Disulfide bonds required to assemble functional von Willebrand factor multimers

The hemostatic functions of human von Willebrand Factor (vWF) depend on the normal assembly of disulfide-linked multimers from approximately 250-kDa subunits. Subunits initially form dimers through disulfide bonds near the COOH terminus. Dimers then form multimers through disulfide bonds near the NH2 terminus of each subunit. Previous studies of plasma vWF and recombinant vWF fragments indicate that 1 or more of the Cys residues at position 459, 462, and 464 form intersubunit disulfide bonds. No evidence has been reported that vWF multimer formation involves additional intersubunit bonds. To probe the disulfide bond requirements for multimer formation, mutant vWF proteins were expressed in which all 3 Cys residues at positions 459, 462, and 464 were changed to either Gly or Ala. Surprisingly, none of these cysteines appears to be necessary for efficient multimer assembly. Furthermore, recombinant vWF with Gly or Ala at all three positions induces platelet aggregation in the presence of ristocetin and binds to platelet glycoprotein Ib, factor VIII, and collagen in a manner similar to wild-type recombinant vWF. These results suggest that other intersubunit disulfide bonds must exist. Direct evidence for such a bond was obtained by characterization of tryptic fragments of vWF. By Edman degradation, amino acid composition, and mass spectrometry, a disulfide bond was demonstrated between Cys379 residues of adjacent vWF subunits. Thus, intersubunit disulfide bonds involving Cys379 and 1 or more of the Cys residues at positions 459, 462, and 464 connect the NH2-terminal ends of the vWF subunits in a parallel orientation.

Effects of 3,4-diaminopyridine on myoplasmic calcium and phosphoinositide hydrolysis in frog sartorius muscle fibers

Using Ca(2+)-selective microelectrodes, the effects of 3,4-diaminopyridine (DAP) on myoplasmic calcium ([Ca2+]i) were studied in frog satorius muscle fibers. DAP induced an increase of [Ca2+]i in a concentration-dependent manner. DAP 1 mmol.L-1 increased the [Ca2+]i from control 166 +/- 41 nmol.L-1 to 416 +/- 69 nmol.L-1 (n = 10). In the absence of external calcium, DAP still enhanced [Ca2+]i. [Ca2+]i of the fibers bathed in Ca(2+)-free Ringer's solution containing DAP 1 mmol.L-1 was 152 +/- 43 nmol.L-1, which was significantly higher than 77 +/- 35 nmol.L-1 of [Ca2+]i of the fibers in Ca(2+)-free Ringer's solution. In addition, DAP promoted the hydrolysis of phosphoinositides, and DAP-induced hydrolysis was more in the presence of external calcium. It is suggested that, through enhancing the hydrolysis of phosphoinositides, DAP released Ca2+ from intracellular Ca2+ store in frog sartorius muscle fibers.

Radioimmunoguided surgery in gastric cancer using 131-I labeled monoclonal antibody 3H11

A clinical trial of radioimmunoguided surgery (RIGS) using 131-I labeled monoclonal antibody (MAb) 3H11 was performed on 25 cases with gastric cancer in this study. The results showed that the route of the administration of radiolabeled MAb is the key point of successful RIGS for gastric cancer. The RIGS in 19 cases in the group of submucosal injection via endoscope was very satisfying. For the detection of metastatic lymph nodes, the sensitivity rate of RIGS was 99.2% (118/119), the specificity, 97.7% (43/44), and the accuracy 98.8% (161/163). The accuracy rate in detecting tumor infiltration of the gastric wall was 95.9% (94/98), the sensitivity 94.6% (35/37), and the specificity 96.7% (59/61). Thus, the use of RIGS may improve the radical resectability rate and possibly the overall survival rate in patients with gastric cancer.

Inverse correlation between expression of inducible nitric oxide synthase activity and production of metastasis in K-1735 murine melanoma cells

The purpose of these studies was to determine whether the induction of NO synthase activity in murine K-1735 melanoma cells correlated with their metastatic potential. Nonmetastatic, metastatic, and somatic cell hybrids (produced by fusion of nonmetastatic and metastatic cells) were injected i.v. into syngeneic C3H/HeN mice. Metastatic cells survived to produce experimental lung metastases, whereas nonmetastatic cells did not. The various clones and somatic cell hybrids were incubated in vitro with combinations of tumor necrosis factor, interleukin 1, gamma-interferon, and lipopolysaccharide. Nonmetastatic cells exhibited high levels of inducible NO synthase activity and NO, whereas metastatic cells did not. Both the cytotoxic effects of the cytokines and NO production were inhibited by the addition of NG-monomethyl-L-arginine, a specific inhibitor of NO synthase. These data demonstrate an inverse correlation between production of endogenous NO and the ability of K-1735 cells to survive in syngeneic mice to produce lung metastases.

Blocking of tumor promoter-induced AP-1 activity inhibits induced transformation in JB6 mouse epidermal cells

AP-1 transcriptional activity is stimulated by the transformation promoters phorbol 12-myristate 13-acetate ("12-O-tetradecanoylphorbol 13-acetate," TPA) and epidermal growth factor (EGF) in promotion-sensitive (P+) but not in promotion-resistant (P-) JB6 mouse epidermal cell lines. Although TPA stimulates expression of the jun and fos family genes, only c-jun expression shows higher elevation in P+ cells than in P- cells. The present study tests the hypothesis that induced AP-1 activity is required for tumor promoter-induced transformation in JB6 P+ cells. Both retinoic acid and the glucocorticoid fluocinolone acetonide inhibited basal and TPA-induced AP-1 activities that were tested with a stromelysin promoter-chloramphenicol acetyltransferase reporter gene in P+ cells. Since both retinoic acid and fluocinolone acetonide are active in inhibiting TPA-induced anchorage-independent transformation of P+ cells in the dose range that blocks TPA-induced AP-1 activity, their antipromoting effects may occur through inhibition of AP-1 activity. To test the hypothesis with a more specific inhibitor, stable clonal transfectants of P+ cells expressing dominant negative c-jun mutant encoding a transcriptionally inactive product were analyzed. All transfectants showed a block in TPA and EGF induction of AP-1 activity. All transfectants also showed inhibition of TPA-induced transformation, and most transfectants showed a block in EGF-induced transformation. These results indicate that AP-1 activity is required for TPA- or EGF-induced transformation. This work demonstrates that a specific block in induced AP-1 activity inhibits tumor promoter-induced transformation.

An efficient route to N-palmitoyl-D-erythro-sphingomyelin and its 13C-labeled derivatives

We describe here a practical and efficient route to a homogeneous N-palmitoyl-D-erythro-sphingomyelin and its 13C-labeled derivatives. (2S,3R,4E)-2-Azido-3-(tert-butyldimethylsilyloxy)-4-octad ecene-1-ol 1 was converted to the sphingosine equivalent 2 by treatment with triphenylphosphine and water. Amine 2 was then coupled with palmitic acid, affording the ceramide derivative 3a. In the following two reactions the phosphorylcholine functional group was generated by using 2-chloro-2-oxo-1,3,2-dioxaphospholane and trimethylamine, respectively. The final deprotection of the secondary hydroxyl group in 5a produced the desired N-palmitoyl-D-erythro-sphingomyelin 6a. The overall yield of this five-step synthesis is 43%. The melting point, 213-215 degrees C, the specific rotation, [alpha]20D = +6.8 (c = 1.3, CH2Cl2/MeOH 1:1) and 1H- and 13C-NMR data indicate that the synthetic sphingomyelin is enantiomerically pure. The 13C-labeled derivatives 6b, 6c and 6d were synthesized by employing the same scheme.

Optimal Rough Machining of Sculptured Parts on a CNC Milling Machine

An optimal approach to the rough machining of sculptured parts with least machining time is presented. The contour map cutting method is used to generate CNC tool paths based on the CAD model of sculptured parts. The part and stock geometry related parameters, including the number of cutting layers and the distributions of cutting depth, and the process parameters of feed rate and depth of cut, are optimized. The method can automate CNC programming for sculptured part rough machining, considerably improve productivity, and lower production costs. Two examples are used to illustrate the approach and its advantages.

Protein tyrosine kinase inhibitors decrease induction of nitric oxide synthase activity in lipopolysaccharide-responsive and lipopolysaccharide-nonresponsive murine macrophages

We investigated tumoricidal activation and induction of nitric oxide synthase (NOS) activity in macrophages from LPS-responsive (C3H/HeN) and -unresponsive mice (C3H/HeJ). Macrophages were incubated in vitro with a synthetic lipopeptide or with LPS and IFN-gamma. LPS and IFN-gamma activated C3H/HeN but not C3H/HeJ macrophages to lyse B16 melanoma cells. In contrast, lipopeptide and IFN-gamma activated macrophages from both strains of mice. Genistein, a specific inhibitor of protein tyrosine kinase, significantly blocked tumoricidal activation of macrophages from both strains of mice. Genistein did not affect tumor cell binding but significantly inhibited the production of nitric oxide. Genistein, herbimycin A, and tyrphostin inhibited the induction of NOS activity in macrophages from both strains of mice. These data suggest that protein tyrosine kinase activity is involved in the signal transduction pathway of LPS and other synthetic bacterial-related immunomodulators at a point preceding triggering of macrophage tumoricidal activation and expression of inducible NOS activity.

Protein tyrosine kinase inhibitors decrease induction of nitric oxide synthase activity in lipopolysaccharide-responsive and lipopolysaccharide-nonresponsive murine macrophages

We investigated tumoricidal activation and induction of nitric oxide synthase (NOS) activity in macrophages from LPS-responsive (C3H/HeN) and -unresponsive mice (C3H/HeJ). Macrophages were incubated in vitro with a synthetic lipopeptide or with LPS and IFN-gamma. LPS and IFN-gamma activated C3H/HeN but not C3H/HeJ macrophages to lyse B16 melanoma cells. In contrast, lipopeptide and IFN-gamma activated macrophages from both strains of mice. Genistein, a specific inhibitor of protein tyrosine kinase, significantly blocked tumoricidal activation of macrophages from both strains of mice. Genistein did not affect tumor cell binding but significantly inhibited the production of nitric oxide. Genistein, herbimycin A, and tyrphostin inhibited the induction of NOS activity in macrophages from both strains of mice. These data suggest that protein tyrosine kinase activity is involved in the signal transduction pathway of LPS and other synthetic bacterial-related immunomodulators at a point preceding triggering of macrophage tumoricidal activation and expression of inducible NOS activity.

Dosage-dependent dominance over wild-type p53 of a mutant p53 isolated from nasopharyngeal carcinoma

Mutational inactivation of p53, a tumor suppressor gene, is the most common genetic alteration found in human cancer. Most mutated p53s either lose tumor suppressor function or gain oncogenic activity. We recently reported the detection of a heterozygous point mutation of p53 at codon 280 in nasopharyngeal carcinoma (NPC) (1), a high-incidence malignancy in southern China and southeast Asia. Given its heterozygous state, in which both wild-type and mutated p53 gene were expressed, p53-thr280 should function dominantly in the presence of the wild-type form if it is to play a role in nasopharynx carcinogenesis. We tested this dominance hypothesis in the cells of two model systems: 1) human Saos-2 cells lacking endogenous p53, and 2) mouse JB6 tumor promotion-resistant cells (P-) expressing endogenous wild-type p53. The results showed dosage-dependent dominance of p53-thr280 in controlling WT p53-driven transcriptional activity; in governing cell growth; and in progressing P- phenotype to tumor promotion-sensitive (P+) phenotype. This dominant negative effect was seen at a 1:1 (WT:MU) ratio and was more striking at a ratio of 1:3. A model is proposed to explain the dominant negative effect of mutant p53. We conclude from this study that p53-thr280 is likely to be dominant in the heterozygous state found in NPC and that this dominant-negative mutated p53 may contribute to the genesis of NPC or of other carcinomas in which both mutant and wild-type p53 are expressed.

DNA polymerase III accessory proteins. IV. Characterization of chi and psi

The gamma complex (gamma delta delta' chi psi) subassembly of the Escherichia coli replicase initiates processive replication upon assembling the ring-shaped beta subunit around DNA. The beta ring acts as a clamp to hold the replicase down to DNA for highly processive synthesis. In this report we characterize the chi and psi subunits of the gamma complex. Both chi and psi are monomeric, and they associate to form a 1:1 complex. The chi subunit does not form a complex with gamma, but psi binds gamma tightly thereby acting as a bridge to assimilate chi into the gamma complex structure. The psi subunit stimulated the ATPase and replication activities of gamma. The chi subunit only stimulated the activities of gamma when the psi subunit was also present thus reflecting the structure where psi bridges the interaction of chi with gamma.

DNA polymerase III accessory proteins. I. holA and holB encoding delta and delta'

The genes encoding the delta and delta' subunits of the 10-subunit Escherichia coli replicase, DNA polymerase III holoenzyme, have been identified and sequenced. The holA gene encoding delta is located downstream of rlpB at 15.2 min and predicts a 38.7 kda protein. The holB gene encoding delta' is located at 24.3 min and predicts a 36.9-kDa protein. Hence the delta and delta' subunits are unrelated proteins encoded by separate genes. The genes have been used to express and purify delta and delta' in quantity. The predicted amino acid sequence of delta' is homologous to the sequences of the tau and gamma subunits revealing a large amount of structural redundancy within the holoenzyme.

DNA polymerase III accessory proteins. III. holC and holD encoding chi and psi

Genes encoding the chi and psi accessory proteins of the DNA polymerase III holoenzyme replicase of Escherichia coli have been identified, sequenced, and used to express and purify both chi and psi in quantity. The holC gene encoding chi is located between the xerB and valS genes at 96.5 min on the chromosome; it encodes a 147-amino acid protein of 16.6 kDa. holD encoding psi lies upstream of rimI at 99.3 min and encodes a 137-amino acid protein of 15.2 kDa. The genes have been cloned into expression vectors, and both chi and psi have been purified in quantity. The accompanying report characterizes the function and physical interactions of chi and psi with other holenzyme subunits (Xiao, H., Dong, Z., and O'Donnell, M. (1993) J. Biol. Chem. 268, 11779-11784).

The immunogenic properties of drug-resistant murine tumor cells do not correlate with expression of the MDR phenotype

Alterations in the immunogenic properties of tumor cells frequently accompany selection for multiple-drug-resistant (MDR) variants. Therefore, studies were performed to examine the hypothesis that overexpression of membrane P-glycoprotein, commonly observed in MDR tumor cells, is associated with enhanced immunogenic properties. Immunogenicity was determined by (a) the ability of drug-sensitive parental UV2237M fibrosarcoma cells and drug-resistant UV2237M variant cells to immunize normal mice against rechallenge with parental tumor cells and (b) the ability of normal syngeneic mice to reject cell inocula that caused progressive tumor growth in immunocompromised mice. Variant UV2237M cell lines included subpopulations selected for a six- to ten-fold increase in mRNA for P-glycoprotein and expression of the MDR phenotype (resistance to doxorubicin) and cells sensitive to doxorubicin (and no expression of MDR properties) but resistant to ouabain. All UV2237M drug-resistant cells were highly immunogenic in immunocompetent mice, regardless of their MDR phenotype. Additional studies showed that CT-26 murine adenocarcinoma cells, sensitive or resistant to doxorubicin (expressing high levels of P-glycoprotein), injected into normal syngeneic Balb/c mice produced rapidly growing tumors. The data do not demonstrate a correlation between the immunogenic properties of drug-resistant tumor cells and the expression of P-glycoprotein.

A rapid colorimetric in situ mRNA hybridization technique using hyperbiotinylated oligonucleotide probes for analysis of mdr1 in mouse colon carcinoma cells

We describe the development of a rapid colorimetric in situ hybridization technique utilizing oligonucleotide probes labeled with six biotin molecules at the 3' end to detect mdr1 in mouse colon cancer cells growing in culture and in vivo. mRNA integrity was verified by the use of a multibiotinylated poly d(T) oligonucleotide, and the specificity of the reaction was confirmed by use of labeled sense and anti-sense probes in serial cryostat sections and cultured cells. The multiple biotin label produced a strong signal after a short hybridization time. Avidin-alkaline phosphatase detection and the capillary technology used in the Microprobe Accelerated System allowed completion of the procedure in less than 5 hr. Excellent correlations with the MDR phenotype of the cells, Northern blot analysis, and immunohistochemistry recommend this procedure for identifying cells that express the MDR phenotype in culture and in vivo.

Tyrosine phosphorylation of mitogen-activated protein kinases is necessary for activation of murine macrophages by natural and synthetic bacterial products

The purpose of these studies was to determine the intracellular signal transduction pathways of bacterial products in murine macrophages from lipopolysaccharide (LPS)-responder C3H/HeN and LPS-nonresponder C3H/HeJ mice. Both LPS and synthetic lipopeptide CGP 31362 (LPP) induced production of tumor necrosis factor alpha (TNF-alpha) in C3H/HeN macrophages. In C3H/HeJ macrophages, however, TNF-alpha was induced only by incubation with LPP. Both LPS and LPP induced tyrosine phosphorylation on proteins with apparent molecular masses of 39, 41, and 45 kD (p35, p41, and p45) in C3H/HeN macrophages, whereas in C3H/HeJ macrophages, tyrosine phosphorylation was induced only by LPP. 20-h incubation with LPS or LPP downregulated TNF-alpha production/secretion and tyrosine phosphorylation in C3H/HeN macrophages induced by additional LPS or LPP. In C3H/HeJ macrophages, however, the downregulation of TNF-alpha production and tyrosine phosphorylation were observed only with LPP. Protein kinase assays, Western blotting analyses, phenyl-Sepharose chromatography, and immunocomplex kinase assay suggested that p45 and p39 were similar or identical to mitogen-activated protein (MAP) kinase 1 and 2, respectively. Pretreatment of macrophages with LPS or LPP did not change the amount of kinase proteins but inhibited the stimulation of kinase activity by the agents. These data suggest that MAP kinases are among target proteins involved in the transduction of LPS and LPP signals that lead to activation of murine macrophages to produce/secrete TNF.

Chiral Sulfur Compounds. XXI. Addition of Azide Ion to β-Aryl-α-phenylsulfinylacrylates

The reaction of methyl β-aryl-α- phenylsulfinylacrylates (1) with sodium azide /acetic acid gives triazoles (6) while the reaction of (1) with sodium azide /hydrochloric acid gives β-azido esters (7a,b). Reaction of (7a) with 1,8-diazabicyclo[5.4.0]undec-7-ene, and (7b) with triethylamine gave the corresponding triazoles (6a) and (6b), respectively. Thus the formation of triazoles from the reaction of acrylates with azide ion most likely involves Michael addition of azide ion followed by cyclization of an incipient β-azido α-anion.

Activation of tumoricidal properties in macrophages by lipopolysaccharide requires protein-tyrosine kinase activity

The purpose of these studies was to determine whether triggering murine peritoneal macrophages to a tumoricidal state by lipopolysaccharide (LPS) requires protein-tyrosine phosphorylation. The LPS-triggered activation of mouse macrophages to lyse syngeneic B16 melanoma cells was significantly inhibited in a dose-dependent manner by the protein-tyrosine kinase (PTK) inhibitors genistein, herbimycin A, and tyrphostin. Genistein was effective only when added to macrophages prior to or simultaneously with LPS. Genistein potently inhibited the productive interaction of macrophages with LPS but had only a minor effect on the action of interferon-gamma. The effects of genistein on LPS-triggered macrophage activation were not due to nonspecific changes in macrophage metabolism or toxicity because genistein did not prevent lysis of tumor cells by activated macrophages, nor did it reduce the capacity of macrophages to phagocytose antibody-opsonized sheep erythrocytes. Western blot analysis with antiphosphotyrosine monoclonal antibody revealed that incubation of macrophages with LPS produced a rapid increase in tyrosine phosphorylation of several proteins and that the induced phosphorylation could be inhibited by effective concentrations of genistein, herbimycin A, or tyrphostin. Taken together, these data indicate that protein-tyrosine phosphorylation plays an important role in LPS-induced tumoricidal activation of macrophages.

No point mutation of Ha-ras or p53 genes expressed in preneoplastic-to-neoplastic progression as modeled in mouse JB6 cell variants

Alterations of the p53 gene have been found in many human and animal tumors, and ras mutations have been seen somewhat less frequently. Loss of p53 tumor suppressor activity has been implicated in multistage tumor promotion/progression after initiation by Ha-ras or Ki-ras activation. Whether p53 or ras alterations occur during promotion of neoplastic transformation in JB6 cells has not been reported. Using a series of mouse JB6 variants representing different stages of progression toward the tumor-cell phenotype, we investigated whether mutational activation of Ha-ras and mutational inactivation and altered expression of the p53 gene occurred during progression. We report here that neither point mutations of the Ha-ras or p53 genes nor p53 structural alterations were involved in this process. Although there was no significant difference in steady-state levels of p53 mRNA, an elevated level of immunoprecipitable p53 protein was observed in a subset of transformed cells. We also report the detection of a second 2.2-kb p53-hybridizing transcript. This transcript was not translated into p53 protein and may be a nuclear precursor of the mature message. Both p53-hybridizing transcripts were downregulated by prolonged 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment (24 h) regardless of the stage of progression. We conclude from this study that in JB6 variants (1) mutational activation of Ha-ras and inactivation of p53 are unlikely to be involved in preneoplastic progression; (2) increased amounts of p53 protein may be involved in a subset of transformed cells, possibly reflecting a longer half-life, as demonstrated in other systems; and (3) late downregulation of p53 mRNA but not protein expression may be a secondary response in the TPA-mediated signal transduction pathway.

B cell deficiency progresses with lineage maturation in nude.X-linked immunodeficient mice B cell deficiency progresses with lineage maturation

Previously we showed that unlike normal, nude, or X-linked immune deficient (xid) mice, nude.xid mice are deficient in bone marrow pre-B cell targets for Abelson murine leukemia virus transformation. We show that nude.xid bone marrow is deficient in both CD45(B220)+ and CD45(B220)- surface (s)IgM- progenitors that give rise to B cell colonies in Whitlock-Witte cultures. CD45(B220)+ precursors had normal differentiation potential in vitro. CD45(B220)- precursors differentiated into CD45(B220)+ cells at the same rate as normal controls, but acquired sIgM at a much slower rate. These results correlated with the observation that in nude.xid mice the severity of B lineage defects correlates with maturity: a profound (ninefold) deficit of sIgM+, CD45(B220)+ mature B cells, a fivefold deficit in the sIgM-, CD45(B220)+ precursors of short term B cell colonies (colonies forming within 4-5 days in Whitlock-Witte cultures), and a moderate (twofold) decrease in the frequency of sIgM-, CD45(B220)- (less mature) precursors of long term B cell colonies (colonies forming after 14 days of Whitlock-Witte culture. Thus the combination of the nude and xid mutations produces a deficiency in early B cell progenitors and the deficiency becomes more profound with further maturation. Therefore the lack of mature B cells is the result of a cascade effect. Inasmuch as bone marrow progenitors are affected, and these are the source of the vast majority of B cells, most B cells are affected by the xid mutation and the xid defect cannot be attributed to a loss of a fetal lineage of B cells. These results suggest that xid affected cells lack the capacity to progress efficiently through differentiation in the absence of an exogenous factor(s) that is dependent on the product of a normal allele at the nude locus. This product might be supplied in vivo by a T cell or T cell-dependent source and/or epithelial elements such as bone marrow stromal cells all of which are known to be affected by the nude mutation.

B cell deficiency progresses with lineage maturation in nude.X-linked immunodeficient mice B cell deficiency progresses with lineage maturation

Previously we showed that unlike normal, nude, or X-linked immune deficient (xid) mice, nude.xid mice are deficient in bone marrow pre-B cell targets for Abelson murine leukemia virus transformation. We show that nude.xid bone marrow is deficient in both CD45(B220)+ and CD45(B220)- surface (s)IgM- progenitors that give rise to B cell colonies in Whitlock-Witte cultures. CD45(B220)+ precursors had normal differentiation potential in vitro. CD45(B220)- precursors differentiated into CD45(B220)+ cells at the same rate as normal controls, but acquired sIgM at a much slower rate. These results correlated with the observation that in nude.xid mice the severity of B lineage defects correlates with maturity: a profound (ninefold) deficit of sIgM+, CD45(B220)+ mature B cells, a fivefold deficit in the sIgM-, CD45(B220)+ precursors of short term B cell colonies (colonies forming within 4-5 days in Whitlock-Witte cultures), and a moderate (twofold) decrease in the frequency of sIgM-, CD45(B220)- (less mature) precursors of long term B cell colonies (colonies forming after 14 days of Whitlock-Witte culture. Thus the combination of the nude and xid mutations produces a deficiency in early B cell progenitors and the deficiency becomes more profound with further maturation. Therefore the lack of mature B cells is the result of a cascade effect. Inasmuch as bone marrow progenitors are affected, and these are the source of the vast majority of B cells, most B cells are affected by the xid mutation and the xid defect cannot be attributed to a loss of a fetal lineage of B cells. These results suggest that xid affected cells lack the capacity to progress efficiently through differentiation in the absence of an exogenous factor(s) that is dependent on the product of a normal allele at the nude locus. This product might be supplied in vivo by a T cell or T cell-dependent source and/or epithelial elements such as bone marrow stromal cells all of which are known to be affected by the nude mutation.