Host Resistance Assays

The goal of immunotoxicity testing is to obtain data useful for immunotoxicity safety assessment. Guidance in the performance of immunotoxicity safety evaluations is provided in documents from the US EPA for chemicals and the ICH S8 document for pharmaceuticals. The ICH S8 document outlines a tiered approach that includes (1) standard toxicity studies with associated hematology, immune system organ weights, and histopathology data; (2) functional assays, such as cytotoxic T lymphocyte (CTL) assays, natural killer (NK) cell assays, respiratory burst, phagocytosis, and T-cell-dependent antibody response (TDAR) assays; and (3) host resistance assays. Host resistance assays are considered the gold standard in immunotoxicity testing and provide a critical overview of the extent to which innate, adaptive, and homeostatic regulatory immune functions are integrated to protect the host. Both comprehensive and targeted host resistance assays are available, each with distinct advantages. This chapter serves to provide a general overview of the various assays that may be used, as well as a summary of procedures.

Effects of Cyclosporin A and Cyclophosphamide on Peyer’s Patches in Rat, Exposed in utero and Neonatally or During Adult Age

The effects of cyclosporin A (CY) and cyclophosphamide (CPS) on Peyer’s patches (PP) were studied in Wistar rats, exposed in utero and neonatally or during adult age. In one study, pregnant dams received 5 or 15 mg/kg bw/day CY from gestation day 6 to day 21 of lactation. In two other studies, animals were exposed at young adult age: female rats received orally 5 or 20 mg/kg/day CY or 5 or 10 mg/kg bw CPS for 4 weeks; males received orally 5 mg/kg bw CPS for 4 weeks, or a single iv injection of 50 mg/kg bw CPS. Upon in utero and neonatal exposure, the numbers of grossly observed PP were increased in male pups from the high-dose CY dams at 70 days of age. Exposure to high-dose CY at adult age only tended to decrease the numbers of PP; germinal center development was reduced in the PP from the middle segment of the small intestines, as examined microscopically. Exposure to both doses CPS at adult age reduced the numbers of PP and reduced germinal centre development and the number of lymphocytes in all compartments of PP. It was concluded that the effects of CPS and CY could be established by counting the number of grossly visible PP and by microscopic observation of PP, provided that regional differences of PP were taken into account. Moreover, the type of effects of an immunotoxic agent may vary with age of exposure.

Viral host resistance studies

A foremost objective of preclinical immunotoxicity testing is to address whether or not a drug or environmental toxicant causes adverse effects on net immune health, expressly the host's ability to mount an appropriate immune response to clear infectious organisms. Given the complex interactions, diverse molecular signaling events, and redundancies of immunity that has itself been subdivided into interdependent arms, namely innate, adaptive, and humoral, the results of single immune parameter testing may not reflect the final outcome of a drug or toxicant's effect on net immune health. The most comprehensive experimental approach to ascertain this information is utilization of host resistance models. Herein, application of viral host resistance models in rodents and non-human primates is described. Although brief descriptions of numerous viral models are discussed including reovirus, Epstein-Barr virus, cytomegalovirus, and lymphocryptovirus, the most well-characterized viral host resistance model, rodent influenza, is emphasized.

Docosahexaenoic acid-enriched fish oil consumption modulates immunoglobulin responses to and clearance of enteric reovirus infection in mice

We hypothesized that consumption of the (n-3) PUFA, docosahexaenoic acid (DHA), modulates the mucosal immune response to enteric infection with respiratory enteric orphan virus (reovirus), a model intestinal pathogen. Mice were fed either AIN-93G control diet, containing 10 g/kg corn oil and 60 g/kg high oleic acid safflower oil, or AIN-93G, containing 10 g/kg corn oil and 60 g/kg DHA-enriched fish oil, for 4 wk and then orally gavaged with reovirus strain Type 1 Lang, (T1/L). Reovirus-specific IgA antibody was first detectable in the feces of mice fed a control diet at 6 d postinfection (PI) and was further elevated at 8 and 10 d PI. IgA responses in DHA-fed mice were similar at 6 and 8 d PI but greater at 10 d PI (P < 0.05). Both reovirus-specific serum IgA and IgG(2a) were comparably induced in mice fed control or DHA diets. Reovirus-specific IgA and IgG(2a) secretion by ex vivo Peyer's patch, lamina propria, and spleen cultures derived from control and DHA groups were comparable. Although both groups carried similar numbers of reovirus plaque forming units per intestine, DHA-fed mice shed nearly 10 times more viral RNA in feces than control mice at 2, 4, and 6 d PI (P < 0.05). However, viral RNA was not detectable in either group at 8 and 10 d. Taken together, these data suggest that DHA consumption did not markedly alter mucosal or systemic Ig responses to reovirus but delayed clearance of the virus from the intestinal tract.

Influenza virus host resistance model

Host resistance (HR) models are used to evaluate the effect of a test article on clearance of an infectious microorganism in order to assess total immunocompetence. HR models serve as biomarkers of net immunological health or immunological well-being. Immunotoxicity can result either in an impaired clearance of an infectious agent, increased susceptibility to an opportunistic microorganism, prevention of immunization, or exacerbation of latent viral infections. The purpose of immunotoxicity testing is to obtain data that is meaningful for safety assessment, and for immunosuppression the major objective is to determine the significance with respect to increased susceptibility to infectious disease. Host resistance models provide the only sure method of examining the influence of test articles on the functional integrity of the immune system and its ability to eliminate pathogenic microorganisms and tumor cells. They provide the means to directly assess the functional reserve of the immune system. Clearance of influenza virus requires an intact and functional immune system that incorporates a cascade of immune responses. Mechanistic studies can be included in the influenza virus host resistance model by measuring the effect of a test article on innate immunity (cytokine and interferon production, macrophage function, and natural killer (NK) cell function) and acquired or adaptive immunity (cytotoxic T lymphocyte (CTL) activity as well as influenza-specific IgM and/or IgG antibody).

T-2 toxin impairment of enteric reovirus clearance in the mouse associated with suppressed immunoglobulin and IFN-gamma responses

Trichothecenes are exquisitely toxic to the gastrointestinal (GI) tract and leukocytes and thus are likely to impair gut immunity. The purpose of this research was to test the hypothesis that the Type A trichothecene T-2 toxin interferes with the gut mucosal immune response to enteric reovirus infection. Mice were exposed i.p. first to 1.75 mg/kg bw T-2 and then 2 h later with 3 × 10⁷ plaque-forming units of reovirus serotype 1, strain Lang (T1/L). As compared to vehicle-treated control, T-2-treated mice had dramatically elevated intestinal plaque-forming viral titers after 5 days and failed to completely clear the virus from intestine by 10 days. Levels of reovirus λ2 core spike (L2 gene) RNA in feces in T-2-treated mice were significantly higher at 1, 3, 5, and 7 days than controls. T-2 potentiated L2 mRNA expression in a dose-dependent manner with as little as 50 μg/kg of the toxin having a potentiative effect. T-2 exposure transiently suppressed induction of reovirus-specific IgA in feces (6 and 8 days) as well as specific IgA and IgG_2a in serum (5 days). This suppression corresponded to decreased secretion of reovirus-specific IgA and IgG_2a in Peyer's patch (PP) and lamina propria fragment cultures prepared 5 days after infection. T-2 suppressed IFN-γ responses in PP to reovirus at 3 and 7 days as compared to infected controls whereas IL-2 mRNA concentrations were unaffected. PP IL-6 mRNA levels were increased 2-fold 2 h after T-2 treatment, but no differences between infected T-2-exposed and infected vehicle-treated mice were detectable over the next 7 days. Overall, the results suggest that T-2 toxin increased both the extent of GI tract reovirus infection and fecal shedding which corresponded to both suppressed immunoglobulin and IFN-γ responses.

Modulation of Murine Host Response to Enteric Reovirus Infection by the Trichothecene Deoxynivalenol

Based on the known capacity of deoxynivalenol (DON) to target gut lymphoid tissue and IgA production, it was hypothesized that this mycotoxin interferes with the immune response to enteric reovirus infection. When mice were orally gavaged, first with 25 mg/kg bw DON, and then with reovirus serotype 1, strain Lang (T1/L) 2 or 12 h later, viral titers in the GI tract were 10-fold higher than control mice after 5 days. Virus was almost completely cleared in both treatment and control groups from intestinal tissue after 10 days. Real-time PCR indicated that, in infected control mice, reovirus lambda2 core spike (L2 gene) RNA per g feces in infected mice that were pretreated with DON was significantly higher at 1, 3, and 5 days than in infected mice only. In reovirus-infected mice, DON at doses of 10 and 25 mg/kg bw but not 2 and 5 mg/kg bw increased fecal L2 RNA, whereas DON doses as low as 2 mg/kg potentiated L2 RNA levels in Peyer's patches (PP). Reovirus-specific IgA levels in feces of mice treated with DON were significantly elevated, as were specific IgA responses in lamina propria and PP fragment cultures. Similar effects were observed for serum IgA and IgG. DON suppressed IFN-gamma responses in PP to reovirus at 3 and 5 days as compared to infected controls, while IL-2 mRNA concentrations were unaffected. Although reovirus alone did not induce Th2 cytokine mRNAs in PP, DON exposure significantly elevated IL-4, IL-6, and IL-10 mRNA expression at various times during the infection. ELISPOT revealed that mRNA expression data corresponded to suppression of IFN-gamma- and enhancement of IL-4-producing cell responses in PP cultures from DON-treated mice. Taken together, these data suggest that DON transiently increased both severity of the reovirus infection and shedding in feces as well as elevated reovirus IgA responses. These effects corresponded to suppressed Th1 and enhanced Th2 cytokine expression.

Mucosal and systemic immunity to intestinal reovirus infection in aged mice

Systemic immunity is progressively impaired in aging, predisposing to morbidity and mortality from neoplasia and infectious disease. However, the effect of aging on mucosal immunity is controversial. To assess intestinal immunity in aging, young and aged mice were orally exposed to reovirus or cholera toxin (CT) and specific antibody and reovirus-specific cytotoxic T-cell (CTL) responses were assessed. As previously reported, aged mice immunized orally with CT mounted diminished intestinal IgA responses to CT compared to young mice. In contrast, aged mice yielded two to three-fold more reovirus-specific IgA-producing cells in the Peyers's patches (PP) compared to young mice, and higher titers of reovirus-specific IgA in fragment culture supernatants. Cytotoxicity and CTL frequencies from aged mice were not different from those of young mice. Together, these results suggest a diminished potential for systemic and intestinal immunity to orally applied protein antigens in aging, but an intact ability to respond to intestinal virus infection. Infection with a replicating virus may induce inflammatory mediators and innate immune factors that potentiate the priming of mucosal immunity; overcoming aging related deficits otherwise observed following oral immunization with non-replicating antigens, and suggests the importance of antigen replication to antigen-specific immunotherapy strategies in the elderly.

Influence of the route of infection on development of T-cell receptor beta-chain repertoires of reovirus-specific cytotoxic T lymphocytes

It is well established that the route of infection affects the nature of the adaptive immune response. However, little is known about the effects of the route of exposure on development of cytotoxic T-lymphocyte (CTL) responses. Alternative antigen-presenting cell populations, tissue-restricted expression of class I major histocompatibility complex-encoded molecules, and unique T-cell receptor (TCR)-bearing cells in mucosal tissues could influence the selection and expansion of responder T cells. This study addresses the question of whether the route of virus infection affects the selection and expansion of subpopulations of virus-specific CTLs. Mice were infected orally or in the hind footpads with reovirus, and the repertoires of TCR beta-chains expressed on virus-specific CD8(+) T cells in Peyer's patches or lymph nodes and spleens were examined. CD8(+) cells expressing the variable gene segment of the TCR beta-chain 6 (Vbeta6) expanded in the spleens of mice infected by either route and in CTL lines established from the spleens and draining lymphoid tissues. Adoptively transferred Vbeta6(+) CD8(+) T cells from orally or parenterally infected donors expanded in reovirus-infected severe combined immunodeficient recipient mice and mediated cytotoxicity ex vivo. Furthermore, recovered Vbeta6(+) cells were enriched for clones utilizing uniform complementarity-determining region 3 (CDR3) lengths. However, sequencing of CDR3beta regions from Vbeta6(+) CD8(+) cells indicated that Jbeta gene segment usage is significantly more restricted in CTLs from orally infected mice, suggesting that the route of infection affects selection and/or subsequent expansion of virus-specific CTLs.