Oxygen derived free radicals and the course of Eimeria vermiformis infection in inbred strains of mice

A selective review of advances in coccidiosis research

Coccidiosis is a widespread and economically significant disease of livestock caused by protozoan parasites of the genus Eimeria. This disease is worldwide in occurrence and costs the animal agricultural industry many millions of dollars to control. In recent years, the modern tools of molecular biology, biochemistry, cell biology and immunology have been used to expand greatly our knowledge of these parasites and the disease they cause. Such studies are essential if we are to develop new means for the control of coccidiosis. In this chapter, selective aspects of the biology of these organisms, with emphasis on recent research in poultry, are reviewed. Topics considered include taxonomy, systematics, genetics, genomics, transcriptomics, proteomics, transfection, oocyst biogenesis, host cell invasion, immunobiology, diagnostics and control.

T cell responses in calves to a primary Eimeria bovis infection: phenotypical and functional changes

The study aimed to characterize T cell responses in calves to a primary E. bovis infection. For this purpose, peripheral blood lymphocytes (PBL) were isolated from six infected calves and three controls during prepatency (Day 12 post infection (p.i.), patency (Day 25 p.i.) and postpatency (Day 35 p.i.). In addition, lymphocytes were isolated from various lymphatic organs (lnn. cervicales superficiales, lnn. jejunales craniales, lnn. jejunales caudales, lnn. caecales, lnn. colici, Peyer's patches (PP) and spleen) at necropsy (Day 35 p.i.). FACS analyses determined the proportions of CD4+-, CD8+-, CD2+-, and gammadelta+-T cells. Proliferative responses of the cells after stimulation with Concanavalin A (Con A) and an E. bovis-merozoite I antigen (EbAg) were measured. Furthermore, in situ hybridization experiments were performed for the detection of IL-2 and IL-4 mRNA in histological sections of lymphatic organs. Proportions of CD4+-, CD8+- and CD2+-expressing PBL were significantly increased 12 days p.i. in infected calves. While the proportions of CD4+- and CD8+-PBL declined until day 25 p.i. and finally reached control values, proportions of activated PBL (CD2+-T cells) remained at a high level throughout the observation period. Those of gammadelta+-PBL, in contrast, remained unaffected. The proportions of CD4+-, gammadelta+- and CD2+-T cells in lymphatic organs were significantly increased in comparison to uninfected controls, when determined 35 days p.i. Concerning the proportions of CD8+-T cells of the organs, however, there were no differences between the groups. PBL and cells from lymphatic organs except those from the PP showed strong proliferative response to the mitogen Con A, without a significant difference between the groups. Reactions to EbAg in contrast differed significantly between controls and E. bovis infected calves. Proliferation responses of PBL of infected animals were highest 12 days p.i.; subsequently they decreased and 35 days p.i. they were found within the ranges of controls. Lymphocytes isolated from lymphatic organs of infected animals reacted significantly stronger than lymphocytes from control animals, whereby most marked differences occured with cells from lymph nodes draining E. bovis infested parts of the intestine and from the spleen. These reactions were accompained by an increased transcription of the IL-2 gene but not of the IL-4 gene in gut associated lymphnodes of infected calves when compared with infected controls. The data suggest strong antigenic stimuli by developing first generation schizonts, and of predominant involvement of (CD4+) Th1 cells in the course of a primary E. bovis infection of calves.

The effect of BCG, zymosan and Coxiella burnetti extract on Eimeria infections

Infection of animals with species of Eimeria induces a hyper-reactivity to endotoxin as manifest by a greatly increased capacity of infected animals to produce TNF in response to LPS in vivo compared with uninfected animals. This finding indicates priming for hyperactivation of macrophages by Eimeria infection and raises the possibility that non-specific triggering of macrophages by agents such as Bacille Calmette-Guerin (BCG), zymosan or Coxiella burnetti extract may be a simple means of control for coccidiosis. However, all of these agents enhanced oocyst excretion in mice, rats or chickens infected with Eimeria vermiformis, Eimeria nieschulzi or Eimeria tenella, respectively, without affecting the patent period.

Cytokines and immunological control of Eimeria spp

Protozoan parasites belonging to the genus Eimeria cause considerable losses in livestock production in which stocking densities are high or environments restricted. The ability of hosts to mount immunological responses which limit parasite reproduction vary according to the particular species of Eimeria. Typically though, immune responses restrict parasite reproduction during primary infection and limit, if not prevent, subsequent infections. Although mechanisms of immunity are unknown, host immune responses have been exploited in the development of a method to control coccidiosis-immunisation with attenuated strains of Eimeria. Limitations of this control method, predominantly the cost of producing the attenuated parasites, necessitates identification of protective immune responses to facilitate selection of antigens for use in non-living vaccines. As in immune responses to many other parasitic infections of the gastrointestinal tract, the role of antibodies is at best minor, whereas T-cells are crucial. Numerous studies have shown that the intestinal mucosal T-cell population is dynamic; the number and phenotype of T-cells changes in response to Eimeria-infection. Specific changes in the intestinal T-cell population have not, however, been correlated with limitation of parasite reproduction. Experiments involving adoptive transfer of T-cell sub-populations and in vivo depletion of specific T-cells have shown that CD4+ T-cells and to a lesser extent CD8+ T-cells are important in immune responses which limit primary infection. In contrast, CD8+ T-cells are more important in subsequent infections with CD4+ T-cells having a lesser role. The effects of T-cells on Eimeria are partially mediated by the cytokines they release. Most attention has concentrated on interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) because these cytokines have been shown to limit other protozoan infections. IFN-gamma is produced in Eimeria-infected hosts but evidence that it is present at the site of infection is limited. Intestinal levels of IFN-gamma increase earlier in response to primary Eimeria-infection in mice which are relatively resistant, than in mice which are relatively susceptible. Neutralisation of endogenously produced IFN-gamma has shown that this cytokine limits oocyst production in either primary or secondary infections depending on the species of Eimeria. Production of TNF-alpha is also increased in infected hosts. In comparison with relatively susceptible mice, TNF-alpha is produced earlier and to a greater extent in the intestines of relatively resistant mice. Unexpectedly, injections of TNF-alpha into infected mice increased oocyst production. It remains to be determined whether the effects of endogenous TNF-alpha are the same as those of exogenous TNF-alpha. Mechanisms by which IFN-gamma and TNF-alpha modulate parasite reproduction have not been identified. A number of lines of experimentation have suggested that it is unlikely that IFN-gamma limits parasite reproduction through induction of the synthesis of reactive oxygen or reactive nitrogen intermediates, since both of these reactive intermediates have the capacity to exacerbate Eimeria-infection.

Nippostrongylus brasiliensis: ability of plasma to prime free radical generation by leukocytes in response to adult worms not due to gamma-interferon or tumour necrosis factor

Plasma-borne factors prime leukocytes from both infected and uninfected rats for radical generation in response to N. brasiliensis. The concentration of these factors is increased following infection and reaches maximal levels on day 8 post-infection (p.i.) as demonstrated by the striking ability of plasma from infected rats to prime leukocytes from uninfected rats to produce free radicals in response to adult worms. The cytokines, gamma-interferon and tumour necrosis factor (TNF) can be detected in plasma during infection with a variety of organisms and several lines of immunological and pathophysiological evidence, including radical generation, weight loss, anaemia and diarrhoea, implicate generation of these proteins in response to infection with N. brasiliensis. We therefore investigated whether gamma-interferon and TNF were detectable in the plasma of rats infected with N. brasiliensis and whether the presence of these cytokines correlated with the ability of plasma to enhance radical generation in response to N. brasiliensis. However, gamma-interferon was not detected in the plasma of rats at any time after infection with N. brasiliensis and neutralizing monoclonal antibody to rat gamma-interferon had no effect on the ability of plasma to prime free radical generation. TNF was detected in the plasma of heavily-infected rats but only at very low levels (< 1 ng/ml), though copius in vivo synthesis of TNF could be induced by treatment of the infected rats with lipopolysaccharide (LPS). However, neither parasite-induced nor parasite plus LPS-induced plasma TNF correlated with the ability of plasma to enhance radical generation in response to N. brasiliensis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokines, free radicals and resistance to Eimeria

The cytokine, gamma-interferon (IFN-gamma), which is produced by CD4(+) T cells, plays a crucial role in host resistance to Eimeria infections. Karen Ovington and Nick Smith propose that free oxygen radical generation by leukocytes in response to infection with Eimeria is the result of activation by IFN-gamma. The functional role of free oxygen radicals is unclear but these highly reactive radicals are produced by the leukocytes that infiltrate the intestine in large numbers during infection, and the parasites,enterocytes and cells of the immune system may all be vulnerable to oxidative damage. Gamma-interferon also appears to induce the enterocytes inhabited by Eimeria to turn against the parasite. The authors draw from literature documenting similar effects on other protozoa, especially Leishmania and Plasmodium, and speculate that reactive nitrogen intermediates produced by enterocytes have a functional role in resistance to Eimeria.

Fasciola hepatica: free radical generation by peritoneal leukocytes in challenged rodents

Free radical generation by peritoneal leukocytes from hosts able to develop resistance to reinfection with Fasciola hepatica (rats) was compared with that of hosts unable to develop resistance (mice). Free radical generation by rat leukocytes was 3.5 times higher per cell and 30 times higher per animal than radical production by mouse leukocytes. The capacity of peritoneal leukocytes to produce free radicals in response to adult fluke crude antigen was increased by the presence of host plasma and was quantitatively greater in challenged rats than in naive or primary infected rats. This was not the case for mice, in which cells from primary infected animals were equally as responsive as cells from challenged mice. Further experiments revealed that challenge infection in rats apparently caused the in vivo activation of peritoneal leukocytes and increased levels of unidentified factors in plasma and that both of these responses were involved in the initiation of free radical generation in response to F. hepatica. Dramatic increases in the number of eosinophils present in the peritoneal cavities of primary infected and challenged rats (but not mice) were observed but the role of eosinophils in the production of free radicals in response to F. hepatica remains to be determined.