Search

The CD44 cell surface molecule has been shown to be the principal cell surface receptor for hyaluronan (or hyaluronic acid), a glycosaminoglycan component of marrow extracellular matrix. However, its affinity for hyaluronan is not constitutive, since it depends on the cell type, the stage of differentiation and on activation by external stimuli including certain anti-CD44 antibodies and phorbol esters. Except for a few lymphoid cell lines, hematopoietic cells do not spontaneously bind hyaluronan and initial studies reported that, contrary to lymphocytes, myeloid cells could not be activated to bind hyaluronan. Because CD44 plays an important role in the initial phases of hematopoiesis, as shown by experiments using blocking anti-CD44 monoclonal antibodies, its capacity to mediate adhesion of primitive myeloid cells has been investigated. It was found that CD44 could mediate spontaneous adhesion to hyaluronan of immature myeloid cell lines KG1, KG1a, and TF1, which serve as a model for hematopoietic progenitors. However, despite expressing high amounts of CD44, no more than 15% of bone marrow progenitors could adhere to hyaluronan. Recent experiments have shown that a very important feature of CD44 is its capacity to be rapidly activated by certain antibodies and cytokines (GM-CSF and KL) from a low affinity to a high affinity state for hyaluronan. These data shed light on striking similarities in the functional regulation of CD44 and of the two integrin receptors VLA-4 (a4b1), and VLA-5 (a5b1), which are also expressed on hematopoietic progenitors. The relevance of these data to the regulation of normal hematopoiesis and mobilization of CD34+ progenitors in the view of cell grafting is analyzed. In addition, we show that in idiopathic myelofibrosis, the amount of hyaluronan is markedly increased in the extracellular matrix from the myeloproliferative spleen. Considering that the production of cytokines is enhanced in this disease, we discuss whether CD44-hyaluronan interaction may have a role in the pathophysiology of this myeloproliferative syndrome.

Receptors (R) are considered as allosteric enzymes whose action on metabolic chains is modulated through binding to the ligand. They play an essential role in the transduction of the multiple signals (e.g. interleukins or CSF) which intervene in the regulation of hematopoiesis. Their ordered interactions are necessary to regulate the growth and differentiation of normal hematopoietic precursors. This paper summarizes recent data concerning the structure-action relationship of growth-factor receptors in the signal transduction and alterations of growth-factor receptors which may play an important role in leukemic transformation. Some therapeutic modulations of growth-factors cascades are also discussed.

Granulocyte and macrophage precursors (GM-CFU-C-), which differentiate in vitro without added granulocyte and macrophage colony stimulating factor (GM-CSF), can be detected in the hematopoietic organs of mice infected with myeloproliferative sarcoma virus (MPSV). Retransplantation experiments have shown that the GM-CFU-C- are incapable of autonomous growth and depend on a factor present in medium conditioned by MPSV spleen cells (MPSV-CM). This factor is not MPSV and is not produced by spleen cells of noninfected mice. Two classical sources of GM-CSF, lung GM-CSF and GM-CSF contained in the plasma of endotoxin-treated mice, cannot replace the MPSV factor. Inversely, MPSV-CM does not stimulate the growth of retransplanted clusters induced in normal bone marrow with lung GM-CSF, whereas lung GM-CSF does. Two conditioned media containing activity promoting the in vitro proliferation and differentiation of hematopoietic stem cells in the mixed colony assay stimulate the growth of MPSV clusters: one medium was conditioned by pokeweed-mitogen-stimulated spleen cells, the other by the WEHI 3 cell line. The implication of the results in the comprehension of MPSV disease is discussed.

A subpopulation of granulocyte and macrophage precursors (GM-CFUc) differentiating in the agar colony technique of Bradley and Metcalf into mature granulocytes and macrophages, without the addition of granulocyte and macrophage colony stimulating factor (GM-CSF), can be detected in MPSV infected mice. These precursors were detected 5 days after virus infection, reaching a maximal concentration of 1/10,000 spleen or bone marrow cells, 25 days after viral infection. The number of the added GM-CSF independent GM-CFUc was linearly correlated with the number of seeded MPSV hematopoietic cells. No GM-CSF producing cells could be detected in the MPSV spleen using normal bone marrow GM-CFUc as responder cells. Study of the GM-CSF sensitivity of the GM precursors has demonstrated the existence of two GM-CFUc populations in the MPSV spleen: a) a GM-CSF dependent population with a GM-CSF sensitivity similar to that of normal GM-CFUc b) a GM-CFUc population which differentiated in the absence of detectable amount of GM-CSF and of which differentiation was not affected by the addition of progressive amounts of GM-CSF. A possible model explaining these results is proposed.