The ligand for CD40 (CD40L) is a membrane glycoprotein on activated T cells that induces B cell proliferation and immunoglobulin secretion. Abnormalities in the CD40L gene were associated with an X-linked immunodeficiency in humans [hyper-IgM (immunoglobulin M) syndrome]. This disease is characterized by elevated concentrations of serum IgM and decreased amounts of all other isotypes. CD40L complementary DNAs from three of four patients with this syndrome contained distinct point mutations. Recombinant expression of two of the mutant CD40L complementary DNAs resulted in proteins incapable of binding to CD40 and unable to induce proliferation or IgE secretion from normal B cells. Activated T cells from the four affected patients failed to express wild-type CD40L, although their B cells responded normally to wild-type CD40L. Thus, these CD40L defects lead to a T cell abnormality that results in the failure of patient B cells to undergo immunoglobulin class switching.

Using a bioassay consisting of the proliferation of a murine B cell line, a cDNA of a gene whose product supports the growth of that cell line was isolated from a thymic stromal cell line. This factor, termed thymic stromal lymphopoietin (TSLP), is a protein of 140 amino acids. The gene encoding TSLP was mapped to murine chromosome 18. Purified recombinant TSLP supported the growth of pre-B cell colonies in vitro, but had no myelopoietic activity. TSLP had comitogenic activity for fetal thymocytes, but was not as potent as interleukin 7 in lobe submersion cultures. Injection of TSLP into neonatal mice induced the expansion of B220+BP-1+ pre-B cells.

Mice homozygous for the viable Si allele steel-Dickie (Si") are sterile, severely anemic, and black-eyed white. The nature of the Sid mutation was investigated at the molecular level and was found to be due to a 4.0-kilobase intragenic deletion in mast cell growth factor (MGF) genomic sequences, providing conclusive evidence that SI encodes MGF.As a consequence ofthis deletion, Sidis only capable ofencoding a soluble truncated growth factor that lacks both transmembrane and cytoplasmic domains. Northern analysis indicates that Sid mRNA is expressed at approximately wild-type levels in adult tissues, and yeast expression studies suggest that the Sid protein is as biologically active as wild-type soluble MGF.These studies provide a molecular basis for explaining the Sid phenotype, a description of a germ-line mutation in the transmembrane and cytoplasmic domains of a membrane-bound growth factor, and in vivo evidence for the importance of membrane-bound forms of growth factors in mammalian development.Mice carrying mutations at the dominant white spotting (W) and steel (So) loci have deficiencies in pigment cells, germ cells, and blood cells (reviewed in ref. 1). While phenotypically similar, W mutations act cell-autonomously whereas Si mutations exert their effects in the extracellular environment. The W locus encodes the c-kit protooncogene product (cKit), a tyrosine kinase receptor (2, 3). A ligand for c-Kit has recently been identified which is variously known as mast cell growth factor (4-6) (subsequently referred to here as MGF), stem cell factor (7-9), and c-Kit ligand (10, 11). This growth factor is a good candidate for the SI gene product (5, 9, 11). Like several other growth factors characterized to date (12-18), MGF exists in both membrane-bound and soluble forms (6,8,11). The primary translation products of these growth factors are membrane-bound, whereas the soluble form(s) is derived by proteolytic cleavage.We and others have shown (5, 9, 11) that MGF coding sequences are totally deleted in mice homozygous for lethal SI alleles, whereas no gross structural alterations are associated with most viable SI alleles. These results suggest, but do not prove, that MGF is encoded by SI. A simple interpretation of these data is that the lethal SI alleles result from the complete absence of MGF protein and that homozygous viable alleles either encode a mutant form ofMGF or produce lower levels of the wild-type gene product. This interpretation is consistent with a previous study showing that an intragenic deletion in c-kit produces a homozygous lethal phenotype (19). It remains possible, however, that MGF deletion mutants are viable but other genes mapping near MGF are deleted in mice carrying lethal Si alleles and that these genes are responsible for the lethality.In this report we show that one of the viable mutant SI alleles, steel-Dickie (Sld), has a 4.0-kilobase (kb) intragenic deletion that truncates the SI coding sequence. These results provide direct evidence that Si encodes MGF and a molecul...

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The E6-E7 region of human papillomavirus types 16 and 18 is selectively retained and expressed in cervical carcinoma cells. In cultured human keratinocytes, expression of the E6 and E7 open reading frames of human papillomavirus type 18, under the control of its homologous promoter, resulted in high-frequency immortalization. Furthermore, by using a system that allows for stratification of keratinocytes in vitro (raft system), we observed that the morphological differentiation of these E6-E7 immortalized cells was altered such that parabasal cells extended throughout most of the epithelium, with abnormal nuclei present in the upper regions. Examination of E6-E7-expressing cell lines in the raft system at a later passage revealed that complete loss of morphological differentiation had occurred. E7 alone was a much less effective immortalizing agent than E6 and E7 together and acted only minimally to alter morphological differentiation in vitro. No such activities were found for E6 alone. High-frequency transformation of human epithelial cells thus appears to require expression of both E6 and E7 gene products.