Immunoreactivity and conformation of the P-P-G-M-R-P-P repetitive epitope of the Sm autoantigen

Minute time scale prolyl isomerization governs antibody recognition of an intrinsically disordered immunodominant epitope

Conformational rearrangements in antibody·antigen recognition are essential events where kinetic discrimination of isomers expands the universe of combinations. We investigated the interaction mechanism of a monoclonal antibody, M1, raised against E7 from human papillomavirus, a prototypic viral oncoprotein and a model intrinsically disordered protein. The mapped 12-amino acid immunodominant epitope lies within a "hinge" region between the N-terminal intrinsically disordered and the C-terminal globular domains. Kinetic experiments show that despite being within an intrinsically disordered region, the hinge E7 epitope has at least two populations separated by a high energy barrier. Nuclear magnetic resonance traced the origin of this barrier to a very slow (t(1/2)∼4 min) trans-cis prolyl isomerization event involving changes in secondary structure. The less populated (10%) cis isomer is the binding-competent species, thus requiring the 90% of molecules in the trans configuration to isomerize before binding. The association rate for the cis isomer approaches 6 × 10(7) M(-1) s(-1), a ceiling for antigen-antibody interactions. Mutagenesis experiments showed that Pro-41 in E7Ep was required for both binding and isomerization. After a slow postbinding unimolecular rearrangement, a consolidated complex with K(D) = 1.2 × 10(-7) M is reached. Our results suggest that presentation of this viral epitope by the antigen-presenting cells would have to be "locked" in the cis conformation, in opposition to the most populated trans isomer, in order to select the specific antibody clone that goes through affinity and kinetic maturation.

A palmitoyl-tailed sequential oligopeptide carrier for engineering immunogenic conjugates

The main guideline in designing effective immunogens as vaccine candidates capable of eliciting potent and specific immune responses is to combine B/T cell epitopes and adjuvants as immunostimulators on the same carrier that links the major histocompatibility complex with T cell receptors. Aiming at contributing to the development of carriers for human usage a helicoid type sequential oligopeptide carrier, SOC(n)-II, formed by the repeating tetrapeptide unit (Aib-Lys-Aib-Gly)(n), n=2-7, elongated from the amino-terminus by the palmitoyl group, known for its adjuvanticity, is now presented. The main B cell epitope, PPGMRPP, of the Sm autoantigen against which the majority of antibodies in patients with systemic lupus erythematosus is directed, was coupled to the Lys-N(epsilon)H(2) groups of the carrier in four copies and the resulting conjugate Palm-SOC(4)-II-Sm(4) was subjected to animal immunizations without utilizing any adjuvant. The induced immune response was comparable with that produced when Ac-SOC(4)-II-Sm(4) was administered in animals following the conventional immunization protocol of complete/incomplete Freund's adjuvant. High titers of anti-Palm-SOC(4)-II-Sm(4) antibodies were generated, which recognize the priming immunogenic conjugate, as well as reconstituted Sm mimics but not the carrier alone. It is concluded that Palm-SOC(n)-II carrier is a valuable tool for engineering immunogens eliciting enhanced and specific humoral immune responses.

Influence of sequential oligopeptide carriers on the bioactive structure of conjugated epitopes: Comparative study of the conformation of aHerpes simplex virus glycoprotein gD-1 epitope in the free and conjugated form, and protein “built-in” crystal structure

Synthetic carriers play an important role in immunogen presentation, due to their ability of inducing improved and specific responses to conjugated epitopes. Their influence on the bioactive conformation of the epitope, though admittedly crucial for relevant in vitro and in vivo applications, is difficult to evaluate, given the usual lack of information on the complex conformational features determined by the nature of the carrier and the mode of ligation. Using the Herpes simplex virus glycoprotein D-1 epitope (Leu(9)-Lys-Nle-Ala-Asp-Pro-Asn-Arg-Phe-Arg-Gly-Lys-Asp-Leu(22)) as a model, we have performed a detailed conformational analysis on the free epitope peptide in solution and on three constructs in which the epitope was conjugated to sequential oligopeptide carriers {Ac-[Lys-Aib-Gly](4)-OH (SOC(4))} (through either a thioether or an amide bond; Ac: acetyl) and polytuftsin oligomers {H-[Thr-Lys-Pro-Lys-Gly](4)-NH(2) (T20)}, (through a thioether bond). The analysis of the epitope conformation in the parent protein, in carrier-conjugated and free form, suggests that the beta-turn structure of the -Asp(13)-Pro-Asn-Arg(16)- segment is highly conserved and independent of the epitope form. However, small conformational variations were observed at the C-terminal part of the epitope, depending on the nature of the carrier.

Peptide mimics of a major lupus epitope of SmB/B'

One of the most distinguishing features of systemic lupus erythematosus is the presence of high concentrations of autoantibodies that recognize a limited number of self-antigens. Even though many lupus autoantigens have been identified, the inciting triggers of these abnormal immune responses are not fully understood. One mechanism that could generate these autoantibodies is a normal immune response toward a foreign epitope that mimics a common antigenic target of an autoantigen. Antibody generated toward the foreign epitope could also bind the autoantigen. This "cross-reactivity" would result in the presentation of the autoantigen to the immune system. Under autoimmune-prone conditions, tolerance toward the native protein is broken and an autoimmune response is initiated. Previously, it was suggested that Epstein-Barr virus might use such a mechanism to initiate an autoimmune response. Cross-reactive epitopes may have a similar amino acid sequence or a similar tertiary structure that is independent of amino acid sequence. A major, and likely initial, target of the lupus anti-SmB' response is a repeated, proline-rich sequence, PPPGMRPP. To identify potential cross-reactive targets, we used affinity-purified autoantibodies specific for PPPGMRPP to screen a random heptapeptide phage display library. Eighty-five clones were isolated and sequenced with eleven distinct sequence motifs being identified. Two of these motifs were homologous to the SmB' epitope, while the other nine were not. Interestingly, one of the peptide motifs that mimicked the SmB' epitope is identical to a peptide sequence found in the Epstein-Barr virus major DNA binding protein.

A diepitopic sequential oligopeptide carrier (SOCn) as mimic of the sm autoantigen: synthesis, conformation and biological assays

Anti-Sm (Sm: U1-U6 RNA-protein complex) antibodies are usually considered highly specific for systemic lupus erythematosus (SLE), while anti-U1RNP (U1RNP: U1RNA-protein complex) are thought of as diagnostic criteria for the mixed connective tissue disease (MCTD). However, both antibody specificities coexist in SLE and MCTD, in varying percentages. Although the anti-Sm/anti-U1RNP immunological cross-reactivity has been initially attributed to a common motif, PPXY(Z)PP (where X, Y, Z are various amino acids), found in the Sm, U1-A and U1-C autoantigens, it appears that the conformational features of the Sm epitopes also play an important role in the immunoreactivity. The PPGMRPP and PPGIRGP main epitopes of the Sm antigen were coupled in duplicate to the tetrameric Ac-(Lys-Aib-Gly)4-OH, SOC4, carrier to form the [(PPGMRPP)2, (PPGIRGP)2]-SOC4 construct as a mimic of the native Sm. It was found that: (i) the 3(10) helical structure of SOC4 allows the epitopes to adopt an exposed orientation, similar to their free forms, that facilitates their recognition from the anti-Sm antibodies, and (ii) the U1-RNP cross-reactivity is minimized.

The position of the LysN epsilon H2-grafted antigens along the sequential oligopeptide carrier, Ac-(Aib-Lys-Aib-Gly)n (SOCn-II), influences the antibody recognition: application to the Sm main autoimmune epitope

A sequential oligopeptide carrier of antigenic peptides is presented, incorporating two Aib residues in each repetitive moiety: Ac-(Aib-Lys-Aib-Gly)(n) (SOC(n) -II; n = 2-4). The conformational study, by (1)H-nmr, CD, and Fourier transform ir spectroscopy, indicated that the SOC(n) -II carrier displays a pronounced 3(10)-helix, compared to the Ac-(Lys-Aib-Gly)(n) (SOC(n) -I) carrier of the same approximately backbone length, previously reported. One of the dominant autoimmune epitopes of the Sm and U1RNP cellular components, the PPGMRPP sequence, was coupled to the Lys-N(epsilon)H(2) groups of the SOC(n) -II carrier and used as antigenic substrate for detecting anti-Sm/U1RNP autoantibodies in ELISA assays. Anti-Sm antibodies are highly specific for systemic lupus erythematosus, while anti-U1RNP are specific for mixed connective tissue disease. The anti-(PPGMRPP)(5)-SOC(n) -II ELISA was compared with the anti-(PPGMRPP)(n) -SOC(n) -I ELISA, provided that both antigenic substrates possess the same amount of the epitope replicates. The significance of the lysine positions along the oligopeptide backbone of the carrier for a favorable antibody recognition of the anchored antigens is also examined.

B-Cell epitope mapping of DNA topoisomerase I defines epitopes strongly associated with pulmonary fibrosis in systemic sclerosis

We hypothesized that B-cell epitope mapping of DNA Topoisomerase I (type-I topoisomerase, or Topo I) may define epitopes strongly associated with pulmonary interstitial fibrosis (PIF) in systemic sclerosis (SSc). B-cell epitope mapping of Topo I was performed using 63 20-mer peptides overlapping by eight residues and spanning the entire length of the Topo I sequence. These peptides, coupled to polystyrene pins, were tested for antibody binding by enzyme-linked immunosorbent assays (ELISAs) using immunoglobulin G fractions from anti-Topo I, anticentromere, anti-U3RNP-positive, and normal sera. Four major epitopes were recognized by anti-Topo I sera, but not from the control sera: WWEEERYPEGIKWKFLEHKG (205-224, epitope I), RIANFKIEPPGLFRGRGNHP (349-368, epitope II), PGHKWKEVRHDNKVTWLVSW (397-416, epitope III), and ELDGQEYVVEFDFLGKDSIR (517-536, epitope IV). Peptide-epitopes were then synthesized in their soluble forms and ELISA systems were developed. Epitopes II to IV are localized at highly exposed sites of the Topo I tertiary structure, whereas epitope I is localized at a less accessible site. In a cohort of 81 patients with SSc with clinical data on the evolution of their disease, patients with antibodies in their sera recognizing at least three of the four epitopes had 3.1 times (P = 0.02) the hazard of developing PIF compared with patients whose sera recognized no epitopes or only one or two of the four epitopes. The discrimination was much stronger than that achieved by the simple determination of Topo I antibodies by counterimmunoelectrophoresis and immunoblot (hazard ratio 1.7, P = 0.30) in the same patients. B-cell epitope mapping of the anti-Topo I response has identified four major epitopes which cumulatively show a strong association with the development of PIF in SSc.

No evidence of epitope spreading after immunization with the major Sm epitope P-P-G-M-R-P-P anchored to sequential oligopeptide carriers (SOCs)

The sequence Pro-Pro-Gly-Meth-Arg-Pro-Pro (PPGMRPP) is the major B-cell epitope of the Sm autoantigen. The aim of the present study was to evaluate the immune response against the native forms of Sm and U1RNP and immune mediated tissue injury after immunization with the sequence PPGMRPP anchored in five copies to a new type helicoid sequential oligopeptide carrier (SOC) formed by the repetitive Lys-Aib-Gly moiety, [(PPGMRPP)(5)SOC(5)]. Rabbits (n=3) were immunized with 0.5 mg of (PPGMRPP)(5)SOC(5)in complete Freud's adjuvant and boosted at days 26, 53, 99; control rabbits were immunized with the PPGMRPP alone (n=3), phosphate buffered saline (PBS) (n=1), SOC(5)alone (n=1), a peptide at aminoacid (aa) position 158-177 of myelin basic protein (MBP aa 158-147) (n=1) and three La/SSB autoantigen B-cell epitopes (n=3). Antibodies to (PPGMRPP)(5)SOC(5)were determined by enzyme linked immunosorbent assay (ELISA); precipitating anti-Sm and anti-U1RNP antibodies were detected by RNA precipitation and western blot on HeLa total cellular and nuclear extract and 12s sucrose gradient fraction of rat liver extracts. High titres of anti-(PPGMRPP)(5)SOC(5)antibodies not recognizing the native forms of Sm or U1RNP antigens were detected in the (PPGMRPP)(5)SOC(5)immunized but not in the control animals. The sera of two (PPGMRPP)(5)SOC(5)immunized but not of the control rabbits recognized a 67 kDa protein in HeLa nuclear extract, distinct from the 70 kDa U1RNP antigen. Diffuse and segmental increase of mesangeal matrix and cells, crescent formation, segmental glomerular necrosis, rarely massive subendothelial deposits occluding the lumen and C3 complement component in the mesangeal area were seen in the kidneys of one (PPGMRPP)(5)SOC(5)immunized, but not of the remaining animals. In conclusion, the immune response induced by (PPGMRPP)(5)SOC(5)was specific for the immunizing epitope but not for the native forms of Sm and U1RNP antigens, but it was associated with immune mediated kidney injury.

Arg side-chain-backbone interactions evidenced in model peptides by 17O-NMR spectroscopy

The guanidinium group of arginine possesses a variety of biochemical functions, either by participating in direct interactions in recognition processes, or by stabilizing secondary structures. Three model compounds, selectively (17)O enriched, Ac-Arg-Ala-[(17)O]Pro-NH(2) (1), Piv-Arg-Pro-[(17)O]Gly-NH(2) (2) (C-terminal segment of the luteinizing hormone releasing hormone), and Piv-Nle-Pro-[(17)O]Gly-NH(2) (3), were prepared and studied by (17)O-nmr spectroscopy. A direct hydrogen-bonded interaction between the Arg side chain and the carbonyl main chain carboxy-terminus was found, thus confirming the tendency of Arg to participate in proton-acceptor functions.

Immunization as a model for systemic lupus erythematosus

BACKGROUND

Immunization of laboratory animals is a new model system for systemic lupus erythematosus (SLE) and the autoimmunity of SLE.

OBJECTIVE

Review the published reports describing immunization as a model of SLE and describe the state of this research as well as future objectives as related to human illness.

METHODS

Medline search for relevant articles as well as review of cited bibliographies.

RESULTS

Either rabbits or mice can be immunized with proteins or oligopeptides that are lupus autoantigens with a resulting immune response not just to the immunogen but instead to a host of other self components that are also SLE-associated autoantigens. Several studies have noted clinical illness in these animals that resembles human SLE. In addition, injection of pristane (a component of mineral oil) also results in SLE-like autoimmunity, even though lupus autoantigens are not present. Pristane injected animals may also develop an SLE-like illness. There are reports of human SLE having its onset after immunization, but there have been no prospective studies.

CONCLUSIONS

Studies are needed to determine whether human SLE tends to begin soon after immunization. Meanwhile, continued study of animal models developed after immunization is needed in order to determine the relevance of this model to human disease.

RELEVANCE

SLE and/or SLE-like autoimmunity can be triggered after immunization of animals. This may be a model for an environmental trigger of human SLE.

Shared early autoantibody recognition events in the development of anti-Sm B/B' in human lupus

Many aspects of the immune maturation are uncharted. For ordinary human autoimmune systems there are no complete descriptions of the progression from an initial antigenic epitope to a maximally complex immune response. In this study we have exploited a large serial collection of human sera to investigate the development of the anti-Sm autoimmune response in systemic lupus erythematosus (SLE). The results suggest a similar, if not virtually identical, stepwise progression in the early humoral immune maturation of anti-Sm. The amino acid sequence PPPGMRPP comprises the first epitope in the anti-Sm B/B'response and its close relative, PPPGMRGP, the second. Epitopes are subsequently enlarged by the incorporation of neighbouring amino acids. The third and fourth epitopes are also recognised by an antibody in a nearly identical sequence in different lupus patients. A column absorption with PPPGMRPP demonstrates that the epitope spreading among the first four early epitopes appears to occur by the sequential generation of cross-reactive antibodies. Unexpectedly, epitope spreading in this system occurs in a predictable fashion by involving essentially the same sequence of antigenic structures from person to person. In addition, these data support the lupus anti-Sm antibodies originating against a single antigenic structure and, hence, strongly support a unifying mechanism in the generation of these autoantibodies.

A new helicoid-type sequential oligopeptide carrier (SOC(n)) for developing potent antigens and immunogens

A new class of sequential oligopeptide carriers (SOC(n)) for anchoring antigenic/immunogenic peptides has been constructed. The carrier, formed by the repetitive Lys-Aib-Gly moiety, is designed to display a predetermined 3D structure, so that the attached peptides would obtain a defined spatial orientation. Conformational analysis showed that SOC(n) adopt a distorted 3(10)-helical structure, while the coupled peptides preserve their original 'active' structure. Coupling to the carrier may also result to the enhancement of one conformer of the anchored peptide. It is concluded that the structure of SOC(n) offers an optimal presentation of the attached peptides, so that potent antigens or immunogens are generated.

Peptide carriers: A helicoid-type sequential oligopeptide carrier (SOC(n)) for multiple anchoring of antigenic/immunogenic peptides

A new peptide carrier with three-dimensional predetermined structural motif has been constructed by the repetitive Lys-Aib-Gly moiety. The sequential oligopeptide carrier (SOC(n)), (Lys-Aib-Gly)(n), adopts a distorted 3(10)-helical conformation and the Lys-N(epsilon)H(2) anchoring groups exhibit defined spatial orientations. Conformational analysis of the SOC(n) conjugates showed that the coupled peptides retain their initial "active" structure, while prevalence of one conformer was also observed. It is concluded that the beneficial structural elements of SOC(n) induce a favorable arrangement of the conjugated peptides, so that potent antigens and immunogens are generated.

A major Sm epitope anchored to sequential oligopeptide carriers is a suitable antigenic substrate to detect anti-Sm antibodies

A sensitive, highly reproducible, solid-phase enzyme immunoassay (ELISA), was developed in order to investigate whether the synthetic heptapeptide PPGMRPP-a major epitope of the Sm autoantigen-anchored in five copies to a sequential oligopeptide carrier (SOC), [(PPGMRPP)5-SOC5] is a suitable antigenic substrate to identify anti-Sm/antibodies. Sera with different autoantibody specificities [45 anti-Sm, 40 anti-U1RNP, 40 anti-Ro (SSA)/La(SSB) positive, 21 Antinuclear antibody positive, but negative for antibodies to extractable nuclear antigens (ANA + /ENA - ) and 75 normal human sera, ANA negative] and 75 sera from patients with rheumatoid arthritis (RA) were tested for anti-(PPGMRPP)5-(SOC)5 reactivity in order to evaluate the specificity and sensitivity of the method to detect anti-Sm antibodies. RNA immunoprecipitation assays for the detection of anti-Sm and anti-U1RNP antibodies and counter immunoelectrophoresis (CIE) for the detection of anti-Ro(SSA) and anti-La(SSB) antibodies were used as reference techniques. The sensitivity of the method was 98% and the specificity was 68% for the determination of anti-Sm antibodies, while for the determination of anti-Sm and/or anti-U1RNP reactivity (antibodies to snRNPs) the corresponding values were 82% and 86%, respectively. In a comparison of the above assay with an ELISA, using Sm/U1RNP purified complex as immobilized antigen it was shown that the sensitivity of the anti-Sm/U1RNP ELISA in detecting anti-snRNPs was 74%; in addition sera with anti-Sm antibodies gave higher binding in the anti-(PPGMRPP)5-(SOC)5 ELISA compared with anti-Sm/U1RNP ELISA. Intra- and inter-assay precision was measured on four sera with reactivities extending into a wide range of absorbance values showed that the intra-assay coefficient of variation (CV%) ranged from 2.7 to 6 and the inter-assay CV% ranged from 9 to 14.5. These results indicate that the PPGMRPP peptide anchored to a pentameric SOC as a carrier is a suitable antigen for detecting anti-Sm antibodies and that the above ELISA is a rapid, reproducible and valuable screening method to test anti-Sm/U1RNP reactivities.