Stimulate Interferon Production Research Articles

Safety and efficacy of lentinan nasal drops in patients infected with the variant of COVID-19: a randomized, placebo-controlled trial.

Objective: Lentinan has antiviral, anti-tumor, immunomodulatory, stimulating interferon production, and other pharmacological effects. Previous animal experiments have shown that lentinan nasal drops can assist [Corona Virus Disease 2019) COVID-19] vaccine to induce high levels of neutralizing antibodies and can effectively resist the invasion of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This study aimed to evaluate the safety and efficacy of lentinan nasal drops in patients infected with Omicron (SARS-CoV-2 variant) through a dose-escalation study and a placebo-controlled trial. Methods: A randomized, placebo-controlled trial. The study was divided into two phases: Phase I: a dose escalation trial in which 24 COVID-19 patients were enrolled, that is, 12 in the escalation dose group (50, 75, and 100µg/day) and 12 in the standard treatment group. The aim was to evaluate the safety and tolerance of lentinan nasal drops. The second stage was a placebo-controlled study. The optimal dose group of the first stage was used as the therapeutic dose, and the sample size was expanded to verify the anti-COVID-19 efficacy of lentinan nasal drops. Results: In the dose-increasing study, lentinan nasal drops showed good safety, and no serious adverse reactions occurred. The virus shedding time of the 100µg dose group was significantly shorter than that in the control group (7.75 ± 1.71 VS 13.41 ± 3.8days) (p = 0.01), and the 100µg/day lentinan nasal drops were tolerated well. The results of the placebo-controlled study showed that compared with that in the placebo group, the time for COVID-19 antigen to turn negative was significantly shorter in the 100µg lentinan nasal drop group (p = 0.0298), but no significant difference was observed in symptom improvement between the two groups. In the placebo-controlled study, two patients experienced mild nasal discomfort with nasal drops, but the symptoms relieved themselves. Conclusion: Lentinan nasal drops are tolerated well and can shorten the time of virus clearance.

SINE RNA of the imprinted miRNA clusters mediates constitutive type III interferon expression and antiviral protection in hemochorial placentas

Hemochorial placentas have evolved defense mechanisms to prevent the vertical transmission of viruses to the immunologically underdeveloped fetus. Unlike somatic cells that require pathogen-associated molecular patterns to stimulate interferon production, placental trophoblasts constitutively produce type III interferons (IFNL) through an unknown mechanism. We demonstrate that transcripts of short interspersed nuclear elements (SINEs) embedded in miRNA clusters within the placenta trigger a viral mimicry response that induces IFNL and confers antiviral protection. Alu SINEs within primate-specific chromosome 19 (C19MC) and B1 SINEs within rodent-specific microRNA cluster on chromosome 2 (C2MC) produce dsRNAs that activate RIG-I-like receptors (RLRs) and downstream IFNL production. Homozygous C2MC knockout mouse trophoblast stem (mTS) cells and placentas lose intrinsic IFN expression and antiviral protection, whereas B1 RNA overexpression restores C2MCΔ/Δ mTS cell viral resistance. Our results uncover a convergently evolved mechanism whereby SINE RNAs drive antiviral resistance in hemochorial placentas, placing SINEs as integral players in innate immunity.

Revealing the characteristics of ZIKV infection through tissue-specific transcriptome sequencing analysis

BackgroundRecently, Zika virus (ZIKV) re-emerged in India and was potentially associated with microcephaly. However, the molecular mechanisms underlying ZIKV pathogenesis remain to be explored.ResultsHerein, we performed a comprehensive RNA-sequencing analysis on ZIKV-infected JEG-3, U-251 MG, and HK-2 cells versus corresponding uninfected controls. Combined with a series of functional analyses, including gene annotation, pathway enrichment, and protein–protein interaction (PPI) network analysis, we defined the molecular characteristics induced by ZIKV infection in different tissues and invasion time points. Data showed that ZIKV infection and replication in each susceptible organ commonly stimulated interferon production and down-regulated metabolic-related processes. Also, tissue-specific immune responses or biological processes (BPs) were induced after ZIKV infection, including GnRH signaling pathway in JEG-3 cells, MAPK signaling pathway in U-251 MG cells, and PPAR signaling pathway in HK-2 cells. Of note, ZIKV infection induced delayed antiviral interferon responses in the placenta-derived cell lines, which potentially explains the molecular mechanism by which ZIKV replicates rapidly in the placenta and subsequential vertical transmission occurs.ConclusionsTogether, these data may provide a systemic insight into the pathogenesis of ZIKV infection in distinct human tissue-derived cell lines, which is likely to help develop prophylactic and therapeutic strategies against ZIKV infection.

TRAF3IP3 mediates the recruitment of TRAF3 to MAVS for antiviral innate immunity.

RIG-I-MAVS antiviral signaling represents an important pathway to stimulate interferon production and confer innate immunity to the host. Upon binding to viral RNA and Riplet-mediated polyubiquitination, RIG-I promotes prion-like aggregation and activation of MAVS. MAVS subsequently induces interferon production by activating two signaling pathways mediated by TBK1-IRF3 and IKK-NF-κB respectively. However, the mechanism underlying the activation of MAVS downstream pathways remains elusive. Here, we demonstrated that activation of TBK1-IRF3 by MAVS-Region III depends on its multimerization state and identified TRAF3IP3 as a critical regulator for the downstream signaling. In response to virus infection, TRAF3IP3 is accumulated on mitochondria and thereby facilitates the recruitment of TRAF3 to MAVS for TBK1-IRF3 activation. Traf3ip3-deficient mice demonstrated a severely compromised potential to induce interferon production and were vulnerable to RNA virus infection. Our findings uncover that TRAF3IP3 is an important regulator for RIG-I-MAVS signaling, which bridges MAVS and TRAF3 for an effective antiviral innate immune response.

Loss of Sendai virus C protein leads to accumulation of RIG-I immunostimulatory defective interfering RNA

Retinoic acid inducible gene (RIG-I)-mediated innate immunity plays a pivotal role in defence against virus infections. Previously we have shown that Sendai virus (SeV) defective interfering (DI) RNA functions as an exclusive and potent RIG-I ligand in DI-RNA-rich SeV-Cantell infected cells. To further understand how RIG-I is activated during SeV infection, we used a different interferon (IFN)-inducing SeV strain, recombinant SeVΔC, which, in contrast to SeV-Cantell is believed to stimulate IFN production due to the lack of the SeV IFN antagonist protein C. Surprisingly, we found that in SevΔC-infected cells, DI RNAs also functioned as an exclusive RIG-I ligand. Infections with wild-type SeV failed to generate any RIG-I-associated immunostimulatory RNA and this correlated with the lack of DI genomes in infected cells, as well as with the absence of cellular innate immune responses. Supplementation of the C protein in the context of SeVΔC infection led to a reduction in the number of DI RNAs, further supporting the potential role of the C protein as a negative regulator of DI generation and/or accumulation. Our findings indicate that limiting DI genome production is an important function of viral IFN antagonist proteins.

Influenza A virus targets a cGAS-independent STING pathway that controls enveloped RNA viruses.

Stimulator of interferon genes (STING) is known be involved in control of DNA viruses but has an unexplored role in control of RNA viruses. During infection with DNA viruses STING is activated downstream of cGAMP synthase (cGAS) to induce type I interferon. Here we identify a STING-dependent, cGAS-independent pathway important for full interferon production and antiviral control of enveloped RNA viruses, including influenza A virus (IAV). Further, IAV interacts with STING through its conserved hemagglutinin fusion peptide (FP). Interestingly, FP antagonizes interferon production induced by membrane fusion or IAV but not by cGAMP or DNA. Similar to the enveloped RNA viruses, membrane fusion stimulates interferon production in a STING-dependent but cGAS-independent manner. Abolishment of this pathway led to reduced interferon production and impaired control of enveloped RNA viruses. Thus, enveloped RNA viruses stimulate a cGAS-independent STING pathway, which is targeted by IAV.

Innate immunity pathways regulate the nephropathy gene Apolipoprotein L1

Apolipoprotein L1 (APOL1) risk variants greatly elevate the risk of kidney disease in African Americans. Here we report a cohort of patients who developed collapsing focal segmental glomerulosclerosis while receiving therapeutic interferon, all of whom carried the APOL1 high-risk genotype. This finding raised the possibility that interferons and the molecular pattern recognition receptors that stimulate interferon production may contribute to APOL1-associated kidney disease. In cell culture, interferons and toll-like receptor agonists increased APOL1 expression by up to 200-fold, in some cases with the appearance of transcripts not detected under basal conditions. PolyI:C, a double-stranded RNA TLR3 agonist, increased APOL1 expression by upregulating interferons directly or through an interferon-independent, IRF-3 dependent pathway. Using pharmacological inhibitors, shRNA knockdown, and chromatin immunoprecipitation, we found that the interferon-independent TLR3 pathway relied on signaling through TBK1, NF-kB, and Jak kinases, and on binding of IRF1, IRF2, and STAT2 at the APOL1 transcription start site. We also demonstrate that overexpression of the APOL1 risk variants is more injurious to cells than overexpression of the wild-type APOL1 protein. Our study illustrates that anti-viral pathways may be an important inducer of kidney disease in individuals with the APOL1 high-risk genotype and identifies potential targets for prevention or treatment.

Novel c-di-GMP recognition modes of the mouse innate immune adaptor protein STING

The mammalian ER protein STING (stimulator of interferon genes; also known as MITA, ERIS, MPYS or TMEM173) is an adaptor protein that links the detection of cytosolic dsDNA to the activation of TANK-binding kinase 1 (TBK1) and its downstream transcription factor interferon regulatory factor 3 (IFN3). Recently, STING itself has been found to be the direct receptor of bacterial c-di-GMP, and crystal structures of several human STING C-terminal domain (STING-CTD) dimers in the apo form or in complex with c-di-GMP have been published. Here, a novel set of structures of mouse STING-CTD (mSTING(137-344)) in apo and complex forms determined from crystals obtained under different crystallization conditions are reported. These novel closed-form structures exhibited considerable differences from previously reported open-form human STING-CTD structures. The novel mSTING structures feature extensive interactions between the two monomers, a unique asymmetric c-di-GMP molecule with one guanine base in an unusual syn conformation that is well accommodated in the dimeric interface with many direct specific interactions and two unexpected equivalent secondary peripheral c-di-GMP binding sites. Replacement of the amino acids crucial for specific c-di-GMP binding in mSTING significantly changes the ITC titration profiles and reduces the IFN-β reporter luciferase activity. Taken together, these results reveal a more stable c-di-GMP binding mode of STING proteins that could serve as a template for rational drug design to stimulate interferon production by mammalian cells.

Usutu virus growth in human cell lines: induction of and sensitivity to type I and III interferons

The mechanisms of Usutu virus (USUV) pathogenesis are largely unknown. The aim of this study was to evaluate the sensitivity of USUV to interferon (IFN) and the capacity of USUV to stimulate IFN production. Initial experiments were conducted to characterize the susceptibility of human cell lines to USUV infection and to evaluate the single-growth cycle replication curve of USUV. Results indicate that USUV is able to infect a variety of human cell lines, completing the replication cycle in Hep-2 and Vero cells within 48 h. Pre-treatment of cells with types I and III IFNs significantly inhibited the replication of USUV. However, the inhibitory effects of IFNs were considerably less if IFN was added after viral infection had been initiated. Also, USUV weakly induced types I and III IFNs.

The Zinc RING Finger of Bovine Herpesvirus 1-Encoded bICP0 Protein Is Crucial for Viral Replication and Virulence

Bovine herpesvirus 1 (BHV-1) infected cell protein 0 (bICP0) stimulates productive infection, in part by activating viral gene expression. The C(3)HC(4) zinc RING finger of bICP0 is crucial for activating viral transcription and productive infection. In this study, we used a bacterial artificial chromosome containing a wild-type (wt) virulent BHV-1 strain to generate a single amino acid mutation in the C(3)HC(4) zinc RING finger of bICP0. This virus (the 51g mutant) contains a cysteine-to-glycine mutation (51st amino acid) in the C(3)HC(4) zinc RING finger of bICP0. A plasmid expressing the 51g mutant protein did not transactivate viral promoter activity as efficiently as wt bICP0. The 51g mutant virus expressed higher levels of the bICP0 protein than did the 51g rescued virus (51gR) but yielded reduced virus titers following infection of permissive bovine cells. The 51g mutant virus, but not the 51gR virus, grew poorly in bovine cells pretreated with imiquimod to stimulate interferon production. During acute infection of calves, levels of infectious virus were 2 to 3 logs lower in ocular or nasal swabs with 51g than with 51gR. Calves latently infected with the 51g mutant did not reactivate from latency because virus shedding did not occur in ocular or nasal cavities. As expected, calves latently infected with 51gR reactivated from latency following dexamethasone treatment. These studies demonstrate that mutation of a single well-conserved cysteine residue in the C(3)HC(4) zinc RING finger of bICP0 has dramatic effects on the growth properties of BHV-1.

Measles Virus Circumvents the Host Interferon Response by Different Actions of the C and V Proteins

Measles is an acute febrile infectious disease with high morbidity and mortality. The genome of measles virus (MV), the causative agent, encodes two accessory products, V and C proteins, that play important roles in MV virulence. The V but not the C protein of the IC-B strain (a well-characterized virulent strain of MV) has been shown to block the Jak/Stat signaling pathway and counteract the cellular interferon (IFN) response. We have recently shown that a recombinant IC-B strain that lacks C protein expression replicates poorly in certain cell lines, and its growth defect is related to translational inhibition and strong IFN induction. Here, we show that the V protein of the MV IC-B strain also blocks the IFN induction pathway mediated by the melanoma differentiation-associated gene 5 product, thus actively interfering with the host IFN response at two different steps. On the other hand, the C protein per se possesses no activity to block the IFN induction pathway. Our data indicate that the C protein acts as a regulator of viral RNA synthesis, thereby acting indirectly to suppress IFN induction. Since recombinant MVs with C protein defective in modulating viral RNA synthesis or lacking C protein expression strongly stimulate IFN production, in spite of V protein production, both the C and V proteins must be required for MV to fully circumvent the host IFN response.

Infectious pancreatic necrosis virus suppresses type I interferon signalling in rainbow trout gonad cell line but not in Atlantic salmon macrophages

RTG-P1 cells are a rainbow trout fibroblastic cell line permanently transfected with the luciferase gene under the control of the Mx promoter. On exposure to interferon (IFN) or IFN inducing agents, the cells produce luciferase. IPNV did not induce luciferase production up to 24 h post-infection but did not suppress constitutive luciferase production. Furthermore, IPNV suppressed luciferase production induced by poly I:C. RT-PCR analysis of IPNV infected cells showed IFN gene transcription from 6 h post-infection with increasing expression up to 24 h. Housekeeping genes β-actin and GAPDH were also expressed along with upregulation of IRF1 and slight upregulation of STAT1. When RTG-P1 cells were stimulated with IFN, Mx transcripts, measured by qRT-PCR, peaked at 3–6 h and thereafter fell to low levels, but in the presence of IPNV, Mx transcription at this time was significantly suppressed but continued to rise gradually. Luciferase production was lower in infected cells at 12 h post-infection but not significantly after 24 h. These results indicate that, in non-stimulated RTG-P1 cells, while IPNV induces IFN transcription, activation of Mx expression is suppressed. Furthermore, when stimulated by IFN, the rate of Mx transcription is significantly suppressed by the virus. This would probably give time for the virus to replicate rapidly in the early phases of infection. Contrary to the fibroblastic cell line, IPNV stimulated IFN production by salmon macrophages in vitro at least as strongly as poly I:C, with no suppression of the IFN response to poly I:C, and the virus persisted for up to 9 days without causing CPE.

PRRS浸潤農場における植物多糖体飼料添加物S-UPによる子豚の呼吸器症状の抑制効果

Porcine reproductive and respiratory syndrome (PRRS) is an economically significant swine disease. Therefore, many studies focus on enhancing the immune response in young pigs. In this study, we investigated the effect of an herbal polysaccharide supplement, S-UP, in post-weaning piglets at a farm chronically infected with PRRSV. S-UP, made from five herbal ingredients that are known to stimulate interferon production was administered in feed at 0.2% for 90 days post-weaning. The growth of the pigs, clinical symptoms and the phagocytic activity of isolated alveolar and peritoneal macrophages were examined. As a result, significant improvements in the growth rate and suppression of respiratory symptoms were observed in pigs fed with S-UP compared to the control, untreated pigs. The mean number of latex beads phagocytizedin vitroby a single macrophage was similar in both the pigs administered S-UP and the control pigs. Macrophages isolated from pigs administered S-UP, however, could internalize a multiple number of beads/cell at higher rates after 30 days (peritoneal) and 60 days (alveolar) of S-UP administration compared to those from the control group. Together, our results suggest that S-UP is effective in promoting the health and macrophage activity in growing piglets exposed to PRRSV.

Current status of immunomodulatory therapies in HCV infection

A number of immunomodulatory agents have been under study that facilitate a Th1 immune response, enhancing Tlymphocyte activity and stimulating interferon production. These include cytokines (interleukin [IL]-12, IL-10, IL-2), histamine, oral interferon-like molecules, ribavirin analogues, interferon gamma, thymosin alpha-1, amantadine, inosine-5′-monophosphate dehydrogenase inhibitors, tumor necrosis factor antagonists, and nonspecific hepatoprotectants (glycyrrhizin and caspase inhibitors). The current available data on these drugs are analyzed with an attempt to determine their usefulness as future hepatitis C virus treatments.

Possible pathomechanism of autoimmune hepatitis.

The hepatotropic viruses, measles, and herpesviruses as well as different drugs were repeatedly shown to act presumably as a trigger in patients with autoimmune hepatitis (AI-H). On the other hand, it is known that viral infections stimulate interferon production, which inactivates the cytochrome P-450 enzymes involved in the metabolism of several endogenous substances and exogenous environmental agents. Moreover, it was reported that several cytokines, including interferons, as well as transforming growth factor beta1 and human hepatocyte growth factor, which are abundantly produced and released in the body during infections, also downregulated expression of major cytochrome P-450 and/or other biotransformation enzymes. It seems that all these factors, in addition to individual immune response and the nature and amount of the neoantigen(s) produced, impair the equilibrium of bioactivation and detoxication pathways, thus leading to the development of AI-H in a genetically predisposed person continually exposed to harmful environmental factor(s). Possible increased/decreased density of lysine residues at position D-related human leukocyte antigen locus (DR)beta71 of the antigen-binding groove may affect the eventual steroid-sparing effect of this critical amino acid at the cellular level. In addition, some food additives, such as monosodium glutamate (MSG) and/or aspartame regularly consumed in excessive amounts, may eventually disturb the delicate balance between a positively charged amino acid residue at position DRbeta71 (lysine or arginine) and a negatively charged amino acid residue at position P4 on the antigenic peptide (glutamic acid or aspartic acid). This may favor formation of a salt bridge between these amino acid residues within the hypervariable region 3 on the alpha-helix of the DRbeta polypeptide and facilitate autoantigen presentation and CD4 T-helper cell activation. MSG and aspartate may also depress serum concentrations of growth hormone, which downregulate the activity of several cytochrome P-450 hepatic and other drug-metabolizing enzymes, thus increasing sensitivity to some environmental agents and possibly influencing efficacy of treatment regimens and final outcome of patients with type 1 AI-H.

Russian Experience in Screening, Analysis, and Clinical Application of Novel Interferon Inducers

This review describes a long-standing experience of screening for interferon (IFN) inducers in Russia. IFN inducers represent a special group of potential antiviral compounds. The main requirements for them are (1) high IFN-inducing activity, (2) absence of side effects, (3) wide spectrum of antimicrobial activity, (4) broad therapeutic security and, (5) good solubility in water and biologic liquids. IFN inducers stimulate IFN production in different cells and organs, and that determines the strategy for their application. Amixin (OOO "Lancepharm," Moscow, Russia) induces IFN-alpha/beta production mostly in T cells. Cycloferon (NTFF "Polysan," St. Petersburg, Russia) stimulates B cells and macrophages to produce almost pure IFN-alpha. Double-stranded RNA (dsRNA) and polyphenols of natural origin stimulate IFN production in different populations of immunocytes. Only polymers, such as Larifan (Riga, Latvia), Kagocel ("NIARnedicplus," Moscow, Russia), and Ragosin (N.F. Gamaleya Institute, Moscow, Russia), induce IFN synthesis in muscles, so they may be effective against rabies. Cycloferon, Larifan, and Kagocel, which induce IFN formation in lungs, may be effective against influenza and rhinoviral infections. Cycloferon and Larifan stimulate IFN production in liver and spleen and may be effective against hepatitis B. Oral compounds (Amixin, Kagocel) that stimulate IFN production in intestines may be effective against hepatitis A and enteroviral infections. Low molecular weight inducers (Amixin, Cycloferon, Kagocel) that penetrate the blood-brain barrier may be active against viral encephalitis. At present, clinical trials of IFN inducers are limited, but in the near future, IFN inducers may be used against very different infections and conditions.

Synthetic oligonucleotides with certain palindromes stimulate interferon production of human peripheral blood lymphocytes in vitro.

We studied the ability of synthetic single‐stranded 30‐mer oligodeoxyribonucleotides oligoDNAs) with three different kinds of hexamer palindromic sequence to induce interferon (IFN) production of human peripheral blood lymphocytes (PBL). When PBL was cultured with oligoDNA having a palindrome of AACGTT or GACGTC, IFN activity was detected by bioassay in the culture fluid after 8 h, and the amount of IFN reached the maximum after 18 h. IFN‐α was predominantly produced, and small amounts of IFN‐β and IFN‐γ were also found. OligoDNA with the palindrome ACCGGT had no effect.

50 INTERFERON PRODUCTION IN NEWBORNS: THE EFFECT OF DIPYRIDAMOLE

The ability of dipyridamole to stimulate interferon production in adults is known since 1978. The aim of our investigation was to reveal the existance of such ability in newborns. 43 newborns with both uncomplicated and complicated respiratory infections were examined. They received dipyridamole (CurantylR) orally in a single dose equal to 2-3 mg/kg body weight. Control group consisted of 50 newborns with identical disease not treated with dipyridamole. Serum level of interferon was determined before treatment and at 24, 48, 72 and 96 hours after the treatment. Interferon titers before treatment in both groups were below 1:8. In newborns treated with dipyridamole interferon titers at 24 and 48 hours were 1:16 and 1:45, respectively, and decreased to the basic level after 72-96 hours. Interferon titers in the control group did not change significantly. Our results suggest that newborns react to interferonogen dipyridamole in similar way as do adults and elder children.

Antibody targeted liposomes containing poly(rI) · poly(rC) exert a specific antiviral and toxic effect on cells primed with interferons α/β or γ

Double-stranded RNA can stimulate interferon production and mediate an antiproliferative effect on certain cell types. We evaluated the possibility of specifically targeting to cells in vitro the RNA duplex poly(rI).poly(rC) in pharmacologically active form after its encapsulation in small, unilamellar liposomes, to which was covalently coupled protein A. These liposomes became bound to and were endocytosed by murine L929 cells in the presence of protein A-binding monoclonal antibodies specific for an expressed cell surface protein, the H-2K molecule. When L929 cells were preincubated in the presence of low doses of interferon alpha/beta or gamma, they could be activated to produce interferon following exposure to either free poly(rI).poly(rC), or specifically bound liposomes poly(rI).poly(rC), but not the same liposomes in the presence of non-cell binding control antibodies. Specifically bound liposome-encapsulated poly(rI).poly(rC) was toxic to L929 cells at dose levels at least three logs lower than free poly(rI).poly(rC). This toxicity was also dependent on pre-treatment with interferon. These results indicate that liposome-encapsulated poly(rI).poly(rC) can survive endocytosis and can be released in active form to specific cell populations, at concentrations much lower than that required for pharmacologic effects of the same molecule in free form. They suggest that introduction into cells of other nucleic acids might benefit from the antibody-targeted liposome technology described here.

Natural killer cell activation and interferon production by peripheral blood lymphocytes after exposure to bacteria

We have previously shown that peripheral blood natural killer (NK) cells have significant levels of cytotoxic activity against Shigella flexneri-infected HeLa cells. In this report, we show that NK cell activity against K562 tumor cells and Shigella flexneri-infected HeLa cells can be greatly enhanced by preincubating peripheral blood lymphocytes (PBL) for 18 h with kanamycin-treated Shigella flexneri or Salmonella typhimurium. Cell-free supernatants obtained from PBL-bacteria cultures contained high levels of interferon (IFN) activity, which was characterized as a mixture of IFN-gamma and IFN-alpha. Cytotoxic activity associated with PBL precultured with shigellae was associated with predominantly CD16+ (Leu-11+) and CD2+ (OKT-11+) cells. Further, IFN production was dependent upon the presence of CD16+ and CD2+ cells at culture initiation. Enhancement of cytotoxic activity associated with PBL-bacteria cultures did not, however, appear to be dependent upon IFN production, since low numbers of bacteria which failed to stimulate IFN production induced high levels of NK cell activity. Lipopolysaccharide appeared not to be involved in bacteria-induced IFN production or enhanced NK cell activity, since Salmonella lipopolysaccharide failed to induce IFN production or enhance NK cell activity. These results suggest that IFN production by NK cells and the killing of bacteria-infected cells play an important role in host defense against facultative intracellular bacterial infections.