HESI/FDA workshop on immunomodulators and cancer risk assessment: Building blocks for a weight-of-evidence approach

Profound immunosuppression (e.g., AIDS, transplant therapy) is epidemiologically associated with an increased cancer risk, and often with oncogenic viruses. It is currently unclear how broadly this association translates to therapeutics that modulate immunity. A workshop co-sponsored by the FDA and HESI examined how perturbing the immune system may contribute to carcinogenesis, and highlighted priorities for improving non-clinical risk assessment of targeted immunomodulatory therapies. Conclusions from the workshop were as follows. 1) While profound altered immunity can promote tumorigenesis, not all components of the immune system are equally important in defense against or promotion of cancer and a similar cancer risk for all immunomodulatory molecules should not be assumed. 2) Rodent carcinogenicity studies have limitations and are generally not reliable predictors of cancer risk associated with immunosuppression. 3) Cancer risk needs to be evaluated based on mechanism-based weight-of-evidence, including data from immune function tests most relevant to tumor immunosurveillance or promotion. 4) Information from nonclinical experiments, clinical epidemiology and immunomodulatory therapeutics show that immunosurveillance involves a complex network of cells and mediators. To support a weight-of-evidence approach, an increased focus on understanding the quantitative relationship between changes in relevant immune function tests and cancer risk is needed.

Th(1)/Th(2) responses to drugs

T lymphocytes can be characterized by their pattern of cytokine secretion and be divided into type I (Th(l)/Tc(l)) and type 2 (Th(2)/Tc(2)) subsets. The involvement of type-1 or type 2-like responses in sensitization has been studied in the mouse, with reference contact and respiratory contact sensitizers. One interesting feature with certain drugs, such as beta-lactam antibiotics, is the diversity of clinical manifestations associated with immune-mediated hypersensitivity reactions in humans: immediate reactions such as urticaria, Quincke oedema and anaphylactic shock, and delayed hypersensitivity reactions, such as maculopapular rashes, allergic contact dermatitis and skin reactions of other types. In the mouse, Th(1) and Th(2) cytokines have been shown to regulate primary and secondary benzylpenicilloyl- (BPO-) specific antibody responses. Peripheral blood lymphocytes isolated from patients with a clear history of beta-lactam allergy were assessed for type-1 and type-2 phenotypes. Immediate reactions involved mixed Th(1), Tc(1), and Tc(2) responses, whereas allergic contact dermatitis involved Tc(1) and Th(1) cells. Other delayed hypersensitivity reactions to beta-lactams were restricted to Th(1) responses. It has been demonstrated that both CD4(+) and CD8(+)-lidocaine-specific T cell clones isolated from patients with allergic contact dermatitis produced IFN-gamma, even though CD8(+) clones only produce IFN-gamma, while IFN-gamma producing CD4(+) cells concomitantly produced IL-5 and IL-4. Together these data illustrate the heterogeneity of drug-specific T-cell responses.

Interaction between the serotoninergic and dopaminergic systems in d-fenfluramine-induced activation of c-fos and jun B genes in rat striatal neurons

To test for the relative contributions of the dopaminergic and serotoninergic systems in the striatum to the effects of d-fenfluramine, an indirect serotonin receptor agonist, we assessed the expression of Fos/Jun proteins induced by d-fenfluramine given alone or in the presence of dopaminergic or serotoninergic agents. To determine the neuronal targets of d-fenfluramine in the striatum, we identified the phenotypes of striatal neurons in which d-fenfluramine induced Fos expression. Our results demonstrated that d-fenfluramine evokes nuclear expression of Fos/Jun B proteins in the striatum, and that the Fos expression was dose-dependent and accompanied by transient induction of c-fos mRNA. Fos expression was blocked by p-chloroamphetamine, a serotoninergic neurotoxin. Pretreatment with SCH 23390, a D1-dopamine receptor antagonist, led to a marked decrease in Fos/Jun B expression in the caudoputamen, but not in the cortex, whereas pretreatment with methiothepin, a nonselective serotonin 5-HT1 receptor antagonist, blocked Fos expression completely in the cortex and only partially in the caudoputamen. The expression of Fos/Jun B in the striatum occurred mainly in dynorphin-containing neurons and in a subpopulation of striatal interneurons that exhibited NADPH-diaphorase activity. Most of the enkephalin-containing neurons of the striatum did not show Fos/Jun B staining. These results suggest that the mechanism by which d-fenfluramine induces c-fos and jun B expression in the rat caudoputamen depends at least in part on activation of the dopaminergic system by serotonin.

Il-4 and IFN-gamma mRNA induction in human peripheral lymphocytes specific for beta-lactam antibiotics in immediate or delayed hypersensitivity reactions

Beta-lactam antibiotics elicit CD4+ and CD8+ T-cell-mediated immune responses that play a central role in allergic reactions. However, the involvement of a type 1- (Th1 or Tc1) or a type 2-like (Th2 or Tc2) differentiation in drug allergy remains unclear. We investigated the expression of interleukin 4 (IL-4) and interferon gamma (IFN-gamma) mRNA by quantitative reverse transcription and polymerase chain reaction (RT-PCR) in patient-derived peripheral blood lymphocytes following specific in vitro stimulation. Samples were collected from a total of 19 patients who had developed immediate or delayed clinical manifestations of hypersensitivity to beta-lactam and from 11 control subjects. Peripheral blood mononuclear cells (PBMCs) were stimulated with either free antibiotics or antibiotic-human serum albumin (HSA) conjugates. Specific induction of IFN-gamma mRNA expression was observed in 11 of 11 allergic patients with immediate reactions, in 6 of 8 patients with delayed reactions, and in 4 of 11 control subjects. IL-4 mRNA expression was induced in 5 of 11 allergic individuals with immediate reactions but in none of the 8 patients with delayed responses and none of the 11 control subjects. IL-4 mRNA expression was only induced following activation with free drugs, while IFN-gamma mRNA expression was predominantly induced in CD4+ T cells following stimulation with HSA-conjugated drugs. Immediate-type hypersensitivity to beta-lactams was not associated with a pure type 2-like response when PBMCs were specifically stimulated in vitro: Some patients with well-documented history of beta-lactam-induced immediate allergic reaction showed a high IFN-gamma response. Contact dermatitis involved Tc1 and Th1 cells and other delayed hypersensitivity reactions to beta-lactams were restricted to Th1 responses.

[Principal cellular and molecular mechanisms of xenobiotic-induced hypersensitivity reactions]

Sensitization to xenobiotic first involves an interaction between the compound and antigen presenting cells such as Langerhans cells within the epidermis. This leads to the transdermal migration of this presenting cells towards draining lymph nodes. A second step consists in activation, proliferation and differenciation of CD4+ T cells, and CD8+ T lymphocytes involved in delayed hypersensitivity reactions such as contact dermatitis. T helper lymphocytes can differenciate into Th1 (producing interferon-gamma and interleukin (IL)-2) or Th2 (producing IL-4, IL-5, IL-10, IL-13) lymphocytes. Some experimental models allowed to demonstrate a link between Th1- or Th2-type responses and different hypersensitivity reactions. Specific antibody production also plays a key role in xenobiotic-induced allergy, especially IgE production involved in mastocytes degranulation and immediate hypersensitivity reactions.

Quantitation of cytokine mRNA expression as an endpoint for prediction and diagnosis of xenobiotic-induced hypersensitivity reactions

Xenobiotic-induced hypersensitivity reactions are immune-mediated effects that involve specific antibodies and/or effector and regulatory T lymphocytes. Cytokines are key mediators of such responses and must be considered as possible endpoints for predicting sensitizing potency of drugs and chemicals, as well as for helping diagnosis of allergy. Detecting cytokine production at the protein level has been shown to not be always sensitive enough. This paper describes three examples of the utilization of semiquantitative or competitive reverse transcription polymerase chain reaction analysis of interleukin-4, interferon gamma, and interleukin-1beta mRNAs as endpoints for assessing T-cell or dendritic cell responses to sensitizing drugs (beta-lactam antibiotics) or chemicals (dinitrochlorobenzene).

The intestinal microflora regulates cytokine production positively in spleen-derived macrophages but negatively in bone marrow-derived macrophages

Besides its role as a barrier against potential pathogens, intestinal flora is presumed to protect the host by priming the immunological defense mechanisms. In this respect, the influence of intestinal flora on macrophage precursors was examined, and its modulating effect was compared on LPS-induced cytokine production by macrophages derived from bone marrow and spleen precursors (BMDM and SDM respectively). The regulation of IL-1, IL-6, TNF-alpha and IL-12 production in macrophages from germ-free and from three groups of flora-associated mice, conventional, conventionalized and E. coli-mono-associated mice, was investigated. The whole flora inhibited IL-1, TNF-alpha and IL-12 secretion by BMDM, whereas it had a stimulatory effect on IL-12 secretion by SDM. Implantation of E. coli alone enhanced cytokine secretion by BMDM but had a more limited effect than whole flora on SDM, enhancing only TNF-alpha and IL-12 secretion. Study of expression of mRNA showed a correlation with protein secretion for IL-6 but not for TNF-alpha and IL-1. IL-12 enhancement in BMDM seemed to be dependent on regulation of p35 mRNA expression while it was correlated to increased p40 mRNA expression in SDM. The results demonstrated that intestinal flora modulated bone marrow and spleen macrophage cytokine production in a differential manner and suggested a role for bacteria other than E. coli among the whole flora. The contrasting effects exerted by the intestinal flora on bone marrow and spleen precursors are an interesting observation in view of the different functions of these organs in immunity. The finding that intestinal flora enhanced IL-12 production in spleen is also potentially important since this cytokine is implicated in the determination of the relative levels of Th1 and Th2 responses and plays a pivotal role in host defense against intracellular microorganisms.

Assessment of apoptosis in xenobiotic-induced immunotoxicity

Generation of immunity is a highly complex process in which proliferation and differentiation of immune-competent cells regulated by cytokines and cell-cell interactions play a major role. Reducing the number of immune-competent cells or altering the function, selection, and differentiation of lymphocytes after xenobiotic treatment may lead to serious adverse effects. Programmed cell death, or apoptosis, is a highly regulated process by which an organism eliminates unwanted cells without eliciting an inflammatory response. However, xenobiotics are also able to trigger unwanted apoptosis or to alter the regulation of programmed cell death. Cytological characteristics of apoptosis are generally different from those seen in acute pathological cell death resulting from cell injury. The morphological characteristics of apoptosis are unique including cell shrinkage, membrane blebbing, chromatin condensation, DNA fragmentation, disruption of the nuclear lamina, nuclear fragmentation, and emergence of apoptotic bodies. It is now established that apoptosis plays a critical role in both development and homeostasis of the immune system: thymic selection, cytotoxicity, deletion of autoreactive cells, and regulation of the size of the lymphoid compartment. Assessment of apoptosis relies on the morphological and biochemical modifications of the dying cells. As a rule, and because an apoptotic cell rarely displays all of the characteristic apoptotic features, several criteria should be monitored in parallel including morphological examination. The techniques described in this paper have been divided into five categories: analysis of cell morphology by microscopy, identification of DNA fragmentation, determination of mitochondrial membrane potential, detection of plasma membrane changes, analysis of caspase activation.

Mechanisms of drug-induced allergic contact dermatitis

Allergic contact dermatitis is induced by a wide variety of drugs that trigger specific immune responses following topical exposure. Identified chemical structures involved in such reactions include the mercuric and thiosalicylic acid groups of thimerosal, the diphenylketone group of the anti-inflammatory drug ketoprofen, the amide or ester structure of local anesthetics, and the side-chain and thiazolidine ring of beta-lactams. The T cell responses to such compounds involve CD4+ and CD8+ alphabeta+ T lymphocytes and also CD4 /CD8 gammadelta+ T cells. Although "T helper 2" cytokine production by drug-specific human T cells from patients with allergic contact dermatitis has been described, T helper 1-like and T cytotoxic 1-like responses clearly play key roles in this cutaneous reaction.

Testing strategies in immunotoxicology

Developing a battery of immune function assays to screen potential immunotoxic compounds has been a major issue these past years. Improving this approach is possible using new probes and parameters that are now available from recent knowledge on how the immune system is working (apoptosis, RT-PCR for cytokine mRNA expression). Immunotoxic outcome generally results in serious adverse effects, thus it seems appropriate to evaluate this risk early in drug development. This is especially true in the context of the emerging combinatorial chemistry techniques and high throughput screening in pharmacology resulting in probably numerous molecules to test in toxicology. In this case, screening for adverse effects (genotoxicity, hepatotoxicity, immunotoxicity) that may compromise definitely the development of a molecule should be a help in the decision process since more than one molecule with equivalent pharmacological properties may be available.

Effect of influenza virus infection on ovalbumin-specific IgE responses to inhaled antigen in the rat

Upper respiratory tract viral infections have been reported in clinical studies to serve as risk factors for allergic sensitization. In order to study the relationship linking influenza virus illnesses to development of allergy, murine models of allergen sensitization were previously employed. These models showed that lethal influenza viruses were able to trigger allergen-specific immunoglobulin E (IgE) production and to inhibit tolerance to repeated exposure to aerosolized allergen in the mouse. The disadvantage of these murine models consists in the utilization of virulant and lethal strains of influenza virus. A nonlethal rat-adapted influenza virus (RAIV) host resistance model has been developed in our laboratory. It was used to evaluate the effect of influenza virus infection on IgE responses to inhaled ovalbumin (OA) in the rat. The high IgE-responder Brown-Norway (BN) rat was chosen for further study after comparing the IgE response to OA in Fischer 344 (F344) and BN rats. On d 1, BN rats were sensitized by administration of 1 mg OA subcutaneously alone or together with aluminum hydroxide (200 mg) and Bordetella pertussis (15 x 10(9) killed bacilli per rat in 1 ml), or only received saline. Rats were either infected with RAIV or sham-infected on d 0 (24 h prior to sensitization) or on d 15, 17, or 57. Rats were exposed for 3 min to aerosolized OA (OA 3% in phosphate-buffered saline) every week, starting on d 18. Serum OA-specific IgE was evaluated by reverse enzyme-linked immunosorbent assay (ELISA) 3 d after each OA challenge. BN rats elicited a detectable OA-specific IgE response that decreased after repeated aerosol exposures. Influenza virus infection transiently increased the OA-specific IgE response when rats were immunized with OA alone and were infected 1 d prior to the first challenge and also when rats received only saline on d 1, were exposed each week to aerosolized OA, and were infected prior to the seventh challenge. These results, with data previously reported in mice, emphasize the importance of upper respiratory tract viral infection in increasing IgE responses to allergens and may be of importance in human disease.

Interleukin-10 and interleukin-4 have similar effects on hapten-specific primary antibody responses to penicillin in vivo

Interleukin (IL)-10 was initially recognized on the basis of its capacity to inhibit production of interferon (IFN)-gamma by T helper (Th)1 lymphocytes; we examined whether this cytokine can bias the primary antibody (Ab) response to the hapten penicillin. We previously reported that BALB/c and SJL mice develop different responses to benzylpenicillin coupled to tetanus toxoid (BPO-TT). The response of SJL mice was characterized as Th2 on the basis of early and high IL-4 mRNA expression and production of BPO-specific Ab of the IgG1 isotype. In contrast, the response of BALB/c mice was characterized as Th1 on the basis of delayed and weaker IL-4 mRNA expression associated with high anti-BPO IgG2a production (Kerdine, S. et al., Mol. Immunol. 1996. 33: 71). In this report, we demonstrate that in naive animals, the level of expression of IL-10 mRNA in LN cells was high in SJL and barely detectable in BALB/c. In addition, injection of BPO-TT resulted in rapid and large increase of IL-10 mRNA expression in SJL. Neutralization of IL-10 in vivo promoted the production of BPO-specific IgG2a in SJL, and injection of IL-10-CHO cells inhibited BPO-specific IgG2a production in BALB/c. Neutralization on administration of IL-10 had effects very similar to neutralization or administration of IL-4. However, co-neutralization of IL-10 and IL-4 in SJL did not evidence additive or synergistic effects of the two cytokines. Administration of IL-10 or IL-4 in BALB/c inhibited BPO-TT-induced expression of IL-12 p40 mRNA without modulating IFN-gamma mRNA. Together, these data demonstrate that endogenous production of IL-10 regulates the production of IgG2a Ab in response to BPO-TT and that IL-10, like IL-4, is critical for controlling primary responses to antibiotics which behave as haptenic compounds.

Interleukin-4 plays a dominant role in Th1- or Th2-like responses during the primary immune response to the hapten penicillin

Despite a large number of studies on the Thl/Th2 balance during immune response to pathogens or protein antigens, little is known concerning the early events which regulate Thl/Th2 differentiation following a single injection of haptenic compounds. In this work, we studied how two mouse strains with different MHC haplotypes, SJL (H-2s) and Balb/c (H-2d), could develop different primary immune responses to subcutaneously injected benzylpenicillin coupled to tetanus toxoid (BPO-TT). The SJL mice showed a high BPO-specific IgG1 response that was maximum on day 10 and no BPO-specific IgG2a response. In contrast, Balb/c mice showed a high BPO-specific IgG2a response on days 15 and 22 and a weak IgG1 production. In SJL mice, the response to BPO-TT was characterized by a very early and high IL-4 mRNA expression. In Balb/c, a delayed and weaker expression of IL-4 mRNA was observed. Kinetics of IL-2 and IFN-gamma mRNA expression were comparable in both strains, but IFN-gamma mRNA expression was higher in SJL than in Balb/c. In vivo neutralization of IL-4 induced a significant BPO-specific IgG2a production and a two-fold reduction of IgG1 production in SJL mice while it accelerated production of BPO-specific IgG2a in Balb/c mice. In addition, studies of IL-12 p4O and IL-10 mRNA expression following immunization with BPO-TT showed a greater IL-12 p4O mRNA expression in Balb/c mice and a slightly higher IL-10 mRNA expression in SJL. Taken together, our data suggest that Th1 or Th2 differentiation in primary immune responses to haptenic compounds such as penicillin may be driven by the kinetics and the level of IL-4 production rather than by the level of IFN-gamma. Additional cytokines such as IL-10 and IL-12 are likely to contribute to the regulation of this response.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on influenza virus host resistance in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes numerous immunotoxic effects including thymic involution and an immunosuppression of nonspecific as well as specific cell- and humoral-mediated immunity. TCDD administration to laboratory animals also results in a decreased resistance to numerous bacteria, viruses, and parasites. Effects on virus host resistance appear to be among the most sensitive effects of TCDD immunotoxicity. However, previous studies have not achieved a no effect level. The present studies utilized an influenza virus host resistance model in mice to quantify the sensitivity of this model to TCDD and to determine the NOAEL (no observed adverse effect level) of TCDD for influenza virus. Results indicated that a single dose of TCDD at 0.10, 0.05, or 0.01 microgram/kg resulted in an increased mortality to Hong Kong influenza virus when mice were challenged 7 days after TCDD administration. Increased mortality was not correlated with increased virus titers in the lungs. TCDD at 0.005 or 0.001 micrograms/kg had no effect on influenza-induced mortality. TCDD alone did not affect thymus weight at any dose administered in this study. TCDD also did not alter the virus-enhanced increase in lung weight:body weight ratio nor the virus-induced decrease in thymus weight. Thus, low levels of TCDD exposure lead to enhanced mortality to influenza virus; however, the mechanism of this effect remains to be elucidated. Nonetheless, enhanced mortality to influenza virus in mice following a single dose of 10 ng TCDD/kg represents the most sensitive adverse effect yet reported for TCDD.

Immunotoxicological investigation using pharmaceutical drugs. In vitro evaluation of immune effects using rodent or human immune cells

In order to evaluate the relevance of in vitro methods for immunotoxicity assessment, the effects of pharmaceutical drugs on lymphoproliferative and cytotoxic functions of mouse splenocytes and human peripheral blood mononuclear cells (hPBMC) were studied. A comparison of sensitivity of immune cells from different origins to an in vitro exposure to different xenobiotics was performed using non-immunosuppressive (cimetidine and furosemide) and immunosuppressive (azathioprine (AZA), cyclosporine A (CSA), and dexamethasone (DEX)) drugs. For CSA, sensitivity of both rat and mouse splenocytes following in vitro exposure was compared to the one of hPBMC. Immune function tests included lymphoproliferative response to mitogenic lectins (concanavalin A (Con A) and phytohemagglutinin (PHA-P)) or to allogeneic cells (mixed leukocyte response (MLR)) and cytotoxicity assays (cytotoxic-T lymphocyte (CTL) and natural killer (NK) cell-mediated cytolysis). Additionally, to evaluate how well in vitro assays represent the in vivo situation, a comparison of the effect of cyclosporine A on the same immune function tests following in vivo or in vitro exposure was performed. The data obtained show numerous similarities in the effects observed following in vitro exposure of rodent or human cells to the drugs and a very similar sensitivity of rat and mouse cells to CSA in vitro. Discrepancies between human and rodent cells such as lymphoproliferative response to PHA-P following exposure to DEX or sensitivity of CTL-mediated cytolysis to CSA do exist. In vitro assays were very representative of the in vivo situation, both in the rat and in the mouse, following CSA exposure, except for NK cell activity in the rat. These data show the usefulness of in vitro systems for immunotoxicity assessment. They allow direct comparison of rodent and human systems, and could be representative, for drugs altering specifically the immune system like CSA does, of the in vivo situation.

Immune parameters are affected differently after cyclosporine A exposure in Fischer 344 rats and B6C3F1 mice: implications for immunotoxicology

Knowledge of interspecies differences, commonly evaluated in other disciplines such as carcinogenesis, is a prerequisite for an appropriate assessment of immunotoxicological risks. The purpose of this study was to assess interspecies differences following exposure of Fischer 344 rats and B6C3F1 mice to cyclosporine A. These animals were exposed daily to cyclosporine A by oral gavage at 0, 5, 10, 25 mg/kg/day for 14 consecutive days. The results showed that splenocytes lymphoproliferation in response to concanavalin A or phytohemagglutinin, and IgM antibody-forming cells to sheep red blood cells, were affected in both species. Cytotoxic T-lymphocyte activity and mixed lymphocyte response were significantly inhibited in the rat following cyclosporine A exposure while they remained unaffected in the mouse. In contrast, natural killer cell activity was significantly depressed in the B6C3F1 mouse but not in the Fischer 344 rat. The discrepancies between the two species in cytotoxic T-lymphocyte activity and mixed lymphocyte response assays could partially be explained by the constantly higher blood level of cyclosporine A in the rat than in the mouse. When these tests were performed using rat and mouse splenocytes exposed to cyclosporin A in vitro (10(-9) to 10(-5) M) it was possible to correlate in vivo and in vitro data for concanavalin A- and phytohemagglutinin-induced lymphoproliferation and for cytotoxic T-lymphocyte activity but not for mixed lymphocyte response. Natural killer activity was 10-fold more sensitive in mice than in rats in vitro but these results did not clarify the in vivo difference. In conclusion, these results emphasize that the utilization of more than one species should be considered when assessing immunotoxicity.

Immunotoxicological investigation using pharmaceutical drugs: in vivo evaluation of immune effects

Traditional methods for toxicological assessment have indicated that the immune system is a frequent target of toxic insult following subchronic or chronic exposure to xenobiotics. However, most of the xenobiotics evaluated in standardized protocols were environmental chemicals and correlation with available clinical data was not possible. The purpose of this work was to evaluate the potential immunosuppressive effects of pharmaceutical drugs using a standardized protocol developed for immunotoxicological assessment. Two groups of pharmaceutical drugs were utilized: (a) drugs without known immunosuppressive effect linked to their utilization in human therapy (cimetidine, furosemide, indomethacin, amoxicillin, and procainamide) and (b) immunosuppressive drugs (azathioprine, cyclosporine A, and dexamethasone). Ex vivo tests using B6C3F1 mice were performed after a 28-day repeat dose regimen and assessed: (a) immunopathology, (b) cell-mediated immunity, (c) humoral immunity, and (d) nonspecific immunity. Host resistance to Listeria monocytogenes was also assessed following exposure to immunosuppressive drugs. The results showed that (a) immunopathology and immune function assays were necessary to detect all immunotoxicants and (b) the effects observed with nonimmunotoxic drugs were sometimes statistically significant but the biological significance of these effects is unlikely.

Influenza virus host resistance models in mice and rats: utilization for immune function assessment and immunotoxicology

Each year influenza viruses are responsible for epidemic respiratory diseases with excess morbidity and mortality. The severity of influenza diseases ranges from mild upper respiratory tract infections to severe lower respiratory tract infections involving pneumonia, bronchiolitis and coincidental bacterial super-infections. The immune response to influenza viruses can be schematically divided into a cascade of non-specific and specific functions. These functions are involved at different well defined time points after infection. We describe in this manuscript three influenza models utilized in our laboratory: (i) a highly virulent influenza virus (influenza A/Hong Kong/8/68 (H3N2) virus) adapted to B6C3F1 mice, (ii) a mouse-adapted influenza A/Port Chalmers/1/73 (H3N2) virus, and (iii) a rat-adapted influenza virus (RAIV) model (influenza A/Port Chalmers/1/73 (H3N2)). This rat-adapted influenza model has been mainly utilized as a model to assess local immunotoxic effects of inhaled environmental pollutants such as phosgene. These host resistance models are also useful for assessing the effect of systemically-induced immunosuppression or immunomodulation by drugs or chemicals on the local pulmonary immune response to influenza virus. The comparison of these different models allowed two major conclusions: (a) viral replication and mortality are two different endpoints and are not necessarily linked (no mortality was observed with Port Chalmers virus in the mouse although the virus replicates to high titers in the lung with a kinetic pattern comparable to the one obtained with Hong Kong virus), (b) mortality, viral replication, and immune function assessment are different endpoints that can be used, depending on the question addressed.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on pulmonary influenza virus titer and natural killer (NK) activity in rats

Suppression of immune function and enhanced susceptibility to infection in mice is one of the more sensitive indicators of TCDD toxicity. Recent efforts to demonstrate similar effects in the rat have shown that there are considerable differences between the two species. The purpose of this study was to determine the effect of TCDD exposure on (a) an influenza virus host resistance model in rats and (b) natural killer cell activity in the lung and spleen. Fischer 344 rats were treated with 10 micrograms TCDD/kg body weight via gavage and infected intranasally with rat-adapted influenza virus (RAIV) 7 days later. Virus-augmented NK activity assessed at 48 hr postinfection in the lung was significantly suppressed in rats treated with 3, 10, or 30 micrograms TCDD/kg body weight. Spontaneous NK activity in either lung or spleen was not affected by TCDD exposure. Significantly higher virus titers were observed on Days 2, 3, and 4 postinfection in the lungs of rats treated with TCDD (10 micrograms/kg). TCDD had no effect on the amount of virus recovered from nasal lavage. Acute exposure to TCDD did not significantly affect lung and body weights in rats infected with RAIV except in the highest dose (30 micrograms/kg) treated rats. Rats exposed to repeated doses of TCDD showed a significant increase of lung weights and L/B ratios when rats were infected with RAIV after TCDD exposure. Virus-augmented pulmonary NK activity in these rats was significantly suppressed; however, the suppression was not more profound than that in rats exposed to a single dose of TCDD.(ABSTRACT TRUNCATED AT 250 WORDS)

Immune modification due to chemical interference with transmembrane signalling: application to polycyclic aromatic hydrocarbons

Triggering of the T-cell antigen receptor complex and some other surface molecules is coupled to the phosphodiesterase (phospholipase C)-mediated hydrolysis of membrane phosphoinositides, in particular, phosphatidylinositol-4,5-biphosphate (PiP2). PiP2 hydrolysis generates two products, inositol 1,4,5-triphosphate and diacylglycerol, which act in concert as second messengers to increase the free intracellular calcium concentration and activate protein kinase C, respectively, thereby stimulating subsequent events leading to cellular activation and proliferation. Transmembrane signalling in T-lymphocytes represents a potential target for designated drugs as well as immunotoxicants. Immunotoxic effects of polycyclic aromatic hydrocarbons are discussed in the view of interaction with transmembrane signalling in the T-lymphocyte.