Small-molecule inhibition of kinesin KIF18A reveals a mitotic vulnerability enriched in chromosomally unstable cancers

Chromosomal instability (CIN) is a hallmark of cancer, caused by persistent errors in chromosome segregation during mitosis. Aggressive cancers like high-grade serous ovarian cancer (HGSOC) and triple-negative breast cancer (TNBC) have a high frequency of CIN and TP53 mutations. Here, we show that inhibitors of the KIF18A motor protein activate the mitotic checkpoint and selectively kill chromosomally unstable cancer cells. Sensitivity to KIF18A inhibition is enriched in TP53-mutant HGSOC and TNBC cell lines with CIN features, including in a subset of CCNE1-amplified, CDK4-CDK6-inhibitor-resistant and BRCA1-altered cell line models. Our KIF18A inhibitors have minimal detrimental effects on human bone marrow cells in culture, distinct from other anti-mitotic agents. In mice, inhibition of KIF18A leads to robust anti-cancer effects with tumor regression observed in human HGSOC and TNBC models at well-tolerated doses. Collectively, our results provide a rational therapeutic strategy for selective targeting of CIN cancers via KIF18A inhibition.

P-glycoprotein Substrate Assessment in Drug Discovery: Application of Modeling to Bridge Differential Protein Expression Across In Vitro Tools

P-glycoprotein (P-gp) efflux assay is an integral part of discovery screening, especially for drugs requiring brain penetration as P-gp efflux ratio (ER) inversely correlates with brain exposure. However, significant variability in P-gp ER generated across cell lines can lead to misclassification of a P-gp substrate and subsequently disconnect with brain exposure data. We hypothesized that the ER depends on P-gp protein expression level in the in vitro assay. Quantitative proteomics and immunofluorescence staining were utilized to characterize P-gp protein expression and localization in four recombinant cell lines, over-expressing human or mouse P-gp isoforms, followed by functional evaluation. Efflux data generated in each cell line was compared against available rodent brain distribution data. The results suggested that the cell line with highest P-gp expression (hMDCK-MDR1 sourced from NIH) led to greatest dynamic range for efflux; thus, proving to be the most sensitive model to predict brain penetration. Cell lines with lower P-gp expression exhibited the greatest tendency for compound-dependent in vitro efflux saturation leading to false negative results. Ultimately, P-gp kinetics were characterized using a compartmental model to generate system-independent parameters to resolve such discrepancy. This study highlights the need for careful choice of well characterized P-gp in vitro tools and utility of modeling techniques to enable appropriate interpretation of the data.

Effects of monoclonal antagonist antibodies on calcitonin gene-related peptide receptor function and trafficking

Background

Monoclonal antibodies against calcitonin gene-related peptide (CGRP) or its receptor are efficacious for the prevention of migraine headaches. The downstream molecular mechanisms following ligand-receptor blockade by which these antibodies prevent CGRP signaling through CGRP receptors have not been demonstrated.

Methods

Here we produced tool monoclonal functional antagonist antibodies against CGRP and its canonical receptor and developed a novel cellular model using fluorogen-activated protein technology that allows detection of CGRP receptor internalization by flow cytometry and, for an extended time course, visualization by confocal microscopy.

Results

Using this cell model we showed that these antagonist antibodies block both CGRP-induced cAMP signaling and CGRP receptor internalization. At least 10-fold higher concentrations of either antibody are necessary to block CGRP receptor internalization compared with cAMP accumulation in our cell model.

Conclusion

These data reinforce our understanding of how monoclonal functional antagonist antibodies interfere with CGRP signaling.

AMG 900, a potent inhibitor of aurora kinases causes pharmacodynamic changes in p-Histone H3 immunoreactivity in human tumor xenografts and proliferating mouse tissues

BACKGROUND

The Aurora family of serine-threonine kinases are essential regulators of cell division in mammalian cells. Aurora-A and -B expression and kinase activity is elevated in a variety of human cancers and is associated with high proliferation rates and poor prognosis. AMG 900 is a highly potent and selective pan-aurora kinase inhibitor that has entered clinical evaluation in adult patients with advanced cancers. In mice, oral administration of AMG 900 blocks the phosphorylation of histone H3 on serine-10 (p-Histone H3), a proximal substrate of aurora-B and inhibits the growth of multiple human tumor xenografts, including multidrug-resistant models.

METHODS

In order to establish a preclinical pharmacokinetic-pharmacodynamic (PK-PD) relationship for AMG 900 that could be translated to the clinic, we used flow cytometry and laser scanning cytometry detection platforms to assess the effects on p-Histone H3 inhibition in terms of sensitivity, precision, and specificity, in human tumor xenografts in conjunction with mouse skin and bone marrow tissues. Mice with established COLO 205 tumors were administered AMG 900 at 3.75, 7.5, and 15 mg/kg and assessed after 3 hours.

RESULTS

Significant suppression of p-Histone H3 in mouse skin was only observed at 15 mg/kg (p <0.0001), whereas in mouse bone marrow and in tumor a dose-dependent inhibition was achieved at all three doses (p ≤ 0.00015). These studies demonstrate that AMG 900 inhibits p-Histone H3 in tumors and surrogate tissues (although tissues such as skin may be less sensitive for assessing PD effects). To further extend our work, we evaluated the feasibility of measuring p-Histone H3 using fine-needle aspirate (FNA) tumor xenograft biopsies. Treatment with AMG 900 significantly inhibited p-Histone H3 (>99% inhibition, p <0.0001) in COLO 205 tumors. Lastly, we illustrate this LSC-based approach can detect p-Histone H3 positive cells using mock FNAs from primary human breast tumor tissues.

CONCLUSION

Phosphorylation of histone H3 is a useful biomarker to determine the pharmacodynamics (PD) activity of AMG 900. FNA biopsies may be a viable approach for assessing AMG 900 PD effects in the clinic.

Motesanib inhibits Kit mutations associated with gastrointestinal stromal tumors

Background

Activating mutations in Kit receptor tyrosine kinase or the related platelet-derived growth factor receptor (PDGFR) play an important role in the pathogenesis of gastrointestinal stromal tumors (GIST).

Methods

This study investigated the activity of motesanib, an inhibitor of vascular endothelial growth factor receptors (VEGFR) 1, 2, and 3; PDGFR; and Kit, against primary activating Kit mutants and mutants associated with secondary resistance to imatinib. Single- and double-mutant isoforms of Kit were evaluated for their sensitivity to motesanib or imatinib in autophosphorylation assays and in Ba/F3 cell proliferation assays.

Results

Motesanib inhibited Kit autophosphorylation in CHO cell lines expressing primary activating mutations in exon 9 (AYins503-504, IC₅₀ = 18 nM) and exon 11 (V560 D, IC₅₀ = 5 nM; Δ552-559, IC₅₀ = 1 nM). Motesanib also demonstrated activity against kinase domain mutations conferring imatinib resistance (V560D/V654A, IC₅₀ = 77 nM; V560D/T670I, IC₅₀ = 277 nM; Y823 D, IC₅₀ = 64 nM) but failed to inhibit the imatinib-resistant D816V mutant (IC₅₀ > 3000 nM). Motesanib suppressed the proliferation of Ba/F3 cells expressing Kit mutants with IC₅₀ values in good agreement with those observed in the autophosphorylation assays.

Conclusions

In conclusion, our data suggest that motesanib possesses inhibitory activity against primary Kit mutations and some imatinib-resistant secondary mutations.

Abstract 3600: Preclinical pharmacokinetic and biomarker analysis of AMG 900, an orally available small molecule inhibitor of aurora kinases, in human xenograft tumor and surrogate tissues

Background: The aurora family of serine-threonine kinases (Aurora-A, -B, -C) regulate cell-cycle progression in mammalian cells. Aurora kinases A and B are essential for proper chromosome congression, segregation, and cytokinesis during mitosis, whereas aurora kinase C function appears restricted to male meiosis. Aurora kinase B is responsible for the direct phosphorylation of histone H3 on serine-10 (p-histone H3). Aurora kinase A and B expression is elevated in a variety of human cancers and is associated with advanced clinical staging and poor prognosis. With the emergence of aurora kinases as key mitotic regulators and their potential role in tumorigenesis, we developed AMG 900, a novel ATP competitive small molecule inhibitor, that is highly potent and selective for aurora kinases A, B, and C. In vivo, AMG 900 inhibits the growth of multiple human tumor xenografts, including multidrug-resistant models.

Objective: The aim of this study was to establish a pharmacokinetic-pharmacodynamic (PK-PD) relationship for AMG 900 in human tumor xenografts and surrogate tissues.

Results: In nude mice, oral administration of AMG 900 inhibited p-histone H3 in tumors and bone marrow in a concentration- and dose-dependent manner. To further refine the AMG 900 PK-PD relationship, p-histone H3 IC50 values for bone marrow and dissociated COLO 205 tumors from mice were associated with plasma concentrations of 294 and 273 ng/mL, respectively. The duration of target coverage required for anti-tumor efficacy was determined to be &gt; 6 hours per day above the in vivo p-histone H3 IC50 using different dosing schedules. To explore whether skin could be a suitable surrogate tissue for measuring p-histone H3 inhibition, tumor-bearing mice were administered AMG 900 at 3.75, 7.5, and 15 mg/kg for 3 hours. Significant suppression of p-histone H3 in skin was only observed at 15 mg/kg (p &lt; 0.0001), whereas in tumor, a dose-dependent inhibition was achieved at all doses (p &lt; 0.0015). To determine the feasibility of measuring p-histone H3 using fine-needle aspirate (FNA) tumor biopsies, tumor-bearing mice were administered AMG 900 at 15 mg/kg for 3 hours. Treatment with AMG 900 significantly inhibited p-histone H3 (&gt; 99%, p &lt; 0.0001) in COLO 205 tumors, suggesting that FNA biopsies may be a viable approach for assessing AMG 900 PD effects in the clinic.

Conclusion: These studies demonstrate that AMG 900 inhibits p-histone H3 in tumors and surrogate tissues, although tissues such as skin may be less sensitive to assess PD effects. The use of p-histone H3 as a biomarker to determine plasma levels of AMG 900 required to inhibit aurora B activity may be useful in clinical studies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3600.

An antibody to IP-10 is a potent antagonist of cell migration in vitro and in vivo and does not affect disease in several animal models of inflammation

IP-10 secretion is induced by pro-inflammatory cytokines and mediates the migration of CXCR3+ cells. Its elevation in clinical samples has been associated with multiple inflammatory diseases and its antagonism has been reported to be effective in several animal models of inflammatory disease. We generated a mouse anti-mouse IP-10 monoclonal antibody (mAb; Clone 20A9) that specifically bound murine IP-10 with high affinity and inhibited in vitro IP-10 induced BaF3/mCXCR3 cell migration with an IC(50) of approximately 4 nM. The 20A9 mAb was completely absorbed in vivo and had dose proportional pharmacokinetic exposure with a serum half life of 2.4-6 days. The 20A9 mAb inhibited IP-10 mediated T-cell recruitment to the airways, indicating that it is effective in vivo. However, administration of the 20A9 mAb had no significant effect on disease in mouse models of delayed type hypersensitivity, collagen induced arthritis, cardiac allograft transplantation tolerance, EAE or CD4+ CD45RBHi T-cell transfer-induced IBD. These data suggest that the 20A9 mAb can antagonize IP-10 mediated chemotaxis in vitro and in vivo and that this is insufficient to cause a therapeutic benefit in multiple mouse models of inflammatory disease.

Calcification inhibitors and Wnt signaling proteins are implicated in bovine artery smooth muscle cell calcification in the presence of phosphate and vitamin D sterols

Administration of active vitamin D sterols to treat secondary hyperparathyroidism in patients with chronic kidney disease receiving dialysis has been associated with elevated serum calcium and phosphorus levels, which may lead to increased risk of vascular calcification. However, calcimimetics, by binding to the parathyroid gland calcium-sensing receptors, reduce serum parathyroid hormone, calcium, phosphorus, and the calcium-phosphorus product. Using cultured bovine aorta vascular smooth muscle cells (BASMCs), an in vitro model of vascular calcification, we compared calcification levels and gene expression profiles after exposure to the phosphate source ss-glycerolphosphate (BGP), the active vitamin D sterols calcitriol and paricalcitol, the calcimimetic R-568, or BGP with the active vitamin D sterols or R-568. Cells exposed to BGP (10 mM) alone or with calcitriol or paricalcitol showed dose-dependent BASMC calcification. No change in calcification was observed in cultures exposed to BGP with R-568, consistent with the observed lack of calcium-sensing receptor expression. Microarray analysis using total cellular RNA from cultures exposed to vehicle or BGP in the absence and presence of 10(-8) M calcitriol or paricalcitol for 7 days showed that cells exposed to BGP with calcitriol or BGP with paricalcitol had virtually identical gene expression profiles, which differed from those of cells treated with BGP or vehicle alone. Several osteoblast- and chondrocyte-associated genes were modulated by BGP and vitamin D exposure. In this study, exposure of BASMCs to phosphate and active vitamin D sterols induced calcification and changes in expression of genes associated with mineralized tissue.

CD4+CD45RBHi T cell transfer induced colitis in mice is accompanied by osteopenia which is treatable with recombinant human osteoprotegerin

BACKGROUND AND AIMS

Transfer of CD4+CD45RBHi T cells into semi syngeneic immunodeficient mice represents a model of inflammatory bowel disease (IBD). As patients with IBD often suffer from osteopenia, we studied if this T cell transfer in mice results in osteopenia in addition to colitis, and if treatment with osteoprotegerin (OPG) has effects on the bone mineral density of T cell transferred mice. We also investigated whether osteopenia was due to malabsorption as a result of a dysregulated digestive tract or as a consequence of the inflammatory process.

METHODS

CD4+CD45RBHi or CD4+CD45RBLo T cells (4 x 10(5)) were sorted from CB6F1 and transferred into C.B.17 scid/scid mice. Recipient mice were treated with human IgG1 Fc (control) or Fc-OPG three times per week in a prophylactic regimen as well as a therapeutic regimen (after 10% body weight loss) and were evaluated for osteopenia and colitis.

RESULTS

Mice that received CD4+CD45RBHi T cells developed osteopenia (as indicated by decreased bone density accompanied by decreased osteoblasts and increased osteoclasts) and colitis (as indicated by histological changes in the large intestine). Mice that received CD4+CD45RBLo T cells developed neither osteopenia nor colitis. All animals consumed, on average, the same amount of food and water over the course of the study. Prophylactic treatment with Fc-OPG increased bone density in mice that received either CD4+CD45RBHi or CD4+CD45RBLo T cells but had no effects on the gastrointestinal tract. Fc-OPG treatment of osteopenic mice with established IBD caused the normalisation of bone density. Osteopenia in CD4+CD45RBHi T cell recipients was accompanied by hypoparathyroidism that was partially normalised by treatment with Fc-OPG. CD4+CD45RBHi T cell recipients also had a bone marrow inflammatory cell infiltrate expressing tumour necrosis factor alpha which was unaffected by treatment with Fc-OPG.

CONCLUSIONS

CD4+CD45RBHi T cell transfer results in osteopenia in addition to colitis. Evidence suggests that this osteopenia was induced by inflammatory cell infiltration and not by malabsorption of calcium. Recombinant human osteoprotegerin effectively treated the osteopenia. OPG may be a useful therapeutic option for treating osteopenia in patients with IBD.

Macroscopic cartilage formation with embryonic stem-cell-derived mesodermal progenitor cells

The totipotent embryonic stem cell generates various mesodermal cells when stimulated with BMP4. Among the resulting cells, those expressing flk-1 and/or PDGFRalpha displayed chondrogenic activity in the presence of TGFbeta3 and expressed cartilage-specific genes in 7 to 16 day pellet cultures. Depositions of cartilage matrix and type II collagen were detected by day 14. TGFbeta-stimulated chondrogenesis was synergistically enhanced by PDGF-BB, resulting in a larger cartilage particle filled with a cartilaginous area containing type II collagen, with a surface cell layer expressing type I collagen. In contrast, noggin inhibited both the TGFbeta- and TGFbeta+PDGF-stimulated cartilage formation, suggesting that a BMP-dependent pathway is involved. In fact, replacement of TGFbeta3 with BMP4 on days 10 to 12 markedly elevated the cartilage matrix deposition during the following 7 to 8 days. Moreover, culture with TGFbeta3 and PDGF-BB, followed by the incubation with BMP4 alone, resulted in a cartilage particle lacking type I collagen in the matrix and the surface layer, which suggests hyaline cartilage formation. Furthermore, such hyaline cartilage particles were mineralized. These studies indicate that the PDGFRalpha+ and/or flk-1+ cells derived from embryonic stem cells possess the full developmental potential toward chondrocytes, in common with embryonic mesenchymal cells.

Transgenic overexpression of human IL-17E results in eosinophilia, B-lymphocyte hyperplasia, and altered antibody production

We have identified and cloned a novel human cytokine with homology to cytokines of the interleukin-17 (IL-17) family, which we have termed human IL-17E (hIL-17E). With the identification of several IL-17 family members, it is critical to understand the in vivo function of these molecules. We have generated transgenic mice overexpressing hIL-17E using an apolipoprotein E (ApoE) hepatic promoter. These mice displayed changes in the peripheral blood, particularly, a 3-fold increase in total leukocytes consisting of increases in eosinophils, lymphocytes, and neutrophils. Splenomegaly and lymphoadenopathy were predominant and included marked eosinophil infiltrates and lymphoid hyperplasia. CCR3(+) eosinophils increased in the blood and lymph nodes of the transgenic mice by 50- and 300-fold, respectively. Eosinophils also increased 8- to 18-fold in the bone marrow and spleen, respectively. In the bone marrow, most of the eosinophils had an immature appearance. CD19(+) B cells increased 2- to 5-fold in the peripheral blood, 2-fold in the spleen, and 10-fold in the lymph nodes of transgenic mice, whereas CD4(+) T lymphocytes increased 2-fold in both blood and spleen. High serum levels of the cytokines IL-2, IL-4, IL-5, granulocyte colony-stimulating factor, eotaxin, and interferon gamma were observed. Consistent with B-lymphocyte increases, serum immunoglobulin (Ig) M, IgG, and IgE were significantly elevated. Antigenic challenge of the transgenic mice with keyhole limpet hemocyanin (KLH) resulted in a decrease in anti-KLH IgG accompanied by increases of anti-KLH IgA and IgE. In situ hybridization of transgenic tissues revealed that IL-17Rh1 (IL-17BR/Evi27), a receptor that binds IL-17E, is up-regulated. Taken together, these data indicate that IL-17E regulates hematopoietic and immune functions, stimulating the development of eosinophils and B lymphocytes. The fact that hIL-17E overexpression results in high levels of circulating eosinophils, IL-4, IL-5, eotaxin, and IgE suggests that IL-17E may be a proinflammatory cytokine favoring Th2-type immune responses.

Polyglutamine repeat length-dependent proteolysis of huntingtin

Amino-terminal fragments of huntingtin, which contain the expanded polyglutamine repeat, have been proposed to contribute to the pathology of Huntington's disease (HD). Data supporting this claim have been generated from patients with HD in which truncated amino-terminal fragments forming intranuclear inclusions have been observed, and from animal and cell-based models of HD where it has been demonstrated that truncated polyglutamine-containing fragments of htt are more toxic than full-length huntingtin. We report here the identification of a region within huntingtin, spanning from amino acids 63 to 111, that is cleaved in cultured cells to generate a fragment of similar size to those observed in patients with HD. Importantly, proteolytic cleavage within this region appears dependent upon the length of the polyglutamine repeat within huntingtin, with pathological polyglutamine repeat-containing huntingtin being more efficiently cleaved than huntingtin containing polyglutamine repeats of nonpathological size.

Characterization of osteoclast precursors in human blood

Osteoclast precursors (OCPs) circulate in the mononuclear fraction of peripheral blood (PB), but their abundance and surface characteristics are unknown. Previous studies suggest that the receptor activator for NF-kappaB (RANK) on cytokine-treated OCPs in mouse bone marrow interacts with osteoprotegerin ligand (OPGL/TRANCE/RANKL/ODF) to initiate osteoclast differentiation. Hence, we used a fluorescent form of human OPGL (Hu-OPGL-F) to identify possible RANK-expressing OCPs in untreated peripheral blood mononuclear cells (PBMCs) using fluorescence-activated cell sorting analysis. Monocytes [CD14-phycoerythrin (PE) antibody (Ab) positive (+) cells, 10-15% of PBMCs] all (98-100%) co-labelled with Hu-OPGL-F (n > 18). T lymphocytes (CD3-PE Ab+ cells, 66% of PBMCs) did not bind Hu-OPGL-F; however, B cells (CD19-PE Ab+ cells, 9% of PBMCs) were also positive for Hu-OPGL-F. All Hu-OPGL-F+ monocytes also co-labelled with CD33, CD61, CD11b, CD38, CD45 and CD54 Abs, but not CD34 or CD56 Abs. Hu-OPGL-F binding was dose dependent and competed with excess Hu-OPGL. When Hu-OPGL-F+, CD14-PE Ab+, CD33-PE Ab+, Hu-OPGL-F+/CD14-PE Ab+ or Hu-OPGL-F+/CD33-PE Ab+ cells were cultured with OPGL (20 ng/ml) and colony-stimulating factor (CSF)-1 (25 ng/ml), OC-like cells readily developed. Thus, all freshly isolated monocytes demonstrate displaceable Hu-OPGL-F binding, suggesting the presence of RANK on OCPs in PB; also, OCPs within a purified PB monocyte population form osteoclast-like cells in the complete absence of other cell types in OPGL and CSF-1 containing medium.

Characterization of a new human B7-related protein: B7RP-1 is the ligand to the co-stimulatory protein ICOS

Optimal T cell activation requires the interactions of co-stimulatory molecules, such as those in the CD28 and B7 protein families. Recently, we described the co-stimulatory properties of the murine ligand to ICOS, which we designated as B7RP-1. Here, we report the co-stimulation of human T cells through the human B7RP-1 and ICOS interaction. This ligand-receptor pair interacts with a K:(D) approximately 33 nM and an off-rate with a t((1/2)) > 10 min. Interestingly, tumor necrosis factor (TNF)-alpha differentially regulates the expression of human B7RP-1 on B cells, monocytes and dendritic cells (DC). TNF-alpha enhances B7RP-1 expression on B cells and monocytes, while it inhibits it on DC. The human B7RP-1-Fc protein or cells that express membrane-bound B7RP-1 co-stimulate T cell proliferation in vitro. Specific cytokines, such as IFN-gamma and IL-10, are induced by B7RP-1 co-stimulation. Although IL-2 levels are not significantly increased, B7RP-1 co-stimulation is dependent on IL-2. These experiments define the human ortholog to murine B7RP-1 and characterize its interaction with human ICOS.

Mitotic separation of daughter cells in the human lymphoma B cell line Daudi involves L-selectin engagement and shedding

BACKGROUND AND OBJECTIVE

A novel role for shedding of the surface molecule L-selectin has been proposed as an adjunctive phenomenon during cell detachment from marrow stroma or vessel endothelium. We wished to examine whether variations in expression of L-selectin on a lymphoma B cell line were linked to shedding.

DESIGN AND METHODS

Mapping of L-selectin expression on the surface of Daudi lymphoma cells was performed by flow cytometry, fluorescence microscopy, and electron microscopy. Levels of shed L-selectin were evaluated by Western blotting of culture supernatants. Evaluation of cell cycle and proliferative activity was performed by flow cytometry.

RESULTS

Large Daudi cells in S+G(2)/M phases were L-selectin positive, whereas small Daudi cells in G(0)/G(1) phase were L-selectin negative. During mitosis, L-selectin was distributed along the cleavage furrow, and gradually lost. Electron microscopy revealed that separating Daudi cells were negative for L-selectin on the entire surface, except minute aggregates of L-selectin within the cleavage furrow. Addition of agents known to interfere with the ligand-binding portion of L-selectin (sulfatides, MoAbs: Lam1.3 and TQ1) results in loss of L-selectin. Removal of L-selectin by digestion with chymotrypsin inhibits Daudi proliferation. The MoAb FMC46 did not interfere with proliferation. Proliferating Daudi cells produced large quantities of shed L-selectin. Inhibition of Daudi proliferation resulted in levels of shed L-selectin below the limit of detection.

INTERPRETATION AND CONCLUSIONS

L-selectin is re-distributed on the cell surface of Daudi cells during the last phase of mitosis, in which plasma membrane invagination occurs between newly formed daughter cells. Shedding of L-selectin is involved in the cytokinesis of Daudi cells.

Dynamics and localization of early B-lymphocyte precursor cells (pro-B cells) in the bone marrow of scid mice

Mice homozygous for the scid (severe combined immunodeficiency) mutation are generally unable to produce B lymphocytes, a condition attributed to defective rearrangement of immunoglobulin genes in precursor B cells. Some early B-lineage cells are present in the bone marrow (BM), however. In scid mice, we defined three subsets of early progenitor B cells lacking mu heavy chains (pro-B cells) based on the expression of terminal deoxynucleotidyl transferase (TdT) and B220 glycoprotein: (a) early pro-B cells (TdT+B220-), (b) intermediate pro-B cells (TdT+B220+), and (c) late pro-B cells (TdT-B220+). Double immunofluorescence labeling of BM cell suspensions has shown normal numbers of early and intermediate pro-B cells, substantially reduced numbers of late pro-B cells, and an absence of pre-B cells and B cells. Early and intermediate pro-B cells accumulated in metaphase in near-normal numbers after intraperitoneal (IP) vincristine administration. B220+ pro-B cells have been localized in BM sections by the binding of intravenously (IV) administered 125I monoclonal antibody (MoAb) 14.8, detected by light and electron microscope radioautography. Many B220+ cells were located peripherally in the bone-lining cell layers associated with stromal reticular cells. More centrally located B220+ cells were frequently associated with macrophages containing prominent cytoplasmic inclusions. Occasional B220+ cells were present in venous sinusoids. These results demonstrate that many pro-B cells in scid mice occupy microenvironments in the BM near the surrounding bone. The pro-B cells maintain normal rates of production during stages of presumptive mu heavy-chain gene rearrangement, apparently unaffected by the absence of a mature B cell pool. Nearly all defective cells then abort at the late pro-B cell stage and are deleted, apparently by macrophages. The findings contribute to models of in vivo differentiation, regulation, localization, and selection of early B-lineage cells in the BM.