HIF signaling in osteoblast-lineage cells promotes systemic breast cancer growth and metastasis in mice.

Bone metastasis involves dynamic interplay between tumor cells and the local stromal environment. In bones, local hypoxia and activation of the hypoxia-inducible factor (HIF)-1α in osteoblasts are essential to maintain skeletal homeostasis. However, the role of osteoblast-specific HIF signaling in cancer metastasis is unknown. Here, we show that osteoprogenitor cells (OPCs) are located in hypoxic niches in the bone marrow and that activation of HIF signaling in these cells increases bone mass and favors breast cancer metastasis to bone locally. Remarkably, HIF signaling in osteoblast-lineage cells also promotes breast cancer growth and dissemination remotely, in the lungs and in other tissues distant from bones. Mechanistically, we found that activation of HIF signaling in OPCs increases blood levels of the chemokine C-X-C motif ligand 12 (CXCL12), which leads to a systemic increase of breast cancer cell proliferation and dissemination through direct activation of the CXCR4 receptor. Hence, our data reveal a previously unrecognized role of the hypoxic osteogenic niche in promoting tumorigenesis beyond the local bone microenvironment. They also support the concept that the skeleton is an important regulator of the systemic tumor environment.

Physical activity in patients with axial spondyloarthritis: a cross-sectional study of 203 patients.

Physical activity is recommended in axial spondyloarthritis (axSpA) but may be insufficiently performed. The objective of this study was to assess physical activity in axial spondyloarthritis and to explore its explanatory factors. This was a cross-sectional study of patients with definite axSpA. The level of physical activity (International Physical Activity Questionnaire-Long form, IPAQ-L), type of aerobic exercise and the Exercise Benefits and Barriers Score were collected. Multivariate logistic regression analyses were performed to explain levels of exercise at least as recommended by the World Health Organization. In all, 203 patients were included: mean age 46.0 ± 11.6 years, 108 (53.2 %) males, mean Bath Ankylosing Spondylitis Activity Index (0-100) 37.8 ± 19.9; 137 (68.8 %) were treated with TNF-inhibitors. In all, 111 patients (54.7 %) were exercising at least as recommended; 96 (47.2 %) were in the 'high physical activity' category. Aerobic exercise >30 min was performed at least once a week by 61 (30.0 %) patients; the most frequent activities were energetic walking (31.0 %) and swimming (21.2 %). Main perceived benefits of exercising were improving physical fitness and functioning of the cardiovascular system, and the main barrier was physical exertion. Patients with paid employment had lower levels of physical activity whereas other demographic variables, disease activity/severity or TNF-inhibitor treatment were not predictive. One half of these patients performed enough physical activity according to the recommendations, similarly to the French population. Levels of physical activity did not appear to be explained by disease-related variables. Physical activity should be encouraged in axSpA.

Relationship between optimism and quality of life in patients with two chronic rheumatic diseases: axial spondyloarthritis and chronic low back pain: a cross sectional study of 288 patients.

BACKGROUND: Axial Spondyloarthritis (AxSpA) and chronic low back pain are rheumatic diseases that impact patients' health-related quality of life (HRQoL). In other chronic conditions, HRQoL was positively associated with dispositional optimism, a personality trait. The objective was to explore the relationship between optimism and HRQoL in these two diseases. METHOD: A cross-sectional study was performed in 2 tertiary care hospitals and 2 private practices in France. Patients had definite AxSpA or chronic low back pain according to the rheumatologist. A generic HRQoL questionnaire (Short Form, SF-12) with physical and mental composite scores (PCS and MCS respectively) and an optimism questionnaire (the Life Orientation Test-revised, LOT-R) were collected. Analyses included non-parametric correlations and multiple regression analyses to study the effect of optimism on PCS and MCS. RESULTS: In all, 288 (199 AxSpA and 89 low back pain) patients were included: mean age, 47.3 ± 11.9 years, 48.6 % were males. Pain levels (0-10) were 4.5 ± 2.4 and 4.3 ± 2.4 in AxSpA and LOW BACK PAIN patients, respectively. HRQoL was similarly altered in both diseases, for both physical and mental composite scores (mean PCS: 43.7 ± 8.2 vs. 41.9 ± 7.1; mean MCS: 45.9 ± 7.8 vs. 46.7 ± 8.1 for AxSpA and low back pain respectively). Optimism was moderate and similar in both populations. Optimism was positively correlated to MCS in both diseases (rho = 0.54 and 0.58, respectively, both p <0.01) and these relations persisted in multivariate analyses (beta = 1.03 and 1.40, both p <0.0001). CONCLUSIONS: Optimism was similar in these 2 chronic diseases and was an explanatory factor of the mental component of HRQoL, but not physical HRQoL. Physical HRQoL may reflect more the disease process than character traits.

CD6 attenuates early and late signaling events, setting thresholds for T-cell activation.

The T lineage glycoprotein CD6 is generally considered to be a costimulator of T-cell activation. Here, we demonstrate that CD6 significantly reduces early and late T-cell responses upon superantigen stimulation or TCR triggering by Abs. Measuring calcium mobilization in single cells responding to superantigen, we found that human T cells expressing rat CD6 react significantly less well compared with T cells not expressing the exogenous receptor. When the cytoplasmic domain of rat CD6 was removed, calcium responses were recovered, indicating that the inhibitory properties of CD6 are attributable to its cytoplasmic domain. Calcium responses, and also late indicators of T-cell activation such as IL-2 release, were also diminished in TCR-activated Jurkat cells expressing human CD6, compared with CD6-deficient cells or cells expressing a cytoplasmic deletion mutant of human CD6. Similarly, calcium signals triggered by anti-CD3 were enhanced in human T lymphocytes following morpholino-mediated suppression of CD6 expression. Finally, the proliferation of T lymphocytes was increased when the CD6-CD166 interaction was blocked with anti-CD166 Abs, but inhibited when anti-CD6 Abs were used. Our data suggest that CD6 is a signaling attenuator whose expression alone, i.e. in the absence of ligand engagement, is sufficient to restrain signaling in T cells.

Lrp5 p.Val667Met Variant Compromises Bone Mineral Density and Matrix Properties in Osteoporosis.

Early-onset osteoporosis (EOOP) has been associated with several genes, including LRP5, coding for a coreceptor in the Wnt pathway. Variants in LRP5 were also described in osteoporosis pseudoglioma syndrome, combining severe osteoporosis and eye abnormalities. Genomewide-association studies (GWAS) showed that LRP5 p.Val667Met (V667M) variant is associated with low bone mineral density (BMD) and increased fractures. However, despite association with a bone phenotype in humans and knockout mice, the impact of the variant in bone and eye remains to be investigated. Here, we aimed to evaluate the bone and ocular impact of the V667M variant. We recruited 11 patients carrying the V667M variant or other loss-of-function variants of LRP5 and generated an Lrp5 V667M mutated mice. Patients had low lumbar and hip BMD Z-score and altered bone microarchitecture evaluated by HR-pQCT compared with an age-matched reference population. Murine primary osteoblasts from Lrp5 V667M mice showed lower differentiation capacity, alkaline phosphatase activity, and mineralization capacity in vitro. Ex vivo, mRNA expression of Osx, Col1, and osteocalcin was lower in Lrp5 V667M bones than controls (all p < 0.01). Lrp5 V667M 3-month-old mice, compared with control (CTL) mice, had decreased BMD at the femur (p < 0.01) and lumbar spine (p < 0.01) with normal microarchitecture and bone biomarkers. However, Lrp5 V667M mice revealed a trend toward a lower femoral and vertebral stiffness (p = 0.14) and had a lower hydroxyproline/proline ratio compared with CTL, (p = 0.01), showing altered composition and quality of the bone matrix. Finally, higher tortuosity of retinal vessels was found in the Lrp5 V667M mice and unspecific vascular tortuosity in two patients only. In conclusion, Lrp5 V667M variant is associated with low BMD and impaired bone matrix quality. Retinal vascularization abnormalities were observed in mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Prediction of Work Impact in Axial Spondyloarthritis by the Work Instability Scale: A Prospective Cohort Study of 101 Patients.

OBJECTIVE: Axial spondyloarthritis (axSpA) may have an effect on work. The Ankylosing Spondylitis Work Instability Scale (AS-WIS) assesses difficulties at work. The objective of this study was to evaluate the predictive value of the AS-WIS on work impact. METHODS: This is a prospective cohort study with 2 timepoints (at baseline and after 1.5 yrs) that included patients with axSpA who had paid employment. Patients completed the AS-WIS at baseline and work instability was scored as moderate/high if ≥ 11 (0-20 scale). At follow-up, adverse work outcomes (AWO) were grouped as moderate (short-term sick leave) or severe AWO (long-term sick leave, disability, unemployment). Univariable and multivariable logistic regression analyses were performed to explain AWO. RESULTS: Of 101 patients, mean age 45 (SD 9) years, 52% male, disease duration was 14 (SD 8) years. The Bath AS Disease Activity Index and the Bath AS Functional Index were 34 (SD 21) and 23 (SD 23), respectively, and 69 (68%) received a tumor necrosis factor inhibitor. At baseline, 46 (46%) patients had moderate/high AS-WIS. At 1.5 years of follow-up, 37 patients (36%) had AWO: 25 patients (25%) a short-term sick leave, and 12 patients (12%, 7/100 patient-years) a severe AWO. Independent baseline factors associated with AWO were a moderate/high AS-WIS score (OR 2.71, 95% CI 1.04-7.22) and shorter disease duration (OR 0.94, 95% CI 0.89-0.99). CONCLUSION: In patients with axSpA, a moderate/high AS-WIS score was predictive of AWO in this population with well-controlled axSpA. This short questionnaire can be helpful to screen for future difficulties at work.

Inadequate response to treatment reveals persistent osteoclast bone resorption in osteoporotic patients.

Several drugs are able to reduce fracture risk in osteoporotic patients. Incident fractures occur despite good adherence to treatment. Inadequate response has been found related to high serum bone biomarkers of bone turnover. We here aimed to analyze bone microarchitecture and cellular profiles of inadequate responders. We retrospectively analyzed bone biopsies from patients with major fractures despite long-term treatment (inadequate responder [IR] n = 31) in comparison to patients with untreated osteoporosis (U-OP, n = 31) and controls without osteoporosis (Ctrl, n = 16). Bone samples were analyzed by histomorphometry and micro-computed tomography. Clinical and bone turnover markers and bone mineral density were assessed. As compared with U-OP patients, IRs were older (mean age 69.7 ± 8.8 vs 63.3 ± 9.3 years, p = 0.007) and had lower mean hip bone mineral density (0.685 ± 0.116 vs 0.786 ± 0.093 g/cm2), p = 0.019 and T-score (-2.3 ± 0.769 vs -1.6 ± 0.900, p = 0.032). BV/TV was lower for IRs than U-OP patients and Ctrls (13.9 ± 3.8% vs 15.2 ± 5.1 and 17.6 ± 5.2%, p = 0.044) as was trabecular thickness (145.6 ± 23.1 vs 160.5 ± 22.7 and 153.7 ± 21.4 µm, p = 0.033). Mean structure model index was lower for IRs than U-OP patients (1.9 ± 0.806 vs 2.4 ± 0.687, p = 0.042) and osteoclast number was higher for IRs than U-OP patients and Ctrls (0.721 ± 0.611 vs 0.394 ± 0.393 and 0.199 ± 0.071 mm-2, p < 0.001). The mean Obl.S/BS was lower for IRs than U-OP patients and Ctrls (1.2 ± 1.3 vs 1.9 ± 1.4 and 3.0 ± 0.638 mm-2, p < 0.0001), and the mean number of labelled surfaces was lower for IRs than U-OP patients (51.6% vs 87%, p = 0.002). Cortical parameters did not significantly differ. We show an imbalance of bone remodeling in favor of bone resorption in IRs. The persistence of high bone resorption suggests insufficient inhibition of bone resorption that could explain the incident fractures with anti-osteoporotic drug use. Adaptation to treatment should be considered to inhibit bone resorption and prevent further fractures.

More severe phenotype of early-onset osteoporosis associated with recessive form of LRP5 and combination with DKK1 or WNT3A.

BACKGROUND: Early-onset osteoporosis (EOOP) is defined by low bone mineral density (BMD), which increases the risk of fracture. Although the prevalence of osteoporosis at a young age is unknown, low BMD is highly linked to genetic background. Heterozygous pathogenic variants in low-density lipoprotein receptor-related protein 5 (LRP5) are associated with EOOP. This study aimed to investigate the genetic profile in patients with EOOP to better understand the variation in phenotype severity by using a targeted gene sequencing panel associated with bone fragility. METHOD AND RESULTS: We used a sequencing panel with 17 genes reported to be related to bone fragility for analysis of 68 patients with EOOP. We found a high positivity rate of EOOP with LRP5 variants (14 patients, 20.6%). The remaining 79.4% of patients with EOOP but without LRP5 variants showed variable disease severity, as observed in patients with at least one variant in this gene. One patient, with multiple fractures and spine L1-L4 BMD Z-score -2.9, carried a novel pathogenic homozygous variant, c.2918T>C, p.(Leu973Pro), without any pseudoglioma. In addition to carrying the LRP5 variant, 2 other patients carried a heterozygous variant in Wnt signaling pathway genes: dickkopf WNT signaling pathway inhibitor 1 (DKK1) [NM_012242.4: c.359G>T, p.(Arg120Leu)] and Wnt family member 3A (WNT3A) [NM_033131.3: c.377G>A, p. (Arg126His)]. As compared with single-variant LRP5 carriers, double-variant carriers had a significantly lower BMD Z-score (-4.1 ± 0.8) and higher mean number of fractures (6.0 ± 2.8 vs. 2.2 ± 1.9). Analysis of the family segregation suggests the inheritance of BMD trait. CONCLUSION: Severe forms of EOOP may occur with carriage of 2 pathogenic variants in genes encoding regulators of the Wnt signaling pathway. Two-variant carriers of Wnt pathway genes had severe EOOP. Moreover, DKK1 and WNT3A genes should be included in next-generation sequence analyses of bone fragility.

DNAM-1 and PVR regulate monocyte migration through endothelial junctions.

DNAX accessory molecule 1 (DNAM-1; CD226) is a transmembrane glycoprotein involved in T cell and natural killer (NK) cell cytotoxicity. We demonstrated recently that DNAM-1 triggers NK cell-mediated killing of tumor cells upon engagement by its two ligands, poliovirus receptor (PVR; CD155) and Nectin-2 (CD112). In the present paper, we show that PVR and Nectin-2 are expressed at cell junctions on primary vascular endothelial cells. Moreover, the specific binding of a soluble DNAM-1-Fc molecule was detected at endothelial junctions. This binding was almost completely abrogated by anti-PVR monoclonal antibodies (mAbs), but not modified by anti-Nectin-2 mAbs, which demonstrates that PVR is the major DNAM-1 ligand on endothelial cells. Because DNAM-1 is highly expressed on leukocytes, we investigated the role of the DNAM-1-PVR interaction during the monocyte transendothelial migration process. In vitro, both anti-DNAM-1 and anti-PVR mAbs strongly blocked the transmigration of monocytes through the endothelium. Moreover, after anti-DNAM-1 or anti-PVR mAb treatment, monocytes were arrested at the apical surface of the endothelium over intercellular junctions, which strongly suggests that the DNAM-1-PVR interaction occurs during the diapedesis step. Altogether, our results demonstrate that DNAM-1 regulates monocyte extravasation via its interaction with PVR expressed at endothelial junctions on normal cells.

FOXO1 regulates L-Selectin and a network of human T cell homing molecules downstream of phosphatidylinositol 3-kinase.

In T cells, the PI3K pathway promotes proliferation and survival induced by Ag or growth factors, in part by inactivating the FOXO transcription factor 1. We now report that FOXO1 controls the expression of L-selectin, an essential homing molecule, in human T lymphocytes. This control is already operational in unprimed T cells and involves a transcriptional regulation process that requires the FOXO1 DNA-binding domain. Using transcriptional profiling, we demonstrate that FOXO1 also increases transcripts of EDG1 and EDG6, two sphingosine-1-phosphate receptors that regulate lymphocyte trafficking. Additionally, FOXO1 binds the promoter of the cell quiescence and homing regulator Krüppel-like factor 2 and regulates its expression. Together, these results reveal a new function of FOXO1 in the immune system and suggest that PI3K controls a coordinated network of transcription factors regulating both cell quiescence and homing of human T lymphocytes.

Anti-Sclerostin Antibodies in Osteoporosis and Other Bone Diseases.

The Wnt pathway is a key element of bone remodeling; its activation stimulates bone formation and inhibits bone resorption. The discovery of sclerostin, a natural antagonist of the Wnt pathway, promoted the development of romosozumab, a human monoclonal antibody directed against sclerostin, as well as other anti-sclerostin antibodies. Phase 3 studies have shown the efficacy of romosozumab in the prevention of fractures in postmenopausal women, against placebo but also against alendronate or teriparatide and this treatment also allows bone mineral density (BMD) increase in men. Romosozumab induces the uncoupling of bone remodeling, leading to both an increase in bone formation and a decrease in bone resorption during the first months of treatment. The effect is attenuated over time and reversible when stopped but transition with anti-resorbing agents allows the maintenance or reinforcement of BMD improvements. Some concerns were raised about cardiovascular events. Therefore, romosozumab was recently approved in several countries for the treatment of severe osteoporosis in postmenopausal women with high fracture risk and without a history of heart attack, myocardial infarction or stroke. This review aims to outline the role of sclerostin, the efficacy and safety of anti-sclerostin therapies and in particular romosozumab and their place in therapeutic strategies against osteoporosis or other bone diseases.

FOXO1, T-cell trafficking and immune responses.

Efficient T-cell adaptive immune response require a faultless coordination between migration of naive T-cells into secondary lymphoid organs and critical biological outcomes driven by antigen such as cell division and cell differentiation into effector and memory cells. Recent works have shown that the phosphoinositide 3-kinase (PI3K) pathway could govern several of these processes. In this control, transcriptional factors of the Forkhead box O (FoxO) family, in particular FOXO1, a downstream effector of PI3K, appears to play a major role by coordinating both cellular proliferation of T-cells after antigen recognition and expression of homing molecules essential for their trafficking in the body.

Accuracy of the HumaSensplus point-of-care uric acid meter using capillary blood obtained by fingertip puncture.

BACKGROUND: The uric acid (UA) level in patients with gout is a key factor in disease management and is typically measured in the laboratory using plasma samples obtained after venous puncture. This study aimed to assess the reliability of immediate UA measurement with capillary blood samples obtained by fingertip puncture with the HumaSensplus point-of-care meter. METHODS: UA levels were measured using both the HumaSensplus meter in the clinic and the routine plasma UA method in the biochemistry laboratory of 238 consenting diabetic patients. HumaSensplus capillary and routine plasma UA measurements were compared by linear regression, Bland-Altman plots, intraclass correlation coefficient (ICC), and Lin's concordance coefficient. Values outside the dynamic range of the meter, low (LO) or high (HI), were analyzed separately. The best capillary UA thresholds for detecting hyperuricemia were determined by receiver operating characteristic (ROC) curves. The impact of potential confounding factors (demographic and biological parameters/treatments) was assessed. Capillary and routine plasma UA levels were compared to reference plasma UA measurements by liquid chromatography-mass spectrometry (LC-MS) for a subgroup of 67 patients. RESULTS: In total, 205 patients had capillary and routine plasma UA measurements available. ICC was 0.90 (95% confidence interval (CI) 0.87-0.92), Lin's coefficient was 0.91 (0.88-0.93), and the Bland-Altman plot showed good agreement over all tested values. Overall, 17 patients showed values outside the dynamic range. LO values were concordant with plasma values, but HI values were considered uninterpretable. Capillary UA thresholds of 299 and 340 µmol/l gave the best results for detecting hyperuricemia (corresponding to routine plasma UA thresholds of 300 and 360 µmol/l, respectively). No significant confounding factor was found among those tested, except for hematocrit; however, this had a negligible influence on the assay reliability. When capillary and routine plasma results were discordant, comparison with LC-MS measurements showed that plasma measurements had better concordance: capillary UA, ICC 0.84 (95% CI 0.75-0.90), Lin's coefficient 0.84 (0.77-0.91); plasma UA, ICC 0.96 (0.94-0.98), Lin's coefficient 0.96 (0.94-0.98). CONCLUSIONS: UA measurements with the HumaSensplus meter were reasonably comparable with those of the laboratory assay. The meter is easy to use and may be useful in the clinic and in epidemiologic studies.

Diagnostic delay in axial spondyloarthritis: A cross-sectional study of 432 patients.

OBJECTIVES: The diagnostic delay of axial spondyloarthritis (axSpA) is usually reported to be more than seven years but may have decreased recently. The objective was to quantify the diagnostic delay in patients with axSpA in France and to explore its associated factors. METHODS: Two cross-sectional observational studies included consecutively patients with axSpA (according to both ASAS criteria and rheumatologist expert opinion). Diagnostic delay was defined as the time interval from the date of first symptoms to the date of diagnosis. Potential predictive factors of diagnostic delay analyzed by multiple linear regression were demographic factors, HLA B27 status, year of diagnosis, clinical presentation and sacroiliitis on MRI or radiography. RESULTS: In all, 432 patients were analyzed: the mean age at diagnosis was 34.2 (standard deviation, 12.5) years, the mean disease duration at the time of the assessment was 11.4 (10.4) years. In all, 66.7% were HLA B27 positive, and 70.2% had radiographic sacroiliitis. The mean diagnostic delay was 4.9 (6.3) years, with a median of 2.0 years (interquartile range, 1-7; range: 0-43). In multivariable analysis, factors independently associated with a longer diagnostic delay were: higher age at diagnosis (beta=0.13; P<0.001), less frequent peripheral arthritis or dactylitis (beta=-1.69; P=0.005), and more frequent entheseal pain (beta=1.46; P=0.015). CONCLUSION: The median diagnostic delay was 2 years indicating diagnostic delay may be for most patients shorter than previously reported. A more "typical" SpA clinical presentation was associated with a shorter diagnostic delay, whereas sacroiliitis and HLA B27 positivity were not associated with this delay.

An unexpected synthesis of ketene monothioacetals.

Some dithiocarbonates (xanthates) can be converted into ketene monothioacetals through extrusion of sulfur upon treatment with base and an alkylating agent.

[PI3-kinase: Linking immunological synapse to T-cell proliferation]. / La PI3-kinase: de la synapse immunologique au contrôle de la prolifération T.

T cell clonal expansion contributing to host defense against pathogens is a tightly controlled process to maintain the homeostasis of the immune system. Our understanding of how T cell growth and proliferation are controlled following antigenic stimulation is therefore a major challenge. Antigen recognition occurs when a naive T lymphocyte contacts an antigen-presenting cell. A specialized junction enriched in T-cell receptors, costimulation molecules and signaling adaptors, called the immunological synapse, is then created for several hours between the two cell types. Recent discoveries now clarify the molecular mechanisms used by this organization to control T cell growth and proliferation. It has been established that the immunological synapse functions in fact as an integrative platform where class Ia phosphoinositide-3-kinases (PI3Ks) are recruited and activated to continuously produce high levels of 3'-phosphoinositides. These lipids regulate the localization and the activation of a wide range of PH-domain containing proteins, among which the serine-threonine kinase Akt, a downstream effector of PI3Ks, appears to be a key player. FoxO (Forkhead subgroup O) family members control in various cell systems genes implicated in apoptosis, stress resistance and cell cycle arrest, thereby contributing to maintain quiescence in unstimulated cells. In naïve T cells contacting antigen-presenting cells a rapid but also very prolonged nuclear exclusion of these transcription factors is observed downstream of Akt. Mainly, this compartmentalization process is mandatory to induce T cell growth triggered by the T cell/antigen-presenting cell interaction. These findings demonstrate that to initiate cell cycle progression the formation of the immunological synapse is an undemanding tactic used by primary T cells to securely maintain the 3'-phosphoinositide-dependent mitotic switch governed by the spatial control of FoxO transcription factors.

Extracellular isoforms of CD6 generated by alternative splicing regulate targeting of CD6 to the immunological synapse.

The great majority of mammalian genes yield multiple transcripts arising from differential mRNA processing, but in very few instances have alternative forms been assigned distinct functional properties. We have cloned and characterized a new isoform of the accessory molecule CD6 that lacks the CD166 binding domain and is expressed in rat and human primary cells. The novel isoform, CD6Deltad3, results from exon 5 skipping and consequently lacks the third scavenger receptor cysteine-rich (SRCR) domain of CD6. Differential expression of the SRCR domain 3 resulted in a remarkable functional difference: whereas full-length CD6 targeted to the immunological synapse, CD6Deltad3 was unable to localize at the T cell:APC interface during Ag presentation. Analysis of expression of CD6 variants showed that, while being more frequent in coexpression with full-length CD6, the CD6Deltad3 isoform constituted the sole species in a small percentage of T cells. In the rat thymus, CD6Deltad3 is less represented in double-positive thymocytes but is detectable in nearly 50% of single-positive CD4 or CD8 thymocytes, suggesting that CD6 switching between full-length and Deltad3 isoforms may be involved in thymic selection. Strikingly, CD6Deltad3 is markedly up-regulated upon activation of T lymphocytes, partially substituting full-length CD6, as evaluated by RT-PCR analysis at the single-cell level, by immunoblotting, and by flow cytometry using Abs recognizing SRCR domains 1 and 3 of human CD6. This elegant mechanism controlling the expression of the CD166 binding domain may help regulate signaling delivered by CD6, through different types of extracellular engagement.