AbstractProliferation and differentiation of hematopoietic progenitor cells are regulated by a network of stimulatory and inhibitory cytokines. An understanding of the molecular mechanisms of growth control may provide a physiologic basis for the innovative therapy of bone marrow disorders. Among various accessory cells, bone marrow T lymphocytes are capable of stimulating, as well as inhibiting, hematopoietic progenitor cells. We have now elucidated molecular mechanisms regulating the differential expression of T cell genes encoding for the stimulatory and inhibitory hematopoietic programs. Stimulation of hematopoiesis requires granulocyte‐macrophage colony stimulating factor (GM‐CSF), whereas inhibition requires interferon‐γ (IFγ). Both cytokines can be induced by interleukin‐2 (IL‐2). The T cell IL2 receptor consists of a 75 kD chain (p75) mainly expressed on a subset of resting T cells and a 55 kD chain (p55) which is strongly expressed upon T cell activation. P55 and p75 associate on activated T cells to form a dimeric receptor molecule exhibiting high affinity for IL2. The p75 monomer has an intermediate affinity for IL2. Expression of p55 in the context of the high affinity IL2 receptor constitutes a requirement for T cell IFg release. In contrast, p75 alone is capable of mediating the production of GM‐CSF. Thus, T cells may be capable of selective production of cytokines with specific effects in hematopoietic growth control. Utilizing a human peripheral blood leukocyte genomic library, we identified various clones containing the entire GM‐CSF gene, including coding and regulatory regions. Cloning of the GM‐CSF gene allowed clinical studies utilizing recombinant DNA‐derived GM‐CSF. Chemotherapy‐induced neutropenia contributes to both complications of cytotoxic therapy as well as increased relapse incidence of underlying disease. In a prospective randomized study, we have demonstrated that GM‐CSF abrogates neutropenia following aplasiogenic chemotherapy in children and adolescents with solid tumors, and that GM‐CSF may reduce the duration of infectious episodes after cytotoxic therapy. Next, we escalated the cumulative doses of cytotoxic therapy in an ablative regimen followed by hematopoietic stem cell transplantation to treat patients with poor prognosis pediatric tumors. Morbidity of this highly toxic ablative regimen depends on the duration of myeloid aplasia. Median duration of aplasia following hyper‐VAMP was 13 days with GM‐CSF and 29 days without GM‐CSF. In addition, we have employed p55 blocking monoclonal antibody for prevention of graft vs. host disease in bone marrow transplantation. The understanding of specific molecular mechanisms of hematopoietic immuno‐regulation can thus be utilized to provide novel approaches to the treatment of bone marrow failure