Expression of heat-stable antigen on tumor cells provides co-stimulation for tumor-specific T cell proliferation and cytotoxicity in mice

Dendritic cell expression of CD24 contributes to optimal priming of T lymphocytes in lymph nodes

CD24 is a GPI anchored cell surface glycoprotein whose function as a co-stimulatory molecule has been implicated. However, the function of CD24 on antigen presenting cells during T cell responses is not well understood. Here we show that in the CD24-deficient host, adoptively transferred CD4+ T cells undergo inefficient expansion and have accelerated cell death in lymph nodes, which results in insufficient priming of T cells. Insufficient expansion of T cells in the CD24-deficient host was not due to host anti-CD24 response by NK, T and B lymphocytes. Transgenic expression of CD24 on DC in CD24-/- mice restored T cell accumulation and survival in draining lymph nodes. Consistent with these findings, MHC II tetramer staining also revealed that an antigen-specific polyclonal T cell response was reduced in lymph nodes of CD24-/- mice. Taken together, we have revealed a novel role of CD24 on DC in optimal T cell priming in lymph nodes. These data suggest that CD24 blockade should lower unwanted T cell responses such as those in autoimmune diseases.

CD24 aggravates acute liver injury in autoimmune hepatitis by promoting IFN-γ production by CD4+ T cells

The T-cell-mediated immune response is implicated in many clinical hepatic injuries, such as autoimmune hepatitis and acute virus hepatitis. CD24 is widely expressed by different immune cells and plays an important role in the pathogenesis of many autoimmune diseases. However, the role of CD24 in T-cell-mediated liver injury has not been elucidated until now. Here we showed that CD24 deficiency protects mice from concanavalin A (ConA)-induced fulminant liver injury by reducing serum interferon-γ (IFN-γ) levels. CD24 expression by hepatic T cells was markedly increased following ConA challenge. Moreover, decreased IFN-γ production by hepatic CD4⁺ T cells in CD24-deficient mice was detected, which was correlated with downregulated phosphorylation of STAT1 in hepatic tissue. In vitro experiments also supported the conclusion that CD24 deficiency impaired IFN-γ production by CD4⁺ T cells following ConA, CD3/CD28 and phorbol myristate acetate/ionomycin stimulation. Our study suggests that CD24 deficiency confers hepatoprotection by decreasing CD4⁺ T-cell-dependent IFN-γ production in vivo, which suggests that CD24 might be a potential target molecule for reducing clinical hepatitis.

Tumor masses support naive T cell infiltration, activation, and differentiation into effectors

Studies of T cell responses to tumors have focused on the draining lymph node (LN) as the site of activation. We examined the tumor mass as a potential site of activation after adoptive transfer of naive tumor-specific CD8 T cells. Activated CD8 T cells were present in tumors within 24 h of adoptive transfer and proliferation of these cells was also evident 4-5 d later in mice treated with FTY720 to prevent infiltration of cells activated in LNs. To confirm that activation of these T cells occurred in the tumor and not the tumor-draining LNs, we used mice lacking LNs. Activated and proliferating tumor-infiltrating lymphocytes were evident in these mice 24 h and 4 d after naive cell transfer. T cells activated within tumors acquired effector function that was evident both ex vivo and in vivo. Both cross-presenting antigen presenting cells within the tumor and tumor cells directly presenting antigen activated these functional CD8 effectors. We conclude that tumors support the infiltration, activation, and effector differentiation of naive CD8 T cells, despite the presence of immunosuppressive mechanisms. Thus, targeting of T cell activation to tumors may present a tool in the development of cancer immunotherapy.

Tumor Therapy Applying Membrane-bound Form of Cytokines

Tumor therapy using cytokines has been developed for last two decades. Several recombinant cytokines and tumor cell vaccines produced by cytokine gene transfer have been in clinical trials, but several side effects hamper routine clinical applications. Many cytokines are originally expressed as membrane-bound form and then processed to secretory form exerting paracrine effects. Though functional differences of these two types of cytokines are elusive yet, the membrane-bound form of cytokine may exert its effects on restricted target cells as a juxtacrine, which are in physical contacts. With the efforts to improve antitumor activities of cytokines in cancer patients, developing new strategies to alleviate life-threatening side effects became an inevitable goal of tumor immunologists. Among these, tumor cell vaccines expressing cytokines as membrane-bound form on tumor cell surface have been developed by genetic engineering techniques with the hope of selective stimulation of the target cells that are in cell-to-cell contacts. In this review, recent progress of tumor cell vaccines expressing membrane-bound form of cytokines will be discussed.

CD24 polymorphisms affect risk and progression of chronic hepatitis B virus infection

T-cell immunity to hepatitis B virus (HBV) is involved in both viral clearance and the pathogenesis of cirrhosis and hepatocellular carcinoma following chronic HBV infection. It is therefore of great interest to analyze whether genetic polymorphism of genes involved in the immune response may determine the outcomes of chronic HBV infection. Here we report that CD24 polymorphisms affect the risk and progression of chronic HBV infection. Thus the CD24 P170(T) allele, which is expressed at a higher level, is associated with an increased risk of chronic HBV infection. Among the chronic HBV patients this allele shows recessive association with more rapid progression to liver cirrhosis and hepatocellular carcinoma in comparison to the P170(C) allele. In contrast, a dinucleotide deletion at position 1527-1528 (P1527(del)), which reduces CD24 expression, is associated with a significantly reduced risk of chronic HBV infection. To confirm the role for CD24 in liver carcinogenesis, we compared the size of liver tumor developed in CD24(-/-) and CD24(+/-) HBV transgenic mice. Our data demonstrate that targeted mutation of CD24 drastically reduced the sizes of spontaneous liver cancer in the HBV transgenic mice.

CONCLUSION

These data demonstrate that genetic variation of CD24 may be an important determinant for the outcome of chronic HBV infection.

CD24 on the Resident Cells of the Central Nervous System Enhances Experimental Autoimmune Encephalomyelitis

CD24 is a cell surface glycoprotein that is expressed on both immune cells and cells of the CNS. We have previously shown that CD24 is required for the induction of experimental autoimmune encephalomyelitis (EAE), an experimental model for the human disease multiple sclerosis (MS). The development of EAE requires CD24 expression on both T cells and non-T host cells in the CNS. To understand the role of CD24 on the resident cells in the CNS during EAE development, we created CD24 bone marrow chimeras and transgenic mice in which CD24 expression was under the control of a glial fibrillary acidic protein promotor (AstroCD24TG mice). We showed that mice lacking CD24 expression on the CNS resident cells developed a mild form of EAE; in contrast, mice with overexpression of CD24 in the CNS developed severe EAE. Compared with nontransgenic mice, the CNS of AstroCD24TG mice had higher expression of cytokine genes such as IL-17 and demyelination-associated marker P8; the CNS of AstroCD24TG mice accumulated higher numbers of Th17 and total CD4+ T cells, whereas CD4+ T cells underwent more proliferation during EAE development. Expression of CD24 in CD24-deficient astrocytes also enhanced their costimulatory activity to myelin oligodendrocyte glycoprotein-specific, TCR-transgenic 2D2 T cells. Thus, CD24 on the resident cells in the CNS enhances EAE development via costimulation of encephalitogenic T cells. Because CD24 is increased drastically on resident cells in the CNS during EAE, our data have important implications for CD24-targeted therapy of MS.

B7-1-HSA (CD80-CD24), a recombinant hybrid costimulatory molecule retains ligand binding and costimulatory functions

Optimal activation of naïve T lymphocyte requires two signals; an antigen-specific signal initiated by engagement of TCR with the antigen-MHC complex and a costimulatory signal independent of the antigen receptor complex. Without the costimulatory signal, T cells become anergic. Various adhesion molecules, such as B7-1 (CD80) and heat stable antigen (HSA, CD24), expressed on antigen presenting cells have been demonstrated to provide costimulatory signals to T cells. It was reported that the combinations of different adhesion molecules could induce even stronger immune response. In this study, we made a hybrid costimulatory molecule, B7-1-HSA, and tested its T cell stimulatory function. Chinese hamster ovary (CHO) cells expressing this hybrid molecule bound both anti-CD80 and anti-CD24 monoclonal antibodies, and induced stronger T cell proliferation than CHO cells expressing B7-1 or HSA alone. These results suggest that the B7-1-HSA hybrid molecule can deliver two costimulatory signals simultaneously that can synergize in inducing T cell proliferation. The purified B7-1-HSA protein reacted with both anti-B7-1 and anti-HSA mAbs in Western blotting and specifically mediated adhesion of Jurkat cells. Furthermore, purified B7-1-HSA molecule spontaneously incorporated onto cell membrane through its glycolipid anchor suggesting that this hybrid costimulatory molecule can be used in protein transfer to develop effective cancer vaccines.

The heat stable antigen (CD24) is not required for the generation of CD4+ effector and memory T cells by dendritic cells in vivo

Previous work has established that CD24 is a costimulatory molecule for T-cell clonal expansion. Studies using CD24 -/- mice demonstrated that CD24 plays a critical role in the CD28-independent immune response against virus and soluble antigens. The role of CD24 on dendritic cells (DCs) has not been reported. Here, we compare the CD24(+/+) and CD24(-/-) DCs in the induction of initial clonal expansion and elicitation of memory CD4(+) T cells in vivo. Our results demonstrate that the CD24 expressed on DCs is essential neither for the induction of initial T-cell clonal expansion nor for elicitation of memory activity of primed T cells in vivo.

CD24 expression on T cells is required for optimal T cell proliferation in lymphopenic host

It is well established that T lymphocytes undergo homeostatic proliferation in lymphopenic environment. The homeostatic proliferation requires recognition of the major histocompatibility complex on the host. Recent studies have demonstrated that costimulation-mediated CD28, 4-1BB, and CD40 is not required for T cell homeostatic proliferation. It has been suggested that homeostatic proliferation is costimulation independent. Here, we report that T cells from mice with a targeted mutation of CD24 have a remarkably reduced rate of proliferation when adoptively transferred into syngeneic lymphopenic hosts. The reduced proliferation cannot be attributed to abnormal survival and homing properties of the CD24-deficient T cells. T cell proliferation in allogeneic hosts is less affected by this mutation. These results demonstrate a novel function of CD24 expressed on T cells. Thus, although distinct costimulatory molecules are involved in antigen-driven proliferation and homeostatic proliferation, both processes can be modulated by costimulatory molecules.

The heat-stable antigen determines pathogenicity of self-reactive T cells in experimental autoimmune encephalomyelitis

Induction of myelin-specific CD4 T cells is a pivotal event in the development of experimental autoimmune encephalomyelitis (EAE). Other checkpoints in EAE pathogenesis have not been clearly defined, although multiple genetic loci are known to influence EAE development. We report here that targeted mutation of the heat-stable antigen (HSA) abrogates development of EAE despite a complete lack of effect on induction of autoimmune T cells. To test whether T-cell expression of HSA is sufficient, we created transgenic mice in which HSA is expressed exclusively in the T-cell lineage. We found that these mice remain resistant to EAE induction. Adoptive transfer studies demonstrate that both T cells and non-T cells must express HSA in order for the pathogenic T cells to execute their effector function. Moreover, HSAIg, a fusion protein consisting of the extracellular domain of the HSA and the Fc portion of immunoglobulin, drastically ameliorates the clinical sign of EAE even when administrated after self-reactive T cells had been expanded. Thus, identification of HSA as a novel checkpoint, even after activation and expansion of self-reactive T cells, provides a novel approach for immunotherapy of autoimmune neurologic diseases, such as multiple sclerosis.

Induction of unresponsiveness to islet xenograft by MMC treatment of graft and blockage of LFA-1/ICAM-1 pathway

BACKGROUND

Induction of unresponsiveness to graft is one of major interest in xenotransplantation. Two different modalities [direct graft treatment by mitomycin C (MMC) and blockage of the lymphocyte function-associated antigen-1/intracellular adhesion molecule-1 (LFA-1/ICAM-1) pathway in recipients by species-specific mAbs] were tested for their ability to produce unresponsiveness to secondary islet xenografts.

METHODS

Collagenase-digested WS (RT1k) rat islets, purified by Ficoll density gradient, were incubated for 30 min with MMC 10 microg/ml, cultured for 20 hr, and transplanted into the renal subcapsular space of streptozotocin-induced diabetic C57BL/6 (H-2b) mice. Recipient mice were divided into experimental groups according to anti-rat ICAM-1 and/or anti-mouse LFA-1 mAb treatment and transplantation of MMC-treated or nontreated islets.

RESULTS

MMC pretreatment alone prolonged graft survival, with a mean survival time (MST) of 23.0+/-7.4 days, compared with that of cultured islets (12.4+/-2.7 days; P<0.01). MMC treatment of islets significantly augmented graft survival, compared with that of crude islet grafts under treatment with anti-donor ICAM-1 mAb (MST: >41.3+/-30 vs. 16.6+/-5.4 days, P<0.01), anti-recipient LFA-mAb (MST: >70.3+/-28.9 vs. 30.4+/-10.4 days, P<0.001), or both mAbs (MST: >88.1+/-24.1 vs. 23+/-7.4 days, P<0.0001). One of six, four of nine, and six of eight animals accepted MMC-treated islet xenografts over 100 days after treatment with anti-rat ICAM-1, anti-mouse LFA-1, or both mAbs treatments, respectively, whereas none of the animals accepted nontreated islets under the same treatment. When the mice bearing long-term functioning xenografts were challenged with the secondary graft from the original donor strain, the animals previously treated with anti-recipient LFA-1 and anti-donor ICAM-1 mAbs were more prone to accept it than animals given anti-recipient LFA-1 mAb alone (MST: 55.8+/-25.7 vs. 15+/-2.4 respectively; P<0.001), although they rejected the third-party xenograft and allograft acutely.

CONCLUSIONS

In the xenogeneic islet transplantation model, MMC graft pretreatment and blockage of the ICAM-1/LFA-1 pathway constitute a potent protocol for inducing unresponsiveness to islet xenografts.

Expression and characterization of glycolipid-anchored B7-1 (CD80) from baculovirus-infected insect cells: protein transfer onto tumor cells

Tumor cells can be modified to express immunostimulatory molecules such as B7-1 by protein transfer using purified glycosylphosphatidylinositol-anchored B7-1 (GPI-B7-1). In this study recombinant baculovirus encoding GPI-B7-1 (vBacB7-1(GPI)) was established to obtain large quantities of purified GPI-B7-1 to modify tumor cells by protein transfer. vBacB7-1(GPI)-infected insect cells showed high-level cell surface expression of GPI-B7-1 that was susceptible to PIPLC treatment. GPI-B7-1 expressed in insect cells (Bac-GPI-B7-1) mediated T cell proliferation, indicating that the GPI-B7-1 retains costimulatory activity. Moreover, Bac-GPI-B7-1 was completely solubilized in Triton X-100 at 4 degrees C compared to 22% solubilization of GPI-B7-1 expressed in CHOK1 cells, suggesting that GPI-anchored proteins expressed in insect cells may not be clustered into the detergent-insoluble fraction. SDS-PAGE analysis of Bac-GPI-B7-1 showed faster mobility (45 kDa) compared to GPI-B7-1 from CHOK1 (68 kDa) and this difference may be due to a difference in glycosylation. Cell binding assays showed that immunoaffinity-purified Bac-GPI-B7-1 retained its functional ability to bind CD28(+) cells. Moreover, when human tumor cells were incubated with this functionally active purified GPI-B7-1, an efficient transfer of B7-1 onto tumor cells was observed. These results demonstrate that GPI-B7-1 can be expressed in insect cells in a functionally active form and can be used to modify tumor cells for immunotherapeutic applications.

Direct antigen presentation through binding of donor intercellular adhesion molecule-1 to recipient lymphocyte function-associated antigen-1 molecules in xenograft rejection

Cellular interactions that lead to graft rejection were examined in a rat-to-mouse xenogeneic combination using species-specific monoclonal antibodies (mAbs) against donor and recipient intercellular adhesion molecule-1 (ICAM-1) and lymphocyte function-associated antigen-1 (LFA-1) molecules, respectively. Although both mAbs displayed moderate blocking activity in an in vitro mixed lymphocyte response assay, strong suppression was observed when anti-donor (rat) ICAM-1 mAb was combined with anti-recipient (mouse) LFA-1 mAb. Likewise, significant prolongation of islet xenograft survival was observed with these mAbs. Thus, 0.05 mg of anti-mouse LFA-1 mAb and anti-rat ICAM-1 mAb given on days 0 and 1 produced significant prolongation of graft survival over the control (51+/-20 days vs. 10+/-3 days, P<0.0001), but not when anti-mouse ICAM-1 mAb was combined with anti-mouse LFA-1 mAb (13+/-3 days). In this species combination, mouse T cells were able to proliferate in the presence of rat antigen-presenting cells (APCs) in a cell number-dependent manner, but not in the presence of mouse APCs. The binding assay showed that LFA-1 molecules on mouse T cells can bind immobilized rat ICAM-1 molecules. These results suggest that rat ICAM-1 molecules on APCs can interact with mouse LFA-1 molecules on T cells across a species barrier and that this binding generates the consequent immune responses leading to rejection. mAb treatment against these adhesion molecules of recipient as well as donor is crucial for preventing rejection in a xenogeneic transplantation model.

CD28-independent induction of T helper cells and immunoglobulin class switches requires costimulation by the heat-stable antigen

It is well established that B7-CD28/CTLA4 interactions play an important role in the induction of T helper cells for T-dependent antibody responses. However, targeted mutation of CD28 does not significantly affect production of IgG and activation of CD4 T helper cells in response to infections by some viruses and nematode parasites. To test whether the CD28-independent induction of Ig class switches requires costimulation by the heat-stable antigen (HSA), we compared T helper cell induction and antibody response in mice deficient for either HSA, CD28, or both genes. We found that after immunization with KLH-DNP, mice deficient for both CD28 and HSA lack DNP-specific IgA and all subtypes of IgG. This deficiency corresponds to a reduced number of effector helper T cells that rapidly produce IL-2, IL-4, and IFN-gamma after in vitro stimulation with carrier antigen KLH. In contrast, priming of T helper cells and Ig class switch are normal in mice deficient with either HSA or CD28 alone. IgM responses are not affected by any of these targeted mutations. These results demonstrate that CD28-independent induction of T helper cells and Ig class-switches requires costimulation by the HSA.

Regulation of the stability of heat-stable antigen mRNA by interplay between two novel cis elements in the 3' untranslated region

The heat-stable antigen (HSA) is a costimulatory molecule for T-cell activation. Its expression is strictly regulated during lymphocyte development and differentiation. Recent studies using HSA-transgenic mice have demonstrated that this regulated expression is critical for normal development of T and B lymphocytes. However, the mechanisms that control the expression of HSA are largely unknown. HSA mRNA is comprised of a 0.23-kb open reading frame and a 1.5-kb 3' untranslated region (3'UTR). The function of the long 3'UTR has not been addressed. Here we investigate the role of the 3'UTR of HSA mRNA. We show that a 160-bp element, located in the region of nucleotides 1465 to 1625 in the 3'UTR of HSA mRNA, promotes RNA degradation and that this effect is neutralized by a 43-bp fragment approximately 1 kb upstream of the negative cis element. Both positive and negative cis elements in the HSA mRNA are distinct from other sequences that are known to modulate mRNA stability. These results provide direct evidence that the interplay between two novel cis elements in the 3'UTR of HSA mRNA determines cell surface HSA expression by modulating its RNA stability.

Homotypic interaction of the heat-stable antigen is not responsible for its co-stimulatory activity for T cell clonal expansion

The heat-stable antigen (HSA) is an important co-stimulatory molecule on antigen-presenting cells (APC). However, the receptor on T cells that receives the co-stimulatory signal from HSA has not been identified. Because the HSA is transiently expressed on T cells after the T cell receptor/CD3 complex is engaged, and because it can bind to itself in a homotypic fashion, it has been proposed that homotypic interaction of HSA is responsible for its co-stimulatory activity. Here we test this hypothesis using mice that have a targeted mutation of the HSA gene, as well as novel transgenic mice that constitutively express HSA on T cells. We show that HSA-deficient T cells remain responsive to co-stimulation by HSA. Furthermore, constitutive expression of HSA does not enhance T cell response to co-stimulatory by HSA. Taken together, our results demonstrate that homotypic interaction of HSA is not responsible for co-stimulation mediated by HSA expressed on APC.

Altered Erythrocytes and a Leaky Block in B-Cell Development in CD24/HSA-Deficient Mice

The heat stable antigen (HSA, or murine CD24) is a glycosyl phosphatidylinositol-linked surface glycoprotein expressed on immature cells of most, if not all, major hematopoietic lineages, as well as in developing neural and epithelial cells. It has been widely used to stage the maturation of B and T lymphocytes because it is strongly induced and then repressed again during their maturation. Terminally differentiated lymphocytes, as well as most myeloid lineages, are negative for HSA. Erythrocytes are an exception in that they maintain high levels of HSA expression. HSA on naive B cells has been shown to mediate cell-cell adhesion, while HSA on antigen-presenting cells has been shown to mediate a costimulatory signal important for activating T lymphocytes during an immune response. Here, we characterize mice that lack a functional HSA gene, constructed by homologous recombination in embryonic stem cells. While T-cell and myeloid development appears normal, these mice show a leaky block in B-cell development with a reduction in late pre-B and immature B-cell populations in the bone marrow. Nevertheless, peripheral B-cell numbers are normal and no impairment of immune function could be detected in these mice in a variety of immunization and infection models. We also observed that erythrocytes are altered in HSA-deficient mice. They show a higher tendency to aggregate and are more susceptible to hypotonic lysis in vitro. In vivo, the mean half-life of HSA-deficient erythrocytes was reduced. When infected with the malarial parasite Plasmodium chabaudi chabaudi, the levels of parasite-bearing erythrocytes in HSA-deficient mice were also significantly elevated, but the mice were able to clear the infection with kinetics similar to wild-type mice and were immune to a second challenge. Thus, apart from alterations in erythrocytes and a mild block in B-cell development, the regulated expression of HSA appears to be dispensable for the maturation and functioning of those cell lineages that normally express it.

Distinct costimulatory molecules are required for the induction of effector and memory cytotoxic T lymphocytes

A successful T cell immune response has two major products: effector T cells which directly or indirectly remove the antigens, and memory T cells, which allow a faster and more efficient recall response when challenged by related antigens. An important issue is whether costimulatory molecules on the antigen-presenting cells are involved in determining whether T cells will differentiate into effector or memory cells after antigenic stimulation. To address this issue, we have produced mice with targeted mutations of either the heat-stable antigen (HSA), or both HSA and CD28. We show that CD28/B7 and HSA provide two alternative costimulatory pathways for induction of immunological memory to influenza virus. Furthermore, our results revealed that B7 is essential for the generation of effector T cells from either naive or memory T cells, while HSA is not necessary for the generation of effector T cells. Our results demonstrate that the induction of memory T cells and effector T cells can utilize distinct costimulatory molecules. These results have important implications on lineage relationship between effector and memory T cells.

Assessment of cytotoxic T-lymphocyte phenotype using the specific markers granzyme B and TIA-1 in cervical neoplastic lesions

Cervical carcinomas are closely associated with high-risk human papillomavirus (HPV) types and are preceded by cervical intraepithelial neoplasia (CIN). Most CIN lesions regress spontaneously and will not evolve to invasive carcinoma. The cellular immune system mediated by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are thought to play an important role in the ultimate decline of CIN lesions. Although TIA-1 is constitutively expressed in the majority of circulating T cells and defines a subpopulation of CD8+ T cells with cytotoxic potential, granzyme B is only expressed in CTLs upon activation. In the present study we have evaluated the expression of these proteins by lymphocytes present in 24 randomly chosen CIN lesions with increasing degree of atypia and in 14 cervical squamous cell carcinomas. As major histocompatibility complex (MHC) class I expression is frequently down-regulated in HPV-induced lesions, thus possibly frustrating tumour cell recognition by infiltrating CTLs, these lesions were also analysed for MHC class I expression. The results indicated that in most CIN lesions only a minority of CTLs are activated, whereas in some carcinomas a massive infiltration of activated, i.e. granzyme B-positive, CTLs were observed. The percentage of activated CTLs was not related to expression of MHC class I on neoplastic cells. These results suggest that in some carcinomas proper activation of CTLs occurs but that most likely local factors or immunoselection of resistant neoplastic cells inhibit a proper response of CTLs to these neoplastic cells.