The importance of concomitant infections in the development of aortic lesions is part of an ongoing debate concerning the pathogenesis of atherosclerosis. To the best of our knowledge, this is the first study correlating T. gondii
infection with the development of atherosclerosis. Our results showed that T. gondii
increased the size of aortic lesions in ApoE KO mice. Although serum cholesterol is a risk factor of atherosclerosis, we did not find a positive relationship between cholesterolemia and atherosclerosis in T. gondii
-infected mice. Infected animals presented a reduction in the serum VLDL cholesterol fraction that represents an important proatherogenic lipoprotein in mice. These results agree with those obtained by Paigen et al. (29
), Dansky et al. (6
), and Grimsditch et al. (19
), who suggested that the correlation between blood cholesterol and atherosclerosis is not evident in cases of concomitant infection.
Specifically, a decrease in serum cholesterol was also described by Sunnemark et al. (37
), who fed CBA/J mice infected with T. cruzi
a high-fat diet, and Doenhoff et al. (10
), who infected ApoE KO mice with Schistosoma mansoni
. Further, systemic chronic infections and inflammations are associated with low cholesterolemia and HDL cholesterol (25
). However, in the examples provided above, the reduction of total cholesterol reflects an unspecific reduction of all lipoprotein fractions. Our results demonstrate a reduction in the VLDL fraction specifically. The reduction in this proatherogenic lipoprotein associated with unchanged levels of HDL cholesterol could suggest a reduction of atherogenesis in those mice. Contrarily, in the present study, this lipoprotein profile was associated with the enhancement of plaque formation in infected mice.
The mechanism for lowering the levels of VLDL during T. gondii
infection is unknown. Nevertheless, it is known that the cholesterol is concentrated in the cell membrane and apical organelles of the tachyzoites (13
) and that T. gondii
is dependent on cholesterol derived from the host (3
). It is well known that lower intracellular levels of cholesterol stimulate the endogenous synthesis of cholesterol and enhance expression of LDL (B/E) receptor expression and, consequently, lipoprotein endocytosis and internalization of blood VLDL and LDL cholesterol. Indeed, in vitro studies demonstrated that LDL cholesterol uptake by infected host cells is two to three times higher than that of noninfected cells (5
). Thus, the enhanced VLDL cholesterol uptake by infected host cells could be one factor determining the lower serum levels of VLDL cholesterol in infected animals. In addition, the hypocholesterolemic effects of cytokine production elicited during T. gondii
infection could further contribute to the reduction of the total VLDL fraction and, consequently, of total cholesterol in infected animals.
High levels of proinflammatory cytokines produced in response to T. gondii
infection have been found to promote atherosclerosis (24
). TNF-α, IL-12, and IL-1β, for example, increase monocyte influx, a possible mechanism of atherosclerosis development. IFN-γ, TNF-α, and IL-12 are associated with the progression of early lesions, as demonstrated in experiments comparing immunodeficient and immunocompetent ApoE KO mice (32
). Gupta et al. (21
) found a 60% reduction in the atherosclerotic lesion size of the aorta when IFN-γ receptor-deficient ApoE KO mice were compared to control ApoE KO mice. Similarly, exogenous IFN-γ administration enhanced the atherosclerotic progression in ApoE KO mice (39
The mechanism by which IFN-γ accelerates the plaque formation is not totally clear. This effect appears to be mediated at least in part by molecules induced by IFN-γ. The results presented here demonstrated that Toxoplasma
infection is associated with higher serum concentrations of nitrite/nitrate and the expression of iNOS and MCP-1 mRNA in aortic lesions. The parasite elicited IFN-γ to trigger iNOS to degrade arginine and produce high levels of NO (8
). Although NO is an important vasodilator component, high levels of NO contribute to the increase of oxidative stress via peroxynitrite formation and LDL oxidation (22
). The latter induces MCP-1 expression and the consequent migration of the monocyte toward the lesion site and lipoprotein uptake (22
). Oxidized LDL is also able to induce IL-12 production from macrophages and favors differentiation of naive T cells to a type 1 immune response (38
), inducing more NO production and LDL oxidation. It is also possible that other immunological factors induced by IFN-γ are involved in the process of enlargement of atherosclerosis lesions in mice infected with T. gondii.
This is an important question that we are currently addressing in our ongoing studies.
Histological analysis indicated that the lesions of T. gondii
-infected mice are larger and contain more-intense inflammatory infiltrate than noninfected ApoE KO mice. The aortic lesions from control and T. gondii
-infected animals were in the early to intermediate stages. In these stages, a pronounced plaque progression associated with Th1 immune responses, oxidative stress, macrophage migration, and lymphocyte influx occurs (2
). Because T. gondii
tachyzoites can invade any nucleated cells from their vertebrate host, one important question raised by our studies is whether local infection was initiating a process of tissue damage, consequent recruitment of inflammatory cells, and development of atherosclerosis. In the present study, we found parasite cysts in the heart tissue close to the aorta in 3 of 11 mice. However, immunocytochemistry studies did not reveal parasite antigens associated with aortic lesions. It is well established that infection with T. gondii
causes a switch from a Th2 response to a Th1 immune response specific to nonrelated antigens (15
). Therefore, we hypothesize that T. gondii
infection accelerates the development of atherosclerosis by promoting Th1 immune response to the components of ongoing aortic lesions.
Our results suggest that T. gondii infection provides a proatherogenic stimulus, inducing Th1 cytokine production and oxidative stress, which culminates in a rapidly progressing atherosclerotic lesion. Given the large population of people infected with T. gondii both in developed and developing countries, we believe T. gondii is a potential risk factor for the development of atherosclerosis in humans and deserves further investigation.