A mutant cholera toxin B subunit that binds GM1- ganglioside but lacks immunomodulatory or toxic activity

GM1-ganglioside receptor binding by the B subunit of cholera toxin (CtxB) is widely accepted to initiate toxin action by triggering uptake and delivery of the toxin A subunit into cells. More recently, GM1 binding by isolated CtxB, or the related B subunit of Escherichia coli heat-labile enterotoxin (EtxB), has been found to modulate leukocyte function, resulting in the down-regulation of proinflammatory immune responses that cause autoimmune disorders such as rheumatoid arthritis and diabetes. Here, we demonstrate that GM1 binding, contrary to expectation, is not sufficient to initiate toxin action. We report the engineering and crystallographic structure of a mutant cholera toxin, with a His to Ala substitution in the B subunit at position 57. Whereas the mutant retained pentameric stability and high affinity binding to GM1-ganglioside, it had lost its immunomodulatory activity and, when part of the holotoxin complex, exhibited ablated toxicity. The implications of these findings on the mode of action of cholera toxin are discussed.

Interferon-β gene therapy inhibits tumor formation and causes regression of established tumors in immune-deficient mice

Despite the potential of type 1 interferons (IFNs) for the treatment of cancer, clinical experience with IFN protein therapy of solid tumors has been disappointing. IFN-β has potent antiproliferative activity against most human tumor cells in vitro in addition to its known immunomodulatory activities. The antiproliferative effect, however, relies on IFN-β concentrations that cannot be achieved by parenteral protein administration because of rapid protein clearance and systemic toxicities. We demonstrate here that ex vivo IFN-β gene transduction by a replication-defective adenovirus in as few as 1% of implanted cells blocked tumor formation. Direct in vivo IFN-β gene delivery into established tumors generated high local concentrations of IFN-β, inhibited tumor growth, and in many cases caused complete tumor regression. Because the mice were immune-deficient, it is likely that the anti-tumor effect was primarily through direct inhibition of tumor cell proliferation and survival. Based on these studies, we argue that local IFN-β gene therapy with replication-defective adenoviral vectors might be an effective treatment for some solid tumors.

Transloading of tumor cells with foreign major histocompatibility complex class I peptide ligand: a novel general strategy for the generation of potent cancer vaccines.

The major hurdle to be cleared in active immunotherapy of cancer is the poor immunogenicity of cancer cells. In previous attempts to overcome this problem, whole tumor cells have been used as vaccines, either admixed with adjuvant(s) or genetically engineered to express nonself proteins or immunomodulatory factors before application. We have developed a novel approach to generate an immunogeneic, highly effective vaccine: major histocompatibility complex (MHC) class I-positive cancer cells are administered together with MHC class I-matched peptide ligands of foreign, nonself origin, generated by a procedure we term transloading. Murine tumor lines of the H2-Kd or the H2-Db haplotype, melanoma M-3 and B16-F10, respectively, as well as colon carcinoma CT-26 (H2-Kd), were transloaded with MHC-matched influenza virus-derived peptides and applied as irradiated vaccines. Mice bearing a deposit of live M-3 melanoma cells were efficiently cured by this treatment. In the CT-26 colon carcinoma and the B16-F10 melanoma, high efficacies were obtained against tumor challenge, suggesting the universal applicability of this new type of vaccine. With foreign peptide ligands adapted to the requirements of a desired MHC class I haplotype, this concept may be used for the treatment of human cancers.

Caffeic acid phenethyl ester is a potent and specific inhibitor of activation of nuclear transcription factor NF-kappa B.

Caffeic acid phenethyl ester (CAPE), an active component of propolis from honeybee hives, is known to have antimitogenic, anticarcinogenic, antiinflammatory, and immunomodulatory properties. The molecular basis for these diverse properties is not known. Since the role of the nuclear factor NF-kappa B in these responses has been documented, we examined the effect of CAPE on this transcription factor. Our results show that the activation of NF-kappa B by tumor necrosis factor (TNF) is completely blocked by CAPE in a dose- and time-dependent manner. Besides TNF, CAPE also inhibited NF-kappa B activation induced by other inflammatory agents including phorbol ester, ceramide, hydrogen peroxide, and okadaic acid. Since the reducing agents reversed the inhibitory effect of CAPE, it suggests the role of critical sulfhydryl groups in NF-kappa B activation. CAPE prevented the translocation of the p65 subunit of NF-kappa B to the nucleus and had no significant effect on TNF-induced I kappa B alpha degradation, but did delay I kappa B alpha resynthesis. The effect of CAPE on inhibition of NF-kappa B binding to the DNA was specific, in as much as binding of other transcription factors including AP-1, Oct-1, and TFIID to their DNA were not affected. When various synthetic structural analogues of CAPE were examined, it was found that a bicyclic, rotationally constrained, 5,6-dihydroxy form was superactive, whereas 6,7-dihydroxy variant was least active. Thus, overall our results demonstrate that CAPE is a potent and a specific inhibitor of NF-kappa B activation and this may provide the molecular basis for its multiple immunomodulatory and antiinflammatory activities.

Binding of thalidomide to alpha1-acid glycoprotein may be involved in its inhibition of tumor necrosis factor alpha production.

In addition to its well known sedative and teratogenic effects, thalidomide also possesses potent immunomodulatory and antiinflammatory activities, being most effective against leprosy and chronic graft-versus-host disease. The immunomodulatory activity of thalidomide has been ascribed to the selective inhibition of tumor necrosis factor alpha from monocytes. The molecular mechanism for the immunomodulatory effect of thalidomide remains unknown. To elucidate this mechanism, we synthesized an active photoaffinity label of thalidomide as a probe to identify the molecular target of the drug. Using the probe, we specifically labeled a pair of proteins of 43-45 kDa with high acidity from bovine thymus extract. Purification of these proteins and partial peptide sequence determination revealed them to be alpha1-acid glycoprotein (AGP). We show that the binding of thalidomide photoaffinity label to authentic human AGP is competed with both thalidomide and the nonradioactive photoaffinity label at concentrations comparable to those required for inhibition of production of tumor necrosis factor alpha from human monocytes, suggesting that AGP may be involved in the immunomodulatory activity of thalidomide.

kappa opioid receptors in human microglia downregulate human immunodeficiency virus 1 expression.

Microglial cells, the resident macrophages of the brain, play an important role in the neuropathogenesis of human immunodeficiency virus type 1 (HIV-1), and recent studies suggest that opioid peptides regulate the function of macrophages from somatic tissues. We report herein the presence of kappa opioid receptors (KORs) in human fetal microglia and inhibition of HIV-1 expression in acutely infected microglial cell cultures treated with KOR ligands. Using reverse transcriptase-polymerase chain reaction and sequencing analyses, we found that mRNA for the KOR was constitutively expressed in microglia and determined that the nucleotide sequence of the open reading frame was identical to that of the human brain KOR gene. The expression of KOR in microglial cells was confirmed by membrane binding of [3H]U69,593, a kappa-selective ligand, and by indirect immunofluorescence. Treatment of microglial cell cultures with U50,488 or U69,593 resulted in a dose-dependent inhibition of expression of the monocytotropic HIV-1 SF162 strain. This antiviral effect of the kappa ligands was blocked by the specific KOR antagonist, nor-binaltrophimine. These findings suggest that kappa opioid agonists have immunomodulatory activity in the brain, and that these compounds could have potential in the treatment of HIV-1-associated encephalopathy.

Inhibition of acute in vivo human immunodeficiency virus infection by human interleukin 10 treatment of SCID mice implanted with human fetal thymus and liver.

To improve the usefulness of in vivo mode for the investigation of the pathophysiology of human immunodeficiency virus (HIV) infection, we modified the construction of SCID mice implanted with human fetal thymus and liver (thy/liv-SCID-hu mice) so that the peripheral blood of the mice contained significant numbers of human monocytes and T cells. After inoculation with HIV-1(59), a primary patient isolate capable of infecting monocytes and T cells, the modified thy/liv-SCID-hu mice developed disseminated HIV infection that was associated with plasma viremia. The development of plasma viremia and HIV infection in thy/liv-SCID-hu mice inoculated with HIV-1(59) was inhibited by acute treatment with human interleukin (IL) 10 but not with human IL-12. The human peripheral blood mononuclear cells in these modified thy/liv-SCID-hu mice were responsive to in vivo treatment with exogenous cytokines. Human interferon gamma expression in the circulating human peripheral blood mononuclear cells was induced by treatment with IL-12 and inhibited by treatment with IL-10. Thus, these modified thy/liv-SCID-hu mice should prove to be a valuable in vivo model for examining the role of immunomodulatory therapy in modifying HIV infection. Furthermore, our demonstration of the vivo inhibitory effect of IL-10 on acute HIV infection suggests that further studies may be warranted to evaluate whether there is a role for IL-10 therapy in preventing HIV infection in individuals soon after exposure to HIV such as for children born to HIV-infected mothers.

Immunoregulatory properties of ISG15, an interferon-induced cytokine.

ISG15 is a 15-kDa protein of unique primary amino acid sequence, which is transcriptionally regulated by interferon (IFN) alpha and IFN-beta. Because it is synthesized in many cell types and secreted from human monocytes and lymphocytes, we postulated that ISG15 might act to modulate immune cell function. ISG15 stimulated B-depleted lymphocyte proliferation in a dose-dependent manner with significant proliferation induced by amounts of ISG15 as low as 1 ng/ml (58 pM). Maximal stimulation of [3H]thymidine incorporation by B-depleted lymphocytes occurred at 6-7 days. Immunophenotyping of ISG15-treated B-depleted lymphocyte cultures indicated a 26-fold expansion of natural killer (NK) cells (CD56+). In cytotoxicity assays, ISG15 was a potent inducer of cytolytic activity directed against both K562 (100 lytic units per 10(6) cells) and Daudi (80 lytic units per 10(6) cells) tumor cell targets, indicating that ISG15 enhanced lymphokine-activated killer-like activity. ISG15-induced NK cell proliferation required coculturing of T and NK cells, suggesting that soluble factor(s) were required. Measurement of ISG15-treated cell culture supernatants for cytokines indicated production of IFN-gamma (> 700 units/ml). No interleukin 2 or interleukin 12 was detected. IFN-gamma itself failed to stimulate lymphocyte proliferation and lymphokine-activated killer cell activation. Further, induced expression of IFN-gamma mRNA was detected by reverse transcription-PCR in T lymphocytes after ISG15 treatment but not in NK cells. Enhancement of NK cell proliferation, augmentation of non-major histocompatibility complex-restricted cytotoxicity, and induction of IFN-gamma from T cells identify ISG15 as a member of the cytokine cascade and suggest that it may be responsible for amplifying and directing some of the immunomodulatory effects of IFN-alpha or IFN-beta.

Interleukin 2 activates STAT5 transcription factor (mammary gland factor) and specific gene expression in T lymphocytes.

Although prolactin and interleukin 2 (IL-2) can elicit distinct physiological responses, we have found that their signal pathways share a common signal transducer and activator of transcription, STAT5. STAT5 was originally identified as a mammary gland factor induced by prolactin in lactating breast cells. Here we demonstrate that STAT5 is activated after IL-2 stimulation of two responsive lymphocyte cell lines, Nb2 and YT. Activation of STAT5 is measured both by IL-2-induced tyrosine phosphorylation and by IL-2-induced DNA binding. The STAT5 DNA recognition site is the same as the interferon gamma-activated site (GAS) in the interferon regulatory factor 1 gene. We demonstrate that the GAS element is necessary and sufficient for transcriptional induction by both IL-2 and prolactin in T lymphocytes. These results indicate that the role of STAT5 in the regulation of gene expression is not restricted to mammary cells or to prolactin, but is an integral part of the signal pathway of a critical immunomodulatory cytokine, IL-2.

Induction of intracellular cAMP by a synthetic retroviral envelope peptide: a possible mechanism of immunopathogenesis in retroviral infections.

A synthetic heptadecapeptide, CKS-17, represents the highly conserved amino acid sequences occurring within the transmembrane envelope protein of many animal and human retroviruses. CKS-17 has been demonstrated to exhibit suppressive properties for numerous immune functions. We have recently shown that CKS-17 acts as an immunomodulatory epitope causing an imbalance of human type 1 and type 2 cytokine production and suppression of cell-mediated immunities. cAMP, an intracellular second messenger, plays an important role in regulation of cytokine biosynthesis--i.e., elevation of intracellular cAMP levels selectively inhibits type 1 cytokine production but has no effect or enhances type 2 cytokine production. Here, we demonstrate that CKS-17 induces dramatic rises in the intracellular cAMP levels of a human monocyte cell line and of human peripheral blood mononuclear cells in a time- and dose-dependent manner. A peptide corresponding to the reverse sequence of CKS-17, used as control, has no effect on intracellular cAMP levels. The cAMP-inducing ability of CKS-17 is significantly blocked by SQ-22536, an inhibitor of adenylate cyclase. These results indicate that CKS-17, a highly conserved component of the transmembrane proteins of immunosuppressive retroviruses, induces increased intracellular levels of cAMP via activation of adenylate cyclase and suggest that this retroviral envelope peptide may differentially modulate type 1 and type 2 cytokine production through elevation of intracellular cAMP levels.

Differential modulation of Th1- and Th2-related cytokine mRNA expression by a synthetic peptide homologous to a conserved domain within retroviral envelope protein.

The influence of a synthetic retroviral peptide, CKS-17, on T helper type 1 (Th1)- or Th2-related cytokines was investigated in human blood mononuclear cells. Cells were stimulated with staphylococcal enterotoxin A, anti-CD3 plus anti-CD28 monoclonal antibodies, or lipopolysaccharide to induce cytokine mRNA. mRNA was detected by a reverse transcription-polymerase chain reaction or Northern blot analysis. CKS-17 down-regulated stimulant-induced mRNA accumulation for interferon gamma (IFN-gamma), interleukin (IL)-2, and p40 heavy and p35 light chains of IL-12, a cytokine that mediates development of Th1 response. CKS-17 up-regulated stimulant-induced mRNA accumulation of IL-10 and did not suppress Th2-related cytokine (IL-4, IL-5, IL-6, or IL-13) mRNA expression. A reverse sequence of CKS-17 peptide, used as a control, showed no such action. Anti-human IL-10 monoclonal antibody blocked ability of CKS-17 to inhibit mRNA accumulation for IFN-gamma but not the CKS-17 suppressive activity of IL-12 p40 heavy chain mRNA. Thus, CKS-17-mediated suppression of IFN-gamma mRNA expression is dependent upon augmentation of IL-10 production by CKS-17. This conserved component of several retroviral envelope proteins, CKS-17, may act as an immunomodulatory epitope responsible for cytokine dysregulation that leads to suppression of cellular immunity.