- Top of page
- 1 Introduction
- 2 Material and methods
- 3 Results
- 4 Discussion
- 5 References
Strong epidemiological evidence indicates that high consumption of fruits and vegetables reduces the risk of chronic disease such as cardiovascular disease (CVD) [], but the actual components of these foods that confer the protective effect and the mechanisms by which they act have yet to be firmly identified. Increased consumption of tomato-rich diets has been reported as associated with a 30% reduction in the relative risk of CVD []. Such potential benefits to vascular health from a tomato-rich diet are often ascribed to high concentrations of lycopene in tomato products that can account for more than 80% of the dietary intake of this carotenoid [].
High lycopene levels in blood and adipose tissue correlate with a reduction in CVD incidence [[4-7]], and low concentrations are associated with early atherosclerosis []. Arterial intimal wall thickness is lower in subjects with higher adipose tissue lycopene concentrations, suggesting decreased risk of arterial occlusion [[5, 9, 10]]. Low plasma lycopene concentrations are also associated with elevated C-reactive protein concentrations, a risk marker for CVD [[11, 12]], and the CARDIA/YALTA study [] found an inverse relationship between serum carotenoids, including lycopene, and markers of inflammation and vascular endothelial dysfunction. Corroborative evidence includes an inverse association between neopterin (a marker for cellular immune activation) and serum lycopene concentrations [], suggesting that lower serum lycopene concentration may relate to higher grade chronic immune activation, a common feature in cardiovascular disorders. Furthermore, it has been suggested that lycopene could modulate the immune response by lowering inflammatory mediator production []. In vitro studies have shown that lycopene modulates the expression of adhesion molecules in human vascular endothelial cells [], while in macrophages it increases the expression of LDL receptors involved in the regulation of cholesterol metabolism, inhibits cholesterol synthesis, and increases LDL degradation []. More recently, lycopene was found to significantly decrease monocyte proliferation [].
The effect of lycopene on lymphocytes, important immune cells actively involved in atherogenesis [], has not yet been studied. The aim of the present study was to determine if lycopene could modulate T-lymphocyte activity, as measured by their ability to proliferate, differentiate, and express adhesion molecules after activation with a mitogen, and to determine whether these effects were dependent on modulation of early activation pathways.
- Top of page
- 1 Introduction
- 2 Material and methods
- 3 Results
- 4 Discussion
- 5 References
Lycopene suppressed proliferation in mitogen-stimulated cells, even at low concentrations, comparable with plasma lycopene concentrations found in populations with low intake of lycopene-rich foods such as tomato-based products []. The decrease in proliferation by lycopene suggests it may be effective in decreasing overall T-cell activation and therefore protective against atheromatous plaque development.
The lycopene concentrations used in this study were physiologically relevant. Maximum inhibitory effects (up to 50%) were observed for the highest lycopene concentrations tested, which are similar to lycopene concentrations found in plasma of subjects with daily consumption of lycopene around 30 mg []. Ex vivo measurements of Con-A- or phytohemagglutinin (PHA)-stimulated PBMCs proliferations were not affected by the consumption of tomato juice [], lycopene supplementation [], or tomato extract []. In line with these findings, both ex vivo PHA- [] and Con-A-stimulated [] PBMCs proliferations were significantly decreased after a high carotenoid diet. However, confounding factors such as other dietary components, age of the subjects, and related variability in absorption between individuals may account for the disparity of results observed. The studies by Corridan [] and Watzl [] were both carried out in the elderly who may have reduced capacity for intestinal absorption and transport of nutrients, such as carotenoids []. Others [[29, 30, 32]] involved periods of carotenoid depletion in the diet that may have already affected the immune response before increasing lycopene intake. However, proliferation and differentiation of a human monocyte/mactrophage cell line (U937) was significantly reduced by lycopene [].
High numbers of T lymphocytes are usually found in atherosclerotic plaques []. During atherogenesis, T cells are recruited and then bound to the site of the damaged endothelium before transmigrating into the intima. The T cells are then activated through binding with antigen-loaded MHC molecules and costimulatory molecules, triggering a cascade of signaling events that result in an immune response including cell proliferation [].
The mechanisms responsible could involve the modulation of the pathways involved in apoptosis and necrosis. Lycopene has previously been shown to affect caspase-3 levels, an enzyme required in the intrinsic apoptotic pathway []. However, apoptosis and necrosis were not affected by lycopene in our study. The signaling cascade process following T-cell activation involves sequential phosphorylation reactions by protein kinases that ultimately regulate the processes of gene transcription. Some evidence suggests that lycopene may affect such signaling pathways, e.g. mitogen-activated protein kinase (MAPK) [], or possibly modulate redox-sensitive transcription factors, thereby affecting gene transcription of proteins required to initiate proliferation through the cell cycle []. However, the cell cycle remained unaffected by lycopene. Cell-cycling phases were measured after 90 h, identical to the time at which proliferation was measured. Therefore, longer incubation time might be required to observe any effects of lycopene on the cell cycle.
CD69 represents a marker of early T-cell activation and acts as a costimulatory molecule that increases T-cell responses following the TCR-ligand interaction []. Lycopene did not significantly affect the proportion of cells expressing CD69. However, its expression was significantly reduced in both Con-A- and anti-CD3-stimulated T-lymphocytes, indicating that lycopene reduces T-cell proliferation through mechanisms involving early activation pathways. After T-cell activation, both IL-2 and the IL-2 receptor (CD25) are rapidly upregulated and therefore represent two additional early activation markers []. In line with the decrease in proliferation observed, IL-2 production was also significantly reduced by lycopene in both Con-A- and anti-CD3-stimulated cells. IL-2 is one of the first cytokines to be released by T cells after activation, and drives the G1-S phase progression and T-cell clonal expansion after binding to the IL-2 receptor (IL-2R) on other T cells, leading to a further stimulation of T-cells proliferation at the site of inflammation []. A previous study reported a decrease in IL-2 due to lycopene in LPS-stimulated PBMCs []. However, ex-vivo studies measuring IL-2 production in stimulated PBMCs after a period of tomato or lycopene nutritional intervention, showed no effect of the dietary interventions [[27-29, 33]]. During an immune challenge, IL-2 is upregulated along with IL-2R expression but our results showed no effect of lycopene on IL-2R. This implies that the upregulation of IL-2 and IL-2R are separately regulated. This is supported by the results of a previous study [] who found that blocking the IL-2R in stimulated PBMCs did not reduce IL-2 expression. Furthermore, blocking the binding of IL-2 to IL-2R in PBMCs does not affect CD69 expression, suggesting that both pathways of activation are independent []. However, T-regulatory cells expressing CD25+ were already present in our samples prior to activation. It is therefore possible that lycopene could modulate the expression of CD25 in some T-cell populations while increasing the proportion of T-regulatory cells or vice versa, resulting overall in no obvious or detectable change. However, this remains to be determined. The potency of lycopene to inhibit proliferation, CD69 expression, and IL-2 secretion to a greater extent in anti-CD3-activated cells compared with Con-A-activated cells suggests that lycopene might be more effective in suppressing TCR-dependent activation processes. A reduction of IL-2 secretion in atherosclerosis would potentially reduce the chronic inflammation occurring in the lesion. It would decrease the proliferation and activation of T cells that would have knock on effects on other immune cells also involved in plaque formation [].
CD11a is part of the receptor leucocyte function antigen 1 (LFA-1), which is expressed on T cells, B cells, macrophages, and neutrophils, and binds to the intercellular adhesion molecule 1 (ICAM-1), expressed on the endothelium, T cells, and other leukocytes. A reduction in LFA-1 expression decreases the potential for T cells to bind to the endothelium and consequently transmigrate into the subintima, a critical step in atherosclerotic plaque progression []. The proportion of CD11a+ cells was slightly reduced only in Con-A-activated cell, not in anti-CD3-activated cell treated with 0.11 μg/mL lycopene. However, such a small effect (less than 1%), even if statistically significant, might result from normal fluctuation in LFA-1 expression independent of lycopene treatment and therefore not physiologically relevant. This is further supported by the lack of change in LFA-1 expression in cells stimulated with anti-CD3. Anti-CD3 would only activate T cells while Con-A would also activate other PBMCs leading to the modulation of CD11a expression. Interestingly, a previous ex vivo study also found no effect of lycopene supplementation for 26 days on CD11a expression in isolated monocytes [].
Overall, our results indicate that low and physiological concentrations of lycopene can inhibit early T-cell activation through the modulation of cell proliferation, CD69 expression, and IL-2 secretion. The mechanism involved remains to be elucidated but could involve the direct [] or indirect activation via oxidized products of lycopene [] of the electrophile response element/antioxidant response element (EpRE/ARE). Dietary lycopene is rapidly metabolized into polar metabolites []. It is possible that these hydrophilic-oxidized products of lycopene generated during lycopene cell metabolism could be responsible for the modulation of lymphocyte activity. Such a reduction in T-cell activation during atherosclerosis would reduce the inflammatory responses involved in atherosclerotic plaque formation and development, and could explain the potential cardioprotective effects of lycopene suggested by observational studies.