Human monoclonal antibodies isolated from spontaneous Epstein-Barr virus-transformed tumors of Hu-SPL-SCID mice and specific for fusion protein display broad neutralizing activity toward respiratory syncytial virus

Human respiratory syncytial virus: pathogenesis, immune responses, and current vaccine approaches

Respiratory syncytial virus continues to pose a serious threat to the pediatric populations worldwide. With a genomic makeup of 15,200 nucleotides, the virus encodes for 11 proteins serving as envelope spikes, inner envelope proteins, and non-structural and ribonucleocapsid complexes. The fusion (F) and attachment (G) surface glycoproteins are the key targets for neutralizing antibodies. The highly variable G with altered glycosylations and the conformational alternations of F create challenges for vaccine development. The metastable F protein is responsible for RSV-host cell fusion and thus infectivity. Novel antigenic sites were identified on this form following its stabilization and solving its crystal structure. Importantly, site ø displays neutralizing activity exceeding those of post-F-specific and shared antigenic sites, such as site II which is the target for Palivizumab therapeutic antibody. Induction of high neutralizing antibody responses by pre-F immunization in animal models promoted it as a major vaccine candidate. Since RSV infection is more serious at age extremities and in individuals with undermining health conditions, vaccines are being developed to target these populations. Infants below three months of age have a suppressive immune system, making vaccines' immunogenicity weak. Therefore, a suggested strategy to protect newborns from RSV infection would be through passive immunity of maternal antibodies. Hence, pregnant women at their third trimester have been selected as an ideal target for vaccination with RSV pre-F vaccine. This review summarizes the different modes of RSV pathogenesis and host's immune response to the infection, and illustrates on the latest updates of vaccine development and vaccination approaches.

Evaluating Human Immune Responses for Vaccine Development in a Novel Human Spleen Cell-Engrafted NOD-SCID-IL2rγNull Mouse Model

The lack of preclinical models able to faithfully predict the immune responses which are later obtained in the clinic is a major hurdle for vaccines development as it increases markedly the delays and the costs required to perform clinical studies. We developed and evaluated the relevance to human immune responses of a novel humanized mouse model, humanized-spleen cells-NOD-SCID-gamma null (Hu-SPL-NSG), in which we grafted human spleen cells in immunodeficient NOD-SCID-IL-2rγnull (NSG) mice. We selected the malaria vaccine candidate, Liver Stage Antigen 3-Full Length, because we had previously observed a major discrepancy between preclinical and clinical results, and compared its immunogenicity with that of a shorter form of the molecule, LSA3-729. NSG mice engrafted with human spleen lymphocytes were immunized with either LSA3-FL or LSA3-729, both adjuvanted with montanide ISA720. We found that the shorter LSA3-729 triggered the production of human antibodies and a T-helper-type 1 cellular immune response associated with protection whereas LSA3-FL did not. Results were consistent in five groups receiving lymphocytes from five distinct human donors. We identified antigenic regions in the full-length molecule, but not in the shorter version, which induced T-regulatory type of cellular responses. These regions had failed to be predicted by previous preclinical experiments in a wide range of animal models, including primates. Results were reproducible using spleen cells from all five human donors. The findings in the Hu-SPL-NSG model were similar to the results obtained using LSA3-FL in the clinic and hence could have been used to predict them. The model does not present graft versus host reaction, low survival of engrafted B lymphocytes and difficulty to raise primary immune responses, all limitations previously reported in humanized immune-compromised mice. Results also point to the shorter construct, LSA3-729 as a more efficient vaccine candidate. In summary, our findings indicate that the Hu-SPL-NSG model could be a relevant and cost-saving choice for early selection of vaccine candidates before clinical development, and deserves being further evaluated.

Prophylactic and therapeutic testing of Nicotiana-derived RSV-neutralizing human monoclonal antibodies in the cotton rat model

Severe lower respiratory tract infection in infants and small children is commonly caused by respiratory syncytial virus (RSV). Palivizumab (Synagis(®)), a humanized IgG1 monoclonal antibody (mAb) approved for RSV immunoprophylaxis in at-risk neonates, is highly effective, but pharmacoeconomic analyses suggest its use may not be cost-effective. Previously described potent RSV neutralizers (human Fab R19 and F2-5; human IgG RF-1 and RF-2) were produced in IgG format in a rapid and inexpensive Nicotiana-based manufacturing system for comparison with palivizumab. Both plant-derived (palivizumab-N) and commercial palivizumab, which is produced in a mouse myeloma cell line, showed protection in prophylactic (p < 0.001 for both mAbs) and therapeutic protocols (p < 0.001 and p < 0.05 respectively). The additional plant-derived human mAbs directed against alternative epitopes displayed neutralizing activity, but conferred less protection in vivo than palivizumab-N or palivizumab. Palivizumab remains one of the most efficacious RSV mAbs described to date. Production in plants may reduce manufacturing costs and improve the pharmacoeconomics of RSV immunoprophylaxis and therapy.

Plasmodium falciparum: production of human antibodies specific for the MSP-3 protein in the Hu-SPL-SCID mouse

Human immunity against Plasmodium falciparum malaria is mediated by IgG antibodies. One of the major targets of protective antibodies is the MSP-3 protein. Anti-MSP-3 human monoclonal antibodies could therefore be valuable for passive immunotherapy, particularly of drug resistant malaria. Human monoclonal antibodies were previously produced in the Hu-SPL-SCID model reconstituted with human splenocytes, immunized by highly immunogenic neo-antigen or a recall antigen. We report here that this model can also be successfully employed to induce human antibody-secreting cells specific of low immunogenicity neo-antigens, such as MSP-3. These cells represent a new and valuable source of human monoclonal anti-malaria antibodies.

A review of modifications to recombinant antibodies: attempt to increase efficacy in oncology applications

Although monoclonal antibodies have high specificity, their usefulness in the clinic, especially against solid tumors, has been limited. This arises in part from the inability of antibody molecules to penetrate into the tumor and kill the tumor cells. In addition, natural cytotoxic effects of antibodies, mediated through complement or Fc receptors, may not be sufficient to kill malignant cells. This review will present some of the antibody modifications used to increase efficacy. Modified recombinant antibodies have been designed to be more cytotoxic (immunotoxins), to increase natural effector functions (bivalent antibodies, antibody-fusion molecules, multimeric antibodies, directed mutations in Fc region), or to pretarget cells for concentration of cytotoxic drugs. This review will also focus on engineering of smaller versions of antibodies that retain specificity (single chain Fvs, Fabs, Fab(2)s, minibodies, domain deleted antibodies) and have increased penetrability of solid tumors. Many of these antibody modifications may result in antigenic compounds which can limit repeat administration. Clinical experiences will be highlighted if information is available.

A humanized anti-human CD154 monoclonal antibody blocks CD154-CD40 mediated human B cell activation

Humanized anti-CD154 antibody, IDEC-131, had a slightly, but reproducibly, better binding affinity for CD154 (Kd = 5.6 nM), compared to the parent antibody 24-31 (Kd = 8.5 nM). Otherwise it was indistinguishable from the murine parent antibody in its ability to bind to CD154, block CD154 binding to CD40 and inhibit T cell-dependent B cell differentiation. The latter activity was independent of FcR binding as the Fab'1 fragment of IDEC-131 had an equivalent biological activity to that of the whole antibody. IDEC-131 blocked soluble CD154 from inducing proliferation of purified B cells, and blocked T cell dependent anti-tetanus toxoid specific antibody production by human B cells in vitro. IDEC-131, gamma1, kappa, had strong Fc gammaRI, Fc gammaRII and C1q binding, but was unable to induce complement dependent (CDC) or antibody dependent cell-cytotoxicity (ADCC) of activated peripheral blood T cells, which express relatively low levels of CD154. IDEC-131 antibody inhibited both primary and secondary antibody responses to ovalbumin in cynomolgus monkeys at a dose of 5 mg/kg. In non-immunized animals, treatment with IDEC-131 at 50 mg/kg weekly for 13 weeks induced no change in any of the measured lymphocyte subsets, including B cells, CD4+ and CD8+ T cells. Similarly, a safety study in chimpanzees showed no discernible safety related issues at 20 mg/kg, including B and T cell subsets. These results show that the humanized anti-CD154 antibody, IDEC-131, has retained the affinity and functional activity of its murine parent antibody, is unlikely to deplete CD154 positive lymphocytes in humans, and is safe and effective in blocking antibody production in monkeys. Based on its safety and efficacy profile, IDEC-131 is being developed for therapy of systemic lupus erythematosus.