The 4-1BB glycoprotein is a member of the tumor necrosis factor receptor superfamily and binds to a high-affinity ligand (4-1BBL) expressed on several antigen-presenting cells such as macrophages and activated B cells. Expression of 4-1BB is restricted to primed CD4+ and CD8+ T cells, and 4-1BB signaling either by binding to 4-1BBL or by antibody ligation delivers a dual mitogenic signal for T-cell activation and growth. These observations suggest an important role for 4-1BB in the amplification of T cell-mediated immune responses. We now show that administration of anti-4-1BB monoclonal antibodies can eradicate established large tumors in mice, including the poorly immunogenic Ag104A sarcoma and the highly tumorigenic P815 masto cytoma. The immune response induced by anti-4- 1BB monoclonal antibodies is mediated by both CD8+ and CD4+ T cells and is accompanied by a marked augmentation of tumor-selective cytolytic T-cell activity. Our data suggest that a similar approach may be efficacious for immunotherapy of human cancer.

The 4-1BB receptor is an inducible type I membrane protein and member of the tumor necrosis factor receptor (TNFR) superfamily that is rapidly expressed on the surface of CD4+ and CD8+ T cells after antigen- or mitogen-induced activation. Cross-linking of 4-1BB and the T cell receptor (TCR) on activated T cells has been shown to deliver a costimulatory signal to T cells. Here, we expand upon previously published studies by demonstrating that CD8+ T cells when compared with CD4+ T cells are preferentially responsive to both early activation events and proliferative signals provided via the TCR and 4-1BB. In comparison, CD28-mediated costimulatory signals appear to function in a reciprocal manner to those induced through 4-1BB costimulation. In vivo examination of the effects of anti-4-1BB monoclonal antibodies (mAbs) on antigen-induced T cell activation have shown that the administration of epitope-specific anti-4-1BB mAbs amplified the generation of H-2d–specific cytotoxic T cells in a murine model of acute graft versus host disease (GVHD) and enhanced the rapidity of cardiac allograft or skin transplant rejection in mice. Cytokine analysis of in vitro activated CD4+ and CD8+ T cells revealed that anti-4-1BB costimulation markedly enhanced interferon-γ production by CD8+ T cells and that anti-4-1BB mediated proliferation of CD8+ T cells appears to be IL-2 independent. The results of these studies suggest that regulatory signals delivered by the 4-1BB receptor play an important role in the regulation of cytotoxic T cells in cellular immune responses to antigen.

We produced human monoclonal antibody that demonstrated specific reactivity to the K1 capsule of Escherichia coli and the group B polysaccharide of Neisseria meningitidis. The antibody was nonreactive with several strains of K1- E. coli and other gram-negative bacteria. All E. coli K1 clinical isolates tested were reactive with the antibody. When assayed for in vitro opsonophagocytic ability, the antibody caused bacterial removal only in the presence of human complement and neutrophils, an observation suggesting a non-bacteriolytic, neutrophil-dependent killing mechanism. Finally, and perhaps most importantly, the antibody was highly protective for infectious disease when used prophylactically in three animal models. The data suggest a potential use for human monoclonal antibodies in preventing and/or treating infections of the blood.

The survival and growth of Yersinia pestis cells within mouse peritoneal cavities and within mouse peritoneal macrophages maintained in vitro was examined. Two strains were used which differed only in that one (KIM) contained the 47-megadalton plasmid associated with virulence and the second (KIM1) lacked this plasmid. The KIM cells, but not the KIM1 cells, acquired some resistance to phagocytosis during growth at 37°C which was not evident when cells were grown at 26°C. Whether previously grown at 26 or 37°C, however, a substantial portion of the cells of either strain which were phagocytized were apparently killed after phagocytosis in vivo, although this was not observed in vitro. KIM cells which survived phagocytosis proliferated within macrophages in vivo, but no increase in viable cells was seen with the KIM1 cells. Growth of the KIM1 cells within macrophages in vitro required that a complex supportive medium be used in which the bacteria could have grown if extracellular. This was not the case for the KIM cells which proliferated within macrophages supported in medium not permissive to bacterial growth. After phagocytosis of cells of either strain by macrophages maintained in vitro, phagolysosome formation occurred normally, as shown by the acridine orange dye staining technique. KIM and KIM1 cells were equally sensitive to hydrogen peroxide and superoxide anion, although the sensitivity in each case varied with growth temperature. The oxidative burst, as determined by the luminol chemiluminescence assay, was low when compared with that seen after phagocytosis of Escherichia coli cells. Chemiluminescence after phagocytosis of yeast cells by macrophages which had engulfed KIM or KIMI was also low. We conclude that survival within macrophages is substantially independent of the 47-megadalton plasmid and may be a consequence, as least in part, of blockage of the oxidative burst or rapid removal of the oxidizing compounds formed. The 47-megadalton plasmid is apparently required for subsequent proliferation within the macrophage.

The influence of valence and heavy chain on antibody activity was investigated using transfectoma-derived, class-switched IgG1 and IgM human monoclonal antibodies (MAbs) reactive with the bacterial pathogens Escherichia coli K1 and group B Streptococcus species. IgG-IgM pairs were compared in vitro for antigen binding and opsonic activities and in vivo for protective efficacy in neonatal rats. For the anti-E. coli pair, the IgM MAb was 1000-fold more potent in all assay formats. Importantly, the 50% protection dose (PD50) of the IgM MAb was 10-20 ng/rat, while 100 micrograms of the IgG MAb was only minimally protective. For the group B streptococcal MAbs, the IgM was 100- and 4500-fold more potent in binding and opsonization assays, respectively. However, while 20 micrograms of IgM protected neonatal rats, 100 micrograms of IgG MAb was partly protective. These experiments demonstrate the utility of recombinant DNA technology for creating a panel of antibodies that may aid in selecting potential immunotherapeutic candidates.