Hematopoietic Stem Cells Are Primarily Involved In Pathogenesis of Chronic Lymphocytic Leukemia.

Abstract 4604

Chronic lymphocytic leukemia (CLL) is characterized by consistent expansion of B cells in peripheral lymphoid organs. CLL B cells express CD5 antigen, and have clonal rearrangement of immunoglobulin heavy chain (IGH) genes with the restricted usage of VH genes. CLL has thus been believed to represent retention or proliferation of abnormal B cell clones presumably with anti-apoptotic potential, or with deregulated response to auto-antigens. In this study, we extensively search for CLL-initiating cells by utilizing the xenogeneic transplantation system, in which human hematopoietic stem cells (HSCs) can normally develop multi-lineage cells including polyclonal B cells. In the xenotransplantation system, neither human (h)CD34^-hCD19⁺ circulating B cells, nor hCD34⁺hCD38⁺ bone marrow (BM) progenitor populations from CLL patients engrafted after injection of >10⁶ cells. We then transplanted hCD34⁺hCD38^- BM HSC population from 12 CLL patients. Mice transplanted with as few as 3×10³ cells of the hCD34⁺hCD38^- BM population from CLL patients exhibited multi-lineage human hematopoietic reconstitution. Surprisingly, monoclonal or oligoclonal hCD5⁺ mature CLL-like B cells were reconstituted in the mice transplanted with HSCs from 11 out of 12 CLL patients. IGH gene rearrangement analyses demonstrated that these clonal abnormal B cells derived from hCD34⁺hCD38^- HSCs of CLL patients (CLL-HSCs) were completely independent from the original leukemia. Interestingly, these clonal B cells showed highly biased usage of VH genes for their IGH rearrangement like primary CLL cells. We then checked the chromosomal abnormalities including del13q14 and del11q22 in CLL-HSCs and the clonal B cells reconstituted in the recipient mice. In 3 cases examined, both of them were negative for the chromosomal abnormalities detected in the original CLL cells, suggesting that the chromosomal abnormalities were acquired as a 2^nd leukemogeneic events during CLL progression in the patients.

The fact that CD34⁺CD38^- HSCs from CLL patients but not those from normal individuals reconstitute monoclonal B cell lymphocytosis (MBL)-like human hematopoiesis in our xenograft model strongly suggests that some genetic abnormalities for mature monoclonal B cell development are acquired already at the HSC level in CLL patients. HSCs in CLL patients are multipotent, but once they commit to the B cell lineage, they use preferentially the VH1, VH3 and VH4 regions for IGH recombination and MBL-like clonal B cell expansion occur. Our hypothesis is that such B cell clones may already be abnormal in that they clonally expand in response, for example, to auto-antigens (xeno-antigens in mice), and these preleukemic B cell clone may possibly sequentially receive additional genetic events including chromosomal abnormalities during development of CLL and finally progress to clinical CLL.

Although this xenograft model may not recapitulate full picture of CLL progression, our data clearly show that primary leukemogenic event occurs at the multipotent HSC stage in human CLL.