A simple point-of-care assay accurately detects anti-spike antibodies after SARS-CoV-2 vaccination

Objective

Lateral flow assays (LFA) are sensitive for detecting antibodies to SARS-CoV-2 proteins within weeks after infection. This study tested samples from immunocompetent adults, and those receiving treatments for chronic inflammatory diseases (CID), before and after mRNA SARS-CoV-2 vaccination.

Methods

We compared results obtained with the COVIBLOCK Covid-19 LFA to those obtained by anti-spike (S) ELISA.

Results

The LFA detected anti-S antibodies in 29 of 29 (100%) of the immunocompetent and 110 of 126 (87.3%) of the CID participants after vaccination. Semiquantitative LFA scores were statistically significantly lower in samples from immunosuppressed participants, and were significantly correlated with anti-S antibody levels measured by ELISA.

Conclusions

This simple LFA test is a practical alternative to laboratory-based assays for detecting anti-S antibodies after infection or vaccination. This type of test may be most useful for testing people in outpatient or resource-limited settings.

Physical Activity Associates With Lower Systemic Inflammatory Gene Expression in Rheumatoid Arthritis

OBJECTIVE

While general population studies have shown inverse associations between physical activity and common inflammatory biomarkers, the effects of physical activity on inflammatory gene expression and signaling pathways in rheumatoid arthritis (RA) remain unknown. We aimed to determine whether physical activity independently associates with expression of inflammatory genes among people with RA.

METHODS

This was a prospective observational study of adults with RA. Physical activity was measured by quantitative actigraphy over 7 consecutive days, and peripheral blood collected during the same time period was used for RNA sequencing followed by differential gene expression, pathway, and network analyses.

RESULTS

Actigraphy and RNA sequencing data were evaluated in 35 patients. The cohort had a mean age of 56 (SD 12) years, and was 91% female, 31% White, 9% Black, 9% Asian, and 40% Hispanic. We found 767 genes differentially expressed (adjusted P < 0.1) between patients in the greatest vs lowest physical activity tertiles, after adjusting for sex, age, race, and ethnicity. The most active patients exhibited dose-dependent downregulation of several immune signaling pathways implicated in RA pathogenesis. These included CD40, STAT3, TREM-1, interleukin (IL)-17A, IL-8, Toll-like receptor, and interferon (IFN) signaling pathways. Upstream cytokine activation state analysis predicted reduced activation of tumor necrosis factor-α and IFN in the most active group. In sensitivity analyses, we adjusted for RA disease activity and physical function and found consistent results.

CONCLUSION

Patients with RA who were more physically active had lower expression of immune signaling pathways implicated in RA pathogenesis, even after adjusting for disease activity, suggesting that physical activity may confer a protective effect in RA.

ACVR1R206H extends inflammatory responses in human induced pluripotent stem cell-derived macrophages

Macrophages play crucial roles in many human disease processes. However, obtaining large numbers of primary cells for study is often difficult. We describe 2D and 3D methods for directing human induced pluripotent stem cells (hiPSCs) into macrophages (iMACs). iMACs generated in 2D culture showed functional similarities to human primary monocyte-derived M2-like macrophages, and could be successfully polarized into a M1-like phenotype. Both M1- and M2-like iMACs showed phagocytic activity and reactivity to endogenous or exogenous stimuli. In contrast, iMACs generated by a 3D culture system showed mixed M1- and M2-like functional characteristics. 2D-iMACs from patients with fibrodysplasia ossificans progressiva (FOP), an inherited disease with progressive heterotopic ossification driven by inflammation, showed prolonged inflammatory cytokine production and higher Activin A production after M1-like polarization, resulting in dampened responses to additional LPS stimulation. These results demonstrate a simple and robust way of creating hiPSC-derived M1- and M2-like macrophage lineages, while identifying macrophages as a source of Activin A that may drive heterotopic ossification in FOP.

Effect of Immunosuppression on the Immunogenicity of mRNA Vaccines to SARS-CoV-2 : A Prospective Cohort Study

BACKGROUND

Patients with chronic inflammatory disease (CID) treated with immunosuppressive medications have increased risk for severe COVID-19. Although mRNA-based SARS-CoV-2 vaccination provides protection in immunocompetent persons, immunogenicity in immunosuppressed patients with CID is unclear.

OBJECTIVE

To determine the immunogenicity of mRNA-based SARS-CoV-2 vaccines in patients with CID.

DESIGN

Prospective observational cohort study.

SETTING

Two U.S. CID referral centers.

PARTICIPANTS

Volunteer sample of adults with confirmed CID eligible for early COVID-19 vaccination, including hospital employees of any age and patients older than 65 years. Immunocompetent participants were recruited separately from hospital employees. All participants received 2 doses of mRNA vaccine against SARS-CoV-2 between 10 December 2020 and 20 March 2021. Participants were assessed within 2 weeks before vaccination and 20 days after final vaccination.

MEASUREMENTS

Anti-SARS-CoV-2 spike (S) IgG⁺ binding in all participants, and neutralizing antibody titers and circulating S-specific plasmablasts in a subset to assess humoral response after vaccination.

RESULTS

Most of the 133 participants with CID (88.7%) and all 53 immunocompetent participants developed antibodies in response to mRNA-based SARS-CoV-2 vaccination, although some with CID developed numerically lower titers of anti-S IgG. Anti-S IgG antibody titers after vaccination were lower in participants with CID receiving glucocorticoids (n = 17) than in those not receiving them; the geometric mean of anti-S IgG antibodies was 357 (95% CI, 96 to 1324) for participants receiving prednisone versus 2190 (CI, 1598 to 3002) for those not receiving it. Anti-S IgG antibody titers were also lower in those receiving B-cell depletion therapy (BCDT) (n = 10). Measures of immunogenicity differed numerically between those who were and those who were not receiving antimetabolites (n = 48), tumor necrosis factor inhibitors (n = 39), and Janus kinase inhibitors (n = 11); however, 95% CIs were wide and overlapped. Neutralization titers seemed generally consistent with anti-S IgG results. Results were not adjusted for differences in baseline clinical factors, including other immunosuppressant therapies.

LIMITATIONS

Small sample that lacked demographic diversity, and residual confounding.

CONCLUSION

Compared with nonusers, patients with CID treated with glucocorticoids and BCDT seem to have lower SARS-CoV-2 vaccine-induced antibody responses. These preliminary findings require confirmation in a larger study.

PRIMARY FUNDING SOURCE

The Leona M. and Harry B. Helmsley Charitable Trust, Marcus Program in Precision Medicine Innovation, National Center for Advancing Translational Sciences, and National Institute of Arthritis and Musculoskeletal and Skin Diseases.

CCR2 deficiency alters activation of microglia subsets in traumatic brain injury

In traumatic brain injury (TBI), a diversity of brain resident and peripherally derived myeloid cells have the potential to worsen damage and/or to assist in healing. We define the heterogeneity of microglia and macrophage phenotypes during TBI in wild-type (WT) mice and Ccr2^−/− mice, which lack macrophage influx following TBI and are resistant to brain damage. We use unbiased single-cell RNA sequencing methods to uncover 25 microglia, monocyte/macrophage, and dendritic cell subsets in acute TBI and normal brains. We find alterations in transcriptional profiles of microglia subsets in Ccr2^−/− TBI mice compared to WT TBI mice indicating that infiltrating monocytes/macrophages influence microglia activation to promote a type I IFN response. Preclinical pharmacological blockade of hCCR2 after injury reduces expression of IFN-responsive gene, Irf7, and improves outcomes. These data extend our understanding of myeloid cell diversity and crosstalk in brain trauma and identify therapeutic targets in myeloid subsets.

By single-cell RNA sequencing of traumatically injured and normal brains from wild-type and Ccr2^−/− mice, Somebang et al. define microglia, macrophage, and dendritic cell phenotypes in TBI. Targeting mouse and/or human CCR2 reduces specific TBI brain CNS myeloid compartments, dampens type I interferon responses, and improves cognition after TBI.

2021 American College of Rheumatology Guideline for the Treatment of Rheumatoid Arthritis

OBJECTIVE

To develop updated guidelines for the pharmacologic management of rheumatoid arthritis.

METHODS

We developed clinically relevant population, intervention, comparator, and outcomes (PICO) questions. After conducting a systematic literature review, the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was used to rate the certainty of evidence. A voting panel comprising clinicians and patients achieved consensus on the direction (for or against) and strength (strong or conditional) of recommendations.

RESULTS

The guideline addresses treatment with disease-modifying antirheumatic drugs (DMARDs), including conventional synthetic DMARDs, biologic DMARDs, and targeted synthetic DMARDs, use of glucocorticoids, and use of DMARDs in certain high-risk populations (i.e., those with liver disease, heart failure, lymphoproliferative disorders, previous serious infections, and nontuberculous mycobacterial lung disease). The guideline includes 44 recommendations (7 strong and 37 conditional).

CONCLUSION

This clinical practice guideline is intended to serve as a tool to support clinician and patient decision-making. Recommendations are not prescriptive, and individual treatment decisions should be made through a shared decision-making process based on patients' values, goals, preferences, and comorbidities.

2021 American College of Rheumatology Guideline for the Treatment of Rheumatoid Arthritis

OBJECTIVE

To develop updated guidelines for the pharmacologic management of rheumatoid arthritis.

METHODS

We developed clinically relevant population, intervention, comparator, and outcomes (PICO) questions. After conducting a systematic literature review, the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was used to rate the certainty of evidence. A voting panel comprising clinicians and patients achieved consensus on the direction (for or against) and strength (strong or conditional) of recommendations.

RESULTS

The guideline addresses treatment with disease-modifying antirheumatic drugs (DMARDs), including conventional synthetic DMARDs, biologic DMARDs, and targeted synthetic DMARDs, use of glucocorticoids, and use of DMARDs in certain high-risk populations (i.e., those with liver disease, heart failure, lymphoproliferative disorders, previous serious infections, and nontuberculous mycobacterial lung disease). The guideline includes 44 recommendations (7 strong and 37 conditional).

CONCLUSION

This clinical practice guideline is intended to serve as a tool to support clinician and patient decision-making. Recommendations are not prescriptive, and individual treatment decisions should be made through a shared decision-making process based on patients' values, goals, preferences, and comorbidities.

Glucocorticoids and B Cell Depleting Agents Substantially Impair Immunogenicity of mRNA Vaccines to SARS-CoV-2

BACKGROUND

Individuals with chronic inflammatory diseases (CID) are frequently treated with immunosuppressive medications that can increase their risk of severe COVID-19. While novel mRNA-based SARS-CoV-2 vaccination platforms provide robust protection in immunocompetent individuals, the immunogenicity in CID patients on immunosuppression is not well established. Therefore, determining the effectiveness of SARS-CoV-2 vaccines in the setting of immunosuppression is essential to risk-stratify CID patients with impaired protection and provide clinical guidance regarding medication management.

METHODS

We conducted a prospective assessment of mRNA-based vaccine immunogenicity in 133 adults with CIDs and 53 immunocompetent controls. Blood from participants over 18 years of age was collected before initial immunization and 1-2 weeks after the second immunization. Serum anti-SARS-CoV-2 spike (S) IgG + binding, neutralizing antibody titers, and circulating S-specific plasmablasts were quantified to assess the magnitude and quality of the humoral response following vaccination.

RESULTS

Compared to immunocompetent controls, a three-fold reduction in anti-S IgG titers (P=0.009) and SARS-CoV-2 neutralization (p<0.0001) were observed in CID patients. B cell depletion and glucocorticoids exerted the strongest effect with a 36- and 10-fold reduction in humoral responses, respectively (p<0.0001). Janus kinase inhibitors and antimetabolites, including methotrexate, also blunted antibody titers in multivariate regression analysis (P<0.0001, P=0.0023, respectively). Other targeted therapies, such as TNF inhibitors, IL-12/23 inhibitors, and integrin inhibitors, had only modest impacts on antibody formation and neutralization.

CONCLUSIONS

CID patients treated with immunosuppressive therapies exhibit impaired SARS-CoV-2 vaccine-induced immunity, with glucocorticoids and B cell depletion therapy more severely impeding optimal responses.

Lower PDL1, PDL2, and AXL Expression on Lung Myeloid Cells Suggests Inflammatory Bias in Smoking and Chronic Obstructive Pulmonary Disease

Lung myeloid cells are important in pulmonary immune homeostasis and in the pathogenesis of chronic obstructive pulmonary disease (COPD). Multiparameter immunophenotypic characterization of these cells is challenging because of their autofluorescence and diversity. We evaluated the immunophenotypic landscape of airway myeloid cells in COPD using time of flight mass cytometry. Cells from BAL, which were obtained from never-smokers (n = 8) and smokers with (n = 20) and without (n = 4) spirometric COPD, were examined using a 44-parameter time of flight mass cytometry panel. Unsupervised cluster analysis was used to identify cellular subtypes that were confirmed by manual gating. We identified major populations of CD68⁺ and CD68^- cells with 22 distinct phenotypic clusters, of which 18 were myeloid cells. We found a higher abundance of putative recruited myeloid cells (CD68⁺ classical monocytes) in BAL from patients with COPD. CD68⁺ classical monocyte population had distinct responses to smoking and COPD that were potentially related to their recruitment from the interstitium and vasculature. We demonstrate that BAL cells from smokers and subjects with COPD have lower AXL expression. Also, among subjects with COPD, we report significant differences in the abundance of PDL1^high and PDL2^high clusters and in the expression of PDL1 and PDL2 across several macrophage subtypes suggesting modulation of inflammatory responses. In addition, several phenotypic differences in BAL cells from subjects with history of COPD exacerbation were identified that could inform potential disease mechanisms. Overall, we report several changes to the immunophenotypic landscape that occur with smoking, COPD, and past exacerbations that are consistent with decreased regulation and increased activation of inflammatory pathways.

Non-catalytic ubiquitin binding by A20 prevents psoriatic arthritis-like disease and inflammation

A20 is an anti-inflammatory protein that is strongly linked to human disease. Here, we find that mice expressing three distinct targeted mutations of A20's zinc finger 7 (ZF7) ubiquitin-binding motif uniformly developed digit arthritis with features common to psoriatic arthritis, while mice expressing point mutations in A20's OTU or ZF4 motifs did not exhibit this phenotype. Arthritis in A20^ZF7 mice required T cells and MyD88, was exquisitely sensitive to tumor necrosis factor and interleukin-17A, and persisted in germ-free conditions. A20^ZF7 cells exhibited prolonged IκB kinase activity that drove exaggerated transcription of late-phase nuclear factor-κB response genes in vitro and in prediseased mouse paws in vivo. In addition, mice expressing double-mutant A20 proteins in A20's ZF4 and ZF7 motifs died perinatally with multi-organ inflammation. Therefore, A20's ZF4 and ZF7 motifs synergistically prevent inflammatory disease in a non-catalytic manner.

Age-related changes to macrophages are detrimental to fracture healing in mice

The elderly population suffers from higher rates of complications during fracture healing that result in increased morbidity and mortality. Inflammatory dysregulation is associated with increased age and is a contributing factor to the myriad of age-related diseases. Therefore, we investigated age-related changes to an important cellular regulator of inflammation, the macrophage, and the impact on fracture healing outcomes. We demonstrated that old mice (24 months) have delayed fracture healing with significantly less bone and more cartilage compared to young mice (3 months). The quantity of infiltrating macrophages into the fracture callus was similar in old and young mice. However, RNA-seq analysis demonstrated distinct differences in the transcriptomes of macrophages derived from the fracture callus of old and young mice, with an up-regulation of M1/pro-inflammatory genes in macrophages from old mice as well as dysregulation of other immune-related genes. Preventing infiltration of the fracture site by macrophages in old mice improved healing outcomes, with significantly more bone in the calluses of treated mice compared to age-matched controls. After preventing infiltration by macrophages, the macrophages remaining within the fracture callus were collected and examined via RNA-seq analysis, and their transcriptome resembled macrophages from young calluses. Taken together, infiltrating macrophages from old mice demonstrate detrimental age-related changes, and depleting infiltrating macrophages can improve fracture healing in old mice.

Female-Specific Role of Progranulin to Suppress Bone Formation

Progranulin (PGRN) is best known as a glial protein for which deficiency leads to the most common inherited form of frontotemporal dementia. Recently, PGRN has been found to be an adipokine associated with diet-induced obesity and insulin resistance. Therefore, PGRN may have homeostatic effects on bone because PGRN is reported to promote the differentiation of bone-resorbing osteoclasts. We investigated the actions of PGRN on bone using PGRN gene (Grn) knockout (KO) mice and transgenic mice with PGRN mutation and surprisingly found that loss of PGRN prevented the bone loss in female mice induced by aging and estrogen deficiency, whereas it had no effect on male bones during aging. Strikingly, bone formation was increased in female (but not male) PGRN KO mice. We also found that loss of PGRN inhibited bone resorption and osteoclastogenesis in both male and female mice and promoted the production of osteogenic factors in osteoclast lineage cells. These results indicate that PGRN serves to uncouple bone turnover in female mice by promoting bone resorption and suppressing bone formation. Furthermore, we demonstrated that microglial cells/macrophages, but not adipocytes, are an important source of PGRN in producing negative skeletal effects in females. Targeting PGRN production by microglial cells/macrophage-lineage cells may provide a therapeutic approach for the treatment of osteoporosis in females.

NF-κB/MAPK activation underlies ACVR1-mediated inflammation in human heterotopic ossification

BACKGROUND

Inflammation helps regulate normal growth and tissue repair. Although bone morphogenetic proteins (BMPs) and inflammation are known contributors to abnormal bone formation, how these pathways interact in ossification remains unclear.

METHODS

We examined this potential link in patients with fibrodysplasia ossificans progressiva (FOP), a genetic condition of progressive heterotopic ossification caused by activating mutations in the Activin A type I receptor (ACVR1/ALK2). FOP patients show exquisite sensitivity to trauma, suggesting that BMP pathway activation may alter immune responses. We studied primary blood, monocyte, and macrophage samples from control and FOP subjects using multiplex cytokine, gene expression, and protein analyses; examined CD14+ primary monocyte and macrophage responses to TLR ligands; and assayed BMP, TGF-β activated kinase 1 (TAK1), and NF-κB pathways.

RESULTS

FOP subjects at baseline without clinically evident heterotopic ossification showed increased serum IL-3, IL-7, IL-8, and IL-10. CD14+ primary monocytes treated with the TLR4 activator LPS showed increased CCL5, CCR7, and CXCL10; abnormal cytokine/chemokine secretion; and prolonged activation of the NF-κB pathway. FOP macrophages derived from primary monocytes also showed abnormal cytokine/chemokine secretion, increased TGF-β production, and p38MAPK activation. Surprisingly, SMAD phosphorylation was not significantly changed in the FOP monocytes/macrophages.

CONCLUSIONS

Abnormal ACVR1 activity causes a proinflammatory state via increased NF-κB and p38MAPK activity. Similar changes may contribute to other types of heterotopic ossification, such as in scleroderma and dermatomyositis; after trauma; or with recombinant BMP-induced bone fusion. Our findings suggest that chronic antiinflammatory treatment may be useful for heterotopic ossification.

A myeloid population potentially corresponding to myeloid monocytic suppressor cells (MoMDSCs) is increased in the blood of Rheumatoid Arthritis patients and associated with osteoclastogenesis

The development of erosive arthritis and joint damage are common and severe complications of rheumatoid arthritis (RA). Erosions are caused by excessive activity of osteoclasts (OCs), the only known bone resorbing cells. OCs are myeloid lineage cells and can be generated in vitro from CD14+ circulating myeloid cells, using RANKL and M-CSF. To characterize circulating myeloid lineage cells in RA, we performed mass cytometry (CyTOF) analysis of blood from 11 RA patients and 10 age and gender-matched healthy controls. Using CITRUS analysis, we found a myeloid cell cluster that is significantly elevated in RA patients compared to healthy subjects. The phenotype of this cluster is CD11b+, CD14+, CD16−, HLA-DR-/lo, CCR2hi, RANKhi, and phosphorylated (p)STAT3hi. The surface phenotype of this cluster is associated with monocytic myeloid derived suppressor cells (MoMDSCs). Analysis by conventional flow of a separate group of 14 RA patients showed that frequency of these myeloid cells correlates with number of blood osteoclasts precursors (OCP) based on the in vitro assay. Baseline level of pSrc and pJNK in these cells also correlated with OCP numbers. Both of these molecules are involved in downstream signaling cascades involved in osteoclastogenesis and osteoclast survival. We also found that intermediate (CD14++CD16+) monocytes are elevated in RA, as previously described, but these cells did not show a positive correlation with OCP numbers. These results indicate that a group of myeloid cells, potentially corresponding to MoMDSCs, are elevated in the blood of RA patients. Their phenotype (high RANK, phosphorylated Src, phosphorylated JNK) and frequency indicate that they may be involved in osteoclastogenesis, as described in mouse models.

A Comprehensive Review of Immunoreceptor Regulation of Osteoclasts

Osteoclasts require coordinated co-stimulation by several signaling pathways to initiate and regulate their cellular differentiation. Receptor activator for NF-κB ligand (RANKL or TNFSF11), a tumor necrosis factor (TNF) superfamily member, is the master cytokine required for osteoclastogenesis with essential co-stimulatory signals mediated by immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptors, DNAX-associated protein 12 kDa size (DAP12) and FcεRI gamma chain (FcRγ). The ITAM-signaling adaptors do not have an extracellular ligand-binding domain and, therefore, must pair with ligand-binding immunoreceptors to interact with their extracellular environment. DAP12 pairs with a number of different immunoreceptors including triggering receptor expressed on myeloid cells 2 (TREM2), myeloid DAP12-associated lectin (MDL-1), and sialic acid-binding immunoglobulin-type lectin 15 (Siglec-15); while FcRγ pairs with a different set of receptors including osteoclast-specific activating receptor (OSCAR), paired immunoglobulin receptor A (PIR-A), and Fc receptors. The ligands for many of these receptors in the bone microenvironment remain unknown. Here, we will review immunoreceptors known to pair with either DAP12 or FcRγ that have been shown to regulate osteoclastogenesis. Co-stimulation and the effects of ITAM-signaling have turned out to be complex, and now include paradoxical findings that ITAM-signaling adaptor-associated receptors can inhibit osteoclastogenesis and immunoreceptor tyrosine-based inhibitory motif (ITIM) receptors can promote osteoclastogenesis. Thus, co-stimulation of osteoclastogenesis continues to reveal additional complexities that are important in the regulatory mechanisms that seek to maintain bone homeostasis.

Brain trauma elicits non-canonical macrophage activation states

Background

Macrophage polarization programs, commonly referred to as “classical” and “alternative” activation, are widely considered as distinct states that are exclusive of one another and are associated with different functions such as inflammation and wound healing, respectively. In a number of disease contexts, such as traumatic brain injury (TBI), macrophage polarization influences the extent of pathogenesis, and efforts are underway to eliminate pathogenic subsets. However, previous studies have not distinguished whether the simultaneous presence of both classical and alternative activation signatures represents the admixture of differentially polarized macrophages or if they have adopted a unique state characterized by components of both classical and alternative activation.

Methods

We analyzed the gene expression profiles of individual monocyte-derived brain macrophages responding to TBI using single-cell RNA sequencing. RNA flow cytometry was used as another single-cell analysis technique to validate the single-cell RNA sequencing results.

Results

The analysis of signature polarization genes by single-cell RNA sequencing revealed the presence of diverse activation states, including M(IL4), M(IL10), and M(LPS, IFNγ). However, the expression of a given polarization marker was no more likely than at random to predict simultaneous expression or repression of markers of another polarization program within the same cell, suggesting a lack of exclusivity in macrophage polarization states in vivo in TBI. Also unexpectedly, individual TBI macrophages simultaneously expressed high levels of signature polarization genes across two or three different polarization states and in several distinct and seemingly incompatible combinations.

Conclusions

Single-cell gene expression profiling demonstrated that monocytic macrophages in TBI are not comprised of distinctly polarized subsets but are uniquely and broadly activated. TBI macrophage activation in vivo is deeply complex, with individual cells concurrently adopting both inflammatory and reparative features with a lack of exclusivity. These data provide physiologically relevant evidence that the early macrophage response to TBI is comprised of novel activation states that are discordant with the current paradigm of macrophage polarization—a key consideration for therapeutic modulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0581-z) contains supplementary material, which is available to authorized users.

Abstract 5427: Inducing tumor apoptosis by redirecting VEGF to activate cell death receptors

Vascular endothelial growth factor (VEGF) is a key tumor angiogenesis factor that is overexpressed by many tumors; the Fas/CD95 death receptor induces apoptosis when activated by its ligand FasL. Agents that inhibit VEGF such as bevacizumab, or activate Fas receptor have been intensively developed as cancer therapeutics. Bevacizumab only modestly improves overall survival or has no effect on overall survival in most cancer patients. More effective VEGF-targeted agents are needed. We conceived of a new strategy that goes beyond VEGF inhibition and converts VEGF to act as a tumor cell death factor. Our aim is to co-opt overexpressed VEGF within the tumor microenvironment to initiate apoptosis via Fas death receptors expressed on tumor cells and/or stromal cells. We created a recombinant homotrimeric fusion protein designated R1FasL that combines the VEGF-binding domain of human VEGFR1 with the trimerization and death receptor-binding domains of human FasL ligand. R1FasL was produced in CHO cells and purified by FLAG antibody affinity chromatography to yield a 135 kD homotrimer. In vitro studies confirmed that VEGF binds to and crosslinks the R1FasL fusion protein to form higher-order complexes that efficiently aggregate and activate Fas death receptors to induce apoptosis. Jurkat cells were killed by R1FasL only when VEGF was added. R1FasL induced apoptosis in diverse human cancer cell lines in vitro, including renal cell, glioblastoma and breast cancers. When VEGF was sequestered by a VEGFR1-Fc trap protein, R1FasL did not induce apoptosis, demonstrating that R1FasL used the VEGF secreted by the cancer cells to kill them. We previously demonstrated that a single intravenous dose of R1FasL induced widespread tumor apoptosis in a xenograft model of human renal cell carcinoma. We have now tested R1FasL in an intracranial xenograft model of human glioblastoma. Nude mice bearing U87 intracranial xenografts were treated with a single intratumoral injection of R1FasL (0.025 to 0.9 mg/kg in 10 microliters). Twenty-four hours later tumors were collected and analyzed for evidence of apoptosis by caspase-3 immunohistochemistry. Mice treated with a single intratumoral dose of R1FasL at 0.1 mg/kg demonstrated significant apoptosis within the tumor but not in surrounding brain. In a safety study, intracranial injection of R1FasL (0.1 or 0.8 mg/kg) into normal brains of nude mice did not cause increased weight loss or death versus PBS injection. In a subsequent survival study, mice treated with a single intratumoral dose of R1FasL (0.8 mg/kg) survived significantly longer than PBS-treated mice (39.5 versus 30 days, p=0.018). Non-invasive imaging demonstrated a median 59% reduction in tumor size 48 hours after R1FasL injection. By converting VEGF to act as a Fas death receptor ligand, the R1FasL fusion protein may be an effective agent to simultaneously target VEGF and death receptors to selectively induce apoptosis in the tumor microenvironment.

Citation Format: Timothy Quinn, YJ Lu, Raquel Santos, Tomoko Ozawa, David James, Mary C. Nakamura. Inducing tumor apoptosis by redirecting VEGF to activate cell death receptors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5427. doi:10.1158/1538-7445.AM2014-5427

CCR2 deficiency impairs macrophage infiltration and improves cognitive function after traumatic brain injury

Traumatic brain injury (TBI) provokes inflammatory responses, including a dramatic rise in brain macrophages in the area of injury. The pathway(s) responsible for macrophage infiltration of the traumatically injured brain and the effects of macrophages on functional outcomes are not well understood. C-C-chemokine receptor 2 (CCR2) is known for directing monocytes to inflamed tissues. To assess the role of macrophages and CCR2 in TBI, we determined outcomes in CCR2-deficient (Ccr2(-/-)) mice in a controlled cortical impact model. We quantified brain myeloid cell numbers post-TBI by flow cytometry and found that Ccr2(-/-) mice had greatly reduced macrophage numbers (∼80-90% reduction) early post-TBI, compared with wild-type mice. Motor, locomotor, and cognitive outcomes were assessed. Lack of Ccr2 improved locomotor activity with less hyperactivity in open field testing, but did not affect anxiety levels or motor coordination on the rotarod three weeks after TBI. Importantly, Ccr2(-/-) mice demonstrated greater spatial learning and memory, compared with wild-type mice eight weeks after TBI. Although there was no difference in the volume of tissue loss, Ccr2(-/-) mice had significantly increased neuronal density in the CA1-CA3 regions of the hippocampus after TBI, compared with wild-type mice. These data demonstrate that Ccr2 directs the majority of macrophage homing to the brain early after TBI and indicates that Ccr2 may facilitate harmful responses. Lack of Ccr2 improves functional recovery and neuronal survival. These results suggest that therapeutic blockade of CCR2-dependent responses may improve outcomes following TBI.

Bone and the innate immune system

The immune system and bone are intimately linked with significant physical and functionally related interactions. The innate immune system functions as an immediate response system to initiate protections against local challenges such as pathogens and cellular damage. Bone is a very specific microenvironment, in which infectious attack is less common but repair and regeneration are ongoing and important functions. Thus, in the bone the primary goal of innate immune and bone interactions is to maintain tissue integrity. Innate immune signals are critical for removal of damaged and apoptotic cells and to stimulate normal tissue repair and regeneration. In this review we focus on the innate immune mechanisms that function to regulate bone homeostasis.

Triggering Receptor Expressed on Myeloid Cells-2 Correlates to Hypothermic Neuroprotection in Ischemic Stroke

Hypothermia is neuroprotective against many acute neurological insults, including ischemic stroke. We and others have previously shown that protection by hypothermia is partially associated with an anti-inflammatory effect. Phagocytes are thought to play an important role in the clearance of necrotic debris, paving the way for endogenous repair mechanisms to commence, but the effect of cooling and phagocytosis has not been extensively studied. Triggering receptor expressed on myeloid cells-2 (TREM2) is a newly identified surface receptor shown to be involved in phagocytosis. In this study, we examined the effect of therapeutic hypothermia on TREM2 expression. Mice underwent permanent middle cerebral artery occlusion (MCAO) and were treated with one of the two cooling paradigms: one where cooling (30°C) began at the onset of MCAO (early hypothermia [eHT]) and another where cooling began 1 hour later (delayed hypothermia [dHT]). In both groups, cooling was maintained for 2 hours. A third group was maintained at normothermia (NT) as a control (37°C). Mice from the NT and dHT groups had similar ischemic lesion sizes and neurological performance, but the eHT group showed marked protection as evidenced by a smaller lesion size and less neurological deficits up to 30 days after the insult. Microglia and macrophages increased after MCAO as early as 3 days, peaked at 7 days, and decreased by 14 days. Both hypothermia paradigms were associated with decreased numbers of microglia and macrophages at 3 and 7 days, with greater decreases in the early paradigm. However, the proportion of the TREM2-positive microglia/macrophages was actually increased among the eHT group at day 7. eHT showed a long-term neurological benefit, but neuroprotection did not correlate to immune suppression. However, hypothermic neuroprotection was associated with a relative increase in TREM2 expression, and suggests that TREM2 may serve a beneficial role in brain ischemia.

Traumatic brain injury induces macrophage subsets in the brain

Traumatic brain injury (TBI) elicits innate inflammatory responses that can lead to secondary brain injury. To better understand the mechanisms involved in TBI-induced inflammation, we examined the nature of macrophages responding to TBI in mice. In this model, brain macrophages were increased >20-fold the day after injury and >77-fold 4 days after injury in the ipsilateral hemisphere compared with sham controls. TBI macrophage subsets were identified by using a reporter mouse strain (YARG) that expresses eYFP from an internal ribosome entry site (IRES) inserted at the 3' end of the gene for arginase-1 (Arg1), a hallmark of alternatively activated (M2) macrophages. One day after TBI, 21 ± 1.5% of ipsilateral brain macrophages expressed relatively high levels of Arg1 as detected by yellow fluorescent protein, and this subpopulation declined thereafter. Arg1(+) cells localized with macrophages near the TBI lesion. Gene expression analysis of sorted Arg1(+) and Arg1(-) brain macrophages revealed that both populations had profiles that included features of conventional M2 macrophages and classically activated (M1) macrophages. The Arg1(+) cells differed from Arg1(-) cells in multiple aspects, most notably in their chemokine repertoires. Thus, the macrophage response to TBI initially involves heterogeneous polarization toward at least two major subsets.

Inflammatory arthritis increases mouse osteoclast precursors with myeloid suppressor function

Increased osteoclastic bone resorption leads to periarticular erosions and systemic osteoporosis in RA patients. Although a great deal is known about how osteoclasts differentiate from precursors and resorb bone, the identity of an osteoclast precursor (OCP) population in vivo and its regulatory role in RA remains elusive. Here, we report the identification of a CD11b(-/lo)Ly6C(hi) BM population with OCP activity in vitro and in vivo. These cells, which can be distinguished from previously characterized precursors in the myeloid lineage, display features of both M1 and M2 monocytes and expand in inflammatory arthritis models. Surprisingly, in one mouse model of RA (adoptive transfer of SKG arthritis), cotransfer of OCP with SKG CD4+ T cells diminished inflammatory arthritis. Similar to monocytic myeloid-derived suppressor cells (M-MDSCs), OCPs suppressed CD4+ and CD8+ T cell proliferation in vitro through the production of NO. This study identifies a BM myeloid precursor population with osteoclastic and T cell-suppressive activity that is expanded in inflammatory arthritis. Therapeutic strategies that prevent the development of OCPs into mature bone-resorbing cells could simultaneously prevent bone resorption and generate an antiinflammatory milieu in the RA joint.

PSTPIP2 deficiency in mice causes osteopenia and increased differentiation of multipotent myeloid precursors into osteoclasts

Missense mutations that reduce or abrogate myeloid cell expression of the F-BAR domain protein, proline serine threonine phosphatase-interacting protein 2 (PSTPIP2), lead to autoinflammatory disease involving extramedullary hematopoiesis, skin and bone lesions. However, little is known about how PSTPIP2 regulates osteoclast development. Here we examined how PSTPIP2 deficiency causes osteopenia and bone lesions, using the mouse PSTPIP2 mutations, cmo, which fails to express PSTPIP2 and Lupo, in which PSTPIP2 is dysfunctional. In both models, serum levels of the pro-osteoclastogenic factor, MIP-1α, were elevated and CSF-1 receptor (CSF-1R)-dependent production of MIP-1α by macrophages was increased. Treatment of cmo mice with a dual specificity CSF-1R and c-Kit inhibitor, PLX3397, decreased circulating MIP-1α and ameliorated the extramedullary hematopoiesis, inflammation, and osteopenia, demonstrating that aberrant myelopoiesis drives disease. Purified osteoclast precursors from PSTPIP2-deficient mice exhibit increased osteoclastogenesis in vitro and were used to probe the structural requirements for PSTPIP2 suppression of osteoclast development. PSTPIP2 tyrosine phosphorylation and a functional F-BAR domain were essential for PSTPIP2 inhibition of TRAP expression and osteoclast precursor fusion, whereas interaction with PEST-type phosphatases was only required for suppression of TRAP expression. Thus, PSTPIP2 acts as a negative feedback regulator of CSF-1R signaling to suppress inflammation and osteoclastogenesis.

Expression of A20 by dendritic cells preserves immune homeostasis and prevents colitis and spondyloarthritis

Dendritic cells (DCs), known to support immune activation during infections, may also regulate immune homeostasis in resting animals. Here we show that mice lacking A20 specifically in DCs spontaneously exhibited DC activation and expansion of activated T cells. DC-specific epistasis experiments using A20^fl/flMyd88^fl/flCd11c-Cre compound mice revealed that A20 restricts both MyD88-independent signals, which drive DC and T cell activation, and MyD88-dependent signals, which drive T cell expansion. In addition, A20^fl/flCd11c-Cre mice spontaneously developed lymphocyte-dependent colitis, sero-negative ankylosing arthritis and enthesitis, conditions stereotypical for human inflammatory bowel disease (IBD). These findings indicate that DCs require A20 to preserve immune quiescence and suggest that A20-dependent DC functions may underlie IBD and IBD-associated arthritides.

Assessment of bone remodelling in childhood-onset systemic lupus erythematosus

OBJECTIVE

To identify predictors of bone remodelling in children and young adults with SLE.

METHODS

Ninety subjects with SLE aged 8-22 years underwent yearly measurements of height, bone age, bone turnover markers, serum Type I IFNs, SLEDAI and BMD. Predictors of bone turnover were examined using serum osteocalcin as a marker of bone formation and both serum tartrate-resistant acid phosphatase (TRAP) and urine N-telopeptide (NTx) as markers of bone resorption.

RESULTS

Subjects demonstrated short stature, high BMI and bone age delay. A spine BMD Z-score of less than -2.0 was seen in 16.1% of subject visits. Serum osteocalcin was negatively correlated with glucocorticoid dose (Spearman rank correlation coefficient R = -0.34, P < 0.0001) but was not associated with SLEDAI after adjustment for confounders. Serum TRAP was negatively associated with SLEDAI, even after controlling for confounders (P = 0.04). Similar results were obtained for urine NTx. There was a negative association between TRAP and serum IFN-β (P = 0.03).

CONCLUSIONS

In this population of children and young adults with moderate lupus disease activity, glucocorticoid dose was a negative predictor of bone formation, whereas lupus disease activity was not. Interestingly, lupus disease activity was a negative predictor of bone resorption, suggesting that lupus disease activity is not the primary factor contributing to the bone deficits of childhood-onset SLE. The potential protective role of IFN-β and the effects of SLE treatment on bone loss require further study.

The loss of Cbl-phosphatidylinositol 3-kinase interaction perturbs RANKL-mediated signaling, inhibiting bone resorption and promoting osteoclast survival

Cbl is an adaptor protein and an E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and is phosphorylated by Syk and Src family kinases. Phosphorylated Cbl Tyr(737) creates a binding site for the p85 regulatory subunit of PI3K, which also plays an important role in the regulation of bone resorption by osteoclasts. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined the knock-in mice (Cbl(YF/YF)) in which the PI3K binding site in Cbl is ablated due to the mutation in the regulatory tyrosine. We report that in Cbl(YF/YF) mice, despite increased numbers of osteoclasts, bone volume is increased due to defective osteoclast function. Additionally, in ex vivo cultures, mature Cbl(YF/YF) osteoclasts showed an increased ability to survive in the presence of RANKL due to delayed onset of apoptosis. RANKL-mediated signaling is perturbed in Cbl(YF/YF) osteoclasts, and most interestingly, AKT phosphorylation is up-regulated, suggesting that the lack of PI3K sequestration by Cbl results in increased survival and decreased bone resorption. Cumulatively, these in vivo and in vitro results show that, on one hand, binding of Cbl to PI3K negatively regulates osteoclast differentiation, survival, and signaling events (e.g. AKT phosphorylation), whereas on the other hand it positively influences osteoclast function.

A role for TREM2 ligands in the phagocytosis of apoptotic neuronal cells by microglia

Following neuronal injury, microglia initiate repair by phagocytosing dead neurons without eliciting inflammation. Prior evidence indicates triggering receptor expressed by myeloid cells-2 (TREM2) promotes phagocytosis and retards inflammation. However, evidence that microglia and neurons directly interact through TREM2 to orchestrate microglial function is lacking. We here demonstrate that TREM2 interacts with endogenous ligands on neurons. Staining with TREM2-Fc identified TREM2 ligands (TREM2-L) on Neuro2A cells and on cultured cortical and dopamine neurons. Apoptosis greatly increased the expression of TREM2-L. Furthermore, apoptotic neurons stimulated TREM2 signaling, and an anti-TREM2 mAb blocked stimulation. To examine the interaction between TREM2 and TREM2-L in phagocytosis, we studied BV2 microglial cells and their engulfment of apoptotic Neuro2A. One of our anti-TREM2 mAb, but not others, reduced engulfment, suggesting the presence of a functional site on TREM2 interacting with neurons. Further, Chinese hamster ovary cells transfected with TREM2 conferred phagocytic activity of neuronal cells demonstrating that TREM2 is both required and sufficient for competent uptake of apoptotic neuronal cells. Finally, while TREM2-L are expressed on neurons, TREM2 is not; in the brain, it is found on microglia. TREM2 and TREM2-L form a receptor-ligand pair connecting microglia with apoptotic neurons, directing removal of damaged cells to allow repair.

Glucocorticoid excess in mice results in early activation of osteoclastogenesis and adipogenesis and prolonged suppression of osteogenesis: a longitudinal study of gene expression in bone tissue from glucocorticoid-treated mice

OBJECTIVE

Glucocorticoid (GC) excess induces alterations in bone metabolism that weaken bone structure and increase fracture risk. The aim of this study was to identify genes associated with bone metabolism in GC-treated mice, by performing a microarray analysis.

METHODS

Long bones from mice exposed to GC excess were collected after 0, 7, 28, and 56 days of treatment, to measure bone microarchitecture and extract RNA for microarray analyses.

RESULTS

Bone loss in this animal model was confirmed by changes in bone turnover markers as well as bone architecture, as measured by microfocal computed tomography. GC excess induced an early up-regulation of genes involved in osteoclast activation, function, and adipogenesis, which peaked on day 7. The expression of genes associated with osteoclast cytoskeletal reorganization and genes associated with matrix degradation peaked on day 28. On day 28 and day 56, the expression of genes associated with osteoblast activation and maturation was decreased from baseline, while the expression of Wnt antagonists was increased. In addition, the expression of genes expressed in osteocytes associated with bone mineralization was significantly higher at the later time points, day 28 and day 56. Reverse transcription-polymerase chain reaction confirmed the results of microarray analysis in selected genes.

CONCLUSION

GC excess is associated with early activation of genes associated with osteoclastogenesis and adipogenesis and a later suppression of genes associated with osteogenesis and mineralization. Novel interventions with agents that modulate either Wnt signaling or mineralization may be effective in GC-induced osteoporosis.

Osteoclasts - the innate immune cells of the bone

Osteopenia and periarticular bony erosion are consequences of chronic inflammatory autoimmune disease due to an imbalance of osteoclast activity relative to new bone formation. Osteoclasts, which are specialized as the only bone resorbing cell type, are differentiated from hematopoietic myeloid precursor cells. Inflammatory signals mediated by multiple types of immune cells and cytokines have significant influence over osteoclast differentiation and function through direct effects on osteoclast precursors and indirect effects via osteoblasts and other cells in the bony microenvironment including synovial cells, stromal cells, osteocytes and chondrocytes. Recent studies have demonstrated that osteoclasts themselves express a number of immune receptors and are regulated similarly to macrophages and dendritic cells, closely related cells in the innate immune system. Though we are only beginning to understand the roles of innate immune receptors in osteoclasts, some of these receptors have been shown to be critical regulators of differentiation and function of osteoclasts. Osteoclasts likely function as the innate immune cells of the bone, thus are highly regulated to appropriately respond to stress and inflammatory changes in their microenvironment.

Bone microenvironment specific roles of ITAM adapter signaling during bone remodeling induced by acute estrogen-deficiency

Immunoreceptor tyrosine-based activation motif (ITAM) signaling mediated by DAP12 or Fcepsilon receptor Igamma chain (FcRgamma) have been shown to be critical for osteoclast differentiation and maturation under normal physiological conditions. Their function in pathological conditions is unknown. We studied the role of ITAM signaling during rapid bone remodeling induced by acute estrogen-deficiency in wild-type (WT), DAP12-deficient (DAP12-/-), FcRgamma-deficient (FcRgamma-/-) and double-deficient (DAP12-/-FcRgamma-/-) mice. Six weeks after ovariectomy (OVX), DAP12-/-FcRgamma-/- mice showed resistance to lumbar vertebral body (LVB) trabecular bone loss, while WT, DAP12-/- and FcRgamma-/- mice had significant LVB bone loss. In contrast, all ITAM adapter-deficient mice responded to OVX with bone loss in both femur and tibia of approximately 40%, relative to basal bone volumes. Only WT mice developed significant cortical bone loss after OVX. In vitro studies showed microenvironmental changes induced by OVX are indispensable for enhanced osteoclast formation and function. Cytokine changes, including TGFbeta and TNFalpha, were able to induce osteoclastogenesis independent of RANKL in BMMs from WT but not DAP12-/- and DAP12-/-FcRgamma-/- mice. FSH stimulated RANKL-induced osteoclast differentiation from BMMs in WT, but not DAP12-/- and DAP12-/-FcRgamma-/- mice. Our study demonstrates that although ITAM adapter signaling is critical for normal bone remodeling, estrogen-deficiency induces an ITAM adapter-independent bypass mechanism allowing for enhanced osteoclastogenesis and activation in specific bony microenvironments.

Inhibition of Toll-like receptor and Fc receptor responses in macrophages by TREM-2 and DAP12 (44.7)

DAP12 is a transmembrane signaling adapter that contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain and is expressed in myeloid cells and natural killer cells. DAP12 associates with receptors that do not have intrinsic signaling ability and therefore use DAP12 to propagate signals. We examined the inflammatory response in macrophages lacking DAP12. Surprisingly, DAP12-deficient macrophages produced increased amounts of inflammatory cytokines in response to Toll-like receptor and Fc receptor stimulation, demonstrating that DAP12 inhibits inflammatory responses in wild-type macrophages. This inhibition was dependent upon the DAP12 ITAM. In vivo, DAP12-deficient mice were more susceptible to LPS/D-galactosamine-induced shock and more resistant to Listeria monocytogenes infection. TREM-2, a DAP12-associated Ig superfamily receptor, was responsible for the DAP12-mediated inhibition of TLR and FcR responses in macrophages. Additionally, a fusion protein containing the extracellular domain of TREM-2 bound to macrophages. These results suggest that the interaction of TREM-2 and its ligand on macrophages results in an inhibitory signal that can reduce the macrophage inflammatory response.

Cutting edge: inhibition of TLR and FcR responses in macrophages by triggering receptor expressed on myeloid cells (TREM)-2 and DAP12

DAP12 is an ITAM-containing adapter that associates with receptors in myeloid and NK cells. DAP12-associated receptors can give activation signals leading to cytokine production; however, in some situations, DAP12 inhibits cytokine production stimulated through TLRs and FcRs. Here we show that Triggering Receptor Expressed on Myeloid cells (TREM)-2 is responsible for the DAP12-mediated inhibition in mouse macrophages. A chimeric receptor composed of the extracellular domain of TREM-2 and the cytoplasmic domain of DAP12 inhibited the TLR- and FcR-induced TNF production of DAP12-deficient macrophages, whereas a TREM-1 chimera did not. In wild-type macrophages, TREM-2 knockdown increased TLR-induced TNF production. A TREM-2 Fc fusion protein bound to macrophages, indicating that macrophages express a TREM-2 ligand. Thus, the interaction of TREM-2 and its ligand results in an inhibitory signal that can reduce the inflammatory response.

TIM-2 is expressed on B cells and in liver and kidney and is a receptor for H-ferritin endocytosis

T cell immunoglobulin-domain and mucin-domain (TIM) proteins constitute a receptor family that was identified first on kidney and liver cells; recently it was also shown to be expressed on T cells. TIM-1 and -3 receptors denote different subsets of T cells and have distinct regulatory effects on T cell function. Ferritin is a spherical protein complex that is formed by 24 subunits of H- and L-ferritin. Ferritin stores iron atoms intracellularly, but it also circulates. H-ferritin, but not L-ferritin, shows saturable binding to subsets of human T and B cells, and its expression is increased in response to inflammation. We demonstrate that mouse TIM-2 is expressed on all splenic B cells, with increased levels on germinal center B cells. TIM-2 also is expressed in the liver, especially in bile duct epithelial cells, and in renal tubule cells. We further demonstrate that TIM-2 is a receptor for H-ferritin, but not for L-ferritin, and expression of TIM-2 permits the cellular uptake of H-ferritin into endosomes. This is the first identification of a receptor for ferritin and reveals a new role for TIM-2.

TREM2, a DAP12-associated receptor, regulates osteoclast differentiation and function

Deficiency of the signaling adapter protein DAP12 or its associated receptor TREM2 is associated with abnormal OC development in humans. Here we examine the role of TREM2 in mouse OC development and function, including migration and resorption in vitro. These results provide new evidence that TREM2 regulates OC function independent of its effects on multinucleated OC differentiation.

INTRODUCTION

TREM2 (triggering receptor expressed in myeloid cells-2) associates with the signaling adapter DAP12 in osteoclasts (OCs). Genetic mutation or deletion of either the TYROBP (DAP12) or TREM2 gene is associated with the human disorder of brain and bone, Nasu-Hakola disease. We and others recently showed the critical requirement for immunoreceptor tyrosine-based activation motif (ITAM) signals through DAP12 and the Fc Receptor gamma chain (FcRgamma) during OC development. Here, we further define the role of TREM2 in OC differentiation and describe a role for TREM2 in OC migration and bone resorption.

MATERIALS AND METHODS

We generated monoclonal anti-mouse TREM2 antibodies (mAb), analyzed pre-osteoclasts and mature OCs for TREM2 surface expression, and determined the effect of antibody ligation on in vitro OC differentiation, resorption, and migration. TREM2 RNA interference (RNAi) was used to disrupt expression of TREM2 in pre-osteoclasts.

RESULTS

Using flow cytometry, our studies reveal that TREM2 is weakly expressed on C57BL/6 bone marrow macrophages (BMMs) and is upregulated during culture with RANKL and macrophage-colony stimulating factor (M-CSF). The expression of TREM2 is unaltered in DAP12-deficient OCs. Using C57BL/6 BMMs or RAW264.7 precursors, anti-TREM2 mAb treatment with RANKL and M-CSF enhances the formation of multinuclear TRACP+ OCs compared with control mAb treatment. In contrast, these agents have no effect on DAP12-deficient precursors. Monoclonal Ab blockade of TREM2 on OCs generated from C57BL/6 BMMs results in decreased resorption of artificial calcium-phosphate substrate and dentine. Reduction of TREM2 expression in RAW264.7 cells by RNAi results in loss of OC formation in response to RANKL and M-CSF. Anti-TREM2 cross-linking enhances migration of C57BL/6 OCs and RAW246.7 OCs in response to M-CSF.

CONCLUSIONS

Our studies indicate that the TREM2 receptor regulates OC multinucleation as well as resorption and migration of mature OCs. Thus, TREM2-DAP12 signals regulate both OC formation and function.

Role of ITAM-containing adapter proteins and their receptors in the immune system and bone

The immunoreceptor tyrosine-based activation motif (ITAM) is a highly conserved region in the cytoplasmic domain of signaling chains and receptors and is a critical mediator of intracellular signals. ITAM-mediated signals depend on the Syk or zeta-associated protein of 70 kDa tyrosine kinases, and ITAM signaling is required for the differentiation and function of B and T cells in adaptive immunity. ITAM-dependent receptors also regulate the function of innate immune cells, including natural killer cells, and myeloid-derived cells such as macrophages, neutrophils, dendritic cells, and mast cells. Myeloid lineage cells also include osteoclasts (OCLs), the cells required for bone resorption, and recent studies show a critical role for the ITAM-containing adapter proteins DAP12 and the FcRgamma chain (Fcepsilon receptor I gamma chain) in OCL differentiation. Mice deficient in both the DAP12 and FcRgamma ITAM-bearing adapters are significantly osteopetrotic with a severe defect in OCL differentiation, demonstrating the requirement for ITAM signals in bone and further implicating this pathway in the development of highly specialized cell functions in hematopoietic cells. Regulation of osteoclastogenesis by ITAM-dependent receptors suggests that OCLs, similar to related myeloid cells, are tightly controlled by arrays of receptors that allow them to sense and respond to their local microenvironment like other innate immune cells.

Mice lacking the integrin beta5 subunit have accelerated osteoclast maturation and increased activity in the estrogen-deficient state

Integrin alphavbeta5 is expressed on osteoclast precursors and is capable of recognizing the same amino acid motif as alphavbeta3. Three-month-old beta5(-/-) female OVX mice had increased osteoclastogenesis ex vivo, and microCT assessment of trabecular bone volume was 53% lower than WT-OVX animals. These preliminary data suggest alphavbeta5 integrin's presence on osteoclast precursors may inhibit of osteoclast formation.

INTRODUCTION

Osteoclasts are unique resorptive skeletal cells, capable of degrading bone on contact to the juxtaposed matrix. Integrin alphavbeta5 is expressed on osteoclast precursors, structurally similar to alphavbeta3, and capable of recognizing the same amino acid motif. Given the structural relationship and reciprocal regulation of alphavbeta3 and alphavbeta5, the purpose of this study was to evaluate how alphavbeta5 might contribute to osteoclast maturation and activity.

MATERIALS AND METHODS

Three-month-old wildtype (WT) and beta5(-/-) female mice had ovariectomy (OVX) or sham operations. The osteoclastogenic capacity of marrow-derived precursors, the kinetic, the circulating, and structural parameters of bone remodeling, was determined after 6 weeks of paired feeding.

RESULTS AND CONCLUSIONS

OVX increased osteoclastogenesis ex vivo and in vivo. Osteoclast formation and prolonged pre-osteoclast survival were substantially enhanced in cultures containing beta5(-/-) cells whether obtained from sham-operated or OVX mice. Expression of cathepsin K, beta3 integrin subunit, and calcitonin receptor were accelerated in cultured beta5(-/-)osteoclasts. beta5(-/-) osteoclasts from OVX animals showed a 3-fold enhancement of net resorptive activity, with quantitative muCT showing trabecular bone volume loss after OVX 53% greater in beta5(-/-) OVX compared with similarly treated WT OVX mice (p < 0.05). alpha5beta3 seems to be an inhibitor of osteoclast formation, in contrast to alphavbeta3. In addition, loss of alphavbeta5 seems to accelerate osteoclast formation in the OVX model. Further examination of alphavbeta5 signaling pathways may enhance our understanding of the activation of bone resorption.

The immunomodulatory adapter proteins DAP12 and Fc receptor gamma-chain (FcRgamma) regulate development of functional osteoclasts through the Syk tyrosine kinase

Osteoclasts, the only bone-resorbing cells, are central to the pathogenesis of osteoporosis, yet their development and regulation are incompletely understood. Multiple receptors of the immune system use a common signaling paradigm whereby phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) within receptor-associated adapter proteins recruit the Syk tyrosine kinase. Here we demonstrate that a similar mechanism is required for development of functional osteoclasts. Mice lacking two ITAM-bearing adapters, DAP12 and the Fc receptor gamma-chain (FcRgamma), are severely osteopetrotic. DAP12(-/-)FcRgamma(-/-) bone marrow cells fail to differentiate into multinucleated osteoclasts or resorb bone in vitro and show impaired phosphorylation of the Syk tyrosine kinase. syk(-/-) progenitors are similarly defective in osteoclast development and bone resorption. Intact SH2-domains of Syk, introduced by retroviral transduction, are required for functional reconstitution of syk(-/-) osteoclasts, whereas intact ITAM-domains on DAP12 are required for reconstitution of DAP12(-/-) FcRgamma(-/-) cells. These data indicate that recruitment of Syk to phosphorylated ITAMs is critical for osteoclastogenesis. Although DAP12 appears to be primarily responsible for osteoclast differentiation in cultures directly stimulated with macrophage-colony stimulating factor and receptor activator of NF-kappaB ligand cytokines, DAP12 and FcRgamma have overlapping roles in supporting osteoclast development in osteoblast-osteoclast cocultures, which mirrors their overlapping functions in vivo. These results provide new insight into the biology of osteoclasts and suggest novel therapeutic targets in diseases of bony remodeling.

The signaling adapter protein DAP12 regulates multinucleation during osteoclast development

Deficiency of the signaling adapter protein DAP12 is associated with bony abnormalities in both mice and humans. We identify specific DAP12-associated receptors expressed by osteoclasts and examine function of DAP12 in murine osteoclasts in vivo and in vitro. These data show a new role for DAP12 signaling in regulating formation of multinucleated osteoclasts.

INTRODUCTION

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors in the myeloid lineage. In other myeloid cell types, the signaling adapter protein DAP12 transmits activating signals on ligation of a DAP12-associated receptor (DAR). The aim of this study was to clarify the role of DAP12 signaling during osteoclast development.

MATERIALS AND METHODS

Osteoclasts from DAP12 -/- or control mice were analyzed in vitro for morphology, function, and for osteoclast markers. DARs were identified in osteoclast cultures through reverse transcriptase-polymerase chain reaction (RT-PCR). Bone density of DAP12 -/- and control mice were analyzed by microcomputed tomography. DAP12 -/- osteoclasts were retrovirally reconstituted with DAP12. RAW264.7 cells were transfected with FLAG-tagged DAP12 or TREM2 and stimulated by anti-FLAG antibody during in vitro osteoclastogenesis.

RESULTS

C57BL/6 DAP12-deficient mice have higher bone mass than C57BL/6 wildtype controls. We verified the presence of DAP12 in pre-osteoclasts and osteoclasts derived from C57BL/6 or the pre-osteoclast line RAW 264.7 and identified the DARs expressed. DAP12 -/- osteoclasts developed in vitro with macrophage colony-stimulating factor (M-CSF) and RANKL formed only intensely TRACP+ mononuclear cells and failed to generate multinuclear osteoclasts. These mononuclear cells are functional osteoclast-like cells because, by RT-PCR, they express other osteoclast markers and generate resorption pits on dentine slices, although quantitative assessment of bone resorption shows decreased resorption by DAP12 -/- osteoclasts compared with C57BL/6 osteoclasts. Restoration of DAP12 expression by retroviral transduction of DAP12 -/- osteoclast precursors rescued in vitro osteoclast multinucleation. Direct stimulation of DAP12 expressed in RAW264.7 during in vitro osteoclastogenesis led to a marked increase in the number of TRACP+ multinucleated osteoclast-like cells formed.

CONCLUSION

Our studies indicate that stimulation of the DAP12 adapter protein plays a significant role in formation of multinuclear osteoclasts and that DAP12 and DARs likely participate in the regulation of bony remodeling.

CMRF-35-like molecule-1, a novel mouse myeloid receptor, can inhibit osteoclast formation

By homology to triggering receptor expressed by myeloid cells-2, we screened the mouse expressed sequence tag database and isolated a new single Ig domain receptor, which we have expressed and characterized. The receptor is most similar in sequence to the human CMRF-35 receptor, and thus we have named it CMRF-35-like molecule (CLM)-1. By screening the mouse genome, we determined that CLM-1 was part of a multigene family located on a small segment of mouse chromosome 11. Each contains a single Ig domain, and they are expressed mainly in cells of the myeloid lineage. CLM-1 contains multiple cytoplasmic tyrosine residues, including two that lie in consensus immunoreceptor tyrosine-based inhibitory motifs, and we demonstrate that CLM-1 can associate with Src-homology 2 containing phosphatase-1. Expression of CLM-1 mRNA is down-regulated by treatment with receptor activator of NF-kappaB ligand (RANKL), a cytokine that drives osteoclast formation. Furthermore, expression of CLM-1 in the osteoclastogenic cell line RAW (RAW.CLM-1) prevents osteoclastogenesis induced by RANKL and TGF-beta. RAW.CLM-1 cells fail to multinucleate and do not up-regulate calcitonin receptor, but they express tartrate-resistant acid phosphatase, cathepsin K, and beta(3) integrin, suggesting that osteoclastogenesis is blocked at a late-intermediate stage. Thus, we define a new family of myeloid receptors, and demonstrate that the first member of this family, CLM-1, is an inhibitory receptor, able to block osteoclastogenesis.

A receptor for vascular endothelial growth factor that stimulates endothelial apoptosis

Vascular endothelial growth factor (VEGF) is a dimeric angiogenic factor that is overexpressed by many tumors and stimulates tumor angiogenesis. VEGF initiates signaling by dimerizing the receptors VEGFR-1 and VEGFR-2. The Fas receptor stimulates apoptosis, and artificial dimerization of the Fas cytoplasmic domain has been shown to induce apoptosis. We constructed a chimeric receptor (VEGFR2Fas) combining the extracellular and transmembrane domains of VEGFR-2 with the cytoplasmic domain of Fas receptor. When VEGFR2Fas was stably expressed in endothelial cells in vitro, treatment with VEGF rapidly induced cell death with features characteristic of Fas-mediated apoptosis. These findings demonstrate that VEGFR2Fas functions as a VEGF-triggered death receptor and raise the possibility that introduction of VEGFR2Fas into tumor endothelium or tumor cells in vivo may convert tumor-derived VEGF from an angiogenic factor into an antiangiogenesis agent.

Immune rejection of a large sarcoma following cyclophosphamide and IL-12 treatment requires both NK and NK T cells and is associated with the induction of a novel NK T cell population

Combined immunotherapy with cyclophosphamide (Cy) and IL-12, but not IL-12 alone, stimulates eradication of a large established solid tumor (20 mm), MCA207, a methylcholanthrene-induced murine sarcoma. In these studies we demonstrate that NK1.1(+) cells and CD1d-dependent NK T cells each play important yet distinct roles in regression of a large tumor in response to Cy and IL-12, and we define a novel NK T cell subset, selectively increased by this treatment. Mice depleted of NK1.1(+) cells demonstrated more rapid initial tumor growth and prolonged tumor regression following treatment, but tumors were eventually eradicated. In contrast, initial tumor regression following therapy was unimpaired in CD1d(-/-) mice, which are deficient in most NK T cells, but tumors recurred. No tumor regression occurred following Cy and IL-12 therapy in CD1d(-/-) mice that were depleted of NK1.1(+) cells. We found that Cy and IL-12 induced the selective increase in liver and spleen lymphocytes of a unique NK T subpopulation (DX5(+)NK1.1(-)CD3(+)). These cells were not induced by treatment in CD1d(-/-) mice. Our studies demonstrate a contribution of both NK and NK T cells to the Cy- and IL-12-stimulated anti-tumor response. We describe the selective induction of a distinct NK T cell subset by Cy and IL-12 therapy, not seen following IL-12 therapy alone, which we suggest may contribute to the successful anti-tumor response induced by this immunotherapeutic regimen.

Ligand interactions by activating and inhibitory Ly-49 receptors

Natural killer (NK) cells express families of homologous receptors, members of which either activate or inhibit NK cells. We demonstrate that mouse Ly-49D is an activating receptor for the MHC antigen H2-Dd, which is also a ligand for the related inhibitory receptor Ly-49A. To compare and contrast their interactions with class I MHC ligand, we studied each of these receptors expressed in a rat NK-cell hne, RNK-16, for their capacity to recognize wild-type or mutated H2-Dd. Our studies with Ly-49A reveal that functional interaction with H2-Dd depends on residues in the floor of the H2-Dd peptide-binding groove. The recent co-crystal of Ly-49A with H2-Dd indicates that these are not contact residues, thus they may contribute to allelic specificity through conformational changes in H2-Dd. We found that structural requirements for functional recognition of H2-Dd by Ly-49D differ markedly from those for recognition by Ly-49A. We note that H2-Dd expression on certain target cells is not sufficient to activate lysis mediated by Ly-49D, though the additional requirements for functional interaction are not yet identified. Here we review recent studies of Ly-49 receptor ligand specificities and their molecular basis. The functions of these related receptors with opposing functions and shared allospecificity remains unclear.

Activating Ly-49D and inhibitory Ly-49A natural killer cell receptors demonstrate distinct requirements for interaction with H2-D(d)

The activating Ly-49D receptor and the inhibitory Ly-49A receptor mediate opposing effects on natural killer (NK) cell cytotoxicity after interaction with the same major histocompatibility complex ligand, H2-D(d). To compare Ly-49D and Ly-49A interactions with H2-D(d), we created mutations in H2-D(d) and examined the functional ability of these mutants to activate lysis through Ly-49D or to inhibit lysis through Ly-49A. Specific single amino acid changes in either the H2-D(d) alpha(1) helix or the alpha(2) helix abrogated Ly-49D-mediated cytotoxicity, but these changes had no significant effect on Ly-49A-dependent inhibition. Each of three alpha(2) domain mutations in the floor of the peptide binding groove reduced functional recognition by either Ly-49D or Ly-49A, but all three were required to fully abrogate inhibition by Ly-49A. Our studies indicate that Ly-49D/H2-D(d) interactions require distinct determinants compared with Ly-49A/H2-D(d) interactions. These differences have important implications for the integration of activating and inhibitory signals in NK cells.