The events involved in the commitment and development of lymphoid lineage cells are poorly understood. We have used a recently described long-term culture system to establish a bioassay that can detect a novel growth factor capable of stimulating the proliferation of lymphoid progenitors. Using direct expression in mammalian cells we have isolated a complementary DNA clone encoding this novel haematopoietic growth factor, designated interleukin-7.

Monocytes are a subpopulation of peripheral blood leukocytes, which when appropriately activated by the regulatory hormones of the immune system, are capable of becoming macrophages--potent effector cells for immune response to tumors and parasites. A complementary DNA for the T lymphocyte-derived lymphokine, granulocyte-macrophage colony-stimulating factor (GM-CSF), has been cloned, and recombinant GM-CSF protein has been expressed in yeast and purified to homogeneity. This purified human recombinant GM-CSF stimulated peripheral blood monocytes in vitro to become cytotoxic for the malignant melanoma cell line A375. Another T cell-derived lymphokine, gamma-interferon (IFN-gamma), also stimulated peripheral blood monocytes to become tumoricidal against this malignant cell line. When IFN-gamma activates monocytes to become tumoricidal, additional stimulation by exogenously added lipopolysaccharide is required. No such exogenous signals were required for the activation of monocytes by GM-CSF.

Hemopoiesis in both mouse and man has been the focus of intense investigation in recent years. Dramatic progress has been achieved in resolving the various growth factors involved in many of the major hemopoietic lineages . The cDNAs encoding growth and differentiation factors active on multipotential cells (1-3) and on the granulocytic (4), eosinophilic (5), monocytic (6), and erythroid (7) lineages have now been cloned and expressed . In contrast, very little is known about the regulatory factors involved in the commitment and differentiation of precursor cells along the pathway of lymphogenesis. The pluripotent lymphoid stem cell has not been identified, nor have the factors or conditions required for commitment and expansion of the B cell lineage. The later stages of B cell growth and differentiation after the appearance of surface Ig and the emergence of the B cell from the bone marrow have been the most well studied and have revealed a number of factors that are active on mature peripheral B cells. These include IL-1 (8), IL-2 (9), IL-4 (10, 11), IL-5 (12), IFN-y (13), IFN-,B2 (14), neuroleukin (15), and transforming growth factor ,8 (16) .Most of the available evidence on B cell generation has been limited to those B cells that are thought to be the immediate precursors of mature functional peripheral B cells. These pre-B cells have been defined as cells containing cytoplasmic 1A chain but no cytoplasmic light chain and no surface Ig (17,18). The information on these cells has primarily involved their phenotypic characterization and localization (19-23), Ig gene rearrangements (24-26), and mitogenic responses (27)(28)(29). It has also been demonstrated that pre-B cells can differentiate to antibody-secreting B cells in vitro (30, 31). In one report (32) IL-1 has been shown to induce the maturation of either pre-B cells from mixed cell cultures or the pre-B cell lymphoma 70 Z/3. In addition, it was reported that a humoral factor from the serum of NZB mice could enhance the maturation of B lineage precursor cells (33). Finally, Landreth et al. (34) have demonstrated the presence of a factor (or factors) that stimulated the formation of pre-B and B cells in cultures of human or mouse bone marrow .Although there has been progress in understanding B cell genesis, the major impediment to the study of lymphocyte development has been the difficulties associated with obtaining highly enriched populations of viable B cell precursors . 988 J. Exp. MED.

Murine spleen cells activated by concanavalin A (Con A) in culture produce a class of lymphokine molecules which possess biological activity in a number of lymphocyte response assays. Lymphokines with a mol wt of 30,000, as estimated from gel filtration studies, can be resolved into two components which differ by charge, with isoelectric point (pI) values of 4.3 and 4.9, respectively. Both components stimulate (a) the growth of established T-cell lines in culture, (b) the proliferation of thymocytes in the presence of Con A under culture conditions where Con A alone is nonmitogenic, (c) the induction of antibody responses to heterologous erythrocyte antigens in athymic (nude) spleen cultures, (d) the generation of cytotoxic T lymphocytes (CTL) in thymocyte cultures, and (e) the generation of CTL in nude spleen cultures. In each of these culture systems we suggest that the assays are detecting a single class of lymphokine which acts directly on activated T cells. Nonactivated T cells must be stimulated by either antigen or mitogen before becoming responsive to lymphokine, but do not require antigen or mitogen for continued growth with lymphokine. The two molecular species, separable by isoelectric focusing are referred to as the T-cell growth factor (TCGF). A lymphokine, similar in size (30,000 daltons) to TCGF but heterogeneous in charge (pI 3.0--4.0), stimulates immune responses to erythrocyte antigens in T-cell-depleted spleen cultures but has no stimulatory activity in the other lymphocyte assay systems described. The data have been interpreted as showing the two molecular forms of murine TCGF (pI 4.3 and 4.9) are responsible for many of the lymphokine activities described elsewhere as thymocyte mitogenic factor, nonspecific T-cell-replacing factor and killer helper factor or costimulator. The other lymphokine, separable from TCGF by charge, appears to have true T-cell-replacing activity.

Human granulocyte/macrophage colonystimulating factor (GM-CSF) is a glycoprotein that is essential for the in vitro proliferation and differentiation of precursor cells into mature granulocytes and macrophages. In this report we have used a mouse GM-CSF cDNA clone to isolate human GM-CSF clones from libraries made from HUT-102 messenger RNA and mitogen-stimulated T-lymphocyte messenger RNA. The human cDNA clones contained a single open-reading frame encoding a protein of 144 amino acids with a predicted molecular mass of 16,293 daltons and showed 69% nucleotide homology and 54% amino acid homology to mouse GM-CSF.One of these cDNA clones was shown to direct the synthesis of biologically active GM-CSF using a yeast expression system.