Chronic Expression of Murine flt3 Ligand in Mice Results in Increased Circulating White Blood Cell Levels and Abnormal Cellular Infiltrates Associated With Splenic Fibrosis

Abstract

The effect of chronic expression of flt3 ligand (FL) on in vivo hematopoiesis was studied. Retroviral vector-mediated gene transfer was used in a mouse model of bone marrow transplantation to enforce expression of mouse FL cDNA in hematopoietic tissues. As early as 2 weeks posttransplantation, peripheral blood white blood cell counts in FL-overexpressing recipients were significantly elevated compared with controls. With the exception of eosinophils, all nucleated cell lineages studied were similarly affected in these animals. Experimental animals also exhibited severe anemia and progressive loss of marrow-derived erythropoiesis. All of the FL-overexpressing animals, but none of the controls, died between 10 and 13 weeks posttransplantation. Upon histological examination, severe splenomegaly was noted, with progressive fibrosis and infiltration by abnormal lymphoreticular cells. Abnormal cell infiltration also occurred in other organ systems, including bone marrow and liver. In situ immunocytochemistry on liver sections showed that the cellular infiltrate was CD3+/NLDC145+/CD11c+, but B220− and F4/80−, suggestive of a mixed infiltrate of dendritic cells and activated T lymphocytes. Infiltration of splenic blood vessel perivascular spaces resulted in vascular compression and eventual occlusion, leading to splenic necrosis consistent with infarction. These results show that FL can affect both myeloid and lymphoid cell lineages in vivo and further demonstrate the potential toxicity of in vivo treatment with FL.

Flt3 Ligand Regulates Dendritic Cell Development from Flt3+ Lymphoid and Myeloid-committed Progenitors to Flt3+ Dendritic Cells In Vivo

“…Flt3 (also termed Flk-2 [1] and STK-1 [2]), a receptor tyrosine kinase with homology to c-Kit (the receptor for steel factor [SLF] * /stem cell factor) and c-fms (the receptor for M-CSF), is highly expressed in hematopoietic progenitor cells (for review see 3). Important information about its function in hematopoiesis has been gained through mice with targeted gene disruption of Flt3 (4) or Flt3 ligand (Flt3L; 5), through in vivo Flt3L injection (6)(7)(8) and through overexpression of Flt3L (9,10) or introduction of constitutively active flt3 mutations (11) in hematopoietic cells. Flt3 Ϫ / Ϫ mice show normal peripheral blood counts, however, pro-B cell numbers are diminished and bone marrow progenitors of these mice display a reduced ability to competitively reconstitute lethally conditioned recipients, most pronounced in the T and myeloid lineages (4).…”

“…Also, it dramatically increases NK Flt3 in Dendritic Cell Development cells (12) and DCs in bone marrow and lymphoid organs (4-27-fold), with up to 30% of spleen cells expressing CD11c and MHC class II (8,13). Mice with Flt3L overexpression or constitutive active Flt3 signaling develop myeloproliferative disease and are prone to develop leukemias with both lymphoid and myeloid marker expression (9)(10)(11). Finally, Flt3L as a single cytokine is able to induce all major DC populations in vitro from whole bone marrow cells (14,15).…”

Flt3 Ligand Induces Monocyte Proliferation and Enhances the Function of Monocyte-Derived Dendritic Cells In Vitro

“…While FL alone failed to support the growth of any type of hematopoietic colony from hematopoietic progenitor cells (HPCs), it potentiates the clonogenic response to other hematopoietic growth factors (HGFs) (Brasel et al, 1996;Juan et al, 1997). Upon systemic injection of FL to human, monocytes as well as DCs are mobilized dramatically (Pulendran et al, 2000).…”

Chronic overexpression of membrane‐bound flt3 ligand by T lymphocytes in severe aplastic anaemia

“…Levels of membrane‐bound FL were also determined in classical autoimmune diseases and in graft‐versus‐host disease (GVHD). As necrosis and fibrosis of the spleen and other organs were described in mice overexpressing FL (Juan et al , 1997), we examined splenectomy specimens and BM biopsies of patients with treatment‐refractory AA for evidence of degenerative changes resulting in fibre increase. Chronic overexpression of membrane‐bound FL was found only in AA, reflecting a long‐lasting defect in haemopoiesis and arguing for the significance of T‐cell‐derived membrane‐bound FL in the pathophysiology of BM failure.…”