Studies on antigenic competition

Haemophilus influenzae Protein D antibody suppression in a multi-component vaccine formulation

Nontypeable Haemophilus influenzae (NTHi) has emerged as a dominant mucosal pathogen causing acute otitis media (AOM) in children, acute sinusitis in children and adults, and acute exacerbations of chronic bronchitis in adults. Consequently, there is an urgent need to develop a vaccine to protect against NTHi infection. A multi-component vaccine will be desirable to avoid emergence of strains expressing modified proteins allowing vaccine escape. Protein D (PD), outer membrane protein (OMP) 26, and Protein 6 (P6) are leading protein vaccine candidates against NTHi. In pre-clinical research using mouse models, we found that recombinantly expressed PD, OMP26, and P6 induce robust antibody responses after vaccination as individual vaccines, but when PD and OMP26 were combined into a single vaccine formulation, PD antibody levels were significantly lower. We postulated that PD and OMP26 physiochemically interacted to mask PD antigenic epitopes resulting in the observed effect on antibody response. However, column chromatography and mass spectrometry analysis did not support our hypothesis. We postulated that the effect might be in vivo through the mechanism of protein vaccine immunologic antigenic competition. We found when PD and OMP26 were injected into the same leg or separate legs of mice, so that antigens were immunologically processed at the same or different regional lymph nodes, respectively, antibody levels to PD were significantly lower with same leg vaccination. Different leg vaccination produced PD antibody levels quantitatively similar to vaccination with PD alone. We conclude that mixing PD and OMP26 into a single vaccine formulation requires further formulation studies.

Antigen- and scaffold-specific antibody responses to protein nanoparticle immunogens

Protein nanoparticle scaffolds are increasingly used in next-generation vaccine designs, and several have established records of clinical safety and efficacy. Yet the rules for how immune responses specific to nanoparticle scaffolds affect the immunogenicity of displayed antigens have not been established. Here we define relationships between anti-scaffold and antigen-specific antibody responses elicited by protein nanoparticle immunogens. We report that dampening anti-scaffold responses by physical masking does not enhance antigen-specific antibody responses. In a series of immunogens that all use the same nanoparticle scaffold but display four different antigens, only HIV-1 envelope glycoprotein (Env) is subdominant to the scaffold. However, we also demonstrate that scaffold-specific antibody responses can competitively inhibit antigen-specific responses when the scaffold is provided in excess. Overall, our results suggest that anti-scaffold antibody responses are unlikely to suppress antigen-specific antibody responses for protein nanoparticle immunogens in which the antigen is immunodominant over the scaffold.

Cocktail Anti-Tick Vaccines: The Unforeseen Constraints and Approaches toward Enhanced Efficacies

Ticks are second to mosquitoes as vectors of disease. Ticks affect livestock industries in Asia, Africa and Australia at ~$1.13 billion USD per annum. For instance, 80% of the global cattle population is at risk of infestation by the Rhipicephalus microplus species-complex, which in 2016 was estimated to cause $22-30 billion USD annual losses. Although the management of tick populations mainly relies on the application of acaricides, this raises concerns due to tick resistance and accumulation of chemical residues in milk, meat, and the environment. To counteract acaricide-resistant tick populations, immunological tick control is regarded among the most promising sustainable strategies. Indeed, immense efforts have been devoted toward identifying tick vaccine antigens. Until now, Bm86-based vaccines have been the most effective under field conditions, but they have shown mixed success worldwide. Currently, of the two Bm86 vaccines commercialized in the 1990s (GavacTM in Cuba and TickGARDPLUSTM in Australia), only GavacTM is available. There is thus growing consensus that combining antigens could broaden the protection range and enhance the efficacies of tick vaccines. Yet, the anticipated outcomes have not been achieved under field conditions. Therefore, this review demystifies the potential limitations and proposes ways of sustaining enhanced cocktail tick vaccine efficacy.

Massive immune response against IVIg interferes with response against other antigens in mice: A new mode of action?

Administration of high dose intravenous immunoglobulin (IVIg) is widely used in the clinic to treat autoimmune and severe inflammatory diseases. However, its mechanisms of action remain poorly understood. We assessed the impact of IVIg on immune cell populations using an in vivo ovalbumin (Ova)-immunization mouse model. High dose IVIg significantly reduced the Ova-specific antibody response. Intriguingly, the results obtained indicate an immediate and massive immune reaction against IVIg, as shown by the activation and expansion of B cells and CD4+ T cells in the spleen and draining lymph nodes and the production of IVIg-specific antibodies. We propose that IVIg competes at the T-cell level with the response against Ova to explain the immunomodulatory properties of IVIg. Two monoclonal antibodies did not succeeded in reproducing the effects of IVIg. This suggests that in addition to the mouse response against human constant domains, the enormous sequence diversity of IVIg may significantly contribute to this massive immune response against IVIg. While correlation of these findings to IVIg-treated patients remains to be explored, our data demonstrate for the first time that IVIg re-directs the immune response towards IVIg and away from a specific antigen response.

A multi-valent vaccine approach that elicits broad immunity within an influenza subtype

Vaccines directed toward individual strains of highly variable viruses like influenza lose efficacy when the circulating viruses no longer resemble the vaccine isolate. Historically, inclusion of more than one isolate per subtype of influenza has been limited by the need to include large doses of antigen with typical protein-based vaccine approaches and by concerns that an immunodominant response to one antigen will limit the response to closely related antigens. Here we provide proof of principle demonstrating that a multi-valent vaccine directed against multiple influenza A virus hemagglutinins (HAs) can elicit broad, neutralizing immunity against multiple strains within a single influenza subtype (H3). We employed a DNA vaccine to direct immunity toward the HA component alone, and a live attenuated influenza virus (LAIV) to assess immunity against the whole virus. Delivery of either HA-DNA or LAIV yielded broad protective immunity across multiple antigenic clusters, including heterologous strains, that was similar to the combined immunity of each antigen assessed separately. Priming with HA-DNA followed by an LAIV boost strengthened and broadened the antibody response toward all three H3 HAs. This prime:boost multi-valent approach was thus able to elicit immunity against multiple strains within the H3 subtype without evidence of immune interference between closely related antigens. Although the trivalent vaccine described here is not a universal vaccine, since protection was limited to circulating viruses from about a two-decade period, these data suggest that full protection within a subtype is possible using this approach with multiple antigens from current and predicted future influenza strains.

Enhancement of the T cell response to a mycobacterial peptide by conjugation to synthetic branched polypeptide

A peptide-based approach towards improving the immunodiagnosis of, and vaccination against, tuberculosis faces the problems of MHC restriction of T cell recognition and the poor immunogenicity of peptides in the absence of adjuvant. We sought to compensate this by the use of synthetic branched polypeptides of the poly[Lys-(Xi-DL-Alam)] type, containing a glutamic acid residue (EAK), and further modified either by succinylation (SucEAK) or acetylation (AcEAK). These carriers were conjugated to two permissively recognized peptides of Mycobacterium tuberculosis. The 38p350 - 369-SucEAK conjugate enhanced IFN-gamma production more than 13-fold (from 22.6 to 294 pg / ml, p = 0.001) in peripheral blood mononuclear cells from healthy subjects, and 8.7-fold (p = 0. 012) in cells from tuberculosis patients. The effect was dependent on the carrier used and on covalent linkage of SucEAK to 38p350 - 369. An increased response occurred best in cells from subjects bearing at least one HLA-DR allele for which 38p350 - 369 had high binding affinity and required cellular processing of the conjugate as inhibitors (chloroquine and wortmannin) blocked the IFN-gamma response. SucEAK conjugation of peptide 16p91 - 110 did not significantly increase IFN-gamma production, indicating that the ability of conjugation to enhance the response was peptide structure dependent. These data indicate that the use of SucEAK polymer coupled with permissively recognized peptides could contribute to the development of an improved immunodiagnostic or vaccine reagent for tuberculosis.

The detection of single cells forming antibodies to defined epitopes on insulin

A solid-phase immunoenzymatic technique was modified to permit the detection and enumeration of antibody-secreting cells recognizing the amino or carboxy terminal of the insulin B chain. The procedure involves coating well surfaces with avidin and binding insulins specifically labelled with biotin at the B1 or B30 residue. On day 15 after immunization with human insulin, 20 outbred NMRI mice had generated cells secreting anti-insulin antibodies. The recognition of B1- or B30-related epitopes differed between individuals, suggesting that there was genetic determination of the epitopes expressed early in the immune response. The method can be used to find strains of mice with a preferential immune response to defined areas on the surface of small peptide molecules. Such strains could then be used to produce specific monoclonal antibodies.

Purified influenza virus hemagglutinin and neuraminidase are equivalent in stimulation of antibody response but induce contrasting types of immunity to infection

BALB/c mice immunized with graded doses of chromatographically purified hemagglutinin (HA) and neuraminidase (NA) antigens derived from A/Hong Kong/1/68 (H3N2) influenza virus demonstrated equivalent responses when HA-specific and NA-specific serum antibodies were measured by enzyme-linked immunosorbent assays (ELISAs). Antibody responses measured by hemagglutination inhibition or neuraminidase inhibition titrations showed similar kinetic patterns, except for more rapid decline in hemagglutination inhibition antibody. Injection of mice with either purified HA or NA resulted in immunity manifested by reduction in pulmonary virus following challenge with virus containing homologous antigens. However, the nature of the immunity induced by the two antigens differed markedly. While HA immunization with all but the lowest doses of antigen prevented manifest infection, immunization with NA was infection-permissive at all antigen doses, although reduction in pulmonary virus was proportional to the amount of antigen administered. The immunizing but infection-permissive effect of NA immunization over a wide range of doses is in accord with results of earlier studies with mice in which single doses of NA and antigenically hybrid viruses were used. The demonstrable immunogenicity of highly purified NA as a single glycoprotein without adjuvant offers a novel infection-permissive approach with potentially low toxicity for human immunization against influenza virus.

Inhibition of allergic encephalomyelitis by the iron chelating agent desferrioxamine: differential effect depending on type of sensitizing encephalitogen

Induction of experimental allergic encephalomyelitis (EAE) in Lewis rats by injection of guinea pig (GP) spinal cord homogenate (SCH) plus adjuvant (SCH-CFA) can be inhibited by treatment with the iron chelating agent desferrioxamine (DFOM). Interestingly, induction of EAE with purified myelin basic protein (BP-CFA) is not inhibited with DFOM. This dichotomy does not appear to be due to any quantitative differences in the two inocula since minimal clinical EAE produced by threshold levels of BP is not inhibited with DFOM. Passive EAE is not inhibited irrespective of the type of encephalitogen used to sensitize the donors. This suggests that the inhibitory effect of DFOM is acting on the afferent limb of the immune response to SCH-CFA. Injection of BP-CFA and SCH-CFA into the same site, mixing BP with central nervous system (CNS) lipids, or incorporating BP into liposomes, all induce EAE which can be partially inhibited by treatment with DFOM. These results support the hypothesis that the close association of lipids with the encephalitogen (i.e. BP) in SCH required extensive lipid breakdown before adequate antigen presentation can occur, and it is at this level that DFOM exerts its inhibitory effect.

Competition between skin-sensitizing chemicals in the mouse

The skin contact sensitivity responses to picryl chloride in CBA mice can be reduced by prior sensitization with oxazolone. Initial experiments showed this reduction to be significant when the interval between skin paintings was 7 days. In further experiments to study the time course of this effect, the depression was found to be maximal when the interval between skin paintings was between 3 and 7 days.

Prior painting with a non-immunogenic chemical irritant, oil of turpentine, did not depress responses to picryl chloride.

The relation of this phenomenon to antigenic competition in antibody production is discussed.

Studies on antigenic competition. 3. Effect on antigenic competition on antibody affinity

The effect of antigenic competition on antibody affinity was studied using a haptenic system in guinea-pigs. A moderate depression in the amount of antibody formed, as a result of antigenic competition, had relatively little effect on affinity. Increasing the dose of the competing antigen resulted in a greater degree of competition. Under these conditions a large amount of low affinity antibody was produced by the animals while essentially no high affinity antibody was detectable. Thus, marked competition appeared to result in a failure to select for high affinity antibody synthesis.

Studies on antigenic competition. II. Abolition of antigenic competition by antibody against or tolerance to the dominant antigen: a model for antigenic competition

Antigenic competition between the Fc and Fab fragments of rabbit IgG in mice could be abolished by passive immunization with antiserum against the dominant antigen (Fc). The induction of tolerance to Fc also eliminated antigenic competition. Anti-Fd production, generally very poor in response to rabbit IgG, was considerably enhanced by these procedures, indicating that it is also subject to antigenic competition with Fc.

A model is proposed to explain antigenic competition which proposes that it is the `co-operative antibody' produced to the dominant antigen which acts as an inhibitor to antibody production to the suppressed antigen by competing for sites —presumably on macrophage membranes—where co-operation occurs.

The merits and difficulties of such an explanation are discussed.

Studies on antigenic competition. II. Evidence for effect at level of antigen 'processing'

Antigenic competition between two haptenic determinants was studied. It was shown that antigenic competition was greater if a 1- or 2-week interval is imposed between injection of the two antigens. Preimmunization with one antigen or with the carrier protein to which one hapten is coupled will decrease the effect of antigenic competition on the antibody response to that antigen or hapten and bring about a greater degree of depression of the antibody response to the second haptenic determinant. Finally, antigenic competition does not occur if the two antigens are injected so as to drain into different groups of regional lymph nodes. This is true even if a 3- or 7-day time interval is imposed between injection of the two antigens.

The results are interpreted as suggesting that competition occurs at the level of the antigen `processing' or `localizing' step in the immune response.

Studies on antigenic competition. I. Antigenic competition between the Fc and Fab fragments of rabbit IgG in mice

In the antibody response to rabbit IgG in BALB/c mice, antigenic competition has been demonstrated between Fc and Fab'. Similar competition was found when a mixture of Fc and Fab' was used as antigen. In both cases the presence of Fc suppressed the antibody response to Fab'. Competition was demonstrable only in the primary antibody response. The effect on competition of varying the routes and the time course of administration of the antigens was investigated.

Suppressive effect of graft versus host reactions on the immune response to heterologous red cells

Graft versus host (GVH) reactions induced by the inoculation of parental spleen cells into adult untreated F₁ recipients caused a marked suppression of the cellular and humoral immune response to sheep red cells (SRBC) and Escherichia coli lipopolysaccharide, provided the GVH reaction was induced 7 days before immunization with SRBC. Adoptive transfer of parental or F₁ spleen cells mixed with SRBC into irradiated F₁ recipients, which had been subjected to a GVH reaction for 7 days, resulted in marked suppression of cellular antibody synthesis to both antigens.

When the GVH reaction was induced by parental spleen cells from donors immunized to SRBC and the immune response to SRBC tested after 7 days, a marked suppression of cellular antibody synthesis occurred when the parental cells were of H-2^b genotype, but not if they were of H-2^a or H-2^k genotypes.

The number of antigen-sensitive cells of parental genotype sensitive to SRBC in animals being subjected to a GVH reaction for 7 days was unaffected when the donors were of H-2^a or H-2^k genotypes, but decreased when the donors were H-2^b. The number of antigen-sensitive cells of F₁ genotype was only slightly decreased by a GVH reaction.

It is suggested that the suppressive effect of a GVH reaction on antibody synthesis to other antigens represents an example of antigenic competition. This phenomenon would be caused by suppressed proliferation of immunocompetent cells of bone-marrow origin and not by competition for pluripotent antigen-sensitive cells. This suppression would be mediated by antigen stimulated thymus-derived lymphocytes.

Antigenic Competition Between and Endotoxic Adjuvant and a Protein Antigen

Antigenic competition between bovine gamma globulin (BGG) and endotoxin from a smooth strain (S-ET) and a rough (R-ET) heptoseless mutant strain of Salmonella minnesota was studied in mice. Both endotoxins acted as adjuvants for enhancing the antibody response to BGG. However, other work showed that the R-ET had minimal antigenicity, and it was used as a control for the competition studies. Antigenic competition between BGG and endotoxin as expressed by a suppression of the antibody response to BGG could not be demonstrated when varying adjuvant doses of S-ET or R-ET were injected simultaneously with a small constant dose of BGG into normal mice. However, mice presensitized with S-ET several weeks before immunization with the S-ET and BGG combination produced anti-BGG levels which were four to eightfold lower than in normal mice. Nearly complete suppression of the anti-BGG response could be obtained in presensitized mice by reducing the BGG dose 10-fold or by increasing the adjuvant dose of endotoxin. Mice pretreated with R-ET and challenged with BGG plus S-ET or R-ET showed no depression of the anti-BGG response. These and other experiments confirmed the immunological basis of the competitive effect.

The production and affinity of anti-hapten antibody under the influence of various inhibitory conditions

The effects of four experimental conditions leading to lower antibody production (minute amount of antigen, excess of antigen, treatment with methotrexate and competition with an unrelated antigen) on the amount and affinity of antibody produced by injecting an optimal immunizing amount of dinitrophenylated bovine serum albumin (DNP-BSA) in adult guinea-pigs were compared.

Adequate use of these four conditions resulted in a rather similar degree of inhibition, antibody production averaging 15 per cent of that produced by optimally stimulated animals. Affinity for the hapten DNP-ε-aminocaproic acid of the antibody produced following stimulation with a minute amount of antigen or following treatment with methotrexate was found to be closely similar to the affinity of antibodies produced by optimally stimulated animals, Both immunization with an excess of antigen and immunization with an optimal amount of DNP-BSA mixed with an excess of an unrelated antigen (bovine γ-globulin), led to the production of antibody of low affinity.

These results seem to indicate that the competing antigen interfered with the test antigen at the level of the antibody producing cell.