The expression of the low-affinity NGF receptor (p75) and the trkA proto-oncogene product was analyzed in a series of human hematopoietic cell lines at protein and RNA levels. We did not detect any form of NGF receptor in cell lines displaying a myelomonocytic phenotype (HL60 and U937). In contrast, cells displaying a more immature erythroleukemic phenotype (TF1 and K562) expressed TrkA in the absence of detectable p75. Scatchard analysis showed a single high-affinity site for NGF (kd = 10(-10) mol/L), with a copy number ranging from 300 to 3,000 sites per cell depending on the studied cell line. In addition, NGF induced autophosphorylation of TrkA and could substitute for granulocyte- monocyte colony-stimulating factor to trigger the proliferation of the TF1 cell line, with a half-maximal signal observed at 50 pmol/L, indicating that p75 is not required for DNA synthesis in this cell line. The physiologic relevance of NGF in early hematopoiesis was confirmed by showing that 12% to 15% of progenitor blood cells from mice treated with 5-fluorouracil expressed TrkA and that these cells could be induced to proliferate and differentiate in response to NGF in association with macrophage colony-stimulating factor. Our study demonstrates for the first time that trkA proto-oncogene expression and activation is not restricted to the nervous system, but is also an important element in early hematopoiesis.

Macrophage colony stimulating factor (CSF-1) is one of several cytokines that control the differentiation, survival, and proliferation of monocytes and macrophages. A set of 11 human T-cell clones, chosen for their phenotypic diversity, were tested for their ability to express CSF-1 mRNA. After 5 hours of stimulation with phorbol myristate acetate (PMA) + calcium ionophore (Cal), all T-cell clones expressed a major 4-kb transcript, a less abundant 2-kb transcript, and several other minor species. This pattern of expression is typical for CSF-1 mRNAs. Furthermore, of the two alloreactive T-cell clones analyzed, only one showed a definitive message for CSF-1 on specific antigenic stimulation, but with delayed kinetics and less efficiency. Both conditions of stimulation induced the release of CSF-1 protein by T cells in the culture medium. Together, these findings demonstrate for the first time that normal T cells are able to produce CSF-1, previous reports being limited to two cases of tumoral cells of the T-cell lineage.

The murine myeloproliferative syndrome induced by the myeloproliferative sarcoma virus (MPSV) has numerous similarities to human primary myelofibrosis. We have shown that medium conditioned by spleen cells of MPSV-infected mice has the capacity to support the growth of primitive blast cell colonies. The detection of this activity associated with MPSV infection stimulated us to characterize the hematopoietins responsible for this activity. Northern blot analysis showed a large increase, or induction, of interleukin-6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), macrophage- CSF (CSF-1), and granulocyte-CSF (G-CSF) transcripts in the hematopoietic organs of MPSV-infected mice; however, no IL-3 transcript could be detected in either MPSV-infected or normal mice. Significant levels of IL-1 alpha, IL-6, G-CSF, and CSF-1 bioactivities were found in the serum of MPSV-infected mice, but not in controls. Additionally, analysis of medium conditioned by spleen cells of MPSV-infected mice showed the presence of tumor necrosis factor alpha bioactivity. The increased production of cytokines that are able to stimulate pluripotent hematopoietic stem cells corroborates the hypothesis of a possible involvement of hematopoietic growth factors in the development of some myeloproliferative disorders.