B cell receptor crosstalk: B cells express osteopontin through the combined action of the alternate and classical BCR signaling pathways

Age-Dependent Effect of Calcitriol on Mouse Regulatory T and B Lymphocytes

The hormonally active vitamin D3 metabolite, calcitriol, functions as an important modulator of the immune system. We assumed that calcitriol exerts different effects on immune cells and cytokine production, depending on the age of the animal; therefore, we analyzed its effects on regulatory T lymphocytes and regulatory B lymphocytes in healthy young and old female C57Bl/6/Foxp3GFP mice. In the lymph nodes of young mice, calcitriol decreased the percentage of Tregs, including tTregs and pTregs, and the expression of GITR, CD103, and CD101; however, calcitriol increased the level of IL-35 in adipose tissue. In the case of aged mice, calcitriol decreased the percentages of tTregs and CD19+ cells in lymph nodes and the level of osteopontin in the plasma. Additionally, increases in the levels of IgG and the lowest levels of IFN-γ, IL-10, and IL-35 were observed in the adipose tissue of aged mice. This study showed that calcitriol treatment had different effects, mainly on Treg phenotypes and cytokine secretion, in young and old female mice; it seemed that calcitriol enhanced the immunosuppressive properties of the lymphatic organs and adipose tissue of healthy young mice but not of healthy aged mice, where the opposite effects were observed.

A Dual Role of Osteopontin in Modifying B Cell Responses

The occurrence of B cell aggregates within the central nervous system (CNS) has prompted the investigation of the potential sources of pathogenic B cell and T cell responses in a subgroup of secondary progressive multiple sclerosis (MS) patients. Nevertheless, the expression profile of molecules associated with these aggregates and their role in aggregate development and persistence is poorly described. Here, we focused on the expression pattern of osteopontin (OPN), which is a well-described cytokine, in MS brain tissue. Autopsied brain sections from MS cases with and without B cell pathology were screened for the presence of CD20⁺ B cell aggregates and co-expression of OPN. To demonstrate the effect of OPN on B cells, flow cytometry, ELISA and in vitro aggregation assays were conducted using the peripheral blood of healthy volunteers. Although OPN was expressed in MS brain tissue independent of B cell pathology, it was also highly expressed within B cell aggregates. In vitro studies demonstrated that OPN downregulated the co-stimulatory molecules CD80 and CD86 on B cells. OPN-treated B cells produced significantly lower amounts of IL-6. However, OPN-treated B cells also exhibited a higher tendency to form homotypic cell aggregates in vitro. Taken together, our data indicate a conflicting role of OPN in modulating B cell responses.

Signal Integration by Translocation and Phosphorylation of PKCδ in the B Cell Alternate Pathway

B cell signaling for activation via the BCR occurs as an isolated event only in vitro; in real life, BCR signaling takes place within a complex milieu that involves interactions with agents that trigger additional receptors. Chief among these is IL-4. We have shown that BCR signaling is reprogrammed by IL-4 receptor engagement and that this reprogramming involves creation of a new, signalosome-independent, Lyn-dependent alternate signaling pathway in B cells isolated from BALB/cByJ mice. A unique aspect of the alternate pathway is protein kinase Cδ (PKCδ) phosphorylation. In dissecting this pathway, we unexpectedly found that Lyn is associated with IL-4Rα, that IL-4 induces Lyn activation, and that Lyn immunoprecipitated from IL-4-treated B cells capably phosphorylates PKCδ in a cell-free system. However, PKCδ phosphorylation does not occur in the absence of BCR triggering in vivo. This raised the question of why IL-4 alone failed to produce PKCδ phosphorylation. We considered the possibility that Lyn and PKCδ may be spatially separated. As expected, before any treatment, Lyn is located primarily in the membrane fraction, whereas PKCδ is located mainly in the cytosol fraction. However, when anti-Ig follows IL-4 treatment, PKCδ is found in the membrane fraction and phosphorylated. This translocation of PKCδ to the membrane fraction is not affected by loss of Lyn, although PKCδ phosphorylation requires Lyn. Thus, PKCδ phosphorylation through the alternate pathway represents the result of signal integration, whereby neither IL-4 nor anti-Ig working alone produces this outcome, but together they achieve this result by Lyn activation (IL-4) and PKCδ translocation (IL-4 followed by anti-Ig).

Analysis of PI3K Pathway Associated Molecules Reveals Dysregulated Innate and Adaptive Functions of B Cells in Early Diffuse Cutaneous Systemic Sclerosis

B cell activation is an early event in the development of systemic sclerosis (SSc). The classical activation of B cells downstream of the B-cell receptor (BCR) involves the phosphatidylinositol-3 kinase (PI3K) pathway that integrates the effects of multiple co-stimulatory receptors. Our analysis of PI3K pathway associated molecules in peripheral blood B cells of early diffuse cutaneous SSc (dcSSc) patients showed altered mRNA expression of Toll-like receptor (TLR) homolog CD180, TLR4, complement component 3, IL-4 receptor and secreted phosphoprotein 1 (SPP1). Parallel to this, we found elevated basal SPP1 secretion in dcSSc B cells, but, with BCR + IL-4 receptor co-stimulation, we could not induce further secretion. CD180 stimulation alone resulted in NF-κB activation in more B cells than CD180 + BCR co-stimulation both in dcSSc and healthy control (HC), but the co-engagement increased the phosphorylation of NF-κB only in dcSSc B cells. Additionally, in contrast with HC B cells, the lower basal production of IL-10 by dcSSc B cells could not be elevated with CD180 stimulation. Furthermore, activation via CD180 increased the percentage of CD86+ switched memory (CD27+IgD−) B cells in dcSSc compared to HC. Our results suggest that alternative B cell activation and CD180 dysfunction cause imbalance of regulatory mechanisms in dcSSc B cells.

The Alternate Pathway for BCR Signaling Induced by IL-4 Requires Lyn Tyrosine Kinase

BCR signaling triggers a cascade of intracellular mediators that eventuates in transcription factor activation. Signaling is proximally mediated by Src family tyrosine kinases, the most abundant being Lyn. Key mediators are grouped together as the signalosome, and failure of any single member of this group leads to failure of signaling via this classical pathway. Recent work has revealed an alternate pathway for BCR signaling, in which signalosome elements are bypassed for downstream events such as ERK and PKCδ phosphorylation. This pathway is created by B cell treatment with IL-4 prior to BCR triggering. After IL-4 treatment, the alternate pathway for pERK operates in parallel with the classical pathway for pERK, whereas PKCδ phosphorylation is specific to the alternate pathway. Remarkably, Lyn is not required for B cell activation via the classical pathway; however, Lyn is indispensable and irreplaceable for B cell activation via the alternate pathway. Thus, Lyn operates at a branch point that determines the nature of the B cell response to BCR activation. The mechanism underlying the absolute dependence of alternate pathway signaling on Lyn is unknown. Here, our current understanding of receptor crosstalk between IL-4R and BCR is summarized along with several possible mechanisms for the role of Lyn in alternate pathway signaling. Further dissection of alternate pathway signaling and the role of Lyn is likely to provide important information relating to normal B cell responses, malignant B cell expansion, and generic principles relating to receptor interactions and crosstalk.

Calcitriol Analogues Decrease Lung Metastasis but Impair Bone Metabolism in Aged Ovariectomized Mice Bearing 4T1 Mammary Gland Tumours

Calcitriol and its analogues are considered drugs supporting the anticancer treatment of breast cancer and preventing the osteoporosis that results from the development of cancer or from chemotherapy or hormone therapy. Following the orthotopic implantation of 4T1 mammary carcinoma cells into aged ovariectomized (OVX) mice, we evaluated the effects of calcitriol and its two analogues, PRI-2191 and PRI-2205, on metastatic spread and bone homeostasis. Calcitriol and its analogues temporarily inhibited the formation of metastases in the lungs. Unexpectedly, only mice treated with calcitriol analogues showed a deterioration of bone-related parameters, such as bone column density, marrow column density and the CaPO₄ coefficient. These findings correlated with an increased number of active osteoclasts differentiated from bone marrow-derived macrophages in mice treated with the analogues. Interestingly, in the tumours from mice treated with PRI-2191 and PRI-2205, the expression of Tnfsf11 (RANKL) was increased. On the other hand, osteopontin (OPN) levels in plasma and tumour tissue, as well as TRAC5b levels in tumours, were diminished by calcitriol and its analogues. Despite a similar action of both analogues towards bone metabolism, their impact on vitamin D metabolism differed. In particular, PRI-2191 and calcitriol, not PRI-2205 treatment significantly diminished the levels of both 25(OH)D₃ and 24,25(OH)₂D₃. In conclusion, though there is evident antimetastatic activity in old OVX mice, signs of increased bone metabolism and deterioration of bone mineralization during therapy with calcitriol analogues were observed.

HE4 suppresses the expression of osteopontin in mononuclear cells and compromises their cytotoxicity against ovarian cancer cells

Ovarian cancers are known to evade immunosurveillance and to orchestrate a suppressive immune microenvironment. Here we examine the role of human epididymis protein 4 (HE4), an ovarian cancer biomarker, in immune evasion. Through modified subtractive hybridization analyses we have characterized the gene targets of HE4 in human peripheral blood mononuclear cells (PBMCs), and established a preliminary mechanism for HE4-mediated immune failure in ovarian tumours. Upon exposure of purified PMBCs to HE4, osteopontin (OPN) and dual-specificity phosphatase 6 (DUSP6) emerged as the most suppressed and up-regulated genes, respectively. SKOV3 and OVCAR8, human ovarian carcinoma cell lines, exhibited enhanced proliferation in conditioned media from HE4-exposed PBMCs, an effect that was attenuated by the addition of recombinant OPN or OPN-inducible cytokines [interleukin (IL)-12 and interferon (IFN)-Ɣ]. Additionally, upon co-culture with PBMCs, HE4-silenced SKOV3 cells were found to be more susceptible to cytotoxic cell death. The relationship between HE4 and OPN was reinforced further through the analysis of serous ovarian cancer patient samples. In these biopsy specimens, the number of OPN⁺ T cells correlated positively with progression free survival (PFS) and inversely with serum HE4 level. Taken together, these findings show that HE4 enhances ovarian cancer tumorigenesis by compromising OPN-mediated T cell activation.

Calcitriol and Its Analogs Establish the Immunosuppressive Microenvironment That Drives Metastasis in 4T1 Mouse Mammary Gland Cancer

In our previous study, calcitriol and its analogs PRI-2191 and PRI-2205 stimulated 4T1 mouse mammary gland cancer metastasis. Therefore, we aimed to analyze the inflammatory response in 4T1-bearing mice treated with these compounds. Gene expression analysis of the splenocytes and regional lymph nodes demonstrated prevalence of the T helper lymphocytes (Th2) response with an increased activity of regulatory T (Treg) lymphocytes in mice treated with these compounds. We also observed an increased number of mature granulocytes and B lymphocytes and a decreased number of TCD4⁺, TCD4⁺CD25⁺, and TCD8⁺, as well as natural killer (NK) CD335⁺, cells in the blood of mice treated with calcitriol and its analogs. Among the splenocytes, we observed a significant decrease in NK CD335⁺ cells and an increase in TCD8⁺ cells. Calcitriol and its analogs decreased the levels of interleukin (IL)-1β and IL-10 and increased the level of interferon gamma (IFN-γ) in the plasma. In the tumor tissue, they caused an increase in the level of IL-10. Gene expression analysis of lung tissue demonstrated an increased level of osteopontin (Spp1) and transforming growth factor β (TGF-β) mRNA. The expression of Spp1 was also elevated in lymph nodes. Calcitriol and its analogs caused prevalence of tumor-conducive changes in the immune system of 4T1 tumor-bearing mice, despite the induction of some tumor-disadvantageous effects.

Osteopontin is a multi-faceted pro-tumorigenic driver for central nervous system lymphoma

Osteopontin (OPN) is the most upregulated gene in primary central nervous system lymphoma (PCNSL) compared to non-CNS diffuse large B cell lymphoma (DLBCL). We show here that OPN is a key mediator of intracerebral tumor growth, invasion, and dissemination in CNS lymphoma, and that these effects depend upon activation of NF-κB. We further show that activation of NF-κB by OPN occurs through a unique mechanism in which intracellular OPN (iOPN) causes transcriptional downregulation of the NF-κB inhibitors, A20/TNFAIP3 and ABIN1/TNIP1, and secretory OPN (sOPN) promotes receptor-mediated activation of NF-κB. We also identify NF-κB-mediated induction of matrix metalloproteinase-8 (MMP-8) as a specific feature of OPN-mediated tissue invasion. These results implicate OPN as a candidate for development of targeted therapy for patients with PCNSL.

Immunomodulation Induced by Stem Cell Mobilization and Harvesting in Healthy Donors: Increased Systemic Osteopontin Levels after Treatment with Granulocyte Colony-Stimulating Factor

Peripheral blood stem cells from healthy donors mobilized by granulocyte colony-stimulating factor (G-CSF) and harvested by leukapheresis are commonly used for allogeneic stem cell transplantation. The frequency of severe graft versus host disease is similar for patients receiving peripheral blood and bone marrow allografts, even though the blood grafts contain more T cells, indicating mobilization-related immunoregulatory effects. The regulatory phosphoprotein osteopontin was quantified in plasma samples from healthy donors before G-CSF treatment, after four days of treatment immediately before and after leukapheresis, and 18–24 h after apheresis. Myeloma patients received chemotherapy, combined with G-CSF, for stem cell mobilization and plasma samples were prepared immediately before, immediately after, and 18–24 h after leukapheresis. G-CSF treatment of healthy stem cell donors increased plasma osteopontin levels, and a further increase was seen immediately after leukapheresis. The pre-apheresis levels were also increased in myeloma patients compared to healthy individuals. Finally, in vivo G-CSF exposure did not alter T cell expression of osteopontin ligand CD44, and in vitro osteopontin exposure induced only small increases in anti-CD3- and anti-CD28-stimulated T cell proliferation. G-CSF treatment, followed by leukapheresis, can increase systemic osteopontin levels, and this effect may contribute to the immunomodulatory effects of G-CSF treatment.

The osteopontin transgenic mouse is a new model for Sjögren's syndrome

Osteopontin (Opn) is a cytokine involved in both physiological and pathological processes, and is elevated in many autoimmune diseases. Sjögren's syndrome (SS) is an autoimmune disease with a strong female predilection characterized by lymphocytic infiltration of exocrine glands. We hypothesized that Opn contributes to SS pathogenesis. We examined an established SS model and found increased Opn locally and systemically. Next, we examined Opn transgenic (Opn Tg) mice for evidence of SS. Opn Tg animals exhibited lymphocytic infiltration of salivary and lacrimal glands, and Opn co-localized with the infiltrates. Moreover, saliva production was reduced, and SS autoantibodies were observed in the serum of these mice. Finally, female Opn Tg mice showed more severe disease compared to males. Taken together, these data support a role for Opn in SS pathogenesis. We identify a new model of spontaneous SS that recapitulates the human disease in terms of sex predilection, histopathology, salivary deficits, and autoantibodies.

Receptor crosstalk: reprogramming B cell receptor signalling to an alternate pathway results in expression and secretion of the autoimmunity-associated cytokine, osteopontin

Receptor crosstalk: reprogramming B cell receptor signalling to an alternate pathway results in expression and secretion of the autoimmunity-associated cytokine, osteopontin (Review). J Intern Med 2009; 265: 632-643.Intracellular signalling emanating from the B-cell antigen receptor is considered to follow a discrete course that requires participation by a set of mediators, grouped together as the signalosome, in order for downstream events to occur. Recent work indicates that this paradigm is true only for naïve B cells. Following engagement of the IL-4 receptor, a new, alternate pathway for B-cell receptor (BCR)-triggered intracellular signalling is established that bypasses the need for signalosome elements and operates in parallel with the classical, signalosome-dependent pathway. Reliance on Lyn and sensitivity to rottlerin by the former, but not the latter, distinguishes these two pathways. The advent of alternate pathway signalling leads to production and secretion by B cells of osteopontin (Opn). As Opn is a polyclonal B-cell activator that is strongly associated with a number of autoimmune diseases including lupus and rheumatoid arthritis, this novel finding is likely to be clinically relevant. Our results highlight the potential role of B-cell-derived Opn in immunity and autoimmunity and suggest that stress-related IL-4 expression might act to strengthen immunoglobulin secretion at the risk of autoantibody formation. Further, these results illustrate receptor crosstalk in the form of reprogramming, whereby engagement of one receptor (IL-4R) produces an effect that persists after the original ligand (IL-4) is removed and results in alteration of the pathway, and outcome, of signalling via a second receptor (BCR) following its activation.