Polyclonal antibodies to the encephalitogenic neighborhoods of myelin basic protein: singular affinity populations neutralized by specific synthetic peptide probes

Prediction of Variable-Length B-Cell Epitopes for Antipeptide Paratopes Using the Program HAPTIC

BACKGROUND

B-cell epitope prediction for antipeptide antibody responses enables peptide-based vaccine design and related translational applications. This entails estimating epitopeparatope binding free-energy changes from antigen sequence; but attempts to do so assuming uniform epitope length (e.g., of hexapeptide sequences, each spanning a typical paratope diameter when fully extended) have neglected empirically established variation in epitope length.

OBJECTIVE

This work aimed to develop a sequence-based physicochemical approach to variablelength B-cell epitope prediction for antipeptide paratopes recognizing flexibly disordered targets.

METHODS

Said approach was developed by analogy between epitope-paratope binding and protein folding modeled as polymer collapse, treating paratope structure implicitly. Epitope-paratope binding was thus conceptually resolved into processes of epitope compaction, collapse and contact, with epitope collapse presenting the main entropic barrier limiting epitope length among nonpolyproline sequences. The resulting algorithm was implemented as a computer program, namely the Heuristic Affinity Prediction Tool for Immune Complexes (HAPTIC), which is freely accessible via an online interface (http://badong.freeshell.org/haptic.htm). This was used in conjunction with published data on representative known peptide immunogens.

RESULTS

HAPTIC predicted immunodominant epitope sequences with lengths limited by penalties for both compaction and collapse, consistent with known paratope-bound structures of flexibly disordered epitopes. In most cases, the predicted association constant was greater than its experimentally determined counterpart but below the predicted upper bound for affinity maturation in vivo.

CONCLUSION

HAPTIC provides a physicochemically plausible means for estimating the affinity of antipeptide paratopes for sterically accessible and flexibly disordered peptidic antigen sequences by explicitly considering candidate B-cell epitopes of variable length.

On the meaning of affinity limits in B-cell epitope prediction for antipeptide antibody-mediated immunity

B-cell epitope prediction aims to aid the design of peptide-based immunogens (e.g., vaccines) for eliciting antipeptide antibodies that protect against disease, but such antibodies fail to confer protection and even promote disease if they bind with low affinity. Hence, the Immune Epitope Database (IEDB) was searched to obtain published thermodynamic and kinetic data on binding interactions of antipeptide antibodies. The data suggest that the affinity of the antibodies for their immunizing peptides appears to be limited in a manner consistent with previously proposed kinetic constraints on affinity maturation in vivo and that cross-reaction of the antibodies with proteins tends to occur with lower affinity than the corresponding reaction of the antibodies with their immunizing peptides. These observations better inform B-cell epitope prediction to avoid overestimating the affinity for both active and passive immunization; whereas active immunization is subject to limitations of affinity maturation in vivo and of the capacity to accumulate endogenous antibodies, passive immunization may transcend such limitations, possibly with the aid of artificial affinity-selection processes and of protein engineering. Additionally, protein disorder warrants further investigation as a possible supplementary criterion for B-cell epitope prediction, where such disorder obviates thermodynamically unfavorable protein structural adjustments in cross-reactions between antipeptide antibodies and proteins.

Identification of a low-affinity subset of protamine-reactive IgM antibodies present in normal, deficient in AIDS, sera: implications for HIV latency

We demonstrate here that the protamine-reactive IgM antibodies previously shown to be present in normal adult sera include two subsets differing in binding affinity. The principal, high-affinity subset was detected in AIDS and ARC as well as normal sera. The secondary, low-affinity subset, however, was absent or markedly deficient in AIDS or ARC sera. Protamine-reactive IgM antibodies were also detected in normal pediatric sera, suggesting that one subset of that class of antibodies may be "natural," i.e., not antigenically induced. The proportionate titer of the low-affinity protamine-reactive IgM antibodies was determined for HIV-positive males who were asymptomatic or mildly immune deficient at specimen collection. Of those who subsequently remained AIDS free for 18 months to 7 years, more than 90% had titers in the range established for the normal sera, while of those diagnosed with AIDS or ARC within 12 months, more than 80% had titers below the normal range. We propose that the low-affinity subset of adult sera corresponds to the natural antibodies of pediatric sera and that a relationship of those natural antibodies to resistance to progression of HIV pathogenesis is suggested.

Role of antibodies in T cell-mediated experimental allergic encephalomyelitis

The role of the humoral phase of the immune response in development of T cell-mediated experimental allergic encephalomyelitis (EAE) had not been clearly defined previously even though studies of the myelin basic protein (MBP) molecule had demonstrated the presence not only of T cell but also B cell epitopes capable of inducing cell-mediated immunity and antibody formation. Particularly relevant to this report are the immunological expressions of the region which induces EAE in the Lewis rat. The development of primary demyelination in Lewis rats is preceded by a cell-mediated immune response as well as antibody formation, both of which are highly specific to the encephalitogenic 14-residue peptide that defines the 69-84 region of the parent MBP. Our results are consistent with the dogma that EAE is a T cell-mediated disease, but they also clearly demonstrate an important role for specific antibodies in the development of these T cells responsible for demyelination. The antibody response, which may be heteroclitic, is necessary for T cells to develop into an effector T cell subset. Without this B cell response the subsequent T cell response does not lead to demyelination. In this report we shall discuss these findings and further show that the T cell and B cell epitopes, which are located within the 14-residue sequence, are physically separated and dependent upon the form of synthetic peptides known to induce T cell-mediated and/or humoral immunity.(ABSTRACT TRUNCATED AT 250 WORDS)

A major epitope of synthetic rabbit encephalitogen S24 disclosed through the use of a tritiated peptide probe

The antibody recognition of synthetic rabbit encephalitogen S24 (TTHYGSLPQKG), which had been inactivated by extensive iodination, was fully restored by tritium-iodine exchange during boron hydride reduction. A highly active form of 3H-S24 was obtained by reverse-phase high-performance liquid chromatography (HPLC) purification of the crude product and could be further separated into three continuous subfractions, the latter two of which were fully active from an immunochemical point of view. A major epitope was disclosed through use of the 3H-S24 probe, which required the intact S24 peptide for its expression. Confirmation of two linear epitopes previously encountered, GSLPQK and THYGSL, was also obtained, the latter through the use of the third HPLC subfraction of 3H-S24.

The polyclonal antibody responses of Lewis rats to the synthetic encephalitogenic neuropeptide S55S (residues 72-84 of guinea pig myelin basic protein) and its analogs

Immunochemical analysis of polyclonal antisera from Lewis rats immunized with peptide analogs and subpeptides of residues 72-84 of guinea pig myelin basic protein (MBP) indicated that there were four main epitopic specificities involved: one represented by residues 76-80, one by residues 72-74, one by 81-84-Gly, and one involving 74-76 in a heteroclitic way even when the immunogen contained a tetraleucine spacer between residues 74 and 75. The affinities were all relatively low, ranging from 3 x 10(4) M-1 to 7.2 x 10(6) M-1, and each affinity population was very restricted with respect to heterogeneity. The results were commensurate with our hypothesis that autoimmune responses to homologous encephalitogenic neuropeptide determinants reflect an evolutionarily imposed degeneration of the overall response, and were also in agreement with our hypothesis that the affinities of autoantibodies against epitopes in the molecular neighborhood of an encephalitogen would be necessarily low. It was also found that peptide 69-81-Gly (S67) in solution would not interact in liquid phase radioimmunoassays with any of the above antibody populations, thus indicating that its preferred conformation did not allow the expression of autoreactive epitopes. There was no evidence that S67 in solution. even in the presence of S53, would express any epitopes reactive with autoantibodies. How this finding for a B cell product correlates with T cell-mediated immunologic responses is not totally understood, but it will be recalled that S67 by itself does not induce T cell-mediated experimental allergic encephalomyelitis (EAE), that S53 (residues 75-84-Gly) induces only clinical symptoms of EAE without attendant pathological disease, but that the two together induce classical EAE.(ABSTRACT TRUNCATED AT 250 WORDS)

Heteroclitic antibodies in Fischer 344 rats to a synthetic encephalitogenic myelin basic protein peptide

Fischer 344 rats, immunized with the synthetic encephalitogenic myelin basic protein peptide YS49 (YGSLPQKAQRPQDENG), produced heteroclitic antibodies that reacted much more extensively and with a much higher affinity with the cross-reacting encephalitogenic guinea pig sequence S49S (GSLPQKSQRSQDENG) than they did with the immunogenic YS49. On the other hand, antisera against S49S reacted in a normal manner with homologous S49S and cross-reacted only poorly with YS49. The phenomenon of heteroclisis in Fischer 344 rats correlated with the greater encephalitogenic potency of the cross-reacting entity. Kibler et al. (J. Exp. Med., 146 (1977) 1323-1331), by comparing the encephalitogenic guinea pig sequence to a less potent analog, had also previously observed what now would be termed a heteroclitic phenomenon at the T cell level in Lewis rats. In their hands, however, as well as in ours Lewis rat antisera against the encephalitogenic peptide region were much too complex to be analyzed with respect to heteroclisis. It was shown in the present experiments that by utilizing the Fischer 344 system one may also readily obtain heteroclisis at the B cell level against encephalitogenic peptides. Neither YS49 nor S49S as immunogen produced detectable antibody in Brown Norway (BN) rats with exception of two immunized with YS49. In those two cases heteroclitic antibodies were obtained that had a very low significant (greater than 3 SD above baseline) antigen binding capacity for S49S and no detectable reactivity for the homologous YS49 ligand.