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.

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.

A cDNA encoding biologically active human interleukin 7 was isolated by hybridization with the homologous murine clone. Nucleotide sequence analysis indicated that this cDNA was capable of encoding a protein of 177 amino acids with a signal sequence of 25 amino acids and a calculated mass of 17.4 kDa for the mature protein. Recombinant human interleukin 7 stimulated the proliferation of murine pre-B cells and was active on cells harvested from human bone marrow that are enriched for B-lineage progenitor cells. Analysis of RNA by blot hybridization demonstrated the presence of two size classes of interleukin 7 mRNA in human splenic and thymic tissue.

A murine cell line (IxN/2b) absolutely dependent upon exogenous IL-7 for continued growth has been obtained that expresses lymphoid precursor and class I MHC antigens and also contains a rearranged mu heavy chain. This cell line has been used to define the binding and structural characteristics of the murine IL-7 receptor using 125I-labeled recombinant murine IL-7. 125I-IL-7 binding to IxN/2b cell was rapid and saturable at both 4 degrees and 37 degrees C. Equilibrium binding studies produced curvilinear Scatchard plots at both temperatures with high and low affinity Ka values of approximately 1 x 10(10) M-1 and 4 x 10(8) M-1, respectively, and a total of 2,000-2,500 IL-7 binding sites expressed per cell. Experiments measuring inhibition of binding of 125I-IL-7 by unlabeled IL-7 also produced data consistent with the existence of two classes of IL-7 receptors. Evidence concerning the possible molecular nature of two classes of IL-7 receptors was provided by dissociation kinetics and affinity crosslinking experiments. The dissociation rate of 125I-IL-7 was markedly increased when measured in the presence of unlabeled IL-7 at both 37 degrees and 4 degrees C, which is diagnostic of a receptor population displaying negative cooperativity. Crosslinking studies showed that under both reducing and nonreducing conditions, the major crosslinked species observed corresponded to a receptor size of 75-79 kD while a less intense higher molecular mass crosslinked species was also seen which corresponded to a receptor size approximately twice as large (159-162 kD). Both types of experiments suggest that the IL-7 receptor may form noncovalently associated dimers in the membrane. The IL-7 receptor was expressed on pre-B cells, but not detected on several murine B cell lines or primary mature B cells. It was also expressed on murine thymocytes, some T lineage cell lines, and on bone marrow-derived macrophage. All cells binding 125I-IL-7 exhibited curvilinear Scatchard plots. No cytokines or growth factors tested were able to inhibit binding of 125I-IL-7 to its receptor. These results define the initial binding and structural characteristics, and the cellular distribution, of the murine IL-7 receptor.

The relationship between endothelial cell growth and surface properties of plasma-deposited films (PDFs) was investigated using partial least-squares regression (PLS). PDFs of oxygen-containing precursors were prepared under various conditions, and bovine arterial endothelial cells (BAECs) were grown on these substrates. Secondary ion mass spectrometry (SIMS) in the static mode was used to characterize the surface chemistry of these substrates. The growth of BAECs on the PDFs was correlated to the positive and negative static SIMS spectra of the PDFs by PLS. A good correlation between the SIMS spectra of PDFs and endothelial cell growth was obtained. Qualitative information was also extracted from the multivariate model, giving some information as to the most important variables influencing BAEC growth.