpha 1) were weakly immunogenic. A conformationally stable 40-residue alpha-alpha fold peptide (alpha 3) was highly immunogenic. The antibodies elicited by alpha 3 reacted strongly with the native molecule by ELISA. Solution-phase binding assays were used to further characterize the specificity of the sera from two mice immunized with alpha 3. Antibodies from one of the mice appeared to recognize the helical portion of the peptides, while antibodies from the other mouse reacted only with the immunogen and may be specific for the non-natural beta bend residues or possibly a topographic determinant peculiar to the anti-parallel helices. Serum from neither mouse was able to recognize the native molecule in solution. Peptides intended to mimic topographic determinants for the purpose of synthetic vaccine development may have to be more complex than those used in this study in order to induce high-affinity antibodies capable of exerting a significant biological effect.

Analysis of T cell reactivities to phosphorylcholine-conjugated hen egg lysozyme in C57BL/6 mice: hapten-conjugate specificity reflects an altered expression of a major carrier epitope

We have selected several different T cell hybridoma clones reactive to hen egg lysozyme (HEL) conjugated to phosphorylcholine (PC) after fusion of PC-HEL-primed C57BL/6 lymphocytes with BW5147 parent cells. These hybridoma clones preferentially recognize PC-HEL over unconjugated HEL, but not other carrier molecules conjugated with the same hapten. All the PC-HEL-reactive clones are similarly responsive to not only p-azobenzenearsonate (ABA)-conjugated HEL (ABA-HEL) but also to a variety of other diazotized hapten-HEL conjugates. However, these clones are not stimulated by fluoresceinated or dinitrophenylated HEL beyond the level of HEL carrier alone. Therefore, the type of hapten linkage (diazonium) to the carrier molecule appears to affect T cell recognition. The hybridoma clones apparently recognize the carrier molecule alone, although the level of stimulation is relatively low compared to that induced by either PC-HEL or ABA-HEL. Interestingly, HEL unfolded by S-carboxymethylation is capable of stimulating the hybridomas to a level comparable to that obtained with PC-HEL. T cell recognition of the unfolded HEL is independent of antigen processing, which is different from that of PC-HEL. The peptide sequence corresponding to the amino acids 81-93 of HEL appears to contain the epitope region for the hybridoma clones based on testing stimulation activity with synthetic peptides. Previously, the peptides including this region (81-96) have been reported as the determinant recognized by T cells derived from C57BL/6 mice after immunization with an HEL peptide (HEL 13-105) but not with native HEL. These results suggest that a hapten conjugation via diazonium linkage modifies antigen presentation and consequently the presentation of the major T cell epitopes similar to that of the HEL fragment.

The distinctive specificity of antigen-specific suppressor T cells

Although suppressor T cells have been cloned in only a few instances, the existence of a functional cadre of T cells that acts to downregulate the immune response is well documented. In this review Eli Sercarz and Urszula Krzych describe studies on suppressor T-cell (TS-cell) specificity that provide some support for the conclusion that the TS cell is a distinctive cell type with an expressed repertoire that is different from that expressed by helper T (TH) cells. They go on to explore the interaction between cells recognizing TS-cell-inducing determinants (SDs) and TH-cell-inducing determinants (HDs), and their relationship to immunogenicity and Ir gene effects.

Identification of T-cell epitopes and use in construction of synthetic vaccines

The T cell is central to the immune system response to foreign antigens, and understanding the mechanism of T cell response to antigen is crucial for vaccine development. Short subpeptides of foreign antigen can prime the T cells to respond to the whole antigen, in some cases as well as or better than immunization with the whole antigen itself. Antigenic sites located first in the murine model are also antigenic in the human, suggesting that the structural features of antigenic sites are species-independent. The amphipathic helix hypothesis has proven useful in developing an algorithm that has successfully located immunodominant sites in important proteins, thus reducing substantially the experimental time and effort required to locate those sites. Other algorithms have also been used successfully, but in all cases there are proven T-cell sites not accounted for by the algorithm. A data base showing T-cell response to collections of peptides uniformly distributed along protein antigens would be very useful in subsequent efforts to characterize the physical and chemical properties of T-cell antigenic sites.

Interaction of an immunodominant epitope with Ia molecules in T-cell activation

The amino acid sequence corresponding to residues 107-116 of hen egg-white lysozyme (HEL) has been identified as containing an immunodominant T-cell epitope recognized in association with the I-Ed molecule. The immunodominance of this epitope in HEL-primed H-2d mice was demonstrated by analysis of the T-cell proliferative response induced by synthetic peptides covering almost the entire HEL sequence. All the T-cell hybridomas from H-2d mice analyzed recognize the HEL sequence 107-116 in association with the I-Ed molecule. Correlating with the restriction of T-cell recognition, HEL-(105-120)-peptide binds to I-Ed but not to I-Ad molecules. Conservative or semiconservative substitutions at positions 113 (Asn----Lys), 114 (Arg----His), or 115 (Cys----Ala) abrogate the ability of HEL-(105-120) to activate T cells. Substitutions at residues 113 and 115 affect T-cell recognition but not the binding to I-Ed molecules, whereas, as shown by binding data and competition experiments, an Arg----His substitution at position 114 profoundly impairs the capacity of the peptide to interact with I-Ed molecules. In agreement with these results, [Lys113]HEL-(105-120)-peptide but not [His114]HEL-(105-120)-peptide was found to be immunogenic in H-2d mice. Thus, a single semiconservative substitution drastically reduces binding capacity and abolishes immunogenicity, suggesting that a strict correlation exists between binding of a peptide to Ia molecules and its immunogenicity.

Multiple T cell antigenic determinants identified within a limited region of the horse cytochrome c molecule

The antigen fine specificity of T cell hybridomas recognizing the horse apocytochrome c fragment 1-65, restricted to the I-Ab molecule, was determined to gain some insight into the molecular nature of T cell antigenic peptides. Two major groups of clones specific for distinct subsites, namely residues 1-38 and 39-65, could be identified. Hybridomas recognizing the latter determinant were further tested with different horse cytochrome c peptides and analogues. This analysis revealed the presence of at least two epitopes encompassed by residues 47-53 and 48-53. Furthermore, clones specific for the amino acid sequence 48-53 showed considerable heterogeneity in respect to the antigen concentration required to obtain 50% of the maximal interleukin 2 secretion. Most prominent was the heteroclitic response towards tuna cytochrome c which differs at positions 44, 46 and 47 from the horse cytochrome c molecule in the relevant region. Comparison of the conformation of the sequence 43-46 between the two cytochrome c suggests that this segment, which forms a 3(10) bend, may be important in maintaining the proper structure of the antigenic determinant. Moreover, the variations up to 180-fold in the concentrations of the cross-reacting cytochrome c and peptides required for stimulation were not always correlated with the maximal interleukin 2 secretion they induced. This indicates that the biological response, that is supposed to be an indication of the affinity of the T cell receptor for its ligand, is not necessarily a function of the antigen concentration.

Immunoregulation of lysozyme-specific suppression. III. Epitope-specific amplification of immunosuppression induced by monoclonal suppressor-T-cell products

The hen egg-white lysozyme (HEL)-specific suppression induced by soluble molecules produced by a monoclonal T-cell lymphoma line (LH8-105) obtained from HEL-specific suppressor T lymphocytes has been examined. Injection of I-J+ molecules from LH8-105 cell culture supernatant (TsFa) in HEL-primed mice during the afferent phase of the response induced Lyt-2+ second order suppressor T (Ts) cells which, upon transfer into HEL-CFA-primed syngeneic recipients, inhibit the delayed-type hypersensitivity (DTH) response to HEL. Transfer of spleen cells from TsFa-injected mice primed with HEL or human lysozyme suppresses the DTH response to HEL in recipient mice whereas this response is not affected by cell transfer from ring-necked pheasant egg-white lysozyme (REL)-primed and TsFa-injected mice, indicating that induction of second order Ts by TsFa is specific for a lysozyme epitope including phenylalanine at position 3. Fine antigenic specificity of second order Ts-cell induction is confirmed by similar results obtained upon injection of TsFa in mice primed with HEL N-terminal synthetic peptide or with an analog in which, as in REL, phenylalanine has been substituted by tyrosine at position 3. The same fine antigenic specificity observed in the induction of second order Ts cells is also present in the expression of TsFe suppressive activity. The similar antigenic specificity of Tsa and Tse suggests that Tse cells could result from amplification of the Tsa cell population or these two cell subsets could reflect different maturation stages of the same cell type rather than distinct T-cell populations activated in cascade.

A microcomputer program for hydrophilicity and amphipathicity analysis of protein antigens

A simple and ready-to-use program to analyze hydrophilicity and amphipathicity characteristics of protein sequences is described. Three of the most commonly used hydrophilicity scales can be selected and the block length can also be varied. The program is written in BASIC language, can be run on any microcomputer and does not require computer experience. The program output is exemplified by a sample of lysozyme hydrophilicity and amphipathicity analysis.