Elimination of an immunodominant CD4+ T cell epitope in human IFN-beta does not result in an in vivo response directed at the subdominant epitope

Pentostatin plus cyclophosphamide safely and effectively prevents immunotoxin immunogenicity in murine hosts

PURPOSE

The success of immunotoxin therapy of cancer is limited by host production of neutralizing antibodies, which are directed toward the Pseudomonas exotoxin A (PE) component. In this proof-of-principle study using a well-established murine model, we hypothesized that a newly developed immune depletion regimen consisting of pentostatin plus cyclophosphamide would abrogate anti-immunotoxin reactivity.

EXPERIMENTAL DESIGN

BALB/c hosts were injected weekly with recombinant immunotoxin (RIT) SS1P, which is an antimesothelin Fv antibody fragment genetically fused to a 38 kDa portion of PE, and has been evaluated in clinical trials. Experimental cohorts received induction chemotherapy consisting of pentostatin (P) plus cyclophosphamide (C) prior to initial RIT exposure; some cohorts received further maintenance PC therapy of varying intensity just prior to each weekly RIT challenge. Cohorts were monitored for T, B, myeloid cell depletion, and for total anti-SS1P antibody (Ab) formation.

RESULTS

Controls uniformly developed anti-SS1P Ab after the third RIT exposure. Induction PC therapy reduced the frequency of hosts with anti-SS1P Ab. Abrogation of antibody generation was improved by maintenance PC therapy: nearly 100% of recipients of intensive PC maintenance were free of anti-SS1P Ab after 9 weekly RIT doses. The most effective PC regimen yielded the greatest degree of host B-cell depletion, moderate T-cell depletion, and minimal myeloid cell depletion.

CONCLUSIONS

Induction and maintenance PC chemotherapy safely prevented anti-immunotoxin antibody formation with uniform efficacy. These data suggest that immunotoxin therapy might be used in combination with pentostatin plus cyclophosphamide chemotherapy to improve the targeted therapy of cancer.

Recombinant immunotoxin against B-cell malignancies with no immunogenicity in mice by removal of B-cell epitopes

Many nonhuman proteins have useful pharmacological activities, but are infrequently effective in humans because of their high immunogenicity. A recombinant immunotoxin (HA22, CAT8015, moxetumomab pasudotox) composed of an anti-CD22 antibody variable fragment fused to PE38, a 38-kDa portion of Pseudomonas exotoxin A, has produced many complete remissions in drug-resistant hairy-cell leukemia when several cycles of the agent can be given, but has much less activity when antibodies develop. We have pursued a strategy to deimmunize recombinant immunotoxins by identifying and removing B-cell epitopes. We previously reported that we could eliminate most B-cell epitopes using a combination of point mutations and deletions. Here we show the location and amino acid composition of all of the B-cell epitopes in the remaining 25-kDa portion of Pseudomonas exotoxin. Using this information, we eliminated these epitopes to produce an immunotoxin (HA22-LR-8M) that is fully cytotoxic against malignant B-cell lines, has high cytotoxic activity against cells directly isolated from patients with chronic lymphocytic leukemia, and has excellent antitumor activity in mice. HA22-LR-8M does not induce antibody formation in mice when given repeatedly by intravenous injection and does not induce a secondary antibody response when given to mice previously exposed to HA22. HA22-LR-8M also has greatly reduced antigenicity when exposed to sera from patients who have produced antibodies to HA22. The properties of HA22-LR-8M make it an excellent candidate for further clinical development.

On the role of aggregates in the immunogenicity of recombinant human interferon beta in patients with multiple sclerosis

Like many other therapeutic proteins, recombinant human interferon beta (rhIFN-β) elicits undesirable immune responses. rhIFN-β-treated multiple sclerosis patients may form binding antibodies and neutralizing antibodies (NAbs), with the latter being responsible for inhibition of the therapeutic effect of the protein. The incidence of binding antibodies and NAbs against rhIFN-β as well as the titer and persistence of NAbs differ among the marketed products. The proportion of patients forming antibodies against rhIFN-β-1b is higher than that against rhIFN-β-1a, which is likely explained by the differences in protein structure and aggregation behavior between the 2 types of rhIFN-β. Here, we summarize the different factors influencing the immunogenicity of rhIFN-β in patients with multiple sclerosis and discuss the role played by rhIFN-β aggregates.

Therapeutic enzyme deimmunization by combinatorial T-cell epitope removal using neutral drift

A number of heterologous enzymes have been investigated for cancer treatment and other therapeutic applications; however, immunogenicity issues have limited their clinical utility. Here, a new approach has been created for heterologous enzyme deimmunization whereby combinatorial saturation mutagenesis is coupled with a screening strategy that capitalizes on the evolutionary biology concept of neutral drift, and combined with iterative computational prediction of T-cell epitopes to achieve extensive reengineering of a protein sequence for reduced MHC-II binding propensity without affecting catalytic and pharmacological properties. Escherichia coli L-asparaginase II (EcAII), the only nonhuman enzyme approved for repeated administration, is critical in treatment of childhood acute lymphoblastic leukemia (ALL), but elicits adverse antibody responses in a significant fraction of patients. The neutral drift screening of combinatorial saturation mutagenesis libraries at a total of 12 positions was used to isolate an EcAII variant containing eight amino acid substitutions within computationally predicted T-cell epitopes--of which four were nonconservative--while still exhibiting k(cat)/K(M) = 10(6) M(-1) s(-1) for L-Asn hydrolysis. Further, immunization of HLA-transgenic mice expressing the ALL-associated DRB1*0401 allele with the engineered variant resulted in significantly reduced T-cell responses and a 10-fold reduction in anti-EcAII IgG titers relative to the existing therapeutic. This significant reduction in the immunogenicity of EcAII may be clinically relevant for ALL treatment and illustrates the potential of employing neutral drift screens to achieve large jumps in sequence space as may be required for the deimmunization of heterologous proteins.

Quantification of the preexisting CD4 T-cell repertoire specific for human erythropoietin reveals its immunogenicity potential

Antibody-mediated pure red cell aplasia is a rare but serious event resulting from the induction of neutralizing erythropoietin (Epo)-specific antibodies provoked by treatment with recombinant Epo. Because of the crucial role of CD4 T cells in humoral response, we have quantified the number of Epo-specific CD4 T cells in the blood of normal donors by in vitro stimulation. An important repertoire of preexisting Epo-specific T cells was observed in almost half of the donors, comparable with that of non-self-proteins. This observation suggests that, at the steady state, endogenous Epo weakly contributes to tolerance induction and may be ignored by the immune system. As a result, circulating Epo-specific CD4 T cells could be prone to be activated by altered batches of Epo, providing them with costimulatory signals. Our data also highlight the relevance of T-cell assays performed with normal donors to evaluate the potential immunogenicity of therapeutic proteins.

Quality and vaccine efficacy of CD4+ T cell responses directed to dominant and subdominant epitopes in ESAT-6 from Mycobacterium tuberculosis

The ESAT-6 (early secretory antigenic target) molecule is a very important target for T cell recognition during infection with Mycobacterium tuberculosis. Although ESAT-6 contains numerous potential T cell epitopes, the immune response during infection is often focused toward a few immunodominant epitopes. By immunization with individual overlapping synthetic peptides in cationic liposomes (cationic adjuvant formulation, CAF01) we demonstrate that the ESAT-6 molecule contains several subdominant epitopes that are not recognized in H-2(d/b) mice either during tuberculosis infection or after immunization with ESAT-6/CAF01. Immunization with a truncated ESAT-6 molecule (Delta15ESAT-6) that lacks the immunodominant ESAT-6(1-15) epitope refocuses the response to include T cells directed to these subdominant epitopes. After aerosol infection of immunized mice, T cells directed to both dominant (ESAT-6-immunized) and subdominant epitopes (Delta15ESAT-6-immunized) proliferate and are recruited to the lung. The vaccine-promoted response consists mainly of double- (TNF-alpha and IL-2) or triple-positive (IFN-gamma, TNF-alpha, and IL-2) polyfunctional T cells. This polyfunctional quality of the CD4(+) T cell response is maintained unchanged even during the later stages of infection, whereas the naturally occurring infection stimulates a response to the ESAT-6(1-15) epitope that consist almost exclusively of CD4(+) effector T cells. ESAT-6 and Delta15ESAT-6 both give significant protection against aerosol challenge with tuberculosis, but the most efficient protection against pulmonary infection is mediated by the subdominant T cell repertoire primed by Delta15ESAT-6.

New approaches to prediction of immune responses to therapeutic proteins during preclinical development

Clinical studies show that immunogenicity observed against therapeutic proteins can limit efficacy and reduce the safety of the treatment. It is therefore beneficial to be able to predict the immunogenicity of therapeutic proteins before they enter the clinic. Studies using deimmunized proteins have highlighted the importance of T-cell epitopes in the generation of undesirable immunogenicity. In silico, in vitro, ex vivo and in vivo methods have therefore been developed that focus on identification of CD4+ T-cell epitopes in the sequence of therapeutic proteins. A case study of existing therapeutic proteins is presented to review these different approaches in order to assess their utility in predicting immunogenic potential.

Structure−Function Engineering of Interferon-β-1b for Improving Stability, Solubility, Potency, Immunogenicity, and Pharmacokinetic Properties by Site-Selective Mono-PEGylation

Recombinant interferon-beta-1b (IFN-beta-1b) is used clinically in the treatment of multiple sclerosis. In common with many biological ligands, IFN-beta-1b exhibits a relatively short serum half-life, and bioavailability may be further diminished by neutralizing antibodies. While PEGylation is an approach commonly employed to increase the blood residency time of protein therapeutics, there is a further requisite for molecular engineering approaches to also address the stability, solubility, aggregation, immunogenicity and in vivo exposure of therapeutic proteins. We investigated these five parameters of recombinant human IFN-beta-1b in over 20 site-selective mono-PEGylated or multi-PEGylated IFN-beta-1b bioconjugates. Primary amines were modified by single or multiple attachments of poly(ethylene glycol), either site-specifically at the N-terminus, or randomly on the 11 lysines. In two alternate approaches, site-directed mutagenesis was independently employed in the construction of designed IFN-beta-1b variants containing either a single free cysteine or lysine for site-specific PEGylation. Optimization of conjugate preparation with 12 kDa, 20 kDa, 30 kDa, and 40 kDa amine-selective PEG polymers was achieved, and a comparison of the structural and functional properties of the IFN-beta-1b proteins and their PEGylated counterparts was conducted. Peptide mapping and MALDI-TOF mass spectrometric analysis confirmed the attachment sites of the PEG polymer. Independent biochemical and bioactivity analyses, including antiviral and antiproliferation bioassays, circular dichroism, capillary electrophoresis, flow cytometric profiling, reversed phase and size exclusion HPLC, and immunoassays demonstrated that the functional activities of the designed IFN-beta-1b conjugates were maintained, while the formation of soluble or insoluble aggregates of IFN-beta-1b was ameliorated. Immunogenicity and pharmacokinetic studies of selected PEGylated IFN-beta-1b compounds in mice and rats demonstrated both diminished IgG responses, and over 100-fold expanded AUC exposure relative to the unmodified protein. The results demonstrate the capacity of this macromolecular engineering strategy to address both pharmacological and formulation challenges for a highly hydrophobic, aggregation-prone protein. The properties of a lead mono-PEGylated candidate, 40 kDa PEG2-IFN-beta-1b, were further investigated in formulation optimization and biological studies.

A beta-lactamase with reduced immunogenicity for the targeted delivery of chemotherapeutics using antibody-directed enzyme prodrug therapy

Antibody-directed enzyme prodrug therapy (ADEPT) delivers chemotherapeutic agents in high concentration to tumor tissue while minimizing systemic drug exposure. beta-Lactamases are particularly useful enzymes for ADEPT systems due to their unique substrate specificity that allows the activation of a variety of lactam-based prodrugs with minimal interference from mammalian enzymes. We evaluated the amino acid sequence of beta-lactamase from Enterobacter cloacae for the presence of human T-cell epitopes using a cell-based proliferation assay using samples from 65 community donors. We observed a low background response that is consistent with a lack of preexposure to this enzyme. beta-Lactamase was found to contain four CD4+ T-cell epitopes. For two of these epitopes, we identified single amino acid changes that result in significantly reduced proliferative responses while retaining stability and activity of the enzyme. The beta-lactamase variant containing both changes induces significantly less proliferation in human and mouse cell assays, and 5-fold lower levels of IgG1 in mice were observed after repeat administration of beta-lactamase variant with adjuvant. The beta-lactamase variant should be very suitable for the construction of ADEPT fusion proteins, as it combines high activity toward lactam prodrugs, high plasma stability, a monomeric architecture, and a relatively low risk of eliciting an immune response in patients.

Clinical link between MHC class II haplotype and interferon-beta (IFN-beta) immunogenicity

Interferon-beta (IFN-beta) is currently the first-line therapy for the treatment of multiple sclerosis (MS). However, a significant percentage of MS patients develop anti-IFN-beta antibodies, which can reduce the efficacy of the drug. We describe an association between a common MHC class II allele (DRB1*0701), present in 23% of the patients studied, and the anti-IFN-beta antibody response. We identified IFN-beta epitopes using a peptide-binding assay with B cell lines expressing this allele. Moreover, epitope-specific activation responses obtained with peripheral blood mononuclear cells (PBMCs) from IFN-beta treated patients with the DRB1*0701 allele indicated a role for T-cell activation in IFN-beta immunogenicity. These results suggest that HLA typing of MS patients may provide an accurate screen for subjects who are likely to develop anti-IFN-beta antibodies and should therefore be considered for alternative therapies. In addition, elucidation of the factors underlying the anti-IFN-beta antibody response should accelerate the engineering of less immunogenic IFN-beta therapeutics.