Rabbits transgenic for human IgG genes recapitulating rabbit B-cell biology to generate human antibodies of high specificity and affinity.

Transgenic animals incorporating human antibody genes are extremely attractive for drug development because they obviate subsequent antibody humanization procedures required for therapeutic translation. Transgenic platforms have previously been established using mice, but also more recently rats, chickens, and cows and are now in abundant use for drug development. However, rabbit-based antibody generation, with a strong track record for specificity and affinity, is able to include gene conversion mediated sequence diversification, thereby enhancing binder maturation and improving the variance/selection of output antibodies in a different way than in rodents. Since it additionally frequently permits good binder generation against antigens that are only weakly immunogenic in other organisms, it is a highly interesting species for therapeutic antibody generation. We report here on the generation, utilization, and analysis of the first transgenic rabbit strain for human antibody production. Through the knockout of endogenous IgM genes and the introduction of human immunoglobulin sequences, this rabbit strain has been engineered to generate a highly diverse human IgG antibody repertoire. We further incorporated human CD79a/b and Bcl2 (B-cell lymphoma 2) genes, which enhance B-cell receptor expression and B-cell survival. Following immunization against the angiogenic factor BMP9 (Bone Morphogenetic Proteins 9), we were able to isolate a set of exquisitely affine and specific neutralizing antibodies from these rabbits. Sequence analysis of these binders revealed that both somatic hypermutation and gene conversion are fully operational in this strain, without compromising the very high degree of humanness. This powerful new transgenic strategy will allow further expansion of the use of endogenous immune mechanisms in drug development.

Induction of regular cytolytic T cell synapses by bispecific single-chain antibody constructs on MHC class I-negative tumor cells.

Certain bispecific single-chain antibody constructs exhibit an extraordinary potency for polyclonal T cell engagement and target cell lysis. Here we studied the structural basis for this potency, using laser scanning confocal microscopy. Cytolytic human T cell synapses could be triggered either by addition of a specific peptide antigen or an Ep-CAM-/CD3-bispecific T cell engager (BiTE). Both kinds of synapses showed a comparable distribution of all protein markers investigated. Two other BiTEs constructed from different Ep-CAM-specific antibodies gave similar results. BiTEs could also induce lytic synapses between human T cells and a MHC class I-negative, Ep-CAM cDNA-transfected cell line resulting in potent target cell lysis. This shows that certain T cell recognition molecules on target cells are dispensable for synapse formation and BiTE activity, and suggests that BiTE-activated polyclonal T cells may ignore major immune evasion mechanisms of tumor cells in vivo, such as loss of MHC class I expression.

MT110: a novel bispecific single-chain antibody construct with high efficacy in eradicating established tumors.

We have developed a novel single-chain Ep-CAM-/CD3-bispecific single-chain antibody construct designated MT110. MT110 redirected unstimulated human peripheral T cells to induce the specific lysis of every Ep-CAM-expressing tumor cell line tested. MT110 induced a costimulation independent polyclonal activation of CD4- and CD8-positive T cells as seen by de novo expression of CD69 and CD25, and secretion of interferon gamma, tumor necrosis factor alpha, and interleukins 2, 4 and 10. CD8-positive T cells made the major contribution to redirected tumor cell lysis by MT110. With a delay, CD4-positive cells could also contribute presumably as consequence of a dramatic upregulation of granzyme B expression. MT110 was highly efficacious in a NOD/SCID mouse model with subcutaneously growing SW480 human colon cancer cells. Five daily doses of 1 microg MT110 on days 0-4 completely prevented tumor outgrowth in all mice treated. The bispecific antibody construct also led to a durable eradication of established tumors in all mice treated with 1 microg doses of MT110 on days 8-12 after tumor inoculation. Finally, MT110 could eradicate patient-derived metastatic ovarian cancer tissue growing under the skin of NOD/SCID mice. MT110 appears as an attractive bispecific antibody candidate for treatment of human Ep-CAM-overexpressing carcinomas.

Epithelial tight junction proteins as potential antibody targets for pancarcinoma therapy.

Recombinant monoclonal antibodies are beginning to revolutionize cancer therapy. In combination with standard chemotherapy, high response rates have been reported with antibodies of the human IgG1 isotype for treatment of non-Hodgkin's lymphoma and breast cancer. It is becoming apparent that targets for antibody-based therapies do not necessarily need to be absent from normal tissues but can be present there either in low copy numbers or with binding epitopes shielded from the therapeutic antibody. Here, we studied whether claudin proteins that form tight junctions in normal epithelia are still expressed on carcinoma cells and whether their extracellular domains can be recognized by antibodies. We show that mRNAs of claudins 1, 3, 4, and 7 are all expressed in different human carcinoma cell lines, while claudin 8 was selectively expressed in breast and pancreas cancer lines. Chicken polyclonal antibodies were raised against peptides contained within predicted extracellular domains of claudins 1, 3, and 4. Affinity-purified IgG fractions for claudins 3 and 4 were monospecific and bound to human breast and colon carcinoma lines, but not to a line of monocytic origin. Claudin 3 antibodies also homogeneously stained human renal cell carcinoma tissue and micrometastatic tumor cells as identified by cytokeratin staining in bone marrow biopsies of breast cancer patients. Fluorescence-activated cell sorting and immunocytochemistry indicated that claudin antibodies bound to the surface of tumor cells. By analogy to other tumor-associated antigens that are differentially accessible to antibodies on tumor vs normal tissue, we propose that certain claudin proteins have potential as targets for novel antibody-based therapies of carcinomas.

Combined transcriptome and genome analysis of single micrometastatic cells.

In human cancer, early systemic spread of tumor cells is recognized as a leading cause of death. Adjuvant therapies are administered to patients after complete resectioning of their primary tumors to eradicate the few residual and latent metastatic cells. These therapeutic regimens, however, are currently designed without direct information about the presence or nature of the latent cells. To address this problem, we developed a PCR-based technique to analyze the transcriptome of individual tumor cells isolated from the bone marrow of cancer patients. From the same cells, genomic aberrations were identified by comparative genomic hybridization. The utility of this approach for understanding the biology of occult disseminated cells and for the identification of new therapeutic targets is demonstrated here by the detection of frequent extracellular matrix metalloproteinase inducer (EMMPRIN; CD147) expression which was verified by immunostaining.

Structural characteristics of halobacterial gas vesicles.

Gas vesicle formation in halophilic archaea is encoded by a DNA region (the vac region) containing 14 different genes: gvpACNO and gvpDEFGHIJKLM. In Halobacterium salinarum PHH1 (which expresses the p-vac region from plasmid pHH1), gas vesicles are spindle shaped, whereas predominantly cylindrical gas vesicles are synthesized by the chromosomal c-vac region of H. salinarum PHH4 and the single chromosomal mc-vac region of Haloferax mediterranei. Homologous complementation of gvp gene clusters derived from the chromosomal c-vac region led to cylindrical gas vesicles in transformants and proved that the activity of the c-gvpA promoter depended on a gene product from the c-gvpE-M DNA region. Heterologous complementation experiments with transcription units of different vac regions demonstrated that the formation of chimeric gas vesicles was possible. Comparison of micrographs of wild-type and chimeric gas vesicles indicated that the shape was not exclusively determined by GvpA, the major structural protein of the gas vesicle wall. More likely, a dynamic equilibrium of several gvp gene products was responsible for determination of the shape. Transmission electron microscopy of frozen hydrated, wild-type gas vesicles showed moiré patterns due to the superposition of the front and back parts of the ribbed gas vesicle envelope. Comparison of projections of model helices with the moiré pattern seen on the cylindrical part of the gas vesicles provided evidence that the ribs formed a helix of low pitch and not a stack of hoops.