Expression and Characterization of a Novel CD6 Ligand in Cells Derived from Joint and Epithelial Tissues

CD6 triggers actomyosin cytoskeleton remodeling after binding to its receptor complex

The T cell marker CD6 regulates both T cells and target cells during inflammatory responses by interacting with its receptors. However, only a few receptors binding to the extracellular domains of CD6 have been identified, and cellular events induced by CD6 engagement with its receptors in target cells remain poorly understood. In this study, we identified CD44 as a novel CD6 receptor by proximity labeling and confirmed the new CD6-CD44 interaction by biochemical and biophysical approaches. CD44 and the other 2 known CD6 receptors, CD166 and CDCP1, were distributed diffusely on resting retinal pigment epithelium (RPE) cells but clustered together to form a receptor complex upon CD6 binding. CD6 stimulation induced dramatic remodeling of the actomyosin cytoskeleton in RPE cells mediated by activation of RhoA, and Rho-associated kinase signaling, resulting in increased myosin II phosphorylation. Such actomyosin activation triggered the disassembly of tight junctions responsible for RPE barrier integrity in a process that required all components of the tripartite CD6 receptor complex. These data provided new insights into the mechanisms by which CD6 mediates T cell-driven disruption of tissue barriers during inflammation.

Chimeric Antigen Receptor T-Cell Therapy: The Light of Day for Osteosarcoma

Simple Summary

As a novel immunotherapy, chimeric antigen receptor (CAR) T-cell therapy has achieved encouraging results in leukemia and lymphoma. Furthermore, CAR-T cells have been explored in the treatment of osteosarcoma (OS). However, there is no strong comprehensive evidence to support their efficacy. Therefore, we reviewed the current evidence on CAR-T cells for OS to demonstrate their feasibility and provide new options for the treatment of OS.

Abstract

Osteosarcoma (OS) is the most common malignant bone tumor, arising mainly in children and adolescents. With the introduction of multiagent chemotherapy, the treatments of OS have remarkably improved, but the prognosis for patients with metastases is still poor, with a five-year survival rate of 20%. In addition, adverse effects brought by traditional treatments, including radical surgery and systemic chemotherapy, may seriously affect the survival quality of patients. Therefore, new treatments for OS await exploitation. As a novel immunotherapy, chimeric antigen receptor (CAR) T-cell therapy has achieved encouraging results in treating cancer in recent years, especially in leukemia and lymphoma. Furthermore, researchers have recently focused on CAR-T therapy in solid tumors, including OS. In this review, we summarize the safety, specificity, and clinical transformation of the targets in treating OS and point out the direction for further research.

CD6 is a target for cancer immunotherapy

Limitations of checkpoint inhibitor cancer immunotherapy include induction of autoimmune syndromes and resistance of many cancers. Since CD318, a novel CD6 ligand, is associated with the aggressiveness and metastatic potential of human cancers, we tested the effect of an anti-CD6 monoclonal antibody, UMCD6, on killing of cancer cells by human lymphocytes. UMCD6 augmented killing of breast, lung, and prostate cancer cells through direct effects on both CD8+ T cells and NK cells, increasing cancer cell death and lowering cancer cell survival in vitro more robustly than monoclonal antibody checkpoint inhibitors that interrupt the programmed cell death 1 (PD-1)/PD-1 ligand 1 (PD-L1) axis. UMCD6 also augmented in vivo killing by human peripheral blood lymphocytes of a human breast cancer line xenotransplanted into immunodeficient mice. Mechanistically, UMCD6 upregulated the expression of the activating receptor NKG2D and downregulated expression of the inhibitory receptor NKG2A on both NK cells and CD8+ T cells, with concurrent increases in perforin and granzyme B production. The combined capability of an anti-CD6 monoclonal antibody to control autoimmunity through effects on CD4+ lymphocyte differentiation while enhancing killing of cancer cells through distinct effects on CD8+ and NK cells opens a potential new approach to cancer immunotherapy that would suppress rather than instigate autoimmunity.

Increased Plasma Levels of the Co-stimulatory Proteins CDCP1 and SLAMF1 in Patients With Autoimmune Endocrine Diseases

Despite that autoimmune diseases share similar immunogenetic mechanisms, studies comparing the protein composition in peripheral blood from patients with autoimmune endocrine diseases are limited. In this study, we applied proximity extension assay to measure proteins related to signaling and interactions within the immune system in peripheral blood from patients with new-onset (N-T1D) and long-standing (L-T1D) type 1 diabetes, Hashimoto's thyroiditis (HT), Graves' disease (GD), and autoimmune Addison's disease in addition to healthy controls (HC). Proteins in plasma and supernatants from cultured PBMC were measured by using a 92-plex Olink® INFLAMMATION panel. Soluble CDCP1 was more abundant in plasma from patients with N-T1D, L-T1D, HT, and GD than in HC. The L-T1D and HT groups had elevated plasma levels of SLAMF1 compared with HC. Patients and HC could not be distinguished by their protein composition in PBMC supernatants. The high-throughput multiplex technology enabled us to detect two low-abundant proteins that have been gradually connected to autoimmune diseases. Our study provides novel associations between CDCP1, SLAMF1, and autoimmune endocrine diseases, which might reflect a higher degree of inflammation and lymphocyte activation.

Attenuation of Murine Collagen-Induced Arthritis by Targeting CD6

OBJECTIVE

CD6 is an important regulator of T cell function that interacts with the ligands CD166 and CD318. To further clarify the significance of CD6 in rheumatoid arthritis (RA), we examined the effects of targeting CD6 in the mouse model of collagen-induced arthritis (CIA), using CD6-knockout (CD6-KO) mice and CD6-humanized mice that express human CD6 in lieu of mouse CD6 on their T cells.

METHODS

We immunized wild-type (WT) and CD6 gene-KO mice with a collagen emulsion to induce CIA. For treatment studies using CD6-humanized mice, mice were immunized similarly and a mouse anti-human CD6 IgG (UMCD6) or control IgG was injected on days 7, 14, and 21. Joint tissues were evaluated for tissue damage, leukocyte infiltration, and local inflammatory cytokine production. Collagen-specific Th1, Th9, and Th17 responses and serum levels of collagen-specific IgG subclasses were also evaluated in WT and CD6-KO mice with CIA.

RESULTS

The absence of CD6 reduced 1) collagen-specific Th9 and Th17, but not Th1 responses, 2) the levels of many proinflammatory joint cytokines, and 3) serum levels of collagen-reactive total IgG and IgG1, but not IgG2a and IgG3. Joint homogenate hemoglobin content was significantly reduced in CD6-KO mice with CIA compared to WT mice with CIA (P < 0.05) (reduced angiogenesis). Moreover, treating CD6-humanized mice with mouse anti-human CD6 monoclonal antibody was similarly effective in reducing joint inflammation in CIA.

CONCLUSION

Taken together, these data suggest that interaction of CD6 with its ligands is important for the perpetuation of CIA and other inflammatory arthritides that are T cell driven.

Anti-CD166/4-1BB chimeric antigen receptor T cell therapy for the treatment of osteosarcoma

BACKGROUND

Chimeric antigen receptor (CAR)-engineered T cells have displayed outstanding performance in the treatment of patients with hematological malignancies. However, their efficacy against solid tumors has been largely limited.

METHODS

In this study, human osteosarcoma cell lines were prepared, flow cytometry using antibodies against CD166 was performed on different cell samples. CD166-specific T cells were obtained by viral gene transfer of corresponding DNA plasmids and selectively expanded using IL-2 and IL-15. The ability of CD166.BBζ CAR-T cells to kill CD166⁺ osteosarcoma cells was evaluated in vitro and in vivo.

RESULTS

CD166 was selectively expressed on four different human osteosarcoma cell lines, indicating its role as the novel target for CAR-T cell therapy. CD166.BBζ CAR-T cells killed osteosarcoma cell lines in vitro; the cytotoxicity correlated with the level of CD166 expression on the tumor cells. Intravenous injection of CD166.BBζ CAR-T cells into mice resulted in the regression of the tumor with no obvious toxicity.

CONCLUSIONS

Together, the data suggest that CD166.BBζ CAR-T cells may serve as a new therapeutic strategy in the future clinical practice for the treatment of osteosarcoma.

CD6, a Rheostat-Type Signalosome That Tunes T Cell Activation

Following T cell receptor triggering, T cell activation is initiated and amplified by the assembly at the TCR/CD3 macrocomplex of a multitude of stimulatory enzymes that activate several signaling cascades. The potency of signaling is, however, modulated by various inhibitory components already at the onset of activation, long before co-inhibitory immune checkpoints are expressed to help terminating the response. CD5 and CD6 are surface glycoproteins of T cells that have determinant roles in thymocyte development, T cell activation and immune responses. They belong to the superfamily of scavenger receptor cysteine-rich (SRCR) glycoproteins but whereas the inhibitory role of CD5 has been established for long, there is still controversy on whether CD6 may have similar or antagonistic functions on T cell signaling. Analysis of the structure and molecular associations of CD5 and CD6 indicates that these molecules assemble at the cytoplasmic tail a considerable number of signaling effectors that can putatively transduce diverse types of intracellular signals. Biochemical studies have concluded that both receptors can antagonize the flow of TCR-mediated signaling; however, the impact that CD5 and CD6 have on T cell development and T cell-mediated immune responses may be different. Here we analyze the signaling function of CD6, the common and also the different properties it exhibits comparing with CD5, and interpret the functional effects displayed by CD6 in recent animal models.

Biophysical Characterization of CD6-TCR/CD3 Interplay in T Cells

Activation of the T cell receptor (TCR) on the T cell through ligation with antigen-MHC complex of an antigen-presenting cell (APC) is an essential process in the activation of T cells and induction of the subsequent adaptive immune response. Upon activation, the TCR, together with its associated co-receptor CD3 complex, assembles in signaling microclusters that are transported to the center of the organizational structure at the T cell-APC interface termed the immunological synapse (IS). During IS formation, local cell surface receptors and associated intracellular molecules are reorganized, ultimately creating the typical bull's eye-shaped pattern of the IS. CD6 is a surface glycoprotein receptor, which has been previously shown to associate with CD3 and co-localize to the center of the IS in static conditions or stable T cell-APC contacts. In this study, we report the use of different experimental set-ups analyzed with microscopy techniques to study the dynamics and stability of CD6-TCR/CD3 interaction dynamics and stability during IS formation in more detail. We exploited antibody spots, created with microcontact printing, and antibody-coated beads, and could demonstrate that CD6 and the TCR/CD3 complex co-localize and are recruited into a stimulatory cluster on the cell surface of T cells. Furthermore, we demonstrate, for the first time, that CD6 forms microclusters co-localizing with TCR/CD3 microclusters during IS formation on supported lipid bilayers. These co-localizing CD6 and TCR/CD3 microclusters are both radially transported toward the center of the IS formed in T cells, in an actin polymerization-dependent manner. Overall, our findings further substantiate the role of CD6 during IS formation and provide novel insight into the dynamic properties of this CD6-TCR/CD3 complex interplay. From a methodological point of view, the biophysical approaches used to characterize these receptors are complementary and amenable for investigation of the dynamic interactions of other membrane receptors.

The anti-CD6 antibody itolizumab provides clinical benefit without lymphopenia in rheumatoid arthritis patients: results from a 6-month, open-label Phase I clinical trial

Itolizumab is a humanized anti-CD6 monoclonal antibody (mAb) that has previously shown encouraging results, in terms of safety and positive clinical effects, in a 6-week monotherapy clinical trial conducted in rheumatoid arthritis (RA) patients. The current Phase I study evaluated the safety and clinical response for a longer treatment of 12 itolizumab intravenous doses in subjects with active RA despite previous disease-modifying anti-rheumatic drug (DMARD) therapy. Twenty-one subjects were enrolled into four dosage groups (0·1, 0·2, 0·4 and 0·8 mg/kg). Efficacy end-points including American College of Rheumatology (ACR)20, ACR50 and ACR70 response rates and disease activity score in 28 joints (DAS28) were monitored at baseline and at specific time-points during a 10-week follow-up period. Itolizumab was well tolerated up to the highest tested dose. No related serious adverse events were reported and most adverse events were mild. Remarkably, itolizumab treatment did not produce lymphopenia and, therefore, was not associated with infections. All patients achieved a clinical response (ACR20) at least once during the study. Eleven subjects (55%) achieved at least a 20% improvement in ACR just 1 week after the first itolizumab administration. The clinical response was observed from the beginning of the treatment and was sustained during 24 weeks. The efficacy profile of this 12-week treatment was similar to that of the previous study (6-week treatment). These results reinforce the safety profile of itolizumab and provide further evidence on the clinical benefit from the use of this anti-CD6 mAb in RA patients.

CD318 is a ligand for CD6

It has been proposed that CD6, an important regulator of T cells, functions by interacting with its currently identified ligand, CD166, but studies performed during the treatment of autoimmune conditions suggest that the CD6-CD166 interaction might not account for important functions of CD6 in autoimmune diseases. The antigen recognized by mAb 3A11 has been proposed as a new CD6 ligand distinct from CD166, yet the identity of it is hitherto unknown. We have identified this CD6 ligand as CD318, a cell surface protein previously found to be present on various epithelial cells and many tumor cells. We found that, like CD6 knockout (KO) mice, CD318 KO mice are also protected in experimental autoimmune encephalomyelitis. In humans, we found that CD318 is highly expressed in synovial tissues and participates in CD6-dependent adhesion of T cells to synovial fibroblasts. In addition, soluble CD318 is chemoattractive to T cells and levels of soluble CD318 are selectively and significantly elevated in the synovial fluid from patients with rheumatoid arthritis and juvenile inflammatory arthritis. These results establish CD318 as a ligand of CD6 and a potential target for the diagnosis and treatment of autoimmune diseases such as multiple sclerosis and inflammatory arthritis.

SK, Montero E, Pai HV, Palanivelu DV, Melarkode R, Nair P. T cell activation and differentiation is modulated by a CD6 domain 1 antibody Itolizumab

CD6 is associated with T-cell modulation and is implicated in several autoimmune diseases. We previously demonstrated that Itolizumab, a CD6 domain 1 (CD6D1) specific humanized monoclonal antibody, inhibited the proliferation and cytokine production by T lymphocytes stimulated with anti-CD3 antibody or when co-stimulated with ALCAM. Aberrant IL-17 producing CD4+ helper T-cells (Th17) have been identified as pivotal for the pathogenesis of certain inflammatory autoimmune disorders, including psoriasis. Itolizumab has demonstrated efficacy in human diseases known to have an IL-17 driven pathogenesis. Here, in in vitro experiments we show that by day 3 of human PBMC activation using anti-CD3 and anti-CD28 co-stimulation in a Th17 polarizing milieu, 15-35% of CD4+ T-cells overexpress CD6 and there is an establishment of differentiated Th17 cells. Addition of Itolizumab reduces the activation and differentiation of T cells to Th17 cells and decreases production of IL-17. These effects are associated with the reduction of key transcription factors pSTAT3 and RORγT. Further, transcription analysis studies in these conditions indicate that Itolizumab suppressed T cell activation by primarily reducing cell cycle, DNA transcription and translation associated genes. To understand the mechanism of this inhibition, we evaluated the effect of this anti-human CD6D1 mAb on ALCAM-CD6 as well as TCR-mediated T cell activation. We show that Itolizumab but not its F(ab')2 fragment directly inhibits CD6 receptor hyper-phosphorylation and leads to subsequent decrease in associated ZAP70 kinase and docking protein SLP76. Since Itolizumab binds to CD6 expressed only on human and chimpanzee, we developed an antibody binding specifically to mouse CD6D1. This antibody successfully ameliorated the incidence of experimental autoimmune encephalitis in the mice model. These results position CD6 as a key molecule in sustaining the activation and differentiation of T cells and an important target for modulating autoimmune diseases.

CD6 as a potential target for treating multiple sclerosis

CD6 was established as a marker of T cells more than three decades ago, and recent studies have identified CD6 as a risk gene for multiple sclerosis (MS), a disease in which autoreactive T cells are integrally involved. Nevertheless, the precise role of CD6 in regulating T-cell responses is controversial and its significance in the pathogenesis of various diseases remains elusive, partly due to the lack of animals engineered to alter expression of the CD6 gene. In this report, we found that CD6 KO mice showed decreased pathogenic T-cell responses, reduced spinal cord T-cell infiltration, and attenuated disease severity in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. CD6-deficient T cells exhibited augmented activation, but also significantly reduced survival and proliferation after activation, leading to overall decreased Th1 and Th17 polarization. Activated CD6-deficient T cells also showed impaired infiltration through brain microvascular endothelial cell monolayers. Furthermore, by developing CD6 humanized mice, we identified a mouse anti-human CD6 monoclonal antibody that is highly effective in treating established EAE without depleting T cells. These results suggest that (i) CD6 is a negative regulator of T-cell activation, (ii) at the same time, CD6 is a positive regulator of activated T-cell survival/proliferation and infiltration; and (iii) CD6 is a potential new target for treating MS and potentially other T-cell-driven autoimmune conditions.

The Cd6 gene as a permissive locus for targeted transgenesis in the mouse

The introduction of a transgene into the genome through homologous recombination or sequence-specific enzymatic modification is a key technique for producing transgenic mice. The Rosa26 gene has been widely used to produce transgenic mice because the gene is transcriptionally active in various cell types and, at many developmental stages, is permissive for constitutive expression of integrated transgenes, and is dispensable for normal development. However, permissive loci other than Rosa26 are needed to generate mice that harbor multiple transgenes for complex studies. Here, we identified the Cd6 locus on mouse chromosome 19 as a transgene integration site in a transgenic mouse strain showing widespread reporter expression. Using this locus, we generated a knock-in mouse line that harbors a CAG promoter-driven reporter transgene, and found that the homozygous transgenic mice are viable and fertile, although transgene insertion disrupted Cd6 gene expression. The transgene on the Cd6 locus expressed reporter genes extensively throughout embryos, neonates, and adults. Combined with the Cre/loxP binary system, blood and lymphatic endothelial cell-specific reporter expression from the transgenic locus was achieved. These results suggest that Cd6 is valuable as an alternative site for targeted transgenesis.

CD6 as a therapeutic target in autoimmune diseases: successes and challenges

The transmembrane surface glycoprotein CD6 was one of the first antigens identified on T lymphocytes. The recognition of its involvement in T-cell signaling processes heralds the potential of CD6 as a target for therapy in a number of pathologies associated with imbalances in T-cell function. Its tissue distribution, cellular expression, and overall molecular structure are well described, and the interaction with its physiological ligand CD166 has been determined to the amino-acid level. Nevertheless, the involvement of CD6 in signaling pathways remains poorly characterized and its biological function is controversial; still unresolved are whether CD6 is a co-stimulatory molecule in T-cell activation or, similar to the related CD5 antigen, a modulator of intracellular signaling. Here we revisit the earliest attempts of modulating immune function using CD6 monoclonal antibodies, and review the current thinking behind the recent developments in immunotherapy targeting CD6. Notwithstanding the promises and hopes brought by monoclonals already in clinical trials, the fact is that very little is known about the mechanism of action of these reagents, whether they enhance the physiological role of the receptor or whether they may induce a completely novel biochemical response that might, nevertheless, be beneficially used to treat human immune pathology.

CD5 costimulation induces stable Th17 development by promoting IL-23R expression and sustained STAT3 activation

IL-17-producing CD4(+) T helper (Th17) cells are important for immunity against extracellular pathogens and in autoimmune diseases. The factors that drive Th17 development in human remain a matter of debate. Here we show that, compared with classic CD28 costimulation, alternative costimulation via the CD5 or CD6 lymphocyte receptors forms a superior pathway for human Th17-priming. In the presence of the Th17-promoting cytokines IL-1β, IL-6, IL-23, and transforming growth factor-β (TGF-β), CD5 costimulation induces more Th17 cells that produce higher amounts of IL-17, which is preceded by prolonged activation of signal transducer and activator of transcription 3 (STAT3), a key regulator in Th17 differentiation, and enhanced levels of the IL-17-associated transcription factor retinoid-related orphan receptor-γt (ROR-γt). Strikingly, these Th17-promoting signals critically depend on CD5-induced elevation of IL-23 receptor (IL-23R) expression. The present data favor the novel concept that alternative costimulation via CD5, rather than classic costimulation via CD28, primes naive T cells for stable Th17 development through promoting the expression of IL-23R.

The conserved scavenger receptor cysteine-rich superfamily in therapy and diagnosis

The scavenger receptor cysteine-rich (SRCR) superfamily of soluble or membrane-bound protein receptors is characterized by the presence of one or several repeats of an ancient and highly conserved protein module, the SRCR domain. This superfamily (SRCR-SF) has been in constant and progressive expansion, now up to more than 30 members. The study of these members is attracting growing interest, which parallels that in innate immunity. No unifying function has been described to date for the SRCR domains, this being the result of the limited knowledge still available on the physiology of most members of the SRCR-SF, but also of the sequence versatility of the SRCR domains. Indeed, involvement of SRCR-SF members in quite different functions, such as pathogen recognition, modulation of the immune response, epithelial homeostasis, stem cell biology, and tumor development, have all been described. This has brought to us new information, unveiling the possibility that targeting or supplementing SRCR-SF proteins could result in diagnostic and/or therapeutic benefit for a number of physiologic and pathologic states. Recent research has provided structural and functional insight into these proteins, facilitating the development of means to modulate the activity of SRCR-SF members. Indeed, some of these approaches are already in use, paving the way for a more comprehensive use of SRCR-SF members in the clinic. The present review will illustrate some available evidence on the potential of well known and new members of the SRCR-SF in this regard.

Rationale for Targeting CD6 as a Treatment for Autoimmune Diseases

CD6 is a 105–130 kDa surface glycoprotein expressed on the majority of T cells and a subset of B cells. The human cd6 gene maps to chromosome 11, and the expression of its protein product is tightly regulated. CD6 mediates cellular adhesion migration across the endothelial and epithelial cells. In addition, it participates in the antigen presentation by B cells and the subsequent proliferation of T cells. CD6 may bind in trans to surface glycoproteins (such as ALCAM and 3A11), or to microbial lipopolysaccharides, and may bind in cis to endogenous ligands (such as CD3 and CD5), and thereby deliver a costimulatory signal. Transinteractions are reinforced during autoimmune diseases (e.g., rheumatoid arthritis (RA), Sjögren's syndrome, and multiple sclerosis) and some cancers. Based on experimental data and on clinical results in RA and psoriasis, we believe that the recent humanized anti-CD6-specific mAb T1h may act as a regulator of the immunological response in addition to its function as an anti-T- and -B cell agent.

CD6 synergistic co-stimulation promoting proinflammatory response is modulated without interfering with the activated leucocyte cell adhesion molecule interaction

The CD6 membrane-proximal scavenger receptor cysteine-rich domain (SRCR3) includes the activated leucocyte cell adhesion molecule (ALCAM) binding site. CD6-ALCAM mediates a low-affinity interaction and their long-term engagement contributes to the immunological synapse. Their ligation may play a dual function, facilitating stable adhesion between the antigen-presenting cells and T cells during the early activation phase and later in the proliferative phase of the immune response. This study explored the strength of the CD6 co-stimulatory effect and whether CD6 co-stimulation with its natural ligand ALCAM also contributes to the lymphocyte effector differentiation. It was found that CD6-ALCAM interaction in vitro induced a synergistic co-stimulation of normal human peripheral blood mononuclear cells, defined by Bliss analysis. CD6 co-stimulation enhanced the CD3 proliferative efficacy by 23-34%. Moreover, a fivefold increment in the CD25 molecules number with a distinct gene transcription profile associated with cell activation, differentiation, survival and adhesion molecules was observed over CD3 single activation. Additionally, CD6 co-stimulation in excess interleukin (IL)-2 promotes a preferentially proinflammatory response. Besides, a CD6 membrane-distal domain (SRCR1)-specific non-depleting monoclonal antibody (mAb) inhibited the induced proliferation in the presence of ALCAM, reducing interferon-γ, IL-6 and tumour necrosis factor-α production. These results suggest that CD6 co-stimulation enhances the intrinsic activity of the CD3 activation pathway and contributes to the T helper type 1 subset commitment, enhancing the IL-2 sensitivity of recent activated human lymphocytes. It supports the role of CD6 as a susceptibility gene for pathological autoimmunity leading to tissue inflammation, and its relevance for targeted therapy.

Towards the definition of a chimpanzee and human conserved CD6 domain 1 epitope recognized by T1 monoclonal antibody

Scavenger receptor cysteine-rich (SRCR) domains are evolutionally conserved modules that display complex structures stabilized by key amino acids, while some other residues have evolved with a relative independence, thus allowing the functional diversity of these receptors. CD6, a highly glycosylated membrane protein predominantly expressed on lymphocytes, contains three SRCR domains. The lack of CD6 domain crystal structure has limited the characterization of the binding sites for the interacting molecules. The interaction between CD6 and its ligand, activated leukocyte-cell adhesion molecule (ALCAM)/CD166, through the membrane-proximal SRCR3 domain, has low affinity and involves conserved sites in both molecules mediating a cross-species binding. The CD6-ALCAM interaction has been involved in cell adhesion, maturation, regulation of activation, and survival processes, suggesting the potential relevance of this target for therapeutic interventions. Several anti-CD6 monoclonal antibodies (MAb) have been described but their affinity and epitope definition remain unclear. We found the murine and humanized T1 MAb versions have similar CD6 recognition profiles and affinity constants of about 6 x 10(8). These antibodies do not block the CD6-ALCAM interaction and recognize a conformational epitope independent of the CD6 N-glycosylation. This epitope was additionally found in the chimpanzee and contains an RXE/Q consensus motif located in the membrane-distal SRCR1. These results, together with the therapeutic evidence previously obtained with these MAbs, suggest a differential contribution of CD6 domains to lymphocyte biology. Potential mechanisms for T1 MAb therapeutic effect different from CD6-CD166 interaction blocking would be dissected.

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.

Cells of the synovium in rheumatoid arthritis. T lymphocytes

Recent findings have substantiated the importance of T lymphocytes to the pathogenesis of rheumatoid arthritis (RA). Here, we review emerging data regarding genetic predisposition, spontaneous animal models of arthritis, and cell-cell interactions that implicate T cells as driving synovial inflammation and joint destruction. Information regarding the proinflammatory role of interleukin-17-producing T cells and the functional state of regulatory T cells both in animal models and in patients with RA is also discussed. In light of the overwhelming evidence that disrupted T-cell homeostasis greatly contributes to joint pathology in RA, the therapeutic potential of targeting activators of pro-inflammatory T cells or their products is compelling.

CD6 regulates T-cell responses through activation-dependent recruitment of the positive regulator SLP-76

Deciphering the role of lymphocyte membrane proteins depends on dissecting the role of a protein in the steady state and on engagement with its ligand. We show that expression of CD6 in T cells limits their responsiveness but that engagement by the physiological ligand CD166 gives costimulation. This costimulatory effect of CD6 is mediated through phosphorylation-dependent binding of a specific tyrosine residue, 662Y, in its cytoplasmic region to the adaptor SLP-76. A direct interaction between SLP-76 and CD6 was shown by binding both to a phosphorylated peptide (equilibrium dissociation constant [K(D)] = 0.5 muM at 37 degrees C) and, using a novel approach, to native phosphorylated CD6. Evidence that CD6 and SLP-76 interact in cells was obtained in coprecipitation experiments with normal human T cells. Analysis of human CD6 mutants in a murine T-cell hybridoma model showed that both costimulation by CD6 and the interaction between CD6 and SLP-76 were dependent on 662Y. The results have implications for regulation by CD6 and the related T-cell surface protein, CD5.

Identification of clinically significant tumor antigens by selecting phage antibody library on tumor cells in situ using laser capture microdissection

Much work has been done to develop tumor-targeting antibodies by selecting a phage antibody library on cancer cell lines. However, when tumor cells are removed from their natural environment, they may undergo genetic and epigenetic changes yielding different surface antigens than those seen in actual cases of cancer. We developed a strategy that allows selection of phage antibodies against tumor cells in situ on both fresh frozen and paraffin-embedded tissues using laser capture microdissection. By restricting antibody selection to binders of internalizing epitopes, we generated a panel of phage antibodies that target clinically represented prostate cancer antigens. We identified ALCAM/MEMD/CD166, a newly discovered prostate cancer marker, as the target for one of the selected antibodies, demonstrating the effectiveness of our approach. We further conjugated two single chain Fv fragments to liposomes and demonstrated that these nanotargeting devices were efficiently delivered to the interior of prostate cancer cells. The ability to deliver payload intracellularly and to recognize tumor cells in situ makes these antibodies attractive candidates for the development of targeted cancer therapeutics.

CD166 expression, characterization, and localization in salivary epithelium: implications for function during sialoadenitis

CD166 is an Ig superfamily molecule that binds homotypically to itself and heterotypically to CD6. Interactions between CD6 and CD166 are important during immune development and in alloreactivity. CD166 is expressed at increased levels in selected cancers and in rheumatoid arthritis synovium. Knowledge that CD166 was expressed in normal human salivary epithelium led to these studies of CD166 and CD6 in diseased mouse salivary glands, that resemble pathology seen in the human disease, Sjögren's syndrome. We showed that in mouse salivary epithelium CD166 was expressed but that expression of CD166 did not necessarily predict its function. Recombinant soluble CD6-Ig bound to CD6 ligands (CD6L) on transformed and freshly isolated salivary epithelial cells. Cross-blocking studies showed that binding of CD6-Ig to salivary epithelium was in part dependent on CD166, but that CD6-Ig binding may also involve additional CD6L. Binding of CD6-Ig was sensitive to trypsin digestion but resistant to digestion by collagenase and sialidase. Anti-CD166 ab precipitated CD166 from salivary epithelium pre- and post-treatment with the pro-inflammatory cytokine IFN-gamma. In contrast CD6-Ig only precipitated CD166 from IFN-gamma treated cells. More extensive colocalization between CD166 and the actin cytoskeleton was observed in sialoadenitis epithelium compared to control. We conclude that during sialoadenitis, CD166 undergoes a gain of function, resulting in closer association with the actin cytoskeleton and increased capacity to bind CD6. We suggest that altered CD166 function may contribute to the pro-inflammatory milieu during sialoadenitis seen in Sjögren's syndrome.

Mitogen-Activated Protein Kinase Pathway Activation by the CD6 Lymphocyte Surface Receptor

CD6 is a cell surface receptor primarily expressed on immature thymocytes and mature T and B1a lymphocytes. Through its binding to activated leukocyte cell adhesion molecule (ALCAM/CD166), CD6 is considered to play an important role in lymphocyte development and activation. Accordingly, CD6 associates with the TCR/CD3 complex and colocalizes with it at the center of the mature immunological synapse on T lymphocytes. Moreover, the CD6-ALCAM interaction has been shown to be critical for proper immunological synapse maturation and T cell proliferative responses. However, the precise biological effects of CD6 ligation and its signaling pathway are still not well understood. The present study shows that CD6 ligation with three different specific mAbs (161.8, SPV-L14.2, and MAE1-C10) induces time- and dose-dependent activation of ERK1/2 on normal and leukemic human T cells. This effect was also observed upon CD6 ligation with a chimerical ALCAM protein (ALCAM-Fc). The C-terminal cytoplasmic region of CD6, as well as Src tyrosine kinases, was critical for CD6-induced ERK1/2 activation. Synergistic effects were observed upon coligation of the TCR/CD3 complex with CD6. The ligation of CD6 induced the transcriptional activation of reporter genes under the control of the c-Fos serum responsive element and AP-1. Accordingly, CD6-mediated activation of p38 and JNK was also observed. These findings indicate that the CD6-ALCAM interaction results in activation of the three MAPK cascades, likely influencing the dynamic balance that determines whether resting or activated lymphocytes survive or undergo apoptosis.

Levamisole synergizes experimental F4ac+ Escherichia coli oral vaccine in stimulating ileal Peyer's patch T cells in weaned pigs

Recent findings demonstrate that priming by levamisole of weaned pigs experimentally vaccinated against postweaning colibacillosis (PWC) contributes to immune protection from challenge-induced clinical disease through stimulation of the mesenteric lymph node cells that participate in cell-mediated immunity. With the objective of better understanding the mechanisms by which levamisole induces protective mucosal cell-mediated immune response to vaccination against PWC, it was tested whether the drug synergizes experimental F4ac+ Escherichia coli oral vaccine in stimulating T cells also in the jejunal lamina propria (JLP) and ileal Peyer's patch (IPP) upon virulent challenge. Commercial crossbred pigs weaned at 4 weeks were allocated into two equal groups. The experimental group was i.m. primed with levamisole at an immunostimulatory dose of 2.5 mg/kg once daily, for 3 consecutive days, and controls received saline. Both groups were vaccinated orally with the vaccinal E. coli strain on day 0 and challenged with the virulent E. coli strain 7 days later. All pigs were killed on postchallenge day 6. The results determined by immunophenotyping of isolated cells indicate that priming by levamisole of the vaccinated weaned pigs selectively recruited and activated T cells in the IPP, a lymphoid organ-generating B lymphocytes. The pig IPP is normally populated with up to 5% of CD3+ T cells and CD6 is an activation antigen expressed exclusively by T cells in swine. Therefore, a significantly higher number of CD3+ (P < 0.01) and CD6+ (P < 0.001) cells observed within the IPP follicles of the primed-vaccinated vs. unprimed-vaccinated challenge-infected pigs suggest enhanced T cell-mediated immunity in this B-cell compartment induced by the potentiating action of the drug and vaccine. The ability of levamisole to influence interaction between activated T cells and B cells in the IPP of primed-vaccinated weaned pigs, and the possibility that this interaction plays a role in regulating B-cell maturation within the IPP follicles, are discussed.

Immunocompetent properties of human osteoblasts: interactions with T lymphocytes

We sought to determine whether osteoblasts (OBs) can serve as accessory cells (ACs) for T-cell activation and whether T cells directly activate OB production of IL-6, using primary human OBs (NHOst), the transformed fetal osteoblast line hFOB1.19, and an osteosarcoma line SaOS-2. Robust, bidirectional activating interactions were shown using each of these three human ostoblast lines.

INTRODUCTION

Osteoblasts (OBs) could come into contact with lymphocytes during inflammatory joint destruction and fracture repair.

MATERIALS AND METHODS

We used several in vitro assays to assess the ability of T cells and OBs to interact in the generation of immune and inflammatory responses.

RESULTS

By flow cytometry, three OB cell lines all were found to express ligands for T-cell co-stimulation. The integrin ligand CD54/ICAM-1 was constitutively expressed by hFOB1.19 and NHOst and was upregulated on SaOS-2 by IFN-gamma. MHC Class II was upregulated on all three lines by IFN-gamma. CD166/ALCAM, a ligand of the T-cell molecule CD6, was constitutively expressed on all three lines. A second putative CD6 ligand designated 3A11 was expressed on hFOB1.19 and NHOst, but not consistently on SaOS-2. The ectoenzyme CD26 (dipeptidyl peptidase IV) was expressed on hFOB1.19 and NHOst, but not on SaOS-2. All three cell lines presented superantigen to T cells, especially after treatment with IFN-gamma. Superantigen presentation was inhibited by antibodies to the leukocyte integrin CD11a/CD18 (LFA-1), MHC Class II, and CD54/ICAM-1. T cells, particularly when cytokine activated for 7 days before co-culture, stimulated all three osteoblast lines to produce interleukin (IL)-6, and this effect was boosted when IL-17 was added to the co-cultures with either resting T cells or cytokine-activated T cells.

CONCLUSIONS

Bidirectional activating interactions are readily shown between human T cells and several types of human OBs. The expression by OBs of ligands for the T cell-specific molecule CD6, as well as other molecules involved in immune interactions, strongly suggests that such in vitro interactions are representative of physiologic or pathologic events that occur in vivo.