Search

Dendritic cells (DCs) are a key part of host defense against microbial pathogens, being part of the innate immune system, but also instructing the adaptive T cell response. This study was designed to evaluate whether human DCs directly contribute to innate immunity by killing intracellular bacteria, using tuberculosis as a model. DCs were detected in bronchoalveolar lavage samples indicating that DCs are available for immediate interaction with Mycobacterium tuberculosis (M. Tb) after inhalation of the pathogen. The phenotype of DC in bronchoalveolar lavage closely resembles monocyte-derived immature DC (iDC) according to the expression of CD1a, CD83, and CCR7. The antimicrobial activity of iDC against intracellular M. Tb inversely correlated with TNF-alpha-release and was enhanced by treatment with anti-TNF-alpha Abs. Differentiation of iDC into mature DC by addition of TNF-alpha or activation via Toll-like receptors further reduced killing of M. Tb. The antibacterial activity against intracellular M. Tb of all DCs was significantly lower than alveolar macrophages. Therefore, the maintenance of a pool of DCs at the site of disease activity in tuberculosis, and the maturation of these DC by TNF-alpha provides a mechanism by which M. Tb escapes the innate immune system.

We have previously shown that a synthetic peptide (CKS-17) homologous to retroviral envelope protein suppresses the accumulation of superantigen staphylococcal enterotoxin-induced TNF-alpha mRNA in human PBMC and in highly purified human monocytes. The present study was designed to examine the underlying mechanism(s) by which CKS-17 down-regulates the TNF-alpha mRNA expression using a human acute monocytic leukemia cell line THP-1 stimulated with the superantigen staphylococcal enterotoxin E. A cyclooxygenase inhibitor indomethacin does not reverse the inhibition of TNF-alpha mRNA expression by CKS-17, suggesting that prostaglandins are not responsible for the suppressive action of CKS-17. The inhibitory effect of CKS-17 is, however, significantly blocked by a protein synthesis inhibitor cycloheximide, indicating that CKS-17 requires de novo protein synthesis to induce the suppressive activity. The mRNA stability assays using actinomycin D show that CKS-17 does not decrease the TNF-alpha mRNA stability. Nuclear run-on transcription assays further reveal that CKS-17 suppresses the TNF-alpha mRNA transcription rate. Taken together, these results suggest that the synthetic retroviral peptide CKS-17 down-regulates TNF-alpha mRNA expression through inhibition of transcriptional activation of the TNF-alpha gene, which requires de novo synthesis of a transcriptional repressor protein(s).

Retroviral infection is associated with immunosuppression, which has been shown to be due, in part, to the action of the envelope protein p15E. We studied a synthetic peptide (CKS-17) homologous to a highly conserved domain of the retroviral envelope protein p15E, which, when conjugated to BSA (CKS-17-BSA), can inhibit IL-1- and phorbol ester-mediated responses in cultured murine thymoma cells, and Ca2(+)- and phosphatidylserine-dependent protein kinase C (PKC) activity of cell homogenates. We characterized the mechanism of inhibition of PKC by the peptide. Using PKC purified from rat brain we found that CKS-17-BSA inhibited PKC-catalyzed Ca2(+)- and phosphatidylserine-dependent histone phosphorylation with an estimated ID50 of 4 microM. CKS-17-BSA did not inhibit the catalytic subunit of cAMP-dependent protein kinase. CKS-17-BSA also inhibited the Ca2(+)- and PS-independent activity of a catalytic fragment of PKC that was generated by limited trypsin treatment. However, CKS-17-BSA did not act as a competitive inhibitor of PKC with respect to ATP or phosphoacceptor substrate, despite the similarity between the CKS-17 sequence and substrates and pseudosubstrates of PKC. We conclude that this peptide homologue of a retroviral envelope protein has a novel mechanism of inhibition of PKC.

A synthetic 17 amino acid peptide (CKS-17) homologous to a highly conserved region of human and animal retroviral transmembrane proteins was investigated for its influence on the generation of murine alloantigen-specific CTL activity in vitro. CKS-17 coupled to a carrier protein, BSA or human serum albumin, inhibited the generation of anti-allo CTL in a dose-dependent manner. Controls consisting of BSA and human serum albumin, which had undergone the coupling procedure or neurotensin, an unrelated peptide, coupled to BSA in an identical manner as CKS-17 showed no such inhibitory action. The suppression was not restricted to the Ag specificity of the CTL activity. CKS-17 exerted inhibitory effects on the early afferent phase of CTL induction. Kinetic studies showed that the suppressive activity occurred when CKS-17 was introduced to the immunologically stimulating culture concomitant with or up to 48 h after initiation of culture. Analysis of the frequency of CTL precursor cells using limiting-dilution assays revealed that CKS-17 did act to reduce the number of precursor cells. Abrogation of the inhibition of CTL activity was observed when IL-2 was introduced to the culture together with the stimulator cells. Other lymphokines, such as IL-4, exerted a similar influence to counteract this suppression.

The present study was designed to characterize the production of chemoattractants by human melanoma lines with high (M4Be, M3Da, NTerDa) or low tumorigenic (Doc8, M1Do) potential when heterotransplanted in nude mice. Supernatants from the Doc8 and M1Do cell lines were strongly chemotactic in vitro for mononuclear phagocytes. Chemotactic activity was destroyed by proteolytic enzymes, and upon gel filtration on Sephadex G75, it eluted in the cytochrome c region corresponding to an apparent m.w. of 12,000. Upon chromatofocusing, the Sephadex-separated tumor-derived chemotactic factor (TDCF) showed an isoelectric point of 5.5 to 6. Cell lines with high tumorigenic potential contained low or no detectable chemotactic activity. When culture supernatants of cell lines with modest (M3Da) or no (M4Be) chemotactic activity were exposed to immobilized monoclonal antibodies directed against the retroviral transmembrane protein P15E, appreciable chemotactic activity was detectable (M4Be) or preexisting levels increased (M3Da). The material eluted from Sepharose-bound anti-P15E antibodies inhibited the migration of monocytes in response to chemoattractants. These findings demonstrate the coexistence in some human melanoma cell line supernatants of factors (TDCF and P15E-related inhibitor) with opposite influence on monocyte chemotaxis. That tumor cell products play a pivotal role in regulating the extravasation of monocytes into neoplastic tissues is suggested by the close correlation observed between macrophage levels in melanomas grown in nude mice and levels of chemotactic activity detectable in culture supernatants.

Two sets of seemingly contradictory evidence have been reported concerning the effects of tumor cell products on the regulation of monocyte migration in vitro and presumably the extravasation of macrophages into tumors in vivo. The present study was designed to explore the relationship between chemotactic and anti-chemotactic products related to tumor cells: a tumor-derived chemotactic factor (TDCF) and retroviral P15E-related inhibitor(s) of chemotaxis. Culture supernatants of the human 8387 sarcoma and SW626 ovarian carcinoma were depleted of P15E-related antigens with immobilized anti-P15E monoclonal antibodies. This treatment produced a significant and consistent increase of the polarizing and chemotactic activity in the tumor cell supernatants. The material eluted from Sepharose-bound anti-P15E antibodies was devoid of chemotactic and polarizing activity and suppressed the polarization and migration of monocytes in response to chemoattractants. These results demonstrate the coexistence in culture supernatants of two human tumor cell lines of factors with opposite influences on monocyte chemotaxis. The data suggest that the entry of monocytes into neoplastic tissue may be regulated by the interplay of chemotactic and anti-chemotactic principals produced by tumor cells.

We studied the mode of action of the synthetic peptide CKS-17, which is a heptadecapeptide homologous to a highly conserved region of the immunosuppressive retroviral envelope protein p15E, as well as to envelope proteins of the human T cell leukemia virus I and II. Previous studies have established that CKS-17 conjugated to BSA (CKS-17-BSA) inhibited IL-1-mediated tumor toxicity in melanoma cells and proliferation in murine Th clones. We examined the effects of CKS-17-BSA on IL-1 action. CKS-17-BSA did not bind to IL-1, nor did it affect the number of IL-1 receptors, their binding affinity, or their ability to internalize IL-1. However, CKS-17-BSA inhibited production of IL-2 by murine thymoma cells treated with IL-1 or with 12-O-tetradecanoyl phorbol-13 acetate. The potent protein kinase C inhibitor, H7, also inhibited IL-1-mediated responses, while HA1004, a weak inhibitor of protein kinase C, did not. Protein kinase C activity in the cytosolic fraction prepared from thymoma cells was found to be inhibited by CKS-17-BSA in a dose-dependent manner. All of these findings are consistent with the idea that CKS-17-BSA inhibits IL-1-mediated responses by interfering with signal transduction through a protein kinase C pathway.

The synthetic peptide CKS-17 has homology to a highly conserved region of the immunosuppressive retroviral envelope protein P15E, to envelope proteins of HTLV I, II, III, and to that encoded by an endogeneous C-type human retroviral DNA. CKS-17 inhibits the immune response of lymphocytes and the respiratory burst of human monocytes. Because P15E-related antigens are present in human malignant cell lines and cancerous effusions, we sought to determine the effect of CKS-17 on monocyte-mediated tumor cell lysis. Lysis of A375 tumor cells by lymphokine or lipopolysaccharide-activated human monocytes was inhibited by 10 microM CKS-17 (control, 79%; CKS-17-treated, 19%). Another synthesized peptide, CS-2, which has partial homology to CKS-17, failed to block monocyte-mediated killing. Thus, the inhibition by CKS-17 appeared to be specific. Because interleukin 1 (IL-1) is a cytocidal factor produced by activated monocytes, we also investigated the effect of CKS-17 on IL-1 production by monocytes and on direct IL-1-mediated cytotoxicity. CKS-17 did not block production or secretion of IL-1 by lipopolysaccharide- or interferon-gamma-activated monocytes. However, the direct cytocidal activity of both recombinant IL-1 alpha and IL-1 beta against A375 tumor cells was blocked by CKS-17. The cytotoxic activity of IL-1 was inhibited by CKS-17 if (a) IL-1 was preincubated with CKS-17 for 1 hr at 37 degrees C or (b) the A375 cells were incubated with CKS-17 for 1 hr prior to the addition of IL-1. CKS-17 also blocked IL-1-induced proliferation of murine thymocytes, the D10 T cell line, and an IL-1-responsive astrocytoma cell line. These data suggest that CKS-17 may be a potent inhibitor of IL-1.

It has been shown previously that the retroviral envelope protein p15E suppresses certain monocyte and lymphocyte functions. In this paper, we describe the effects on natural killer (NK) activity of a synthetic peptide (CKS-17) with homology to a region of p15E conserved among numerous retroviruses. Enriched human NK cells were assayed against K562 tumor target cells in a 51Cr-release cytotoxicity assay. Pretreatment of NK cells with CKS-17 at concentrations as low as 1.5 microM, but not with equivalent concentrations of control materials, markedly and reproducibly suppressed NK lytic activity. Prior exposure of NK cells to interferon-alpha (IFN-alpha) at 1000 U/ml did not alter their sensitivity to CKS-17-induced inhibition. Pretreating NK cells with CKS-17 almost entirely diminished their responsiveness to IFN-alpha and IFN-gamma, but not to interleukin 2 (IL 2). Kinetics experiments demonstrated that CKS-17-mediated suppression of both endogenous and activated NK cells was reversible after 18 hr at 37 degrees C. Experiments designed to examine the CKS-17 mechanism of action revealed that the peptide bound to all Leu-11+ lymphocytes, as shown by two-color flow cytometry. CKS-17 did not, however, inhibit effector cell/target cell conjugate formation. These data suggest a new mechanism for immune suppression mediated by retroviruses; inhibition of NK function. They moreover imply that the CKS-17 peptide interferes with the lytic phase of NK cytolysis.

A synthetic 17 amino acid peptide (CKS-17) homologous to a highly conserved region of human and animal retroviral transmembrane proteins was investigated for its influence on the in vitro production of IFN-gamma from human peripheral mononuclear cells. The results showed that CKS-17 coupled to a carrier protein, BSA, inhibited production of IFN-gamma in a dose-dependent manner. Controls, consisting of BSA, which had undergone the coupling procedure or neurotensin coupled to BSA in an identical manner as CKS-17, showed no such inhibition. Reduction in IFN-gamma production could not be attributed to decreased viability of cells, delay of IFN-gamma production or to involvement of suppressor cells. Moreover, inhibition of IFN-gamma production was not related to the inhibition of DNA synthesis. The inhibition appeared to be a direct effect of CKS-17 on IFN-gamma-producing cells. Kinetic studies revealed that this suppression occurred when CKS-17 was introduced to the culture concurrent with or within 48 h after introduction of IFN inducers. Preincubation experiments showed that the presence of CKS-17 in the culture medium was not necessary to exert its inhibitory effect. These results suggest that a portion of retroviral envelope proteins possess important immunomodulatory actions.

Low m.w. extracts from three known oncogenic viruses, Friend, Moloney, and Rauscher, inhibited the accumulation of macrophages at sites of delayed inflammatory reactions in mice. The potential biologic significance of these proteins is suggested by their potency: as little as 1.2 ng of viral protein inhibited (p less than 0.02) macrophage accumulation when injected at a site distant to the inflammatory reaction. A virus envelope protein fraction of 15,000 daltons (p15E) was likewise found to inhibit macrophage accumulation and may in part represent the active factor of the virus extracts. Certain oncogenic viruses may thus exert their immunosuppressive activity by release of potent inhibitors of systemic macrophage function.

The effects of IL-1 on vascular endothelium result in a complex set of alterations which are potentially disruptive of vessel wall and underlying tissue integrity. The present study was aimed at investigating possible regulation of such potentially destructive responses elicited by IL-1 on endothelial cells. Culture supernatants of IL-1-treated human umbilical vein endothelial cells (HEC) were depleted of retroviral p15E-related Ag with immobilized anti-p15E mAb. The monocyte chemotactic and polarizing activity of supernatants of IL-1-treated HEC (presumably related to colony-stimulating factors being released by HEC) was markedly augmented by absorption on immobilized anti-p15E antibodies. Irrelevant IgG had no effect and anti-p15E antibodies did not affect the chemotactic activity of supernatants from unstimulated HEC. The material eluted from Sepharose-bound anti-p15E antibodies was devoid of chemotactic and polarizing activity and suppressed the polarization and migration of monocytes in response to chemoattractants. The alpha and beta molecular species of IL-1 were equally effective in inducing the production of p15E-related inhibitor. The production of a p15E-related inhibitor of chemotaxis induced by IL-1 in HEC may represent a negative signal in the regulation of the potentially destructive responses to pro-inflammatory cytokines.