Protein-calorie malnutrition (PCM) is still the most common type of undernutrition and approximately 800 million people in the world present some kind of malnutrition 1. This deficiency is usually complex, frequently involving both protein calorie and varying degrees of micronutrient deficiency of vitamin A, vitamin E, vitamin B6, folate, zinc, iron, copper, and selenium. PCM leads to atrophy of the lymphoid organs, profound T-lymphocyte deficiency, and increased susceptibility to pathogens, reactivation of viral infections, and development of opportunistic infections 2. The immune response to infection involves a complex process, including synthesis of acute-phase proteins, cytokines and immunoglobulins and also clonal expansion and cellular differentiation 3. Clearly this requires an appropriate supply of nutrients to optimize the response and consequently the nutritive status of the host critically determines the outcome of infection.

Effects of nutritional depletion can be found in the innate immune system, for example, lysozyme production by monocytes and polymorphonuclear cells is decreased, complement factors are diminished in both concentration and activity and macrophage functions are also impaired 4. Multiple abnormalities in specific immunity have also been frequently described in connection with malnutrition. These studies indicate decrease in T-cell function, cytokine production and also in the ability of lymphocytes to respond appropriately to cytokines 5. T cells have been characterized as Th1 and Th2, depending on their cytokine profile. Th1-type responses are dominated by the production of IFN-γ and are associated with cell-mediated immunity, whereas Th2-type responses are characterized by IL-4 production and more related to humoral responses 6. In general, innate and cell-mediated immunity are more sensitive to undernutrition than humoral immunity 7. Nevertheless, more recent investigations also indicate a reduced Th2 activity 8.

Tuberculosis is a disease caused by Mycobacterium tuberculosis that is historically known to be particularly influenced by undernutrition. It is a major cause of morbidity and mortality in developing countries where PCM is also prevalent 9. Even though some reports suggest contribution of humoral immunity against M. tuberculosis, it is believe that celular immune response is much more relevant 101112. Therefore, the design of all the new vaccines to control TB is based on induction of a predominant cellular immune response. The attenuated BCG strain of Mycobacterium bovis has been extensively used as a vaccine against tuberculosis. However, well documented trials showed that the protective efficacy of BCG varies from 0 to 80%. This highly variable and poorly protective efficacy in certain countries has been attributed to the various BCG strains used as vaccines, environmental factors as well as host genetic characteristics 13. In addition, experimental studies showed that animals were adequately protected by BCG vaccine when properly nourished but exhibited significant weight loss and tuberculin anergy when maintained on a protein-deficient diet 9. Despite BCG vaccination, malnourished children developed serious and often fatal types of tuberculosis such as miliary, meningitic and disseminated tuberculosis 14.

DNA vaccines represent a promising new approach to vaccination in which the gene for a foreign antigen is expressed within the host's cells. These vaccines generated humoral and cell-mediated immune responses followed by protective efficacy in different experimental models of infectious diseases including tuberculosis. DNA vaccination has been proposed as a hope for better vaccination programs in developing countries 15.

Our group has been working with DNA vaccines constructed by inserting the heat shock protein 65 gene from Mycobacterium leprae (hsp65) into plasmid vectors (DNAhsp65). Theoretically, this construction could protect against TB because hsp65 family is one of the most conserved families of proteins presenting more than 97% homology among prokaryotes 16. In addition, hsp65 and other molecular chaperones are highly immunogenic. Around 10 to 20% of all T cells specifically stimulated are reactive with hsp65 in mice immunized with M. tuberculosis 17. Indeed, this construction displayed both, prophylactic and therapeutic effect in experimental tuberculosis 1819. These evaluations were done with mice or guinea pigs submitted to normal chow. Malnutrition could affect both, antigen synthesis and the immune response itself, as they rely on the host's metabolism. Based on this scenario, we hypothesized that immune response induced by a genetic vaccine (pVAXhsp65) could be jeopardized in malnourished mice.

Experimental dietary restriction by deprivation of variable percentages of food intake is being used to explore effects of PCM on immunity and susceptibility to infectious agents 20. In this study, we first characterized the immunological status of mice submitted to a dietary restriction protocol for 40 days and then evaluated the effect of this restriction on their ability to mount an immune response against a DNA vaccine containing the mycobacterial hsp65 gene.

A significant weight loss was already observed at the fourth day of diet and this was maintained until the end of the experiment that was at the 40^th day. Weight losses are described in many studies with undernourished animals and used as a criteria to characterize undernutrition. A striking decrease in leucocyte number that selectively affected lymphocytes was also observed.

Alterations in body and spleen weights were compatible with the findings from the histopathological analysis that showed evident alterations in lymphoid organs. Thymus sections from dietary restricted group revealed severe atrophy that was reinforced by a 52% reduction in their weights (not shown). These findings are highly supported by the literature in both, experimental and human malnutrition 21. Peyer's patches and inguinal lymph nodes were clearly atrophic (not shown). The deleterious effect over mucosal immune system was attested by the evident villous atrophy observed in the small intestine. Sulivan et al. 22 have shown that poor dietary protein has a direct effect on mucosal IgA, secretory component, number of IgA-containing cells and IgG levels in rats.

As cytokines are the major effectors and regulators of the immune response, we next evaluated the ability of spleen cells to produce IFN-γ and IL-4 that are considered key cytokines in the development of Th1 and Th2 cells, respectively 23. As IFN-γ can be directly induced by polyclonal activation of T cells, the spleen cells were stimulated with ConA, LPS and SAC were additionally used because they indirectly induce IFN-γ production by NK cells, i.e, via IL-12 production 2425. In ConA stimulated cultures there was no difference between normal and dietary restricted groups. However, IFN-γ production was significantly compromised in cultures stimulated with LPS or S. aureus (SAC). This decreased IFN-γ production is consistently described in humans and experimental models with malnutrition 2627.

The mechanism involved in this differential IFN-γ response associated with distinct stimuli was not investigated. However, we could think that the decreased T cell number was associated with a higher degree of apoptosis as was clearly demonstrated by Pires et al. 28. In this context, the remaining T cells, i.e., the ones spared from apoptosis, could still be able to produce this cytokine if adequately stimulated. This was hypothesized from the additional fact that ConA is a strong stimulus that directly and strongly interacts with glycoproteins from T cell surface 29. On the other hand, the reduced IFN-γ levels induced by LPS and SAC could indicate that other cell functions or cytokine synthesis are compromised by dietary restriction. IL-12 availability is considered the dominant factor in driving the development of Th1 cells that are characterized by IFN-γ synthesis 30. Therefore, lower levels of this cytokine could profoundly impair IFN-γ production. It is also well described that IL-12 is involved in IFN-γ production in protocols where LPS and SAC are used to stimulate human cells 31. The possibility that reduced IFN-γ production is associated with a deficit in IL-12 supply is reinforced by a recent publication in which the authors demonstrated a significant reduction in both, IL-12p70 and IFN-γ synthesis in mice whose diet was reduced to 70% of the amount of food consumed by the corresponding control group 20.

The effect of these alterations on the immune response induced by the pVAXhsp65 vaccine was devastating. In comparison to the control group that produced significant amounts of both, IgG1 and IgG2a anti-hsp65 antibodies, undernourished mice did not produce even basal levels of these antibodies. As Th1 cells are characterized by IFN-γ production and, in mice, the selective switching to IgG2a whereas Th2 cells produce IL-4 and trigger switch to IgG1 and IgE 6 these results indicate that this degree of diet restriction is highly deleterious for both, cellular and humoral components of the immune response.

The effect of the nutritional status during conventional vaccination has been investigated. Measles vaccines did not show efficacy in undernourished children in Africa and India 32. On the other hand, Moore et al. 33 studying the immune response to different vaccines in undernourished children in Gambia, concluded that the secretion of antibodies was not altered even by different degrees of nutritional deficiencies. Only a few reports addressed the consequences of a nutritional deficiency on DNA vaccines. Recently, Sakai et al. 34 found a selective impairment of T cells with no effect over B lymphocytes, in a protein deficiency model.

This complete abrogation of the immune response towards a DNA vaccine in undernourished mice could be explained by the double role of the host submitted to this kind of vaccination. In this case, in addition of cellular interactions that are necessary to mount the immune response, the host cells also need to synthesize the antigen. Therefore, it is expected that the immunity to DNA vaccines is even more compromised than the response to conventional vaccines.

Further investigations will be necessary to answer very relevant questions in this area. It will be important to establish if this finding will apply to other plasmids, if other delivery vectors will behave the same way and also if the immunization route can affect the final immune response.

Together these results demonstrate that a 20% reduction in the amount of food intake was able to significantly alter the immune system. The physiological relevance of these alterations was demonstrated by the abrogation of the immune response induced by a DNA vaccine against tuberculosis. These results alert for the fundamental role of the nutritional state, which is frequently affected in developing countries, in vaccine programs.

The authors declare that they have no competing interests.

LLWI, TGDF and AS are the main investigators in this study. FCM, SFGZP and NMM largely contributed with the immunological experiments. PCMP and CLS provided critical input.