Interleukin-4 receptor expression by human B cells: functional analysis with a human interleukin-4 toxin, DAB389IL-4

Recombinant toxins in haematologic malignancies and solid tumours

Recombinant toxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. They are fusion proteins, which contain toxin and ligand regions, and are produced in Escherichia coli. The ligand may be a growth factor or a fragment of an antibody, and the toxin is usually one of the two bacterial toxins: Pseudomonas exotoxin and diphtheria toxin. Compared to the earlier generation chemical conjugates of ligands and toxins, recombinant toxins have many advantages, including homogeneity with respect to the connection between the ligand and toxin, ease and yield of production and small size. A variety of chemotherapy-resistant haematologic and solid tumours have been targeted with recombinant toxins, and clinical trials with many of them have recently demonstrated their effectiveness. Moreover, their unwanted toxic effects are different from those of most chemotherapeutic agents, supporting the expectation that they can be combined with existing modalities to improve the clinical resources available to treat cancer in humans.

Novel diversity in IL-4-mediated responses in resting human naive B cells versus germinal center/memory B cells

Recent studies have defined several phenotypic and molecular changes associated with the maturation of naive human B cells within the milieu of germinal centers. Although naive B cells serve as natural precursors to germinal center (GC)/memory (M) subpopulations, little is known about the physiological requirements for the survival of the naive B cell pool in the absence of cell-cell contact or Ag-mediated activation. Because IL-4 induces expression of several membrane receptors such as CD23 which are uniquely present on resting human naive B lymphocytes, we hypothesized that these cells might be intrinsically programmed to respond to IL-4 in the absence of cell division. Using buoyant density-dependent isolation and further enrichment by negative/positive selection of human naive and GC/M subpopulations, we characterized cytokine receptor moieties on these cells and analyzed their survival and growth in the presence of IL-4 or IL-10. Resting naive B cells expressed significantly higher IL-4 receptor alpha-chain on their cell surface than the combined GC/M subpopulation. The IL-10 receptor and the IL-2 receptor gammac chain were almost equally expressed on both subpopulations. When cultured in vitro, the addition of IL-4, but not IL-10, protected naive B cells from apoptosis in the absence of activation and growth. However, IL-4 exerted no such effect on resting GC/M B cells. These data support the hypothesis that IL-4 plays a pivotal role in the survival and maintenance of resting human naive B cells.

Molecular mechanisms of IgE regulation

IgE antibody plays an important role in allergic diseases. IgE synthesis by B cells requires two signals. The first signal is delivered by the cytokines IL-4 or IL-13, which target the Cepsilon gene for switch recombination. The second signal is delivered by interaction of the B cell surface antigen CD40 with its ligand (CD40L) expressed on activated T cells. This activates deletional switch recombination. We review the molecular mechanisms of IL-4 and CD40 signaling that lead to IgE isotype switching and discuss the implications for intervening to abort or suppress the IgE antibody response.

Specific enzyme-linked immunosorbent assays for quantitation of antibody-cytokine fusion proteins

Preliminary testing has shown in vitro and in vivo that antitumor activity can be obtained with fusion proteins linking tumor-reactive monoclonal antibodies to cytokines, such as granulocyte-macrophage colony-stimulating factor or interleukin 2 (IL-2). Preclinical and clinical testing of these reagents requires their in vitro and in vivo quantitation and pharmacokinetic evaluation. We have focused on the detection of a fusion protein which links one human IL-2 molecule to the carboxy terminus of each heavy chain of the tumor-reactive human-mouse chimeric anti-GD2 antibody, ch14.18. We have developed enzyme-linked immunosorbent assays (ELISAs) to evaluate intact tumor-reactive fusion proteins. By these ELISAs we can reliably measure nanogram quantities of intact ch14.18-IL-2 fusion protein and distinguish the intact protein from its components (ch14.18 and IL-2) in buffer, mouse serum, and human serum with specificity and reproducibility. The measurement of intact ch14.18-IL-2 fusion protein is not confounded by free IL-2 or free ch14.18 when 100 ng or less of total immunoglobulin per ml is used during the assay procedure. Our results indicate that these ELISAs are suitable for preclinical and clinical testing and with slight modifications are applicable to the analysis of a variety of other fusion proteins.

IL-4R expression in AIDS-KS cells and response to rhIL-4 and IL-4 toxin (DAB389-IL-4)

Interleukin-4 (IL-4) is a pleiotropic cytokine affecting growth and differentiation of various cell types as well as regulating other cytokines. To study the effect of IL-4 on AIDS-related Kaposi's sarcoma (AIDS-KS) cells, we first examined the tumor cells for IL-4 receptor (IL-4R) expression. KS cells express a single 4 kB IL-4R-specific mRNA and 1828 +/- 408 high affinity IL-4 binding sites per cell with a dissociation constant (Kd) of 154 +/- 37 pM. Addition of recombinant human IL-4 (rIL-4) minimally inhibited AIDS-KS cell growth and expression of IL-6. We then studied the effects of a chimeric fusion toxin DAB389-IL-4 which exerts cellular toxicity only on cells expressing IL-4R. DAB389-IL-4 inhibited protein synthesis in AIDS-KS cells at low concentrations (IC50 of 5 x 10(-11) M). This effect was abrogated by neutralizing antibody to IL-4 (25D2). We conclude that KS cells express a functional IL-4R and this receptor could serve as a target for novel therapy with agents such as DAB389-IL-4.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.