Identification of a costimulatory molecule rapidly induced by CD40L as CD44H

Increased expression of surface CD44 in hypoxia-DCs skews helper T cells toward a Th2 polarization

A low partial oxygen pressure (hypoxia) occurs in many pathological environments, such as solid tumors and inflammatory lesions. Understanding the cellular response to hypoxic stress has broad implications for human diseases. As we previously reported, hypoxia significantly altered dendritic cells (DCs) to a DC2 phenotype and promoted a Th2 polarization of naïve T cells with increased IL-4 production. However, the underlying mechanisms still remain largely unknown. In this study, we found the over-expression of surface CD44 in DCs was involved in this process via ligand binding. Further investigation showed hypoxia could reduce the surface expression of membrane type 1 metalloprotease (MT1-MMP) via down-regulating the kinesin-like protein KIF2A, which subsequently alleviated the shedding of CD44 from DCs. Moreover, KIF2A expression was found negatively regulated by HIF-1α in hypoxic microenvironment. These results suggest a previously uncharacterized mechanism by which hypoxia regulates the function of DCs via KIF2A/MT1-MMP/CD44 axis, providing critical information to understand the immune response under hypoxia.

Differences in CD44 Surface Expression Levels and Function Discriminates IL-17 and IFN-γ Producing Helper T Cells

CD44 is a prominent activation marker which distinguishes memory and effector T cells from their naïve counterparts. It also plays a role in early T cell signaling events as it is bound to the lymphocyte-specific protein kinase and thereby enhances T cell receptor signalling. Here, we investigated whether IFN-γ and IL-17 producing T helper cells differ in their CD44 expression and their dependence of CD44 for differentiation. Stimulation of CD4+ T cells with allogeneic dendritic cells resulted in the formation of three distinguishable populations: CD44+, CD44++ and CD44+++. In vitro and in vivo generated allo-reactive IL-17 producing T helper cells were mainly CD44+++ as compared to IFN-γ+ T helper cells, which were CD44++. This effect was enhanced under polarizing conditions. T helper 17 polarization led to a shift towards the CD44+++ population, whereas T helper 1 polarization diminished this population. Furthermore, blocking CD44 decreased IL-17 secretion, while IFN-γ was barely affected. Titration experiments revealed that low T cell receptor and CD28 stimulation supported T helper 17 rather than T helper 1 development. Under these conditions CD44 could act as a co-stimulatory molecule and replace CD28. Indeed, rested CD44+++CD4+ T cells contained already more total and especially phosphorylated zeta-chain-associated protein kinase 70 as compared to CD44++ cells. Our results support the notion, that CD44 enhances T cell receptor signaling strength by delivering lymphocyte-specific protein kinase, which is required for induction of IL-17 producing T helper cells.

CD44-deficiency on hematopoietic cells limits T-cell number but does not protect against atherogenesis in LDL receptor-deficient mice

OBJECTIVE

Vascular and inflammatory cells express adhesion molecule CD44. We demonstrated previously that enhanced CD44 localizes in human atherosclerotic lesions. Apolipoprotein E/cd44 double-deficient mice and apolipoprotein E-deficient mice transplanted with CD44-deficient bone marrow (BM) exhibit reduced atherosclerosis. Since CD44 is a novel factor in atherogenesis, it is imperative that it is investigated in more than one animal model to conclusively determine its role in this particular disease pathology. To test the hypothesis that CD44 expressed by hematopoietic cells plays a critical role in atherogenesis in the low density lipoprotein (LDL) receptor-deficient mouse model, we performed BM reconstitution experiments.

METHODS

Lethally irradiated LDL receptor-deficient mice were transplanted with either CD44-deficient or wild-type BM. Beginning 10 weeks after successful reconstitution, mice consumed a cholesterol-enriched atherogenic diet for 6 or 11 weeks.

RESULTS

Surprisingly, CD44-deficiency on BM-derived inflammatory cells did not affect lesion size. Additionally, neither group displayed differences in smooth muscle cell, macrophage, collagen, or elastin content as well as lipoprotein levels. However, lesions in CD44-deficient BM-recipient mice contained fewer T-cells compared to wild-type BM mice. Interestingly, CD44-deficient T-cells expressed less chemokine receptor-5 mRNA. Furthermore, in vivo leukocyte adhesion decreased in CD44-deficient mice compared to wild-type mice.

CONCLUSION

This study surprisingly revealed that atherogenesis does not require CD44 expression on hematopoietic cells in the LDL receptor-deficient mouse model. However, CD44 promotes T-cell recruitment, downregulates chemokine receptor-5, and participates critically in leukocyte adhesion in vivo. Consequently, the anti-atherogenic role of CD44 may require CD44-deficiency on cell types other than inflammatory cells in the LDL receptor-deficient mouse model.

Why can't the immune system control HIV-1? Defining HIV-1-specific CD4+ T cell immunity in order to develop strategies to enhance viral immunity

Globally, at least 60 million people have been infected with the human immunodeficiency virus type 1 (HIV-1), the majority of whom will develop the acquired immunodeficiency syndrome (AIDS) leading to tremendous morbidity and the mortality. Understanding the immunopathogenesis of AIDS and the immune correlates of viral protection are necessary to develop effective vaccines and immunotherapies. A major focus of our laboratory has been to understand the CD4+ T cell immune response directed against HIV- 1, and to determine mechanisms of T cell dysfunction that lead to viral escape. In addition, we are interested in evaluating the TNF-TNFR family members as potential molecular adjuvants that could be incorporated into vaccines which could be used to further boost T cell immunogenicity in healthy or HIV-1-infected individuals, as many of these molecules have been shown to replace the functions of CD4+ T cell help.

OX40 ligation of CD4+ T cells enhances virus-specific CD8+ T cell memory responses independently of IL-2 and CD4+ T regulatory cell inhibition

We have previously shown that CD4(+) T cells are required to optimally expand viral-specific memory CD8(+) CTL responses using a human dendritic cell-T cell-based coculture system. OX40 (CD134), a 50-kDa transmembrane protein of the TNFR family, is expressed primarily on activated CD4(+) T cells. In murine models, the OX40/OX40L pathway has been shown to play a critical costimulatory role in dendritic cell/T cell interactions that may be important in promoting long-lived CD4(+) T cells, which subsequently can help CD8(+) T cell responses. The current study examined whether OX40 ligation on ex vivo CD4(+) T cells can enhance their ability to "help" virus-specific CTL responses in HIV-1-infected and -uninfected individuals. OX40 ligation of CD4(+) T cells by human OX40L-IgG1 enhanced the ex vivo expansion of HIV-1-specific and EBV-specific CTL from HIV-1-infected and -uninfected individuals, respectively. The mechanism whereby OX40 ligation enhanced help of CTL was independent of the induction of cytokines such as IL-2 or any inhibitory effect on CD4(+) T regulatory cells, but was associated with a direct effect on proliferation of CD4(+) T cells. Thus, OX40 ligation on CD4(+) T cells represents a potentially novel immunotherapeutic strategy that should be investigated to treat and prevent persistent virus infections, such as HIV-1 infection.

Mouse B cell activation is inhibited by CD44 cross-linking

Lymphocyte activation and trafficking are indispensable to the immune system. CD44 is an adhesion molecule with known importance in T cell activation, lymphocyte trafficking, and tumor metastasis. Although CD44 has been shown to participate in the activation, rolling and adhesion, and homing of T cells, the role of CD44 on B cells is relatively unknown. The effects of CD44 cross-linking on murine B cell activation via CD40L was explored using the anti-CD44 mAbs RK3G9 and IM7. When immobilized on a plate, both RK3G9 and IM7 were found to strongly inhibit B cell proliferation and Ig production, especially at lower cell input concentrations. IgE inhibition was especially prominent. In contrast, soluble RK3G9 added to the B cell cultures had no effect. The inhibitory effect of anti-CD44 on B cell activation was not influenced by the addition of the anti-FcgammaRII, indicating that Fc cross-linking did not play a role in this inhibition. As Ig production requires several days for both B cell proliferation and differentiation to occur, the effects of delayed addition of immobilized anti-CD44 mAbs were studied, and the results indicated no inhibition after 96 hrs of culture. Finally, B cells were activated by either LPS or anti-IgM F(ab')2. While LPS-induced B cell activation was inhibited by immobilized anti-CD44 mAbs, anti-IgM activation was refractory. Interestingly, addition of both anti-IgM and CD40L or LPS resulted in some modulation of the inhibitory activity. These results suggest that CD44 cross-linking could control polyclonal B cell activation by CD40L, but allow sIgM/CD40L activation to continue.

The regulation and activation of CD44 by natural killer (NK) cells and its role in the production of IFN-gamma

Natural killer (NK) cells can express high levels of CD44, and signaling through CD44 has been shown to enhance NK cell cytotoxic activity. However, little is known about the factors that regulate CD44-mediated activation of NK cells. The studies reported here reveal that resting NK cells constitutively express CD44 that is in an inactive form that does not bind to hyaluronan (HA), the principal known ligand for CD44. After infection of mice with the intracellular parasite Toxoplasma gondii, however, a population of NK cells that expressed activated CD44 emerged. To determine how expression and activation of CD44 by resting NK cells were regulated, the role of cytokines in these events was assessed. These studies revealed that whereas stimulation of resting NK cells with interleukin-12 (IL-12) or IL-18 caused increased expression of CD44, only IL-2 or IL-15 led to the upregulation and activation of CD44. The cytokine-induced upregulation and activation of CD44 was independent of NK cell proliferation. To determine the functional consequences of CD44 activation, the effects of low molecular weight HA (LMWHA) on the production of interferon-gamma (IFN-gamma) by IL-2-activated NK cells were assessed. These studies showed that HA alone had little effect on the production of IFN-gamma, but when used in combination with IL-2, IL-12, or IL-18, LMWHA was a potent enhancer of IFN-gamma production. Together, these studies indicate an important role for proinflammatory cytokines in the activation of CD44 on NK cells and identify a novel pathway to enhance the ability of activated NK cells to produce IFN-gamma.

Role of CD44 and hyaluronic acid (HA) in activation of alloreactive and antigen-specific T cells by bone marrow-derived dendritic cells

In the current study, the role played by hyaluronic acid (HA) and its receptor CD44 on the activation and functions of dendritic cells (DCs) was investigated. Activation of DCs with HA enhanced their ability to stimulate allogeneic and antigen (Ag)-specific T cells markedly. HA treatment upregulated the expression of costimulatory molecules such as CD40, CD80, and CD86 on DCs. Cell mixing experiments using DC or T cells from CD44 wild-type or CD44 knockout mice as well as blocking studies with anti-CD44 monoclonal antibodies revealed that CD44 expression on T cells but not DC played a critical role in Ag-specific T-cell responsiveness. Also, the HA-induced activation of DC was independent of CD44. When conjugate formation between Ag-pulsed DCs and Ag-specific T cells was studied, the deficiency of CD44 on T cells rather than on DCs was found to play a key role in T-cell-DC interaction. Together, these data demonstrated that HA can activate DC independently of CD44; however, CD44 expressed on Ag-specific T cells plays a critical role in its interaction with DC and resultant expansion of T cells.

The Role of the p38 Mitogen-Activated Protein Kinase, Extracellular Signal-Regulated Kinase, and Phosphoinositide-3-OH Kinase Signal Transduction Pathways in CD40 Ligand-Induced Dendritic Cell Activation and Expansion of Virus-Specific CD8+T Cell Memory Responses

Mature dendritic cells (DCs) are central to the development of optimal T cell immune responses. CD40 ligand (CD40L, CD154) is one of the most potent maturation stimuli for immature DCs. We studied the role of three signaling pathways, p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), and phosphoinositide-3-OH kinase (PI3K), in CD40L-induced monocyte-derived DC activation, survival, and expansion of virus-specific CD8(+) T cell responses. p38 MAPK pathway was critical for CD40L-mediated up-regulation of CD83, a marker of DC maturation. CD40L-induced monocyte-derived DC IL-12 production was mediated by both the p38 MAPK and PI3K pathways. CD40L-mediated DC survival was mostly mediated by the PI3K pathway, with smaller contributions by p38 MAPK and ERK pathways. Finally, the p38 MAPK pathway was most important in mediating CD40L-stimulated DCs to induce strong allogeneic responses as well as expanding virus-specific memory CD8(+) T cell responses. Thus, although the p38 MAPK, PI3K, and ERK pathways independently affect various parameters of DC maturation induced by CD40L, the p38 MAPK pathway within CD40L-conditioned DCs is the most important pathway to maximally elicit T cell immune responses. This pathway should be exploited in vivo to either completely suppress or enhance CD8(+) T cell immune responses.

G-CSF as immune regulator in T cells expressing the G-CSF receptor: implications for transplantation and autoimmune diseases

Results from experimental models, in vitro studies, and clinical data indicate that granulocyte colony-stimulating factor (G-CSF) stimulation alters T-cell function and induces Th2 immune responses. The immune modulatory effect of G-CSF on T cells results in an unexpected low incidence of acute graft-versus-host disease in peripheral stem cell transplantation. However, the underlying mechanism for the reduced reactivity and/or alloreactivity of T cells upon G-CSF treatment is still unknown. In contrast to the general belief that G-CSF acts exclusively on T cells via monocytes and dendritic cells, our results clearly show the expression of the G-CSF receptor in class I- and II- restricted T cells at the single-cell level both in vivo and in vitro. Kinetic studies demonstrate the induction and functional activity of the G-CSF receptor in T cells upon G-CSF exposure. Expression profiling of T cells from G-CSF-treated stem cell donors allowed identification of several immune modulatory genes, which are regulated upon G-CSF administration in vivo (eg, LFA1-alpha, ISGF3-gamma) and that are likely responsible for the reduced reactivity and/or alloreactivity. Most importantly, the induction of GATA-3, the master transcription factor for a Th2 immune response, could be demonstrated in T cells upon G-CSF treatment in vivo accompanied by an increase of spontaneous interleukin-4 secretion. Hence, G-CSF is a strong immune regulator of T cells and a promising therapeutic tool in acute graft-versus-host disease as well as in conditions associated with Th1/Th2 imbalance, such as bone marrow failure syndromes and autoimmune diseases.

CD44v7 interferes with activation-induced cell death by up-regulation of anti-apoptotic gene expression

Blockade of CD44v7 was described to cure trinitrobenzene sulfonic acid-induced colitis, a disease not developed by mice with targeted deletion of the CD44v7 exon. There was evidence for a reduction in activation-induced cell death on lamina propria lymphocytes of control as compared with CD44v7-deficient mice. To elucidate the mechanism underlying the relative apoptosis resistance of CD44v7-competent as compared with CD44v7-deficient lymphocytes, T cell activation and induction of apoptosis were analyzed on mesenteric lymph node cells and Peyer's patch lymphocytes of CD44v7-deficient and CD44v4-v7-transgenic mice, which overexpress rat CD44v4-v7 on T lymphocytes. CD44v7 deficiency was characterized by an increase in the percentage of apoptotic cells after stimulation, increased numbers of CD95L- and CD152-positive cells, low levels of the anti-apoptotic proteins Bcl-2 and Bcl-Xl, and decreased phosphorylation of the pro-apoptotic protein BAD. Also, lymphocytes from CD44v4-v7-transgenic mice displayed reduced levels of CD95L, low numbers of apoptotic cells, and constitutively elevated levels of Bcl-Xl. When stimulating lymphocytes by CD3 cross-linking, CD44v7 was not recruited toward the immunological synapse and preferentially associated with the cytoskeletal-linker protein ezrin. Thus, as opposed to the CD44 standard isoform, CD44v7 does not function as an accessory molecule; instead, it supports survival of activated T cells by interfering with activation-induced cell death.

Targeting dendritic cells with CD44 monoclonal antibodies selectively inhibits the proliferation of naive CD4+ T-helper cells by induction of FAS-independent T-cell apoptosis

CD44 is a multifunctional adhesion molecule that has been shown to be a costimulatory factor for T-cell activation in vitro and in vivo. The aim of the present study was to expand these findings by characterizing the role of CD44 during dendritic cell (DC) antigen presentation to naive, resting T cells. Certain monoclonal antibodies (mAbs) directed against all CD44 isoforms (pan CD44), or against the epitope encoded by the alternatively spliced exon v4 (CD44v4), dose-dependently inhibited the capacity of murine DC to induce proliferation of naive alloreactive T cells. Preincubation of the T cells or DC with these CD44 mAbs revealed that the effect was dependent upon mAb binding to DC, but not to T cells. DC treated with anti-pan CD44 and anti-CD44v4 mAbs induced CD4+ T-cell apoptosis, as shown by annexin V staining and TdT-mediated biotin-dUTP nick-end labelling (TUNEL) assays. However, CD4+ T-cell apoptosis was not dependent on the Fas/Fas ligand (Fas/FasL) system, as DC from FasL-deficient (Gld) mice and T cells from Fas-deficient (Lpr) mice were still susceptible to apoptosis induced by CD44-treated DC. To investigate whether CD44 treatment of DC affects early T-cell/DC interactions, time-lapse video microscopy was performed using peptide-specific T cells from T-cell receptor (TCR) transgenic mice. Interestingly, calcium signalling in CD4+ T cells was significantly diminished following interaction with CD44 mAb-treated DC, but this was not observed in CD8+ T cells. Taken together, we found that perturbation of distinct epitopes of CD44 on DC interfere with early Ca2+ signalling events during the activation of CD4+ T cells, resulting in T-cell apoptosis.

T cell costimulatory pathways: blockade for autoimmunity

The activation of T lymphocytes is thought to require at least two signals, one delivered by the T cell receptor (TCR) complex after antigen recognition, and one provided on engagement of costimulatory receptors. The B7-1/B7-2-CD28/cytotoxic T lymphocyte antigen-4 (CTLA-4) and CD154-CD40 pathways have been shown to be crucial in regulating T cell activation and tolerance. Novel members of the B7-CD28 superfamily have recently been discovered and they seem to be particularly important for regulating the responses of previously activated T cells. Superimposition of inhibitory signals like those delivered by CTLA-4 and programmed death (PD)-1-PD-1-ligand (PD-L1) pathway leads to a complex network of positive and negative costimulatory signals, the integration of which modulates immune responses. Furthermore, expression of several B7 homologues on cells other than professional antigen-presenting cells (APCs), indicate new mechanisms for regulating T cell responses in peripheral tissues. This review focuses on our current understanding of the members of the B7-CD28 superfamily and discusses their therapeutic potential.

CD44-stimulated human B cells express transcripts specifically involved in immunomodulation and inflammation as analyzed by DNA microarrays

A number of studies have implicated a role for the cell surface glycoprotein CD44 in several biologic events, such as lymphopoiesis, homing, lymphocyte activation, and apoptosis. We have earlier reported that signaling via CD44 on naive B cells in addition to B-cell receptor (BCR) and CD40 engagement generated a germinal center-like phenotype. To further characterize the global role of CD44 in B differentiation, we examined the expression profile of human B cells cultured in vitro in the presence or absence of CD44 ligation, together with anti-immunoglobulin (anti-Ig) and anti-CD40 antibodies. The data sets derived from DNA microarrays were analyzed using a novel statistical analysis scheme created to retrieve the most likely expression pattern of CD44 ligation. Our results show that genes such as interleukin-6 (IL-6), IL-1alpha, and beta(2)-adrenergic receptor (beta(2)-AR) were specifically up-regulated by CD44 ligation, suggesting a novel role for CD44 in immunoregulation and inflammation.

Cooperation of TNF family members CD40 ligand, receptor activator of NF-kappa B ligand, and TNF-alpha in the activation of dendritic cells and the expansion of viral specific CD8+ T cell memory responses in HIV-1-infected and HIV-1-uninfected individuals

Members of the TNF superfamily have been shown to be instrumental in enhancing cell-mediated immune responses, primarily through their interactions with dendritic cells (DCs). We systematically evaluated the ability of three TNF superfamily molecules, CD40 ligand (CD40L), receptor activator of NF-kappaB ligand (RANKL), and TNF-alpha, to expand ex vivo EBV-specific CTL responses in healthy human individuals and ex vivo HIV-1-specific CTL responses in HIV-1-infected individuals. In both groups of individuals, we found that all three TNF family molecules could expand CTL responses, albeit at differing degrees. CD40L treatment alone was better than RANKL or TNF-alpha alone to mature DCs and to expand CTL. In healthy volunteers, TNF-alpha or RANKL could cooperate with CD40L to maximize the ability of DCs to expand virus-specific CTL responses. In HIV-1 infection, cooperative effects between TNF-alpha or RANKL in combination with CD40L were variable. TNF-alpha and RANKL cooperated with CD40L via differing mechanisms, i.e., TNF-alpha enhanced IL-12 production, whereas RANKL enhanced survival of CD40L-stimulated DCs. These findings demonstrate that optimal maturation of DCs requires multiple signals by TNF superfamily members that include CD40L. In HIV-1 infection, DCs may only require CD40L to maximally expand CTL. Finally, CTL responses were higher in CD4(+) T cell-containing conditions even in the presence of TNF family molecules, suggesting that CD4(+) T cells can provide help to CD8(+) T cells independently of CD40L, RANKL, or TNF-alpha.

The transmembrane form of TNF-alpha drives autoantibody production in the absence of CD154: studies using MRL/Mp-Fas(lpr) mice

It is generally accepted that the interaction between CD40 and its ligand (CD154) plays a decisive role in contact-dependent help for T and B cells. In CD154-deficient MRL/Mp-Fas(lpr) (MRL/lpr) mice, however, high titres of IgG2a-type autoantibodies against small nuclear ribonucleoproteins (snRNPs) are observed. We successfully isolated two CD154-deficient MRL/lpr Th1 lines, which could provide B cell help for anti-snRNP antibody production. The proliferative responses of the Th1 cell lines were MHC class II (I-Ek)-restricted. Although syngeneic B cell proliferation was induced by Th1 lines in both a contact-dependent and -independent manner, the soluble form of TNF-alpha (sTNF-alpha) was not involved in contact-independent B cell proliferation. On the other hand, both anti-TNF-alpha and TNF-receptor 2 (TNF-R2, p75) monoclonal antibody (MoAb) blocked contact-dependent B cell proliferation, suggesting that the transmembrane form of TNF-alpha (mTNF-alpha)-TNF-R2 co-stimulation participates in B cell activation. Similarly, anti-TNF-alpha and TNF-R2 MoAb inhibited anti-snRNP antibody production in vitro, but anti-CD154 or TNF-R1 MoAb did not. These results indicate that the interaction of mTNF-alpha on activated Th1 cells with TNF-R2 on B cells may be involved in the autoimmunity seen in MRL mice, and that the blockade of CD40-CD154 co-stimulation may not always be able to suppress some Th1-related manifestations of lupus.

Nitric oxide limits the expansion of antigen-specific T cells in mice infected with the microfilariae of Brugia pahangi

Infection of BALB/c mice with the microfilariae (Mf) of the filarial nematode Brugia pahangi results in an antigen-specific proliferative defect that is induced by high levels of NO. Using carboxyfluorescein diacetate succinimydl ester and cell surface labeling, it was possible to identify a population of antigen-specific T cells from Mf-infected BALB/c mice that expressed particularly high levels of CD4 (CD4(hi)). These cells proliferated in culture only when inducible NO synthase was inhibited and accounted for almost all of the antigen-specific proliferative response under those conditions. CD4(hi) cells also expressed high levels of CD44, consistent with their status as activated T cells. A similar population of CD4(hi) cells was observed in cultures from Mf-infected gamma interferon receptor knockout (IFN-gammaR(-/-)) mice. Terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling staining revealed that the CD4(+) T cells from Mf-infected wild-type mice were preferentially susceptible to apoptosis compared to CD4(+) T cells from IFN-gammaR(-/-) mice. These studies suggest that the expansion of antigen-specific T cells in Mf-infected mice is limited by NO.

A role for inducible costimulator protein in the CD28- independent mechanism of resistance to Toxoplasma gondii

Long-term resistance to Toxoplasma gondii is dependent on the development of parasite-specific T cells that produce IFN-gamma. CD28 is a costimulatory molecule important for optimal activation of T cells, but CD28(-/-) mice are resistant to T. gondii, demonstrating that CD28-independent mechanisms regulate T cell responses during toxoplasmosis. The identification of the B7-related protein 1/inducible costimulator protein (ICOS) pathway and its ability to regulate the production of IFN-gamma suggested that this pathway may be involved in the CD28-independent activation of T cells required for resistance to T. gondii. In support of this hypothesis, infection of wild-type or CD28(-/-) mice with T. gondii resulted in the increased expression of ICOS by activated CD4(+) and CD8(+) T cells. In addition, both costimulatory pathways contributed to the in vitro production of IFN-gamma by parasite-specific T cells and when both pathways were blocked, there was an additive effect that resulted in almost complete inhibition of IFN-gamma production. Although in vivo blockade of the ICOS costimulatory pathway did not result in the early mortality of wild-type mice infected with T. gondii, it did lead to increased susceptibility of CD28(-/-) mice to T. gondi associated with reduced serum levels of IFN-gamma, increased parasite burden, and increased mortality compared with the control group. Together, these results identify a critical role for ICOS in the protective Th1-type response required for resistance to T. gondii and suggest that ICOS and CD28 are parallel costimulatory pathways, either of which is sufficient to mediate resistance to this intracellular pathogen.

Transient CD44 variant isoform expression and reduction in CD4(+)/CD25(+) regulatory T cells in C3H/HeJ mice with alopecia areata

Alopecia areata, an autoimmune disease affecting anagen stage hair follicles, can be induced by grafting spontaneous alopecia areata affected skin to normal-haired C3H/HeJ mice. As the onset of alopecia areata can be significantly retarded by anti-CD44 variant isoform 10 treatment, it was interesting to explore the underlying disease mechanism. Two weeks after transplanting alopecia areata affected skin, expression of CD44 variant isoforms 3, 6, 7, and 10 was strikingly upregulated as compared with sham-grafted mice. By 6 wk after grafting, CD44 variant isoform levels had returned to normal, whereas in draining lymph nodes, CD44 variant isoform expression was slightly decreased. Leukocytes in the skin of mice with chronic alopecia areata expressed a hematopoietic isoform of CD44 and CD44 variant isoform 6 at an elevated level, but CD44 variant isoform 3 expression was reduced. Cytokine expression in leukocytes of chronic alopecia areata affected skin was higher than in normal-haired controls. Cytokine expression also increased postsurgery in sham and alopecia areata grafted mice, but remained elevated only in mice receiving alopecia areata affected skin. Finally, from the skin of mice with chronic alopecia areata and of mice transplanted with alopecia areata affected skin, an increased number of CD4(+) and CD8(+) cells, but a strongly decreased number of CD4(+)/CD25(+) regulatory T cells was recovered. Thus, expression of CD44 variant isoforms is important for the migration of leukocytes during the initial period of alopecia areata. CD44, however, is apparently not involved in the maintenance of the disease state, which is characterized by high cytokine expression levels, an increased number of CD4(+) and CD8+ cells, but a low level of CD4(+)/CD25(+) suppressor cells.

CD40/CD40L interaction is essential for the induction of EAE in the absence of CD28-mediated co-stimulation

CD28 provides a co-stimulatory signal critical for optimal T cell activation. We and others have shown that the B7/CD28 co-stimulatory pathway is a major regulatory pathway for the control of immune responses. Experimentally induced models of autoimmunity have been shown to be prevented or reduced in intensity in mice deficient for CD28. Here, we show that EAE and accompanying neuroantigen-specific immune responses are drastically reduced in the absence of CD28. However, we go on to show that EAE can be induced in CD28-deficient mice following two immunizations. After re-immunization, CD28-deficient mice develop severe EAE with myelin-specific responses equal to those of wildtype controls, and extensive demyelination in the spinal cord. Treatment of CD28-deficient mice with anti-CD40L at the time of immunization significantly reduced DTH responses and prevented the development of EAE following two immunizations, indicating a critical role for CD40/CD40L signaling in the absence of CD28. Taken together, our results indicate that CD28-mediated co-stimulation does not regulate immunological anergy. Instead, CD28 appears to adjust the threshold for activation and expansion of autoreactive cells.

Cis elements for transporter associated with antigen-processing-2 transcription: two new promoters and an essential role of the IFN response factor binding element in IFN-gamma-mediated activation of the transcription initiator

Expression of cell surface MHC class I:peptide complex requires coordinated expression of multiple genes such as MHC class I heavy chain, beta(2)-microglobulin (beta(2)m), transporters associated with antigen-processing (TAP)-1 and TAP-2, and proteosomal components low-molecular weight polypeptide (LMP)-2 and LMP-7. All of these genes are expressed at defined and distinct levels in normal tissues, and are inducible by IFN-gamma. While the cis elements involved in transcription of the MHC class I heavy chain, beta(2)m, TAP-1 and LMP-2 have been analyzed extensively, those for TAP-2 and LMP-7 have not been well studied. Here we systematically analyzed the cis elements for TAP-2 transcription. We found at least two independent elements that are sufficient to activate transcription of a reporter gene. One (hereby called TAP-2 P1) is located 5' to the TAP-2 exon 1, while the other (hereby called TAP-2 P2) is a transcription initiator residing in intron 1. Analysis of the 5' sequence of TAP-2 mRNA indicates that both promoters are active. Moreover, while the TAP-2 promoter region contains cis elements that can mediate TAP-2 induction by IFN-gamma, such as gamma-activation site and IFN response factor binding element (IRFE), only the IRFE is required for IFN-gamma induction of TAP-2 promoter in vitro. The IRFE appears to work as an enhancer for the initiator (P2). Together with another promoter recently identified by others, TAP-2 therefore has three independent promoters that can be differentially regulated.

Decreased CD44v6 expression in lamina propria lymphocytes of patients with inflammatory bowel disease

Splice variants of the glycoprotein CD44 are transiently expressed on lymphocytes during T cell activation. Increased expression of CD44v6 on peripheral blood lymphocytes (PBL) of patients with inflammatory bowel disease (IBD) was described recently. The aim of this study was therefore to characterize CD44v6 expression on CD4(+) lamina propria lymphocytes (LPL) of patients with active IBD in comparison to controls. CD44v6 expression on CD4(+) LPL (n = 19) of controls and patients with active IBD (Crohn's disease n = 14, ulcerative colitis n = 15) was analyzed by flow cytometry and compared to that on autologous PBL. Thereby, in vitro regulation of CD44v6 on LPL and PBL via CD3 and CD2 and the costimulatory signal B7-1 was examined. In addition, the role of protein kinase C (PKC) in CD44v6 expression was tested. CD44v6 expression was increased in CD4(+) LPL (median, 45%) compared to PBL (median, 38%). Surprisingly, in IBD CD44v6 was downregulated on CD4(+) lamina propria T cells, irrespective of their state of inflammation (median, 28%). CD44v6 expression on LPL was not upregulated upon CD3 activation alone but following costimulation with B7-1. However, CD2-mediated T cell activation sufficiently induced upregulation of CD44v6 on LPL and PBL. In our study, downregulation of CD44v6 on LPL of patients with IBD was not due to defective PKC activation. Taken together, these data indicate that decreased CD44v6 expression on LPL in IBD might be a feature of an inappropriate costimulatory signal in T cell activation.

Characterization of endogenous Chinese hamster ovary cell surface molecules that mediate T cell costimulation

Chinese hamster ovary (CHO) cells are commonly used in the generation of transfectants for use in in vitro costimulation assays. However, we have noted that nontransfected CHO cells can themselves provide a low-level B7/CD28 independent costimulatory signal for CD3-mediated murine T cell activation and IL-2 production. This study set out to identify those molecules that contribute to this CHO-dependent costimulatory activity. We describe a CHO subline capable of delivering potent CD28-independent costimulation to murine T cells and the generation of monoclonal antibodies against these CHO cells that inhibited this costimulatory activity. These blocking antibodies do not affect CHO cell-independent costimulation or bind mouse cells, suggesting an effect mediated by their target molecules on the costimulatory competent CHO cells. Immunoprecipitation and expression cloning revealed that these antibodies bound the hamster homologues of Crry (CD21/35), CD44, CD54 (ICAM-1), CD63, CD87, CD147, and an 80- to 90-kDa protein which could not be cloned. Expression of these hamster genes on COS cells demonstrated that hamster CD54 was able to costimulate both CD3-mediated IL-2 secretion and T cell proliferation by naive murine T cells independent of the other molecules identified.

The adhesion receptor CD44 promotes atherosclerosis by mediating inflammatory cell recruitment and vascular cell activation

Atherosclerosis causes most acute coronary syndromes and strokes. The pathogenesis of atherosclerosis includes recruitment of inflammatory cells to the vessel wall and activation of vascular cells. CD44 is an adhesion protein expressed on inflammatory and vascular cells. CD44 supports the adhesion of activated lymphocytes to endothelium and smooth muscle cells. Furthermore, ligation of CD44 induces activation of both inflammatory and vascular cells. To assess the potential contribution of CD44 to atherosclerosis, we bred CD44-null mice to atherosclerosis-prone apoE-deficient mice. We found a 50-70% reduction in aortic lesions in CD44-null mice compared with CD44 heterozygote and wild-type littermates. We demonstrate that CD44 promotes the recruitment of macrophages to atherosclerotic lesions. Furthermore, we show that CD44 is required for phenotypic dedifferentiation of medial smooth muscle cells to the "synthetic" state as measured by expression of VCAM-1. Finally, we demonstrate that hyaluronan, the principal ligand for CD44, is upregulated in atherosclerotic lesions of apoE-deficient mice and that the low-molecular-weight proinflammatory forms of hyaluronan stimulate VCAM-1 expression and proliferation of cultured primary aortic smooth muscle cells, whereas high-molecular-weight forms of hyaluronan inhibit smooth muscle cell proliferation. We conclude that CD44 plays a critical role in the progression of atherosclerosis through multiple mechanisms.

Inhibition of the function of the FcgammaRIIB by a monoclonal antibody to thymic shared antigen-1, a Ly-6 family antigen

Thymic shared antigen-1 (TSA-1) is a member of the Ly-6 family of glycosyl-phosphatidylinositol (GPI)-linked proteins. While it has been proposed that TSA-1 may play a role in thymic development, a physiological ligand for this antigen has not been identified. Here we report that a monoclonal antibody (mAb) to TSA-1, generated by immunizing a hamster with CD40 ligand (CD40L)-activated B cells, interferes with the function of FcgammaRIIB on splenic B cells and the B-cell lymphoma cell line, M12, by binding to TSA on the same cells. The interaction of anti-TSA with FcgammaRIIB resulted in an inhibition of the ability of the FcgammaRIIB to cross-link and/or aggregate soluble anti-CD3 or soluble anti-Cbeta T-cell receptor (TCR), leading to an inhibition of induction of expression of CD25 and CD69, interleukin (IL)-2 production and proliferation of naive T cells. Cross-blocking studies with mAbs strongly suggested that a physical association exists between TSA-1 and the FcgammaRIIB on the surface of activated B cells and favour the view that a functional intermolecular association exists between these two distinct membrane antigens.

A role for CD44 in the production of IFN-gamma and immunopathology during infection with Toxoplasma gondii

The interaction of activated CD44 with its ligand, low m.w. hyaluronan, is involved in inflammation, but no role has been identified for this interaction in the regulation of an immune response to infection. In these studies, infection of C57BL/6 mice with Toxoplasma gondii resulted in increased expression of CD44 on T cells, B cells, NK cells, and macrophages, and a small percentage of CD4(+) T cells express an activated form of CD44. Administration of anti-CD44 to infected mice prevented the development of a CD4(+) T cell-dependent, infection-induced inflammatory response in the small intestine characterized by the overproduction of IFN-gamma. The protective effect of anti-CD44 treatment was associated with reduced production of IFN-gamma, but not IL-12, in vivo and in vitro. Furthermore, the addition of low m.w. hyaluronan to cultures of splenocytes or purified CD4(+) T cells from infected mice resulted in the production of high levels of IFN-gamma, which was dependent on IL-12 and TCR stimulation. Together, these results identify a novel role for CD44 in the regulation of IFN-gamma production by CD4(+) T cells during infection and demonstrate a role for CD44 in the regulation of infection-induced immune pathology.

Validity of the two-signal model for activation of CD28-deficient T lymphocytes: quantitative characterization of an alternative costimulatory function of dendritic cells

The observation that primary T-dependent immune responses are generated in mice lacking CD28, the only receptor definitively shown to costimulate naive T cells, has led to ambiguity as to whether costimulation is absolutely required for initiation of T-cell responses. In this report, in vitro analysis of the relationship between cell density and proliferation demonstrates that activation of CD28-/- T cells to immobilized T-cell receptor (TCR)-alpha monoclonal antibodies (MoAb) depends on costimulatory signals provided by other cells in culture and occurs only at cell densities sufficient to permit these intercellular interactions. These signals are necessary even under TCR triggering conditions that obviate the CD28 requirement. Dendritic cells (DCs) provide the necessary costimulation in vitro and prime T cells in vivo in CD28-/- mice. Single-cell and limiting dilution analyses indicate that individual T cells from normal and CD28-/- mice produce equivalent interleukin (IL)-2 in response to DCs. However, half as many T cells produce IL-2 when only the CD28-independent pathway is used. Nonetheless, CD28-/- T cells produce sufficient IL-2 to support clonal expansion comparable to that of CD28+/+ T cells, which may account for the equally robust in vivo responses initiated by DCs in normal and CD28-deficient animals.

Combining G-CSF with a blockade of adhesion strongly improves the reconstitutive capacity of mobilized hematopoietic progenitor cells

OBJECTIVE

Mobilization of hematopoietic progenitor cells is achieved mainly by application of growth factors and, more recently, by blockade of adhesion. In this report, we describe the advantages of a combined treatment with granulocyte colony-stimulating factor (G-CSF) and anti-VLA4 (CD49d)/anti-CD44 as compared to treatment with the individual components.

MATERIALS AND METHODS

Mobilization by intravenous injection of anti-CD44, anti-VLA4, or G-CSF was controlled in spleen and bone marrow with regard to frequencies of multipotential colony-forming unit (C-CFU), marrow repopulating ability, long-term reconstitution, recovery of myelopoiesis, and regain of immunocompetence.

RESULTS

Mobilization by anti-CD44 had a strong effect on expansion of early progenitor cells in the bone marrow, while the recovery in the spleen was poor. In anti-CD49d-mobilized noncommitted and committed progenitors, progenitor expansion was less pronounced, but settlement in the spleen was quite efficient. Thus, anti-CD44 and anti-CD49d differently influenced mobilization. Accordingly, mobilization and recovery after transfer were improved by combining anti-CD44 with anti-CD49d treatment. Mobilization by G-CSF was most efficient with respect to recovery of progenitor cells in the spleen. However, when transferring G-CSF-mobilized cells, regain of immunocompetence was strongly delayed. This disadvantage could be overridden when progenitor cells were mobilized via blockade of adhesion and when expansion of these mobilized progenitor cells was supported by low-dose G-CSF only during the last 24 hours before transfer.

CONCLUSION

Mobilization of pluripotent progenitor cells via antibody blockade of CD44 or CD49d or via G-CSF relies on distinct mechanisms. Therefore, the reconstitutive capacity of a transplant can be significantly improved by mobilization regimens combining antibody with low-dose G-CSF treatment.

CD28-B7 costimulation: a critical role for initiation and development of experimental autoimmune neuritis in C57BL/6 mice

CD28 provides a critical costimulatory signal for antigen-specific T cell activation. Because CD28 is an important factor in the development of autoimmune diseases, we investigated its role in T cell-mediated experimental autoimmune neuritis (EAN), an animal model of Guillain-Barré syndrome in humans. CD28-deficient mutant (CD28-/-) C57BL/6 mice and corresponding wild-type mice were immunized with P0 peptide 180-199, a purified component of peripheral nerve myelin, and Freund's complete adjuvant. As a result, all wild-type mice developed severe EAN, in contrast, none of the CD28-/- mice manifested clinical signs of disease. Additionally, CD28-/- mice had fewer IL-12 producing cells in sciatic nerve sections and fewer IFN-gamma secreting splenic cells than wild-type mice on day 24 post immunization, i.e., at the peak of clinical EAN. At that time point, CD28-/- mice had milder infiltration of such inflammatory cells as macrophages, CD4+ T cells and monocytes into sciatic nerve tissues and less demyelination than wild-type mice. Moreover, the CD28-deficiency led to reduced production of specific anti-P0 peptide 180-199 antibodies compared with wild-type mice. Evidently, CD28 is required for interaction with B7 to regulate the activation of T and B cells that initiates development of EAN.

Activated T lymphocytes regulate hyaluronan binding to monocyte CD44 via production of IL-2 and IFN-gamma

Interactions of the cell surface proteoglycan CD44 with the extracellular matrix glycosaminoglycan hyaluronan (HA) are important during inflammatory immune responses. Our previous studies indicated that monocyte HA binding could be induced by TNF-alpha. Moreover, monocyte HA binding could be markedly up-regulated by culturing PBMC with anti-CD3 (TCR complex) mAbs. The present study was undertaken to identify soluble factors and/or cell surface molecules of activated T lymphocytes that might regulate HA binding to monocytes. Abs to IL-1 alpha, IL-1 beta, IL-2, IL-3, IL-10, IL-15, GM-CSF, IFN-gamma, and TNF-alpha were tested for their effects on anti-CD3 mAb-, Con A-, and PMA/ionomycin-mediated monocyte HA binding in PBMC cultures. Anti-TNF-alpha, anti-IL-2, and anti-IFN-gamma Abs, when added together to PBMC cultures, completely blocked Con A- and partially blocked anti-CD3- and PMA/ionomycin-induced monocyte HA binding. Furthermore, when added together to PBMC cultures, IL-2 and TNF-alpha induced high levels of monocyte HA binding. Likewise, IFN-gamma augmented TNF-alpha-induced monocyte HA binding. To investigate the role of T cell-monocyte direct contact in induction of monocyte HA binding, we studied PMA/ionomycin-activated, paraformaldehyde-fixed CD4(+) T cells in these assays. Fixed, PMA/ionomycin-activated CD4(+) T lymphocytes induced monocyte HA binding, but direct T cell-monocyte contact was not required. Moreover, anti-IFN-gamma and anti-TNF-alpha Abs blocked fixed PMA/ionomycin-activated CD4(+) T cell-induced monocyte HA binding. Taken together, these studies indicate roles for soluble T lymphocyte-derived factor(s), such as IL-2 and IFN-gamma, and a role for monocyte-derived TNF-alpha in Con A-, TCR complex-, and PMA/ionomycin-induced HA binding to monocyte CD44.

The role of CD4+ T cell help and CD40 ligand in the in vitro expansion of HIV-1-specific memory cytotoxic CD8+ T cell responses

CD4(+) T cells have been shown to play a critical role in the maintenance of an effective anti-viral CD8(+) CTL response in murine models. Recent studies have demonstrated that CD4(+) T cells provide help to CTLs through ligation of the CD40 receptor on dendritic cells. The role of CD4(+) T cell help in the expansion of virus-specific CD8(+) memory T cell responses was examined in normal volunteers recently vaccinated to influenza and in HIV-1 infected individuals. In recently vaccinated normal volunteers, CD4(+) T cell help was required for optimal in vitro expansion of influenza-specific CTL responses. Also, CD40 ligand trimer (CD40LT) enhanced CTL responses and was able to completely substitute for CD4(+) T cell help in PBMCs from normal volunteers. In HIV-1 infection, CD4(+) T cell help was required for optimal expansion of HIV-1-specific memory CTL in vitro in 9 of 10 patients. CD40LT could enhance CTL in the absence of CD4(+) T cell help in the majority of patients; however, the degree of enhancement of CTL responses was variable such that, in some patients, CD40LT could not completely substitute for CD4(+) T cell help. In those HIV-1-infected patients who demonstrated poor responses to CD40LT, a dysfunction in circulating CD8(+) memory T cells was demonstrated, which was reversed by the addition of cytokines including IL-2. Finally, it was demonstrated that IL-15 produced by CD40LT-stimulated dendritic cells may be an additional mechanism by which CD40LT induces the expansion of memory CTL in CD4(+) T cell-depleted conditions, where IL-2 is lacking.

Intercellular adhesion molecule 1 is critical for activation of CD28-deficient T cells

Presentation of Ag to T lymphocytes in the absence of the requisite costimulatory signals leads to an Ag-specific unresponsiveness termed anergy, whereas Ag presentation in conjunction with costimulation leads to clonal expansion. B7/CD28 signaling has been shown to provide this critical costimulatory signal and blockade of this pathway may inhibit in vitro and in vivo immune responses. Although T cells from CD28-deficient mice are lacking in a variety of responses, they nonetheless are capable of various primary and secondary responses without the induction of anergy expected in the absence of costimulation. This suggests that there may be alternative costimulatory pathways that can replace CD28 signaling under certain circumstances. In this paper, we show that ICAM-1becomes a dominant costimulatory molecule for CD28-deficient T cells. ICAM-1 costimulates anti-CD3-mediated T cell proliferation and IL-2 secretion in CD28-deficient murine T cells. Furthermore, splenocytes from ICAM-1-deficient mice could not activate CD28-deficient T cells and splenocytes lacking both ICAM and CD28 fail to proliferate in response to anti-CD3-induced T cell signals. This confirms that not only can ICAM-1 act as a CD28-independent costimulator, but it is the dominant, requisite costimulatory molecule for the activation of T cells in the absence of B7/CD28 costimulation.

Abrogation of experimental colitis correlates with increased apoptosis in mice deficient for CD44 variant exon 7 (CD44v7)

Experimental colitis in mice is characterized by infiltration of activated T helper (Th) cells and macrophages into the lamina propria. Particularly, these cells expressed CD44 variant exon 7 (CD44v7)-containing isoforms. Upregulation of CD44v7 isoforms was induced by CD40 ligation, an inflammation-driving interaction between activated Th cells and macrophages. To define the role of CD44v7 in colitis, mice bearing a targeted deletion for exon v7 were generated. In trinitrobenzene sulfonic acid-induced colitis, wild-type mice developed severe signs of persistent inflammation. Mice lacking CD44v7 initially showed unspecific inflammation, then recovered completely. The pathogenic origin was shown to reside in bone marrow-derived CD44v7(+) cells, because adoptive transfer experiments demonstrated an absolute requirement for CD44v7 on hematopoietic cells for maintenance of colitis. Interleukin (IL)-10-deficient mice, which develop a chronic Th1-driven enterocolitis, were crossbred with CD44v6/v7 null mice. In IL-10 x CD44v6/v7 double deficient mice, intestinal inflammation developed only weakly and at an older age. Analysis of cell death in the inflamed lesions revealed that mononuclear cells in the CD44v7 null infiltrates had higher rates of apoptosis than those from wild-type mice. Thus, the region encoded by CD44v7 appears to be essential for survival of effector lymphocytes, resulting in persistence of inflammation.

Resting B lymphocytes as APC for naive T lymphocytes: dependence on CD40 ligand/CD40

Although resting B cells as APC are tolerogenic for naive T cells in vivo, we show here that they can provide all the costimulatory signals necessary for naive T cell proliferation in vivo and in vitro. In the absence of an activating signal through the B cell Ag receptor, T cell proliferation after Ag recognition on resting B cells depends on CD40 expression on the B cells, implying that naive T cells use the membrane-bound cytokine, CD40 ligand (CD154), to induce the costimulatory signals that they need. Induction of B7-1 (CD80) and increased or sustained expression of CD44H, ICAM-1 (CD54), and B7-2 (CD86) are dependent on the interaction of CD40 ligand with CD40. Transient expression (12 h) of B7-2 is T cell- and peptide Ag-dependent, but CD40-independent. Only sustained (>/=24 h) expression of B7-2 and perhaps increased expression of ICAM-1 could be shown to be functionally important in this system. T cells cultured with CD40-deficient B cells and peptide remain about as responsive as fresh naive cells upon secondary culture with whole splenic APC. Therefore, B cells, and perhaps other APC, may be tolerogenic not because they fail to provide sufficient costimulation for T cell proliferation, but because they are deficient in some later functions necessary for a productive T cell response.

The CD44 variant isoforms CD44v6 and CD44v7 are expressed by distinct leukocyte subpopulations and exert non-overlapping functional activities

We have described recently that anti-CD44s, anti-CD44v6 and anti-CD44v7 interfere with delayed-type hypersensitivity (DTH) reactions. Yet, TNBS-induced colitis can be cured only by anti-CD44v7. To clarify the mechanisms underlying the divergent functional activities of CD44v6 and CD44v7 we explored their contribution to lymphocyte activation in vivo and in vitro. CD44v6 and CD44v7 are distinctly expressed on subpopulations of activated lymphocytes. Expression of CD44v6 is mainly restricted to T cell blasts. CD44v7 has been detected on CD4(+) cells, B cells and monocytes. Mitogenic and antigenic stimulation of lymphocytes in vitro was impaired in the presence of anti-CD44v6 and anti-CD44v7. Accordingly, anti-CD44v6 and anti-CD44v7 mitigated the DTH reaction in 2,4-dinitro-1-fluorobenzene-sensitized and challenged mice. However, the seemingly similar effects of CD44v6- and CD44v7-specific antibodies resulted from different activities. Anti-CD44v6 treatment led to a down-regulation of IL-2 and IFN-gamma production predominantly by CD8(+) cells. In anti-CD44v7-treated mice expression of IL-12 was decreased. Elevated levels of IL-10 accompanied this reduction. The latter resulted from an anti-CD44v7-mediated blockade of interactions between CD4(+) cells and monocytes as well as an active triggering of B cells. Thus, anti-CD44v6 and anti-CD44v7 interfere with lymphocyte activation at very specific points. CD44v6 functions predominantly at the T cell level. CD44v7 influences production of proinflammatory cytokines by B cells as well as an interaction between CD4(+) cells and antigen-presenting cells. As CD44 isoforms do not differ in their intracytoplasmatic tail, the distinct activities must result from expression on different leukocyte subsets and interactions with distinct ligands.

Expression of CD44 on bile ducts in primary sclerosing cholangitis and primary biliary cirrhosis

AIM

To examine expression of CD44, a transmembrane glycoprotein involved in lymphocyte homing and activation, in inflammatory liver diseases.

METHODS

Formalin fixed, paraffin embedded tissues were obtained from normal, uninvolved liver from patients undergoing partial hepatectomy for metastatic carcinoma (9) and transplant hepatectomy specimens from patients with primary biliary cirrhosis (12), primary sclerosing cholangitis (8), autoimmune hepatitis (3), hepatitis C (3), and secondary sclerosing cholangitis (1). Expression of CD44 (using antibodies to three core epitopes), HLA-DR, and lymphocyte phenotypic markers was studied by immunohistochemistry.

RESULTS

CD44 expression was not detected in either hepatocytes or biliary epithelial cells in normal livers. In sections from all 27 transplant hepatectomy specimens, CD44 was positive in bile duct epithelial cells but not in hepatocytes. The proportion of CD44+ ducts was much higher in biliary disease than in chronic hepatitis. By contrast, expression of HLA-DR was detected in a relatively small percentage of bile ducts. Activated, memory phenotype CD4+ T lymphocytes were increased in the parenchyma of all diseased livers and an infiltrate of activated CD8+ cells within the biliary epithelium was evident in inflammatory biliary disease.

CONCLUSIONS

CD44 appears to play an important role in the development of autoimmune biliary disease by promoting lymphoepithelial interactions, whereas HLA-DR may be involved in the subsequent progression of these conditions.

Role of CD28 in the Generation of Effector and Memory Responses Required for Resistance to Toxoplasma gondii

CD28 deficient (CD28−/−) mice were used to study the role of costimulation in the T cell-mediated, IFN-γ-dependent mechanism of resistance to Toxoplasma gondii. These mice were resistant to infection with the ME49 strain of T. gondii. Analysis of the immune response of acutely infected CD28−/− mice revealed that IL-12 was required for T cell production of IFN-γ and this was independent of the CD40/CD40 ligand interaction. A similar mechanism of IL-12-dependent, CD28/B7 independent production of IFN-γ by T cells was also observed in wild-type mice. Interestingly, although chronically infected wild-type mice were resistant to rechallenge with the virulent RH strain of T. gondii, chronically infected CD28−/− mice were susceptible to rechallenge with the RH strain. This deficiency in the protective memory response by CD28−/− mice correlated with a lack of IL-2 and IFN-γ in recall responses and reduced numbers of CD4+ T cells expressing a memory phenotype. Together, our findings demonstrate that CD28 is not required for the development of a protective T cell response to T. gondii, but CD28 is required for an optimal secondary immune response.

The immunobiology of bile and biliary epithelium

Long thought to be just a simple pipe involved in the delivery of bile from hepatocytes to the gallbladder and intestine, bile ducts are now regarded as highly dynamic structures consisting of cell populations involved in formation, transport and modification of bile by both secretory and absorptive processes. In fact, both bile and biliary epithelium appear to have active immunologic roles in both innate and adaptive immune responses. These roles are becoming increasingly clear as techniques have been developed allowing for the study of bile and biliary epithelial cells (BECs) in mucosal immunity. Bile is actively involved in the transport of immunoglobulin to the intestine, while BECs secrete chemokines and cytokines and serve to localize the immune response by expressing critical cell adhesion molecules. Evidence suggests that BECs may also function as professional antigen-presenting cells (APC) and, in the process, contribute to the modulation of inflammatory reactions. Bile ducts and, in particular, BECs, are the primary site of damage in several immunologically mediated liver diseases. Progress in these important areas has been rapid and forms the basis of this review.

Blocking CD40 - CD154 and CD80/CD86 - CD28 interactions during primary allogeneic stimulation results in T cell anergy and high IL-10 production

Although CD28 triggering provides an important co-stimulatory signal to T cells, blocking the CD80/CD86 - CD28 interaction with CTLA-4lg fusion protein is not sufficient for tolerance induction in vivo or in vitro. According to more recent data, interruption of the CD40 - CD154 interaction might complement the effect of CTLA-4lg and induce graft acceptance. We studied the effects of a blocking anti-CD40 monoclonal antibody (mAb) and/or blocking anti-CD80/anti-CD86 mAb in cultures of human peripheral blood mononuclear cells (PBMC) stimulated with allogeneic PBMC. T cells activated by alloantigens in the presence of anti-CD80, anti-CD86 and anti-CD40 entered a state of alloantigen-specific non-responsiveness as evidenced upon restimulation by lack of proliferation, cytotoxic activity, and IL-2, IL-5 and IL-13 production. IFN-gamma production during restimulation was less than in the control cultures, while the production of IL-10 was enhanced. Addition of recombinant IL-2 during the restimulation rescued alloantigen-specific activity. We conclude that the simultaneous blocking of the CD40 - CD154 and CD80/CD86 - CD28 interaction during allogeneic T cell activation induces T cell anergy. Since anergic cells induced by this treatment still produce high levels of IL-10, the latter could contribute to modulation of antigen-presenting cell activity and to bystander suppression of residual reactive T cells.

Mitigation of delayed-type hypersensitivity reactions by a CD44 variant isoform v3-specific antibody: blockade of leukocyte egress

In allergic alterations of human skin the majority of infiltrated leukocytes express CD44v3, but no other CD44 variant isoform. Vessel endothelium, too, is brightly stained with a CD44v3-specific antibody. Being concerned about therapeutic intervention, it became of importance to define whether expression of CD44v3 on the endothelial cells or on the leukocytes or on both is of functional importance. As expression of CD44v3 in the mouse on activated endothelium and on subpopulations of activated CD4+ cells, B cells and monocytes was similar to the expression in the human, we answered the question in a mouse delayed-type hypersensitivity model. The effect of anti-CD44v3 was compared with the effect of anti-CD44s and anti-CD44v10, both known to suppress delayed-type hypersensitivity reactions. Anti-CD44v3 mitigated the delayed-type hypersensitivity reaction in dinitrofluorobenzene sensitized and challenged mice comparable with anti-CD44s and anti-CD44v10. The seemingly similar effects of CD44 isoform-specific antibodies, however, resulted from a distinct modulation of response. Anti-CD44s mainly suppressed T cell activation and interleukin-2 as well as interferon-gamma expression. Anti-CD44v10 inhibited the activation of monocytes in the draining lymph nodes and in the infiltrate, which led to a strong reduction in the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-12 and in edema formation. Anti-CD44v3 had only a weak effect on cytokine expression by isolated subpopulations of leukocytes, but suppressed cytokine production by helper T cells when cocultured with antigen-presenting cells, i.e., blocked an interaction between antigen-presenting cells and helper T cells. The dominating effect of anti-CD44v3, however, relied on a blockade of leukocyte extravasation. As leukocytes transferred into dinitrofluorobenzene sensitized, anti-CD44v3-treated and lethally irradiated mice did not infiltrate the sensitized skin, anti-CD44v3 most likely prevented leukocyte extravasation by blocking CD44v3 on endothelial cells.

A new human prostate carcinoma cell line, 22Rv1

A cell line has been derived from a human prostatic carcinoma xenograft, CWR22R. This represents one of very few available cell lines representative of this disease. The cell line is derived from a xenograft that was serially propagated in mice after castration-induced regression and relapse of the parental, androgen-dependent CWR22 xenograft. Flow cytometric and cytogenetic analysis showed that this cell line represents one hyper DNA-diploid stem line with two clonal, evolved cytogenetic sublines. The basic karyotype is close to that of the grandparent xenograft, CWR22, and is relatively simple with 50 chromosomes. In nude mice, the line forms tumors with morphology similar to that of the xenografts, and like the parental CWR22 and CWR22R xenografts, this cell line expresses prostate specific antigen. Growth is weakly stimulated by dihydroxytestosterone and lysates are immunoreactive with androgen receptor antibody by Western blot analysis. Growth is stimulated by epidermal growth factor but is not inhibited by transforming growth factor-beta1.

Immunological self/nonself discrimination: integration of self vs nonself during cognate T cell interactions with antigen-presenting cells

The hypothesis is presented that immunological integration of nonefficacious vs efficacious T cell antigen receptor (TCR) signals are foundational for self/nonself discrimination and that multiple integrative mechanisms are intrinsic to the molecular to molar organization of an adaptive immune response. These integrative mechanisms are proposed to adaptively regulate expression of costimulatory signals, such that foreign proteins are associated with the expression of costimulatory signals, whereas self-proteins are associated with the lack of costimulatory signaling. Overall, this model offers several unique contributions to the study of immunology. First, this model postulates that cognate TCR/major histocompatibility complex (MHC) interactions are sufficient to adaptively mediate immunological self/nonself discrimination. This model thereby offers a unique alternative to models that largely rely on innate immunity to prime immune discrimination. Second, the integrative model argues that the immune system can simultaneously reinforce self-tolerance and promote immunity to foreign organisms at the same time and in the same location. Many alternative models presume that pathogenic self-reactive T cells do not exist at the outset of an immune response against foreign agents. Third, the integrative model uniquely predicts relationships between immunodeficiency and autoimmune pathogenesis. Fourth, this model illustrates the regulatory advantages of cognate antigen presenting cell (APC) systems (i.e., T cell or B cell APC) compared to nonspecific APC. Cognate APC systems together with the respective clonotypic responders may comprise a fundamental "network" of lymphoid cells. Such networks would have clone-specific regulatory capabilities and may be central for immunological self/nonself discrimination. Fifth, this model provides an explanation for "infectious" tolerance without creating specialized subsets of "suppressor" or "regulatory" T cells. Each mature T cell retains the potential to reinforce tolerance or mediate immunity, depending on the specific antigenic cues present in the immediate environment.

CD28-Independent Costimulation of T Cells by OX40 Ligand and CD70 on Activated B Cells

OX40 and its ligand (OX40L) have been implicated in T cell-dependent humoral immune responses. To further characterize the role of OX40/OX40L in T-B cell interaction, we newly generated an anti-mouse OX40L mAb (RM134L) that can inhibit the costimulatory activity of OX40L transfectants for anti-CD3-stimulated T cell proliferation. Flow cytometric analyses using RM134L and an anti-mouse OX40 mAb indicated that OX40 was inducible on splenic T cells by stimulation with immobilized anti-CD3 mAb in a CD28-independent manner, while OX40L was not expressed on resting or activated T cells. OX40L was inducible on splenic B cells by stimulation with anti-IgM Ab plus anti-CD40 mAb, but not by either alone. These activated B cells exhibited a potent costimulatory activity for anti-CD3-stimulated T cell proliferation and IL-2 production. Anti-CD80 and anti-CD86 mAbs partially inhibited the costimulatory activity, and further inhibition was obtained by their combination with RM134L and/or anti-CD70 mAb. We also found the anti-IgM Ab- plus anti-CD40 mAb-stimulated B cells exhibited a potent costimulatory activity for proliferation of and IL-2 production by anti-CD3-stimulated CD28− T cells from CD28-deficient mice, which was substantially inhibited by RM134L and/or anti-CD70 mAb. These results indicated that OX40L and CD70 expressed on surface Ig- and CD40-stimulated B cells can provide CD28-independent costimulatory signals to T cells.