DL is an emergent and severe form of L. braziliensis infection 1,10,37. While the mechanism underlying parasite dissemination has yet to be identified, it is clear that this does not occur at the time of infection. Indeed, multiple acneiform and papular lesions suddenly appear only days or weeks after a primary ulcerated lesion; dissemination has been associated with fever and chills lasting 1–2 days.10 As L. braziliensis is polymorphic, distinct genotypic characteristics among isolates have been linked to different clinical forms of disease, e.g. cutaneous, mucosal, DL or atypical CL 27,28,33. Here we attempted to evaluate whether infection with DL isolates provokes different behavior compared to CL in terms of parasite internalization and multiplication, as well as monocyte activation and the production of inflammatory molecules. The present results indicate that L. braziliensis isolates from DL patients exhibit a greater ability to penetrate and multiply in monocytes compared to CL, in spite of higher respiratory burst induction and enhanced proinflammatory cytokine production. Additionally, host factors also play a role in the parasite dissemination, as monocytes from DL is more permissive to the infection and allow a greater parasite multiplication than CL cells upon infection with a DL isolate. Moreover, cells from DL expressed more TLR-2 and TLR-4 than cells from CL patients infected with a DL isolate, and produce higher levels of PGE2 than CL cells upon stimulation with SLA.
In the first 48 hours after in vitro infection with L. braziliensis, the percentage of infected monocytes/macrophages and the number of intracellular parasites increases, reaches a plateau and then begins to decrease 19,38. Parasite internalization is observed via the quantification of these parameters in the first two hours of infection. After this time, the percentage of infected cells and numbers of parasites reflect the ability of leishmania to multiply inside host cells, as well the capacity of these infected cells to kill parasites. Here, we demonstrate that experimental infection using isolate from DL patient enables greater monocyte penetration and survival compared to isolates from CL, regardless of the origin of source cells, i.e. DL or CL monocytes, providing evidence of the capability of genotypic differences among L. braziliensis to interfere in parasite internalization and multiplication.
Parasite proliferation inside phagocytic cells leads to cell lysis and the release of leishmania 36. Our results document the greater viability of DL isolate compared to CL in the supernatants of DL, CL and HS monocyte cultures. Interestingly, and perhaps more important than genotypic differences among parasites, monocytes from DL patients were observed to allow enhanced leishmania multiplication, as higher numbers of viable promastigotes were observed in the supernatants of cultured monocytes from DL patients, in comparison to CL, following infection with a DL isolate. This results provides evidence that DL monocytes were more permissive to leishmania survival, regardless of the source of L. braziliensis, i.e. isolates from DL or CL patients.
SOD enhances parasite multiplication in macrophages and SOD is highly expressed in host tissue during CL infection by L. braziliensis 32. In addition, in CL caused by L. amazonensis, SOD plasma levels constitute a predictor of failure to meglumine antimoniate therapy 24. More recently, PGE-2 has been associated with leishmania proliferation and survival 25,26,39. Here we found no differences in SOD serum levels but observed that, upon stimulation with SLA, monocytes from DL patients produced higher levels of PGE-2 than cells from CL patients. This finding lends support to the role of monocytes, since origin, i.e. DL vs. CL, as was shown to influence parasite survival.
Monocytes from CL patients present higher oxidative burst and also produce more reactive oxygen species (ROS) and nitric oxide (NO) than cells from HS; still, these phenomena are not sufficient to control parasite multiplication and may still lead to pathology 19. The importance of NO in the killing of L. infantum and L. amazonensis has been documented, as well as that of ROS in the control of L. braziliensis proliferation 15,40,41. Ávila et al. showed that L. braziliensis promastigotes and amastigotes isolated from CL and ML patients produced similar amounts of NO in culture. However, promastigotes from ML isolates were found to be more resistant to NO and H2O2 than CL parasites 42. Our results document that infection with DL isolates induces higher respiratory burst in monocytes from both DL and CL patients compared to isolates from CL, yet the observed enhancement in oxidative burst did not inhibit parasite multiplication, which suggests that DL isolates are less susceptible to monocyte killing than CL.
While TLRs are known to participate in host defense mechanisms, the expression of TLRs has also been associated with inflammatory and autoimmune diseases 43. Our results show that monocytes from DL patients infected with DL isolates express more TLR2 and TLR4 compared to CL isolates. However, we did not similarly document this in CL monocytes. As cells from DL expressed more TLRs upon infection with DL isolates, but also exhibited less capability to kill leishmania, we suggest that the exaggerated inflammatory response observed in DL may likely be more closely related to pathology than protection.
Proinflammatory cytokines, such as TNF, CXCL9 and CXCL10, are mainly produced by monocytes/macrophages. In CL patients, these cells produce higher levels of cytokines than healthy subjects 38. Elevated systemic production of CXCL9 has been documented in sera from DL patients compared to CL 10. Additionally, upon stimulation in PBMCs of patients with both forms of disease, SLA obtained from isolates of DL patients induced higher TNF and IFN-γ expression than SLA from CL 30. The present work expanded on these observations by demonstrating that DL isolate induce higher levels of TNF, CXCL9 and CXCL10 expression than CL isolate in cultured monocytes from both CL and DL patients. Indeed, it appears controversial that infection using DL isolate induces a pronounced proinflammatory response in monocytes, yet at the same time permits parasite survival inside phagocytic cells. However, it is important to consider that proinflammatory cytokine production by monocytes has not been definitively linked to parasite killing in CL caused by L. braziliensis 38.
Our data indicate that parasite dissemination in DL occurs due to parasite multiplication, cell death and the release of amastigotes that infect monocytes at different sites of the skin. However, we cannot rule out the possibility that parasite dissemination may also occur through the metastasis of infected monocytes from the original lesion site to other areas of the body. In cancer, inflammation influences metastasis. More specifically, the production of CXCL9 and CXCL10, among other proinflammatory cytokines, has been associated with the severity of melanoma and metastasis 44,45. Here we expanded on a previous report that documented higher pro-inflammatory cytokine production induced by DL isolates compared to CL isolates 30. In light of the lack of evidence that inflammation correlates with leishmania killing in L. braziliensis infection, it is important to consider the possibility that inflammation may hold influence over parasite dissemination.
DL, a severe and emergent form of L. braziliensis infection, has been associated with high rates of failure to antimony therapy. Our results show that parasite dissemination can be influenced by host and parasite factors, and that parasite multiplication in macrophages is highly linked to parasite dissemination. As inflammation has been associated with the pathology of L. braziliensis infection, emphasis has been placed on a combined regimen of chemotherapy with immunomodulatory agents in the treatment of TL 46,47. In addition to offering insight into the pathogenesis of DL, the present results point to the necessity of identifying novel therapeutic agents capable of enhancing leishmania killing by macrophages in order to enable the control leishmaniasis.