Macrophage Migration Inhibitory Factor (MIF) Promotes the Development of Murine Chronic Lymphocytic Leukemia (CLL)

Introduction: Tumor formation results from a complex interplay between genetic/epigenetic alterations, cell cycle dysregulation and promotion by the tumor environment. Stimulation by extracellular survival factors is important for chronic lymphocytic leukemia (CLL), since the leukemic cells undergo spontaneous apoptosis when removed from their normal milieu. Since preliminary experiments demonstrated that macrophage migration inhibitory factor (MIF), a chemokine-like proinflammatory mediator and an intracellular regulator of growth and apoptosis, is overexpressed in human CLL, we investigated whether MIF participates in the pathogenesis of murine CLL.

Methods: We studied the role of MIF in CLL by crossing the Eμ-TCL1-transgenic mouse model with MIF knockout (MIF^−/−) mice. B-cell-specific overexpression of T cell leukemia-1 (TCL1) leads to accumulation and proliferation of IgM⁺/CD5⁺ mature B-cells via activation of AKT. This results in a CLL-like disease with peripheral lymphocytic leukemia, lymphadenopathy, splenomegaly, BM infiltration and premature death after 8–15 months. TCL1^+/wtMIF^−/− and TCL1^+/wtMIF^+/+ mice were compared with respect to leukemia development, tumor burden, cytogenetics and survival.

Results: The MIF receptors CD74/CD44 and CXCR2 are expressed on murine B-cells. TCL1^+/wtMIF^+/+ mice exhibited increased numbers of IgM⁺/CD5⁺ B-cells already in the preleukemic phase at month 3 and developed overt leukemia (WBC > 20G/l) 3 months earlier than their MIF^−/− counterparts (p = 0.02). Leukemia load at 12 months of age as measured by hepatosplenomegaly was increased in TCL1^+/wtMIF^+/+ animals and lymphatic organs were densely infiltrated by small, mature lymphocytes. The accelerated disease progression in the presence of MIF translated into a median survival which was 60 days shorter than in the absence of MIF (TCL1^+/wtMIF^+/+ 400 days, TCL1^+/wtMIF^−/− 460 days, p = 0.04). SKY analysis in leukemic splenocytes yielded various complex genetic aberrations with trisomies (e.g. +15), tetraploidy, translocations and deletions. Overexpression of tp53 due to the presence of an inactivating mutation in the p53 gene was found more frequently in TCL1^+/wtMIF^+/+ than in TCL1^+/wtMIF^−/− animals. Although the rates of DNA-damage-induced apoptosis in pre-leukemic and leukemic mice ex vivo were not significantly different between the genotypes, this defect in the p53-dependent apoptosis pathway corresponded with a reduced rate of spontaneous apoptosis in spleens of leukemic TCL1^+/wtMIF^+/+ animals.

Conclusions: Our experience with the Eμ-TCL-1-transgenic mice shows that this model is suitable for the identification of novel regulators of CLL-like disease. We provide genetic proof that MIF acts to promote the early preleukemic and the leukemic phase of TCL1-induced CLL and thereby identify MIF as a novel regulator of CLL pathogenesis. Ongoing efforts are focussing on further characterizing the differences in pathology, the activation of the AKT pathway and cell cycle control between TCL1^+/wtMIF^−/− and TCL1^+/wtMIF^+/+ mice.