Oxidized LDL in the Guinea Pig Tuberculosis Model Figure 6. Lung lesions in M. tuberculosis infected guinea pigs have increased expression of LOX1. A. The overall expression of LOX1 in the lungs increases with time in guinea pigs sham-vaccinated with saline or BCG as determined by scoring of immunostained sections. The bars represent median values plus range (when present) for each group (n = 5). The asterisks denote statistically significant increase compared to the naive animals (* = p,0.05 and ** = p,0.01). The photomicrographs B and C represent immunostaining of LOX1 predominantly within lung granulomas at 30 and 60 days of infection respectively (406). The inserts (10006) show intracellular staining within macrophages. doi:10.1371/journal.pone.0034148.g006 OxLDL, especially within primary lesion with necrosis. Even though CD36 and LOX1 bind multiple ligands, the elevation of OxLDL levels during M. tuberculosis infection suggests that these receptors may play an important role in oxidized lipid uptake and accumulation. Unlike the other macrophage scavenger receptor types A-I and II (SR-AI/II) that recognize copper-oxidized and acetylated-LDL, CD36 can also recognize LDL modified by myeloperoxidase-hydrogen peroxide-nitrite system (MPO- OxLDL) which also has physiological relevance associated with chronic inflammation [27]. Recently we have showed that the ability of M. tuberculosis to resist copper accumulation is critical for virulence and that necrotic granulomas in guinea pigs accumulate significant amounts of the free radical generating transition metals copper and iron in vivo [28,29]. These data support the biological relevance and potential role free radical generating transition metal may have in modifying host and possibly bacilli macromol- ecules. In an earlier study, we showed that malondialdehyde (MDA), a byproduct of lipid peroxidation, accumulated in a variety of cell types early in M. tuberculosis infected guinea pigs [4]. Further evidence of excessive ROS generation during infection was a marked depletion of serum total antioxidant capacity and specifically the critical antioxidant tripeptide glutathione [4]. Tuberculosis is a chronic inflammatory disease characterized by systemic oxidative stress with elevated markers of free radical damage in patients with active disease [2,3]. Considering a variety of host macromolecules such as proteins, lipids and nucleic acids that are susceptible to ROS mediated damage, it is not surprising that oxidatively modified LDL is among the host macromolecules that accumulate during experimental M. tuberculosis infection in guinea pigs. In addition, OxLDL contains other lipid peroxidation products that may also contribute directly to the pathogenesis of granuloma necrosis [30,31]. Unlike LDL which is quickly degraded, and does not accumulate within cells, OxLDL is stored and accumulates within cells especially macrophages [32]. The lipid components of OxLDL includes esterified and non-esterified cholesterol, phos- pholipids and triglycerides that make up approximately 80% of the total LDL content [33,34]. As cholesterol and other host lipids may serve as a carbon source for M. tuberculosis [35], we determined whether these oxidized lipids were among the stored products associated with vacuolated macrophages in vitro and in vivo. A unique subpopulation of macrophages with abundant, vacuolated cytoplasm is a prominent feature of chronic inflam- matory lesions like tuberculosis and atherosclerosis. These data indicate however that OxLDL expression was not restricted to foamy macrophages since epithelioid cells had similar staining characteristics as demonstrated by immunohistochemistry. This PLoS ONE | www.plosone.org 7 March 2012 | Volume 7 | Issue 3 | e34148