Insulin-like growth factor-I serum levels and their biological effects on Leishmania isolates from different clinical forms of American tegumentary leishmaniasis
The pathogenesis of American tegumentary leishmaniasis currently is considered multifactorial, due to not only specific immune response [24] but also parasite diversity [4, 17] and other factors related to the host [25, 26]. In this scenario, we have been studying the role of IGF-I in leishmaniasis mainly in vitro and in experimental models of leishmaniasis. The role of IGF-I in human leishmaniasis has not been established.
Based on our previous experimental data, IGF-I can play a role in the progression of Leishmania (Leishmania) amazonensis infection by (i) decreasing NO production in macrophages and allowing parasite multiplication, (ii) modulating immune responses by increasing TGF-? and decreasing IFN? production in promastigote infected macrophages, and (iii) inducing apoptotic mimicry characterized by exposure of phosphatidyl serine on amastigotes but without leading to death [15, 16]. However, it is unknown whether similar effects of IGF-I would occur in other Leishmania species.
Because tegumentary leishmaniasis in Brazil is mostly caused by L. (V.) braziliensis [1] and different biological effects may occur upon IGF-I stimulus, we initially studied its effect on promastigotes. Focusing on the increase of arginase activity of the parasite, which was seen as main effect of IGF-I [16], we confronted more complex outcomes that were dependent on Leishmania isolates, whether coming from patients with CL, ML or DL. IGF-I induced higher basal arginase activity in L. (V.) braziliensis promastigote isolates from CL and DL but not in ML. Conversely, in Leishmania isolate from ML the arginase activity was already increased in basal conditions [17]. In the present study, we analyzed the effect of IGF-I on intracellular amastigotes in sequence.
The intracellular amastigote growth was evaluated here using human macrophage cell line THP-1 rather than macrophages derived from peripheral blood monocytes to avoid the variation in cellular response that would occur if obtained from different individuals. IGF-I induced an increase in parasitism in CL and ML isolates but a decrease in parasitism in DL isolates. In CL and ML these data suggest that the presence of IGF-I may contribute to the persistence of the parasite in the skin. In DL isolates, basal arginase activity was also lower than that of CL. However, arginase activity was similar with and without IGF-I stimulus, leaving mechanisms that need to be further explored.
To proceed with the study of the involvement of IGF-I in different clinical manifestations, IGF-I and IGFBP3 serum levels were evaluated in patients with CL, ML and DL. In CL, both of these levels were similar to those in healthy controls, while in ML and DL, both levels were decreased. Altogether, these data indicate that IGF-I cannot be interpreted as having similar effects in patients presenting different clinical manifestations and that the effects may encompass healing and inflammatory processes beyond parasite growth.
American tegumentary leishmaniasis is characterized by an intense inflammation with a high production of tumor necrosis factor-? (TNF-?) and interferon-? (IFN-?), molecules that are necessary to protect the host against Leishmania but can also cause tissue damage even with a scarce number of parasites in the lesions [25, 27]. In the early phase of the disease when patients have a small cutaneous lesion or when is evolving to heal [28], this T helper type 1 immune response is known to be down modulated [29]. IGF-I is known to be increased in the initial phase of skin injury because this molecule is necessary for epidermis maintenance [30, 31], which would occur also in CL. In CL, because IGF-I is suggested by the present findings to have a parasite growth promoting effect on Leishmania isolates and has its serum level maintained in the level of controls, IGF-I would probably contribute to the progression of parasite growth and establishment of Leishmania in the host skin.
Cutaneous ulcer is the first lesion of tegumentary leishmaniasis that later can be complicated by the appearance of mucosal or disseminated disease. After the initial phase of L. (V.) braziliensis infection, a very strong Th1 immune response occurs that is associated with a decrease in IL-10 production [32]. IFN-? and TNF-? are also produced in even higher concentrations in ML patients [33], and this increased production of cytokines may relate to the lower level of IGF-I in patients with ML observed in the present study.
In DL patients, a slight decrease in IFN-? production was observed when analyzing peripheral blood mononuclear cells, which may result from the migration of activated T cells to the multiple lesions and to the mucosal tissue because up to 40 % of DL patients have mucosal disease [34]. In DL, IFN-? production is still high that may relate to the decreased IGF-I serum levels found here in this form of the disease.
IGF-I was shown to decrease vascular inflammatory process in mouse model of atherosclerosis [14]. Because the pathogenesis of American tegumentary leishmaniasis is based on an exaggerated immune response [35, 36], a decrease in IGF-I in ML and DL may contribute to the persistence of the inflammatory response and further on the delayed healing of the lesion.
Having pathogenic mechanism involving IGF-I in mind we can speculate the use of IGF-I to stimulate healing and control of inflammation in chronic and severe cases of ATL where IGF-I level is decreased. There are studies on IGF-I use in other dermatological lesions. In diabetes mellitus ulcer it is suggested that the healing is delayed due to a decrease in IGF-I level in the skin [12] and some approaches have been tried. In diabetes mellitus patients the skin ulcer healed with an increase in local IGF-I level upon the use of hyperbaric oxygen therapy [37]. In experimental diabetes model local use of IGF-I cream has also improved the healing [38]. In ATL IGF-I may increase the parasite growth but it would be in the tissue environment where strong Th1-type immune response is present and thus the parasite growth would be restrained.