The traditional concept of ROS function is that they indiscriminately destroy cell components.
However, exciting research has more recently elucidated the role of these reactive species in signal transduction, gene regulation, and disease etiology.
This has infused new excitement and challenges into research on the possible role of carotenoids as antioxidants in disease prevention.
This discussion will attempt to address the complex interaction of carotenoids and the immune response, and how this interaction may relate to cancer etiology.
Early studies demonstrated that dietary β-carotene prevented bladder, kidney, ear and gut infection in vitamin A deficient rats and reduced ear infection in young children.
Because of the provitamin A activity of β-carotene, these studies raised the possibility that the action of the carotenoid is due to its prior conversion to vitamin A.
To circumvent this problem, the specific role of carotenoids can be demonstrated either by using carotenoids without provitamin A activity (e.g., lutein, lycopene, canthaxanthin, astaxanthin) or by using animals that cannot convert or are poor converters of carotenoids to vitamin A (e.g., cats).
Numerous studies using nonprovitamin A carotenoids and, more recently, using cats as the animal model have demonstrated the immuno-modulatory action of dietary carotenoids.
It is recognized that cats can convert carotenoids to vitamin A, albeit very inefficiently.
Many earlier studies focused on β-carotene.
Seifter et al. reported a marked stimulatory action of β-carotene on the growth of the thymus gland and a large increase in the number of thymic small lymphocytes.
The stimulatory activity of β -carotene on lymphocyte blastogenesis has similarly been demonstrated in rats, pigs, and cattle.
Increased numbers of Th and T inducer lymphocytes have been reported in human adults given oral β -carotene supplementation.
The number of lymphoid cells with surface markers for NK cells and for IL-2 and transferring receptors also was increased substantially in peripheral blood mononuclear cells (PBMC) from individuals supplemented with β-carotene.
Enhanced NK cell cytotoxicity was observed in human subjects given oral β-carotene.
Similarly, long-term β-carotene supplementation to elderly but not middle-age men increased NK cell activity.
In vitro, β-carotene induced hamster macrophages to produce TNFα.
Activation of TNFα by ROS increases the dissociation of IκB from NFκB, and the subsequent translocation of this transcription factor to the nucleus, resulting in the production of cytokines, chemokines, cell adhesion molecules, and acute phase proteins; this activation also produces an anti-apoptotic effect.
Alternatively, intracellular ROS may directly increase NFκB.
Therefore, ROS are important in primary immune response; conversely, antioxidants can produce the opposite effect.
In fact, Verhasselt et al. reported that the antioxidant molecule N-acetyl-L-cysteine can inhibit NFκB, and consequently down-regulate the production of cytokines (IL-6, IL-8, IL-12, and TNFα), as well as down-regulate the expression of surface molecules (HLA-DR, B7-2 and CD40) in human dendritic cells.
Therefore, an antioxidant may impair the generation of primary immune responses through its inhibitory action on dendritic cells.
While this scenario occurs in a normal cell, under conditions of high oxidative stress, excess ROS may be produced, resulting in the inhibition of NFκB.
Excess ROS is known to cause abnormal cell proliferation and to decrease apoptosis; both are undesirable responses in tumor cells.
Therefore, antioxidants are desirable under conditions of high oxidative stress.
Analogous to this situation, high concentrations of intracellular nitric oxide induced oxidative killing of isolated rat hepatocytes while low nitric oxide concentrations was protective.
Besides cell-mediated and humoral immune responses, β-carotene has been shown to regulate nonspecific cellular host defense.
Blood neutrophils isolated from cattle fed β-carotene had higher killing ability during the peripartum period.
The increased bacterial killing could be accounted for partly by increased myeloperoxidase activity in the neutrophils.
Tjoelker et al. reported that dietary β-carotene stimulated phagocytic and bacterial killing ability of neutrophils from dairy cows during the stressful drying off period.
In contrast, retinol and retinoic acid generally decreased phagocytosis and had no effect on killing activity.
A specific role of carotenoids on immune response was first reported by Bendich and Shapiro.
They showed that rats fed canthaxanthin, a carotenoid with no provitamin A activity, had a heightened mitogen-induced lymphocyte proliferation; dietary β-carotene showed similar action.
Subsequent studies have similarly reported the immuno-enhancing action of carotenoids without provitamin A activity, notably lutein, lycopene, astaxanthin and canthaxanthin.
Canthaxanthin enhanced the expression of activation markers for Th and NK cells in human PBMC in vitro.
Jyonouchi et al. reported that lutein and astaxanthin increased the ex vivo antibody response of mouse splenocytes to T-cell antigens.
Schwartz et al. reported increased cytochrome oxidase and peroxidase activities in macrophages incubated with canthaxanthin, β-carotene, and β-carotene compared with incubation with 13-cis retinoic acid.
The stimulatory activity of canthaxanthin was greater than that observed with β-carotene and β-carotene.
Phagocytosis also was stimulated by these carotenoids, even though to a lower degree.
All of these changes indicate increased respiratory bursts by the macrophages when they are exposed to carotenoids.
The domestic dog and cat have recently been used in parallel studies using similar experimental designs to compare the immuno-modulatory role of carotenoids.
These studies thus provide direct comparisons between carotenoids with (β-carotene) or without (lutein) provitamin A activity, and also between species that can (dogs) or that are very inefficient converters (cats) of β-carotene to vitamin A.
Dietary β-carotene and lutein stimulated DTH response, the number of CD4+Th cells, and IgG production in dogs, thus demonstrating that lutein, a carotenoid without provitamin A activity, exerts a similar immuno-modulating action as β-carotene.
In contrast, lutein but not β-carotene enhanced mitogen- induced lymphocyte proliferation in dogs, indicating species differences in the lymphocyte proliferation response to a given dietary carotenoid.
Cats fed β-carotene or lutein also showed heightened DTH response, higher Th and B cell subpopulations, and increased plasma IgG concentrations.
It can be concluded that the actions of both β-carotene and lutein in cats are not due to their prior conversion to vitamin A because cats are poor converters of β-carotene to vitamin A.