Osteoclasts Derive Predominantly from Bone Marrow-Resident CX3CR1+ Precursor Cells in Homeostasis, whereas Circulating CX3CR1+ Cells Contribute to Osteoclast Development during Fracture Repair

Osteoclasts (OC) originate from either bone marrow (BM)-resident or circulating myeloid OC progenitors (OCP) expressing the receptor CX₃CR1. Multiple lines of evidence argue that OCP in homeostasis and inflammation differ. We investigated the relative contributions of BM-resident and circulating OCP to osteoclastogenesis during homeostasis and fracture repair. Using CX₃CR1-EGFP/TRAP tdTomato mice, we found CX₃CR1 expression in mononuclear cells, but not in multinucleated TRAP⁺ OC. However, CX₃CR1-expressing cells generated TRAP⁺ OC on bone within 5 d in CX₃CR1CreERT2/Ai14 tdTomato reporter mice. To define the role that circulating cells play in osteoclastogenesis during homeostasis, we parabiosed TRAP tdTomato mice (CD45.2) on a C57BL/6 background with wild-type (WT) mice (CD45.1). Flow cytometry (CD45.1/45.2) demonstrated abundant blood cell mixing between parabionts after 2 wk. At 4 wk, there were numerous tdTomato⁺ OC in the femurs of TRAP tdTomato mice but almost none in WT mice. Similarly, cultured BM stimulated to form OC demonstrated multiple fluorescent OC in cell cultures from TRAP tdTomato mice, but not from WT mice. Finally, flow cytometry confirmed low-level engraftment of BM cells between parabionts but significant engraftment in the spleens. In contrast, during fracture repair, we found that circulating CX₃CR1⁺ cells migrated to bone, lost expression of CX₃CR1, and became OC. These data demonstrate that OCP, but not mature OC, express CX₃CR1 during both homeostasis and fracture repair. We conclude that, in homeostasis mature OC derive predominantly from BM-resident OCP, whereas during fracture repair, circulating CX₃CR1⁺ cells can become OC.

IL-15-deficient mice develop enhanced airway allergic responses in a mouse model of allergic airway disease

Interleukin-15 is a pleiotropic cytokine critical for the development of multiple hematopoietic lineages. Mice lacking IL-15 have selective defects in populations of several pro-allergic immune cells including natural killer (NK) cells, NKT cells, and memory CD8+T cells. We therefore hypothesized that IL-15−/− mice will have reduced inflammatory responses during the development of allergic airway disease (AAD). Here we report that IL-15−/− mice developed enhanced allergic responses in an OVA-induced model of AAD. C57BL/6 wild-type (WT) and IL-15−/− mice were sensitized and challenged with ovalbumin (OVA) and the development of AAD was ascertained. In the absence of IL-15, OVA-challenged mice exhibited enhanced bronchial eosinophilic inflammation, elevated IL-13 production, and severe lung histopathology in comparison with WT mice. In addition, increased numbers of CD4+T and B cells in the spleens and broncholaveolar lavage were also observed. Examination of OVA-challenged IL-15R−/− animals revealed a similar phenotype resulting in enhanced airway eosinophilia compared to WT mice. Adoptive transfer of splenic CD8+T cells from OVA-sensitized WT mice suppressed the enhancement of eosinophilia in IL-15−/− animals to levels observed in WT mice, but had no further effects. These data demonstrate that mice with an endogenous IL-15 deficiency are susceptible to the development of severe, enhanced Th2-mediated AAD. Furthermore, the development of disease as well as allergen-specific Th2 responses occurs despite deficiencies in several IL-15-dependent cell types including NK and NKT cells, suggesting that these cells or their subsets are dispensable for the induction of AAD in IL-15-deficient mice.

IL-15-deficient mice develop enhanced allergic responses to airway allergen exposure

BACKGROUND

Interleukin-15 is a pleiotropic cytokine that is critical for the development and survival of multiple haematopoietic lineages. Mice lacking IL-15 have selective defects in populations of several pro-allergic immune cells including natural killer (NK) cells, NKT cells, and memory CD8⁺ T cells. We therefore hypothesized that IL-15^-/- mice will have reduced inflammatory responses during the development of allergic airway disease (AAD).

OBJECTIVE

To determine whether IL-15^-/- mice have attenuated allergic responses in a mouse model of AAD.

METHODS

C57BL/6 wild-type (WT) and IL-15^-/- mice were sensitized and challenged with ovalbumin (OVA), and the development of AAD was ascertained by examining changes in airway inflammatory responses, Th2 responses, and lung histopathology.

RESULTS

Here, we report that IL-15^-/- mice developed enhanced allergic responses in an OVA-induced model of AAD. In the absence of IL-15, OVA-challenged mice exhibited enhanced bronchial eosinophilic inflammation, elevated IL-13 production, and severe lung histopathology in comparison with WT mice. In addition, increased numbers of CD4⁺ T and B cells in the spleens and bronchoalveolar lavage (BAL) were also observed. Examination of OVA-challenged IL-15Rα^-/- animals revealed a similar phenotype resulting in enhanced airway eosinophilia compared to WT mice. Adoptive transfer of splenic CD8⁺ T cells from OVA-sensitized WT mice suppressed the enhancement of eosinophilia in IL-15^-/- animals to levels observed in WT mice, but had no further effects.

CONCLUSION AND CLINICAL RELEVANCE

These data demonstrate that mice with an endogenous IL-15 deficiency are susceptible to the development of severe, enhanced Th2-mediated AAD, which can be regulated by CD8⁺ T cells. Furthermore, the development of disease as well as allergen-specific Th2 responses occurs despite deficiencies in several IL-15-dependent cell types including NK, NKT, and γδ T cells, suggesting that these cells or their subsets are dispensable for the induction of AAD in IL-15-deficient mice.

Serum Amyloid A3 Secreted by Preosteoclasts Inhibits Parathyroid Hormone-stimulated cAMP Signaling in Murine Osteoblasts

Continuous parathyroid hormone (PTH) blocks its own osteogenic actions in marrow stromal cell cultures by inducing Cox2 and receptor activator of nuclear factor κB ligand (RANKL) in the osteoblastic lineage cells, which then cause the hematopoietic lineage cells to secrete an inhibitor of PTH-stimulated osteoblast differentiation. To identify this inhibitor, we used bone marrow macrophages (BMMs) and primary osteoblasts (POBs) from WT and Cox2 knock-out (KO) mice. Conditioned medium (CM) from RANKL-treated WT, but not KO, BMMs blocked PTH-stimulated cAMP production in POBs. Inhibition was reversed by pertussis toxin (PTX), which blocks Gαi/o activation. Saa3 was the most highly differentially expressed gene in a microarray comparison of RANKL-treated WT versus Cox2 KO BMMs, and RANKL induced Saa3 protein secretion only from WT BMMs. CM from RANKL-stimulated BMMs with Saa3 knockdown did not inhibit PTH-stimulated responses in POBs. SAA added to POBs inhibited PTH-stimulated cAMP responses, which was reversed by PTX. Selective agonists and antagonists of formyl peptide receptor 2 (Fpr2) suggested that Fpr2 mediated the inhibitory actions of Saa3 on osteoblasts. In BMMs committed to become osteoclasts by RANKL treatment, Saa3 expression peaked prior to appearance of multinucleated cells. Flow sorting of WT marrow revealed that Saa3 was secreted only from the RANKL-stimulated B220(-) CD3(-)CD11b(-/low) CD115(+) preosteoclast population. We conclude that Saa3 secretion from preosteoclasts, induced by RANKL in a Cox2-dependent manner, inhibits PTH-stimulated cAMP signaling and osteoblast differentiation via Gαi/o signaling. The induction of Saa3 by PTH may explain the suppression of bone formation when PTH is applied continuously and may be a new therapeutic target for osteoporosis.

Pro-inflammatory role of natural killer cells in the development of allergic airway disease

BACKGROUND

Natural Killer (NK) cells have been implicated in the development of allergic airway inflammation. However, the in vivo role of NK cells has not been firmly established due to the lack of animal models with selective deficiencies in NK cells.

OBJECTIVE

To determine the specific contribution of NK cells in a murine model of allergic airway disease (AAD).

METHODS

The role of NK cells in AAD was studied using NK-deficient (NKD) mice, perforin(-/-) mice, and mice depleted of Ly49A/D/G(+) NK cell subsets in an ovalbumin-induced model of allergic airway disease (OVA-AAD).

RESULTS

Induction of OVA-AAD in C57BL/6 wild-type (WT) mice resulted in the expansion of airway NK cells and the development of pronounced airway eosinophilia. In the absence of NK cells or specific subsets of NK cells, either in NKD mice, or after the depletion of Ly49A/D/G(+) NK cells, the development of OVA-AAD was significantly impaired as seen by decreased airway inflammation and eosinophilia, decreased secretion of the Th2 cytokines IL-4, IL-5 and IL-13 and diminished OVA-specific antibody production. Furthermore, while OVA-exposure induced a dramatic expansion of dendritic cells (DCs) in WT mice, their induction was significantly attenuated in NKD mice. Development of OVA-AAD in perforin(-/-) mice suggested that the proinflammatory role of NK cells is not dependent on perforin-mediated cytotoxicity. Lastly, induction of allergic disease by OVA-specific CD4 T cells from WT but not NK-depleted or NKD mice in RAG(-/-) recipients, demonstrates that NK cells are essential for T cell priming.

CONCLUSIONS AND CLINICAL RELEVANCE

Our data demonstrate that conventional NK cells play an important and distinct role in the development of AAD. The presence of activated NK cells has been noted in patients with asthma. Understanding the mechanisms by which NK cells regulate allergic disease is therefore an important component of treatment approaches.

Pannexin1 channels act downstream of P2X 7 receptors in ATP-induced murine T-cell death

Death of murine T cells induced by extracellular ATP is mainly triggered by activation of purinergic P2X 7 receptors (P2X 7Rs). However, a link between P2X 7Rs and pannexin1 (Panx1) channels, which are non-selective, has been recently demonstrated in other cell types. In this work, we characterized the expression and cellular distribution of pannexin family members (Panxs 1, 2 and 3) in isolated T cells. Panx1 was the main pannexin family member clearly detected in both helper (CD4+) and cytotoxic (CD8+) T cells, whereas low levels of Panx2 were found in both T-cell subsets. Using pharmacological and genetic approaches, Panx1 channels were found to mediate most ATP-induced ethidium uptake since this was drastically reduced by Panx1 channel blockers (10Panx1, Probenecid and low carbenoxolone concentration) and absent in T cells derived from Panx1-/- mice. Moreover, electrophysiological measurements in wild-type CD4+ cells treated with ATP unitary current events and pharmacological sensitivity compatible with Panx1 channels were found. In addition, ATP release from T cells treated with 4Br-A23187, a calcium ionophore, was completely blocked with inhibitors of both connexin hemichannels and Panx1 channels. Panx1 channel blockers drastically reduced the ATP-induced T-cell mortality, indicating that Panx1 channels mediate the ATP-induced T-cell death. However, mortality was not reduced in T cells of Panx1-/- mice, in which levels of P2X 7Rs and ATP-induced intracellular free Ca2+ responses were enhanced suggesting that P2X 7Rs take over Panx1 channels lose-function in mediating the onset of cell death induced by extracellular ATP.

In vivo fate mapping identifies mesenchymal progenitor cells

Adult mesenchymal progenitor cells have enormous potential for use in regenerative medicine. However, the true identity of the progenitors in vivo and their progeny has not been precisely defined. We hypothesize that cells expressing a smooth muscle α-actin promoter (αSMA)-directed Cre transgene represent mesenchymal progenitors of adult bone tissue. By combining complementary colors in combination with transgenes activating at mature stages of the lineage, we characterized the phenotype and confirmed the ability of isolated αSMA(+) cells to progress from a progenitor to fully mature state. In vivo lineage tracing experiments using a new bone formation model confirmed the osteogenic phenotype of αSMA(+) cells. In vitro analysis of the in vivo-labeled SMA9(+) cells supported their differentiation potential into mesenchymal lineages. Using a fracture-healing model, αSMA9(+) cells served as a pool of fibrocartilage and skeletal progenitors. Confirmation of the transition of αSMA9(+) progenitor cells to mature osteoblasts during fracture healing was assessed by activation of bone-specific Col2.3emd transgene. Our findings provide a novel in vivo identification of defined population of mesenchymal progenitor cells with active role in bone remodeling and regeneration.

Osteoblast-specific overexpression of human interleukin-7 rescues the bone mass phenotype of interleukin-7-deficient female mice

Interleukin-7 is a critical cytokine for lymphoid development and a direct inhibitor of in vitro osteoclastogenesis in murine bone marrow cultures. To explore the role of IL-7 in bone, we generated transgenic mouse lines bearing the 2.3-kb rat collagen 1α1 promoter driving the expression of human IL-7 specifically in osteoblasts. In addition, we crossed these mice with IL-7-deficient mice to determine if the alterations in lymphopoiesis, bone mass, and osteoclast formation observed in the IL-7 knockout (KO) mice could be rescued by osteoblast-specific overexpression of IL-7. Here, we show that mice overexpressing human IL-7 in the osteoblast lineage showed increased trabecular bone volume in vivo by µCT and decreased osteoclast formation in vitro. Furthermore, targeted overexpression of IL-7 in osteoblasts rescued the osteopenic bone phenotype and B-cell development of IL-7 KO mice but did not have an effect on T lymphopoiesis, which occurs in the periphery. The bone phenotypes in IL-7 KO mice and targeted IL-7-overexpressing mouse models were observed only in females. These results likely reflect both direct inhibitory effects of IL-7 on osteoclastogenesis in vivo and sex-specific differences in responses to IL-7.

A novel population of cells expressing both hematopoietic and mesenchymal markers is present in the normal adult bone marrow and is augmented in a murine model of marrow fibrosis

Bone marrow (BM) fibrosis is a feature of severe hyperparathyroidism. Consistent with this observation, mice expressing constitutively active parathyroid hormone (PTH)/PTH-related peptide receptors (PPR) in osteoblasts (PPR*Tg) display BM fibrosis. To obtain insight into the nature of BM fibrosis in such a model, a double-mutant mouse expressing constitutively active PPR and green fluorescent protein (GFP) under the control of the type I collagen promoter (PPR*Tg/GFP) was generated. Confocal microscopy and flow cytometry revealed the presence of a cell population expressing GFP (GFP(+)) that was also positive for the hematopoietic marker CD45 in the BM of both PPR*Tg/GFP and control animals. This cell population was expanded in PPR*Tg/GFP. The existence of cells expressing both type I collagen and CD45 in the adult BM was confirmed by IHC and fluorescence-activated cell sorting. An analysis of total RNA extracted from sorted GFP(+)CD45(+) cells showed that these cells produced type I collagen and PTH/PTH-related peptide receptor and receptor activator for NF-κB mRNAs, further supporting their features of being both mesenchymal and hematopoietic lineages. Similar cells, known as fibrocytes, are also present in pathological fibroses. Our findings, thus, indicate that the BM is a permissive microenvironment for the differentiation of fibrocyte-like cells and raise the possibility that these cells could contribute to the pathogenesis of BM fibrosis.

CD13 is dispensable for normal hematopoiesis and myeloid cell functions in the mouse

While the myeloid marker CD13 has been implicated in numerous myeloid cell functions, its genetic ablation reveals a nominal contribution of CD13 to these functions.

The robust and consistent expression of the CD13 cell surface marker on very early as well as differentiated myeloid hematopoietic cells has prompted numerous investigations seeking to define roles for CD13 in myeloid cells. To address the function of myeloid CD13 directly, we created a CD13 null mouse and assessed the responses of purified primary macrophages or DCs from WT and CD13 null animals in cell assays and inflammatory disease models, where CD13 has been implicated previously. We find that mice lacking CD13 develop normally with normal hematopoietic profiles except for an increase in thymic but not peripheral T cell numbers. Moreover, in in vitro assays, CD13 appears to be largely dispensable for the aspects of phagocytosis, proliferation, and antigen presentation that we tested, although we observed a slight decrease in actin‐independent erythrocyte uptake. However, in agreement with our published studies, we show that lack of monocytic CD13 completely ablates anti‐CD13‐dependent monocyte adhesion to WT endothelial cells. In vivo assessment of four inflammatory disease models showed that lack of CD13 has little effect on disease onset or progression. Nominal alterations in gene expression levels between CD13 WT and null macrophages argue against compensatory mechanisms. Therefore, although CD13 is highly expressed on myeloid cells and is a reliable marker of the myeloid lineage of normal and leukemic cells, it is not a critical regulator of hematopoietic development, hemostasis, or myeloid cell function.

Macrophage migration inhibitory factor inhibits osteoclastogenesis

MIF is an important regulator of innate and adaptive immunity, which is produced by a variety of cell types including activated T cells and macrophages. We examined the effects of MIF on osteoclastogenesis in bone marrow (BM) cultures from WT and MIF-deficient (KO) mice as well as the bone mass of MIF KO mice. Exogenous MIF inhibited osteoclast formation in BM cultures by decreasing fusion in cells that were treated with M-CSF and RANKL. However, inhibition of OCL formation by MIF treatment was not mediated by fusion-related molecules in heterogeneous bone marrow cultures. BM cultures from MIF KO mice that were treated with M-CSF and RANKL, PTH or vitamin D had significantly increased OCL number compared to cells from WT mice. MIF also significantly inhibited OCL formation in cultures of RAW 264.7 cells that were treated with RANKL. In addition, the number of CFU-GM and Mac-1(+) cells in the BM of MIF KO mice was greater than in WT controls. Trabecular bone volume (TBV) in the femurs and vertebrae of MIF KO mice was decreased compared to WT mice. In addition, serum bone resorption and formation markers were decreased in MIF KO mice compared to WT mice. These results demonstrate that MIF has inhibitory effects on OCL formation in vitro. We also found that BM cell cultures from MIF KO mice had an increased capacity to form osteoclasts. Furthermore, MIF KO animals had significantly decreased TBV with low turnover. We conclude that MIF is an inhibitor of osteoclastogenesis in vitro, which may regulate bone turnover via indirect mechanism in vivo.

Defining a visual marker of osteoprogenitor cells within the periodontium

BACKGROUND AND OBJECTIVE

Cells with osteoprogenitor potential are present within periodontal tissues during development and in postnatal life. To identify an osteoprogenitor population, this study utilized a transgenic model in which an alpha-smooth muscle actin (alphaSMA) promoter directed green fluorescent protein (GFP) expression.

MATERIAL AND METHODS

Observation of GFP expression was complemented with analysis of osteogenic differentiation by determining the expression of RNA of bone markers, by histochemical staining for alkaline phosphatase and by the detection of mineralized nodules using xylenol orange. Flow cytometry was utilized to determine the proliferative potential and cell-surface phenotype of cultured alphaSMA-positive cells.

RESULTS

alphaSMA-GFP expression was detected within the dental follicle and in the apical region of the root (i.e. areas rich in vascularization) but not in mature bone. alphaSMA-GFP expression was observed during the early stages of primary cultures derived from the dental follicle and periodontal ligament and was diminished in areas undergoing mineralization. Intense alkaline phosphatase activity and the presence of mineralized nodules was observed 2 wk after osteogenic induction. Consequently, the expression of bone sialoprotein, osteocalcin and dentin matrix protein-1 was increased. Flow cytometry revealed that in vitro expansion enriched for an alphaSMA-GFP-positive population in which 55-65% of cells expressed the cell-surface markers Thy1(+) and Sca1(+). The alphaSMA-GFP-positive population exhibited high proliferative and osteogenic potentials when compared with an alphaSMA-GFP-negative population.

CONCLUSION

Our data indicate that the alphaSMA promoter can be used to identify a population of osteoprogenitor cells residing within the dental follicle and periodontal ligament that can differentiate into mature osteoblasts.

Self-antigen prevents CD8 T cell effector differentiation by CD134 and CD137 dual costimulation

We compared how CD4 vs CD8 cells attain the capacity to express the effector cytokine IFN-gamma under both immunogenic and tolerogenic conditions. Although the Ifng gene locus was epigenetically repressed in naive Ag-inexperienced CD4 cells, it had already undergone partial remodeling toward a transcriptionally competent configuration in naive CD8 cells. After TCR stimulation, CD8 cells fully remodeled the Ifng locus and gained the capacity to express high levels of IFN-gamma more rapidly than CD4 cells. Enforced dual costimulation through OX40 and 4-1BB redirected CD8 cells encountering soluble exogenous peptide to expand and differentiate into IFN-gamma and TNF-alpha double-producing effectors rather than becoming tolerant. Despite this and the stronger tendency of CD8 compared with CD4 cells to differentiate into IFN-gamma-expressing effectors, when parenchymal self-Ag was the source of tolerizing Ag, enforced dual costimulation selectively boosted expansion but did not push effector differentiation in CD8 cells while both expansion and effector differentiation were dramatically boosted in CD4 cells. Notably, enforced dual costimulation was able to push effector differentiation in CD8 cells encountering cognate parenchymal self-Ag when CD4 cells were simultaneously engaged. Thus, the ability of enforced OX40 plus 4-1BB dual costimulation to redirect CD8 cells to undergo effector differentiation was unexpectedly influenced by the source of tolerizing Ag and help was selectively required to facilitate CD8 cell effector differentiation when the tolerizing Ag derived from self.

Use of an alpha-smooth muscle actin GFP reporter to identify an osteoprogenitor population

Identification of a reliable marker of skeletal precursor cells within calcified and soft tissues remains a major challenge for the field. To address this, we used a transgenic model in which osteoblasts can be eliminated by pharmacological treatment. Following osteoblast ablation a dramatic increase in a population of alpha-smooth muscle actin (alpha-SMA) positive cells was observed. During early recovery phase from ablation we have detected cells with the simultaneous expression of alpha-SMA and a preosteoblastic 3.6GFP marker, indicating the potential for transition of alpha-SMA+ cells towards osteoprogenitor lineage. Utilizing alpha-SMAGFP transgene, alpha-SMAGFP+ positive cells were detected in the microvasculature and in the osteoprogenitor population within bone marrow stromal cells. Osteogenic and adipogenic induction stimulated expression of bone and fat markers in the alpha-SMAGFP+ population derived from bone marrow or adipose tissue. In adipose tissue, alpha-SMA+ cells were localized within the smooth muscle cell layer and in pericytes. After in vitro expansion, alpha-SMA+/CD45-/Sca1+ progenitors were highly enriched. Following cell sorting and transplantation of expanded pericyte/myofibroblast populations, donor-derived differentiated osteoblasts and new bone formation was detected. Our results show that cells with a pericyte/myofibroblast phenotype have the potential to differentiate into functional osteoblasts.

Estradiol rapidly inhibits osteoclastogenesis and RANKL expression in bone marrow cultures in postmenopausal women: a pilot study

We examined RANKL expression and OCL formation in cultured bone marrow cells from eight postmenopausal women in response to E(2). E(2) treatment inhibited the ability of hematopoietic cells to form OCLs in response to RANKL, and decreased RANKL production. These changes are likely involved in the ability of E(2) to influence the development of osteoporosis.

INTRODUCTION

Estrogen (E(2)) deficiency at menopause increases osteoclast (OCL) formation and bone resorption, predisposing women to osteoporosis. We examined receptor activator of NF-kappa B-ligand (RANKL) expression and in vitro OCL formation in cultured bone marrow cells from eight postmenopausal women before and after 3 weeks of E(2) therapy and three untreated premenopausal women.

METHODS

TRAP staining and resorption pit assay determined OCL number and function. Flow cytometry measured the distribution of marrow cell types and expression of RANKL in the macrophage-enriched fraction (R1) and a lymphocyte-enriched fraction (R2).

RESULTS

RANKL (3-100 ng/ml) produced a dose-dependent increase in in vitro OC formation and E(2) therapy significantly (p < 0.01) inhibited OCL formation by 33 to 50%. A small proportion of marrow cells bound anti- RANKL Ab (0.2-4.3%). There was no effect of E(2) on the percentage of cells binding the anti-RANKL Ab in the R1 fraction. In the R2 fraction E(2) treatment decreased the percentage of cells binding anti-RANKL Ab by 68 +/- 9% (p < 0.01).

CONCLUSION

Three weeks of E(2) treatment had a dual action. It inhibited the ability of hematopoietic cells to form OCLs in response to RANKL, and decreased the production of RANKL in cells of the bone marrow. The observed changes in the osteoclastic potential of bone marrow cells are likely involved in the ability of E(2) to regulate bone mass and influence the development of osteoporosis.

The 3.6 kb DNA fragment from the rat Col1a1 gene promoter drives the expression of genes in both osteoblast and osteoclast lineage cells

The type I collagen promoter has been used to develop transgenic constructs that are able to mark different stages of osteoblastic differentiation. The pOBCol3.6 promoter is active in early mesenchymal progenitors, including preosteoblasts and osteoblasts, while the pOBCol2.3 promoter is more restricted, showing expression in mature osteoblasts and osteocytes. Transgenic mouse lines have been created that express various GFP reporters under the control of both promoters. These transgenic mice permit the tracking of osteoblastic lineage progression in vitro. They also represent a system to test lineage progression in vivo after the transplantation of progenitors. A parabiosis system was used in which pOBCol3.6GFP transgenic mice were surgically joined with mice bearing a Col2.3DeltaTK transgene. The Col2.3DeltaTK transgenic mouse bears a herpes thymidine kinase gene driven by the pOBCol2.3 promoter, and upon treatment with gancyclovir (GCV) displays extensive destruction of the bone lining cells. After a common circulation was established, parabiotic pairs were treated with GCV for 15 days. Histological analysis of their bones showed the clear presence of GFP positive cells in the Col2.3DeltaTK parabionts, around trabecular bone and on the endosteal and periosteal surfaces. Stromal cell cultures from these Col2.3DeltaTK parabionts did not display mineralized colonies coexpressing GFP. In contrast, scattered GFP positive clusters that contained large cells with morphology similar to osteoclast like cells (OCLs) were observed. These cells were also TRAP positive. They were readily detected in Col2.3DeltaTK mice treated with GCV and transplanted with purified hematopoietic stem cells (HSCs) isolated from pOBCol3.6GFP mice. OCLs were also generated in vitro from osteoclast progenitor cells obtained from pOBCol3.6GFP mice that were defined by the B220- CD3- CD11b- c-fms+ phenotype. Molecular analysis showed that OCLs did not express type I collagen indicating that the Col3.6 promoter contains elements that are active during osteoclastogenesis and are not strictly related to collagen transcription. In summary, we demonstrate that pOBCol3.6 unexpectedly directs the expression of transgenes in the osteoclast lineage and this effect must be considered when utilizing this promoter to study of mesenchymal progenitor cells.

T lymphocyte-deficient mice lose trabecular bone mass with ovariectomy

We examined OVX-induced bone loss in three TLD mouse models. In TLD mice, OVX caused trabecular bone loss equivalent to that of WT. In contrast, cortical bone loss with OVX was variable. We conclude that T lymphocytes do not influence OVX-induced trabecular bone loss.

INTRODUCTION

We examined ovariectomy (OVX)-induced bone loss in three T lymphocyte-deficient (TLD) mouse models: nude mice, recombination activating gene 2-deficient (RAG2 KO) mice, and T cell receptor alpha chain-deficient (TCRalpha KO) mice.

MATERIALS AND METHODS

Bone mass was examined by DXA, microCT, and histomorphometry. We also examined the effect of OVX on T lymphocytes in the bone marrow and spleens of wildtype (WT) mice and on in vitro osteoclastogenesis and colony forming unit-granulocyte macrophage (CFU-GM) activity in the bone marrow of WT and nude mice.

RESULTS

In WT mice, OVX did not alter T lymphocyte number in the bone marrow but did increase T lymphocytes in the spleen. Comparison of bone mass in nude, RAG2 KO, and TCRalpha KO mice with WT as measured by DXA showed decreased femoral bone mass in nude mice and increased vertebral bone mass in RAG2 KO mice. In TCRalpha KO mice, femoral, tibial, and vertebral bone mass were decreased. In vertebrae and long bones, bone loss with OVX was consistently present in WT mice but variably present in TLD mice as measured by DXA. In contrast, microCT and histomorphometry showed similar trabecular bone loss after OVX in all mice. However, femoral cortical bone loss occurred only in WT and RAG2 KO mice. OVX produced similar trabecular bone loss in WT and TCRalpha KO mice and also induced cortical bone loss in both. Histomorphometry showed that TRACP(+) area in bones was increased by OVX in femurs from both WT and nude mice as was in vitro osteoclast-like cell formation and CFU-GM activity.

CONCLUSIONS

These results show that OVX caused similar trabecular bone loss in both WT and TLD mice. The ability of DXA and measurement of cortical bone loss to show OVX-induced effects on bone mass was variable. It seems that T lymphocytes are not critical for OVX-induced trabecular bone loss in these mouse models.

IL-18 bridges innate and adaptive immunity through IFN-gamma and the CD134 pathway

IL-18 induces inflammation resulting in either enhanced protection from pathogens or exacerbation of autoimmunity, and T cells are profoundly activated during these responses. How IL-18 influences T cell activation is unknown, but this study in mice shows that IL-18 boosted Ag-specific T cell clonal expansion of effector T cells and induced a subpopulation of IFN-gamma superproducing T cells. Commitment to IFN-gamma production through IL-18 was independent of NK cells and IL-12 but dependent on host-derived IFN-gamma. To determine how expansion of these effectors occurred, IL-18 was shown to induce OX40L on dendritic cells, whereas peptide stimulation induced CD134 (OX40) on specific T cells. CD134 blockade inhibited T cell effector expansion thereby reducing the number of IFN-gamma superproducers by 12-fold. Thus, independent of IL-12, IL-18 impacts T cell immunity throughout lymphoid and nonlymphoid tissue by bridging the innate and adaptive arms of the immune system through IFN-gamma and the CD134 costimulatory pathway.

Identification of multiple osteoclast precursor populations in murine bone marrow

Murine BM was fractionated using a series of hematopoietic markers to characterize its osteoclast progenitor populations. We found that the early osteoclastogenic activity in total BM was recapitulated by a population of cells contained within the CD11b(-/low) CD45R- CD3- CD115high fraction.

INTRODUCTION

Osteoclasts are of hematopoietic origin and they have been shown to share the same lineage as macrophages. We further characterized the phenotype of osteoclast progenitor populations in murine bone marrow (BM) by analyzing their cell surface markers.

MATERIALS AND METHODS

We used fluorescence-activated cell sorting (FACS) to identify the subsets of BM cells that contained osteoclast progenitors. We fractionated BM according to several markers and cultured the sorted populations for a period of 2-6 days with macrophage-colony stimulating factor (M-CSF) and RANKL. The numbers of multinucleated osteoclast-like cells (OCLs) that formed in the cultures were counted.

RESULTS

We found that the CD45R- CD11b(-/low) population recapitulated the early osteoclastogenic activity of total BM. In addition, although previous experiments indicated that osteoclastogenic activity was enriched within the CD45R+ population, we found that highly purified CD45R+ BM was incapable of differentiating into osteoclasts in vitro. We also found that CD45R- CD11b(high) BM cells were an inefficient source of osteoclast progenitors. However, CD11b was transiently upregulated by cells of the CD45R- CD11b(-/low) fraction early (within 24 h) during culture with M-CSF. Finally, further fractionation of BM using CD115 and CD117 showed that, as osteoclast precursor cells matured, they downregulate CD117 but remain CD115+. Curiously, pure populations of CD117- (CD115high) cells isolated fresh from BM have low osteoclastogenic activity in vitro.

CONCLUSIONS

We provided a refined analysis of the precise subpopulations of murine BM that are capable of differentiating into OCLs in vitro when treated with M-CSF and RANKL.

Distinct cell types control lymphoid subset development by means of IL-15 and IL-15 receptor alpha expression

IL-15 and the IL-15 receptor (IL-15R)alpha chain are essential for normal development of naive CD8 T cells, intestinal intraepithelial lymphocytes (IEL), and natural killer (NK)/NK/T cells. However, whether IL-15R alpha expression by these subsets is necessary for their production and which cell type needs to produce IL-15 to drive development are unknown. We analyzed the requirements for IL-15 and IL-15R alpha expression by bone marrow-derived or parenchymal cells for mediating lymphocyte subset development. Naive CD8 T cell development required IL-15R alpha expression by both bone marrow-derived and parenchymal cells, whereas memory-phenotype CD8 T cells required IL-15R alpha expression only by hematopoietic cells. In contrast and surprisingly, the development of IEL subsets, particularly CD8 alpha alpha Thy1(-)V gamma 5(+) T cell antigen receptor gamma delta and the CD8 alpha alpha Thy1(-) T cell antigen receptor alpha beta IEL populations, depended completely on parenchymal cell expression of IL-15R alpha and IL-15 but not IL-15R beta. In the case of NK and NK/T cell generation and maturation, expression of IL-15 and IL-15R alpha by both parenchymal and hematopoietic cells was important, although the latter played the greatest role. These results demonstrated dichotomous mechanisms by which IL-15 regulated lymphoid development, interacting with distinct cell types depending on the developmental pathway.

Hematopoiesis is severely altered in mice with an induced osteoblast deficiency

We previously reported a transgenic mouse model expressing herpesvirus thymidine kinase (TK) gene under the control of a 2.3-kilobase fragment of the rat collagen alpha1 type I promoter (Col2.3 Delta TK). This construct confers lineage-specific expression in developing osteoblasts, allowing the conditional ablation of osteoblast lineage after treatment with ganciclovir (GCV). After GCV treatment these mice have profound alterations on bone formation leading to a progressive bone loss. In addition, treated animals also lose bone marrow cellularity. In this report we characterized hematopoietic parameters in GCV-treated Col2.3 Delta TK mice, and we show that after treatment transgenic animals lose lymphoid, erythroid, and myeloid progenitors in the bone marrow, followed by decreases in the number of hematopoietic stem cells (HSCs). Together with the decrease in bone marrow hematopoiesis, active extramedullary hematopoiesis was observed in the spleen and liver, as measured by an increase in peripheral HSCs and active primary in vitro hematopoiesis. After withdrawal of GCV, osteoblasts reappeared in the bone compartment together with a recovery of medullary and decrease in extramedullary hematopoiesis. These observations directly demonstrate the role of osteoblasts in hematopoiesis and provide a model to study the interactions between the mesenchymal and hematopoietic compartments in the marrow.

In vivo evidence for a dependence on interleukin 15 for survival of natural killer cells

Cellular homeostasis requires a balance between cell production, cell survival, and cell death. Production of natural killer (NK) cells from bone marrow precursor cells requires interleukin 15 (IL-15); however, very little is known about the factors controlling survival of mature NK cells in vivo. Because mice deficient in IL-15 (IL-15(-/-) mice) fail to develop NK cells, it is not known whether mature NK cells can survive in an environment lacking IL-15. We hypothesized that IL-15 might indeed be required for survival of mature NK cells in vivo. Freshly isolated NK cells labeled with 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester (CFSE) were adoptively transferred into IL-15(-/-) mice and littermate control (IL-15(+/-)) mice. Within 36 hours after transfer, NK cells were detected in both IL-15(-/-) and IL-15(+/-) mice; however, significantly more (P <.003) CFSE-positive (CFSE(+)) NK cells were found in control mice than in IL-15(-/-) mice. By 5 days, similar numbers of CFSE(+) NK cells were still easily detected in IL-15(+/-) mice, whereas no CFSE(+) NK cells survived in IL-15(-/-) mice. Furthermore, mice with severe combined immunodeficiency treated with the Fab fragment of a blocking antibody recognizing a signaling subunit of the IL-15 receptor, IL-2/15Rbeta, had a significant ( approximately 90%) loss of NK cells compared with control mice. Finally, NK cells from Bcl-2 transgenic mice that were adoptively transferred into IL-15(-/-) mice did survive. These results show conclusively that IL-15 is required for mature NK cell survival in vivo and suggest that IL-15 mediates its effect on NK cell survival by means of Bcl-2.

Conditional ablation of the osteoblast lineage in Col2.3deltatk transgenic mice

Two transgenic mouse lines were generated with a DNA construct bearing a 2.3-kilobase (kb) fragment of the rat alpha1 type I collagen promoter driving a truncated form of the herpes thymidine kinase gene (Col2.3Atk). Expression of the transgene was found in osteoblasts coincident with other genetic markers of early osteoblast differentiation. Mice treated with ganciclovir (GCV) for 16 days displayed extensive destruction of the bone lining cells and decreased osteoclast number. In addition, a dramatic decrease in bone marrow elements was observed, which was more severe in the primary spongiosum and marrow adjacent to the diaphyseal endosteal bone. Immunostaining for transgene expression within the bone marrow was negative and marrow stromal cell cultures developed normally in the presence of GCV until the point of early osteoblast differentiation. Our findings suggest that the early differentiating osteoblasts are necessary for the maintenance of osteoclasts and hematopoiesis. Termination of GCV treatment produced an exaggerated response of new bone formation in cortical and trabecular bone. The Col2.3deltatk mouse should be a useful model to define the interrelation between bone and marrow elements as well as a model to analyze the molecular and cellular events associated with a defined wave of osteogenesis on termination of GCV treatment.

In vivo natural killer cell activities revealed by natural killer cell-deficient mice

Studies of natural killer (NK) cell function in vivo have been challenging primarily due to the lack of animal models in which NK cells are genetically and selectively deficient. Here, we describe a transgenic mouse with defective natural killing and selective deficiency in NK1.1(+) CD3(-) cells. Despite functionally normal B, T, and NK/T cells, transgenic mice displayed impaired acute in vivo rejection of tumor cells. Adoptive transfer experiments confirmed that NK1.1(+) CD3(-) cells were responsible for acute tumor rejection, establishing the relationship of NK1.1(+) CD3(-) cells to NK cells. Additional studies provided evidence that (i) NK cells play an important role in suppressing tumor metastasis and outgrowth; (ii) NK cells are major producers of IFNgamma in response to bacterial endotoxin but not to interleukin-12, and; (iii) NK cells are not essential for humoral responses to T cell-independent type 2 antigen or the generalized Shwartzman reaction, both of which were previously proposed to involve NK cells.

From stem cells to lymphocytes: biology and transplantation

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

Hematopoietic stem cells are not direct cytotoxic targets of natural killer cells

Bone marrow (BM) transplants from one individual to an irradiated histoincompatible individual of the same species are rejected. In mice, the primary host barrier cells that recognize bone marrow grafts bearing hematopoietic histocompatibility antigens bear surface markers of natural killer (NK) lymphocytes. Because of the innate ability of NK cells to kill susceptible targets, it has been proposed that the cytotoxic bone marrow graft rejection. To test this hypothesis, we purified hematopoietic stem cells from mice and incubated them with purified populations of actively cytotoxic allogeneic and semisyngeneic NK cells, followed by analysis of the ability of the treated hematopoietic stem cells (HSCs) to rescue lethally irradiated syngeneic animals. Such rescue was unimpaired. Also, HSC allografts were transplanted into transgenic mice deficient in NK and killer T-cell cytotoxicity generated by expressing diphtheria toxin A chain under the control of granzyme A promoter. Allogeneic HSCs were susceptible to allogeneic restriction in these mice, implying that the effector functions of NK marker-positive cells do not require NK cell cytotoxicity.

Transgenic mice carrying the diphtheria toxin A chain gene under the control of the granzyme A promoter: expected depletion of cytotoxic cells and unexpected depletion of CD8 T cells

We have generated transgenic mice bearing the diphtheria toxin A chain (DTA) gene under the control of granzyme A (GrA) promoter sequences (GrA-DTA). GrA is expressed in activated cytotoxic cells but not in their immediate progenitors. These GrA-DTA mice are deficient in cytotoxic functions, indicating that most cytotoxic cells express GrA in vivo. Surprisingly, one founder strain containing a multicopy GrA-DTA insert show a marked and selective deficiency in CD8+ cells in peripheral lymphoid organs. This depletion was not observed in thymus, where the distribution of CD4+ and CD8+ cells is normal. Moreover, the emigration of T cells from thymus is normal, indicating that the depletion occurs in the periphery. GrA-DTA mice should be useful as models to dissect the role of cytotoxic cells in immune responses and as recipients of normal and neoplastic hematopoietic cells. The selective depletion of CD8+ cells in one founder strain could have implications for postthymic T-cell development.

Rapid and sustained hematopoietic recovery in lethally irradiated mice transplanted with purified Thy-1.1lo Lin-Sca-1+ hematopoietic stem cells

Hematopoietic stem cells (HSCs) are believed to play a critical role in the sustained repopulation of all blood cells after bone marrow transplantation (BMT). However, understanding the role of HSCs versus other hematopoietic cells in the quantitative reconstitution of various blood cell types has awaited methods to isolate HSCs. A candidate population of mouse HSCs, Thy-1.1lo Lin-Sca-1+ cells, was isolated several years ago and, recently, this population has been shown to be the only population of BM cells that contains HSCs in C57BL/Ka-Thy-1.1 mice. As few as 100 of these cells can radioprotect 95% to 100% of irradiated mice, resulting long-term multilineage reconstitution. In this study, we examined the reconstitution potential of irradiated mice transplanted with purified Thy-1.1lo Lin-Sca-1+ BM cells. Donor-derived peripheral blood (PB) white blood cells were detected as early as day 9 or 10 when 100 to 1,000 Thy-1.1lo Lin-Sca-1+ cells were used, with minor dose-dependent differences. The reappearance of platelets by day 14 and thereafter was also seen at all HSC doses (100 to 1,000 cells), with a slight dose-dependence. All studied HSC doses also allowed RBC levels to recover, although at the 100 cell dose a delay in hematocrit recovery was observed at day 14. When irradiated mice were transplanted with 500 Thy-1.1lo Lin-Sca-1+ cells compared with 1 x 10(6) BM cells (the equivalent amount of cells that contain 500 Thy-1.1lo Lin-Sca-1+ cells as well as progenitor and mature cells), very little difference in the kinetics of recovery of PB, white blood cells, platelets, and hematocrit was observed. Surprisingly, even when 200 Thy1.1lo Lin-Sca-1+ cells were mixed with 4 x 10(5) Sca-1- BM cells in a competitive repopulation assay, most of the early (days 11 and 14) PB myeloid cells were derived from the HSC genotype, indicating the superiority of the Thy-1.1lo Lin-Sca-1+ cells over Sca-1- cells even in the early phases of myeloid reconstitution. Within the Thy-1.1lo Lin-Sca-1+ population, the Rhodamine 123 (Rh123)hi subset dominates in PB myeloid reconstitution at 10 to 14 days, only to be overtaken by the Rh123lo subset at 3 weeks and thereafter. These findings indicate that HSCs can account for the early phase of hematopoietic recovery, as well as sustained hematopoiesis, and raise questions about the role of non-HSC BM populations in the setting of BMT.

Clonal variants of hybridoma cells that switch isotype at a high frequency

As B cells differentiate under the influence of antigen and T cells, they frequently switch from the expression of IgM antibody to the expression of other isotypes. This is accomplished by rearranging the expressed variable region gene to downstream constant region genes and deleting the intervening sequences. Some B-cell lines that represent early stages in development switch constitutively in culture at frequencies that approach those of lipopolysaccharide- or lymphokine-stimulated normal B cells. Hybridoma cells represent a later stage of development and rarely switch in culture. In contrast to early B-cell lines, hybridomas produce large amounts of immunoglobulin, and single cells can be assayed easily for the expression of new isotypes. We have used the ELISA spot assay and fluctuation analysis to determine the rate of switching of two hybridoma cell lines. By identifying subclones that switched more frequently, we have progressively enriched for cells that switch spontaneously at higher rates. These cells, like normal cells, switch by rearrangement and deletion, and the frequency of switched cells in some of the clones is comparable to that which has been observed in less differentiated B-cell lines and in normal B cells.

Bcl-2 prevents death of factor-deprived cells but fails to prevent apoptosis in targets of cell mediated killing

'Programmed cell death' has been used to describe the death of cells killed by cytotoxic T cells or growth factor deprivation. Although bcl-2 can prevent death of cells deprived of growth factor, it failed to protect cells against T cell killing. In spite of bcl-2 expression, the DNA of targeted cells was degraded into nucleosome-sized fragments. Therefore the early steps in apoptosis induced by factor deprivation differ from those triggered by cytotoxic T cells, although they share a common final pathway featuring degradation of the DNA and loss of cytoplasmic membrane integrity.

The molecular and biochemical characterization of mutant monoclonal antibodies with increased antigen binding

Mutant mAb with increased Ag binding were generated from a hybridoma cell line, 36-65, that secretes an IgG1,kappa anti-p-azophenylarsonate-(Ars) specific antibody. The mutant antibodies were identified using an Ars-specific ELISA and sib selection so that approximately 10(6) cells could be analyzed. The ELISA used as Ag a low ratio of Ars coupled to BSA and was set up so that only those antibodies that had higher binding than the parent would be detected. Seven mutant producing cell lines were isolated from five independent clones of 36-65. The mutant antibodies bind Ag 20 to more than 200-fold better than the parent and have wild type V region sequences. All have C region mutations that result in an increased avidity. At least five different genetic events are responsible for the C region mutations.

The molecular and biochemical characterization of mutant monoclonal antibodies with increased antigen binding

Mutant mAb with increased Ag binding were generated from a hybridoma cell line, 36-65, that secretes an IgG1,kappa anti-p-azophenylarsonate-(Ars) specific antibody. The mutant antibodies were identified using an Ars-specific ELISA and sib selection so that approximately 10(6) cells could be analyzed. The ELISA used as Ag a low ratio of Ars coupled to BSA and was set up so that only those antibodies that had higher binding than the parent would be detected. Seven mutant producing cell lines were isolated from five independent clones of 36-65. The mutant antibodies bind Ag 20 to more than 200-fold better than the parent and have wild type V region sequences. All have C region mutations that result in an increased avidity. At least five different genetic events are responsible for the C region mutations.

Amino terminal sequence of heavy and light chains from ratfish immunoglobulin

The ratfish, Callorhinchus callorhinchus, a representative of the Holocephali, has a natural serum hemagglutinin (Mr 960,000), composed of heavy (Mr 71,000), light (Mr 22,500), and J (Mr 16,000) chains. To approach the mechanisms that generate diversity at this level of evolution, the amino terminal sequence of the heavy and light chains was determined by automated microsequencing. The chains are unblocked and have modest internal sequence heterogeneity. The heavy chains show sequence similarity with the terminal region of the heavy chain from the horned shark, Heterodontus francisci, and other species. In contrast to the heavy chain, the ratfish light chains display low sequence similarity with their shark kappa counterparts. However, their similarity with the variable region of the chicken lambda light chains is about 75%.

T15 PC-binding monoclonal antibodies retain specificity when they switch from IgM to IgG

The expression of some Ag-binding sites and their associated H and L chain V region genes are often dominated by one or another of the IgG subclasses. This is true of T15 anti-phosphorylcholine (PC) response in which the dominant T15 H (V1, DFL11.6 JH1) and L (VK22, JK5) chain V regions are seldom found associated with the gamma 2b or gamma 2a C regions in the circulation of BALB/c mice. In the present study we describe the characterization of gamma 2b, gamma 2a, and gamma 1 anti-PC antibodies obtained from IgM-producing hybridomas by Ig switching in culture. All switch variants retained the parental T15 idiotype, PC binding, and fine specificity. We thus conclude that the apparent in vivo restriction on the expressed T15 PC binding gamma 2 antibodies is not due to a conformational limitation on the expression of the anti-PC T15 binding site. Further, these studies confirm that in vitro switching can be used to generate mAb that are not readily available in vivo.

T15 PC-binding monoclonal antibodies retain specificity when they switch from IgM to IgG

The expression of some Ag-binding sites and their associated H and L chain V region genes are often dominated by one or another of the IgG subclasses. This is true of T15 anti-phosphorylcholine (PC) response in which the dominant T15 H (V1, DFL11.6 JH1) and L (VK22, JK5) chain V regions are seldom found associated with the gamma 2b or gamma 2a C regions in the circulation of BALB/c mice. In the present study we describe the characterization of gamma 2b, gamma 2a, and gamma 1 anti-PC antibodies obtained from IgM-producing hybridomas by Ig switching in culture. All switch variants retained the parental T15 idiotype, PC binding, and fine specificity. We thus conclude that the apparent in vivo restriction on the expressed T15 PC binding gamma 2 antibodies is not due to a conformational limitation on the expression of the anti-PC T15 binding site. Further, these studies confirm that in vitro switching can be used to generate mAb that are not readily available in vivo.

Studies on the somatic instability of immunoglobulin genes in vivo and in cultured cells

We have examined the molecular mechanism and impact of somatic diversification on the T15 heavy chain variable region gene in vivo and in vitro. Somatic point mutation appears to be responsible for the changes we have observed in both hybridomas from early and late in the immune response and in the S107 myeloma cell line in culture. By identifying S107 mutants with decreases in antigen binding, we have shown that a single point mutation can cause the loss of binding to the eliciting antigen and the acquisition of binding to another antigen. Furthermore, in this case a point mutation of the T15 heavy chain variable region gene caused the conversion of an important protective antibody to an autoantibody. While the S107 cell line frequently generates both constant and variable region mutants, hybridomas appear to have relatively stable variable region genes and unstable constant region genes which in some cases result in mutants with increased binding.