Generation and Characterization Of a CD30 Positive T-Cell Lymphoma Mouse Model Resembling Human Anaplastic Large Cell Lymphoma

Anaplastic large cell lymphomas (ALCL) are a subgroup of aggressive Non-Hodgkin-Lymphomas mainly affecting children and young adults. In 60 % of systemic ALCLs, a translocation t(2;5) (p23;q35) resulting in NPM-ALK fusion gene expression is found. The constitutively activation of ALK tyrosine kinase expressed from the NPM-promoter causes increased proliferation and inhibition of apoptosis thereby promoting cell survival and tumorigenesis. Immunphenotypic characterization of human ALCLs revealed highly CD30-positive cells of T- or Null-Cell-origin and resulted in promising clinical trials with CD30-coupled antibodies. However, the impact of CD30 on diseases development as well as NPM-ALK signal transduction in course of disease remains unclear and appropriate mouse models to answer these questions are missing.

In this regard, we established a retroviral murine bone marrow (BM) transplantation model resembling a human ALCL-like T-cell neoplasia. Therefore we use an inducible Cre/loxP system where NPM-ALK expression is controlled and expressed in a special type of early T-cells. For generation of this vector, we inserted a floxed translational ‘stop-cassette’ between the retroviral promoter MSCV-LTR and the NPM-ALK cDNA, which guaranties specific expression of NPM-ALK only in cells, where the enzyme Cre-recombinase is expressed. Recognition of the loxP-sites by Cre-recombinase leads in our system to deletion of the stop-cassette and consequently NPM-ALK expression. Using different Cre-expressing cell types allowed us to study pathogenesis of ALCL in more detail.

In our recent study, we infected bone marrow of transgenic mice expressing Cre-recombinase under the control of the Lck-promotor with our MSCV-Stop-NPM-ALK-IRES-EGFP (MSNAIE) vector and transplanted it into lethally irradiated C57Bl6 recipient mice. With a latency of 4-5 months, these mice developed Thy1.2-positive lymphomas and died from neoplastic infiltration of bone marrow and lymphatic organs with T-cells. Immunphenotypic analyses confirmed T-Cell origin of the lymphomas and showed importantly highly CD30-expression. Staining of the different T-cell-subpopulations demonstrated highest NPM-ALK expression in immature CD4/CD8 double negative T-cells and not fully differentiated CD4/CD8 double positive T-cells. Interestingly, FACS-staining of the proliferation marker Ki-67 revealed highest expression in CD4/CD8 double negative T-cells, in contrast to the other subpopulations where Ki-67 is less detected. Therefore we hypothesized, that the lymphoma initiating cell (LIC) must be within this early T-cell population. Most interestingly we found highest CD30-expression just in the same CD4/CD8 negative T-cell population, pointing to a crucial role of CD30 in lymphoma initiation.

To further substantiate our hypothesis we performed secondary and tertiary transplantations with different sorted T-Cell subpopulation and indeed, the immature CD4/CD8 double negative population was able to initiate lymphoma growth in recipient mice. Further transplantations by limited dilution will help to identify the leukemia initiating cell in this model.

Taken together, our murine LckCre-NPM-ALK bone marrow transplantation model represents a precise and versatile tool to study disease initiation and development resembling human ALCL. Moreover, the impact of specific proteins (e.g. CD30) in the course of disease can be addressed by combining Knockout (e.g. CD30)/LckCre transgenic mice with our model. To this end we crossed CD30/Lck-Cre mice, and preliminary analysis indicate that CD30 expression seems not to be required for the initial onset of disease. Further characterization of the role of CD30 in ALCL is ongoing.