Date Published: May 15, 2013
Publisher: Public Library of Science
Author(s): Zhi-Jun Zhao, Jia Zhang, Jun Wei, Zhi Li, Tao Wang, Si-Qi Yi, Ji-Long Shen, Ting-Bao Yang, Geoff Hide, Zhao-Rong Lun, Paul G. Thomas. http://doi.org/10.1371/journal.pone.0063650
Rats are naturally resistant to Toxoplasma gondii infection, particularly the RH strain, while mice are not. Previous studies have demonstrated that inducible nitric oxide synthase (iNOS) and arginase-1 of rodent peritoneal macrophages are linked to the mechanism of resistance. As an increasing number of studies on human and animal infections are showing that pulmonary toxoplasmosis is one of the most severe clinical signs from T. gondii infection, we are interested to know whether T. gondii infection in alveolar macrophages of rats is also linked to the levels of iNOS and arginase-1 activity. Our results demonstrate that T. gondii could grow and proliferate in rat alveolar macrophages, both in vitro and in vivo, at levels higher than resistant rat peritoneal macrophages and at comparable levels to sensitive mouse peritoneal macrophages. Lower activity and expression levels of iNOS and higher activity and expression levels of arginase-1 in rat alveolar macrophages were found to be linked to the susceptibility of T. gondii infection in these cells. These novel findings could aid a better understanding of the pathogenesis of clinical pulmonary toxoplasmosis in humans and domestic animals.
Toxoplasma gondii is an obligate intracellular parasitic protozoan causing toxoplasmosis in infected humans and animals. In most cases, T. gondii causes asymptomatic infection in healthy individuals, but severe clinical presentations can be found in congenital toxoplasmosis, ocular toxoplasmosis and in immunocompromised people including AIDS patients . Pulmonary toxoplasmosis has been reported from immunocompromised or immunodeficient patients –, pregnant women  and immunocompetent individuals –. Additionally, animals with toxoplasmic pneumonia have also been reported in a large number of studies –. However, little attention has been focused on this disease, due to the difficulties of diagnosis, leading to the reporting of relatively few cases , . It has been recognized that the lungs are one of the most susceptible organs (following the CNS) to T. gondii infection  and there are considerable concerns especially when considering lung transplantation .
For a long time, pneumonia caused by T. gondii has been overlooked as a severe problem in immunocompetent hosts and thus few studies have been concerned with the disease. However, more and more reports have indicated that toxoplasmic pneumonia can be frequently found in individuals with normal immune function including both humans – and animals –. It was considered that rats, like humans, have a higher level of resistance to T. gondii infection in comparison to other mammals such as mice, guinea pigs and hamsters which all show a higher degree of susceptibility to this parasite –. Therefore, rats are recognized as a good animal model for the understanding of human toxoplasmosis , –. Previous studies from our laboratory demonstrated that the higher expression and activity of iNOS and lower expression and activity of arginase in RPMs are strongly linked with the resistance to the T. gondii RH strain infection in resistant (rats) and susceptible (mice) species . This highlights the mechanism of species specificity to pathogen infection at least in T. gondii. Furthermore, these studies demonstrated that individual variation in susceptibility, based on differences in inbred lines of rats, could also be linked to the balance of expression of iNOS and arginase-1 . Although RPMs were considered to be resistant to T. gondii, particularly the RH strain, interestingly, we have found that RAMs are much more susceptible to this parasite. This phenomenon was also reported by Chinchilla et al.  and Badger et al.  but the mechanism was not known. They did, however, report that interferon-γ was found to be involved in activating rat alveolar macrophages which in turn appeared to induce antimicrobial activity against T. gondii in vitro. As far as we can ascertain, no other studies have investigated this phenomenon more recently. A key question that has not been addressed is the reason why RPMs show different susceptibility levels to RAMs during T. gondii RH strain infection. Previous studies  showed that iNOS and arginase-1 expression are strongly linked to resistance and susceptibility by peritoneal macrophages. This raises the question as to whether the expression and activity levels of iNOS and arginase-1 differ between RAMs and RPMs and could therefore account for differences in susceptibility. Our results clearly demonstrate that, in comparison to RPMs, there is a much lower level of expression of iNOS (and, consequently, a lower production of NO) but higher expression levels of arginase-1 (and, consequently, more recorded urea) in RAMs (Figure 3 and Figure 4). This could explain why the T. gondii RH strain grows well in RAMs but not in RPMs. In comparison with the peritoneal macrophages of mice, which are also highly sensitive to infection with T. gondii, RAMs also produce less NO. The presence of high levels of NO is considered to confer great benefit in preventing the growth of T. gondii and the consequent destruction of host cells . This study shows a strong link between the balance of iNOS and arginase-1 activity in RAMs and their higher susceptibility to T. gondii than RPMs in vitro and in vivo. As far as we can ascertain, this has not been reported before and few studies have considered the interactions between T. gondii and RAMs. The presence of the phylogenetically unrelated parasite, Trypanosoma lewisi, has been shown to have an immunosuppressant effect on T. gondii infection in RAMs . The mechanism for this is not known but may be triggering key generic pathways that suppress the proliferation of T. gondii. A recent study, using indoleamine 2,3-dioxygenase (IDO) gene knockout mice, has shown that increased IDO attenuates acute toxoplasmic infection in lung tissue . This enzyme, which depletes local stocks of L-tryptophan is stimulated by effectors such as interferon-γ. Previous studies have reported associations between NO synthesis, interferon-γ and reduced microbial activity , suggesting that the roles of these molecules may also be linked to the regulation of T. gondii proliferation. More research is required to explore such associations.