Editorial Article

Immunopathogenesis of Leishmaniasis: Some Challenging Aspects

Luis Aliaga1,*, Fernando Cobo2
1Departamento de Medicina. Facultad de Medicina. Universidad de Granada. Avda. de la Investigación, 11. 18071 Granada , Spain
2Servicio de Microbiología. Hospital Universitario Virgen de las Nieves. Avda. Fuerzas Armadas, s/n. 18014 Granada, Spain
*Corresponding author:

Luis Aliaga, Departamento de Medicina. Facultad de Medicina. Universidad de Granada. Avda. de la Investigación, 11. 18071 Granada , Spain, Email: laliaga@ugr.es

The term leishmaniasis (or ‘leishmaniosis’)1 refers to a diverse group of syndromes caused by >20 different species of intracellular protozoan of the genus Leishmania, belonging to the Leishmania and Viannia subgenera1-3. The parasite infection is widely distributed across the tropical, subtropical, and temperate regions in 88 countries1,2. Three hundred fifty million people are at risk in widely scattered regions, including the Mediterranean basin. An estimated 12 million people suffer from leishmaniasis worldwide, with 1.5 to 2 million new cases per year2,3.

            The clinical manifestations of leishmaniasis depend on the interplay between the virulence factors of the infecting Leishmania species and the genetically determined immune responses of their human hosts1-4. Leishmaniasis in humans has been divided traditionally into 3 major clinical syndromes: visceral (VL), cutaneous (CL), and mucosal leishmaniasis (ML). In addition, several other less common presentations have been described, such as diffuse cutaneous leishmaniasis, disseminated cutaneous leishmaniasis, leishmaniasis recidivans and post-kala-azar dermal leishmaniasis (PKDL)2,3.  A single Leishmania species can produce more than one clinical syndrome, and each syndrome is caused by multiple species1,2. On the other hand, in endemic regions, the majority of individuals infected with Leishmania spp. never develop the disease, they only will show antileishmanial antibodies and/or cutaneous delayed type hypersensitivity (DTH) to leishmanial antigens1-3.

            Leishmania parasites are transmitted to the mammalian host by the bite of female sand flies of the genera Phlebotomus (Old World) and Lutzomya (New World)1-3. Peptides in sandfly saliva (e.g., maxadilan) cause vasodilation and erythema and help establish infection in the dermal layer of the skin2. Once inoculated into the skin, neutrophils are the earliest cells recruited to the bite site, ingest parasites and infected neutrophils are then taken up by dendritic cells and macrophages2,3. Now, the parasite in the metacyclic promastigote stage infect macrophages and transform into the amastigote form. The parasite lives and divide within an endosomal compartment of the macrophage2.  The infective forms have some virulence factors2,3. They have a glycoprotein coat (lipophosphoglycan) which interferes with macrophage and dendritic cell function, and surface metalloprotease gp63, which protects the parasite against innate complement mediated lysis and enables entry into macrophages2,15.

            After these events, the infection may be controlled or it may progress to a localized self-resolving skin lesion (CL) to more diffuse forms that disseminate through the reticuloendotelial system of the body (VL). Cytokines and chemokines play key roles in mediating the outcome of infection, and there is evidence of both protective and disease-enhancing immune responses. The progress of the disease depends on the parasite species and host responses, but the precise reasons by which some patients infected by a certain leishmania species (e.g., Leishmania infantum) developing form of the disease or another are not yet understood.  

            For both VL and CL, disease progression depends on the maintenance of a parasite-specific immunosuppressive state. However, skin lesions at the site where promastigotes were inoculated are usually not apparent in persons with VL2. Host cell macrophages are in a deactivated state; but under optimum conditions, macrophages are eventually activated to a leishmanicidal state. Cure follows when macrophages become activated and kill the parasites. Resolution of disease, after the activation of macrophages, is mediated by a T-helper cell TH1 response after interaction of antigen-presenting cells (e.g., dendritic cells) with CD4+ and CD8+ T cells and subsequent secretion of pro-inflammatory cytokines  [e.g., interleukin 2 (IL-2), interferon-g (IFN-g), tumor necrosis factor-a(TNF-a)]2,3. IL-12 also plays an important role in the development of protective immune responses2. At the cellular level, IFN-g activates macrophages to kill amastigotes through L-arginine-dependent nitric oxide production, which follows induction of nitric oxide synthase, and oxidative killing mechanisms in the phagolysosomal compartment2. IL-1 and TNF-a prime macrophages for activation by IFN-g.  Thus, persons with self-resolving infection with Leishmania spp. and those who have undergone successful chemotherapy develop protective immune responses, characterized by a TH1 profile response and the development of cutaneous DTH. Recovery of CL is associated with a high level of resistance to reinfection by the homologous Leishmania spp.2This same TH1 response also prevents recrudescence of latent infection and VL can reactivate after cure years later only if the infected person becomes immunocompromised2,3.

            However, in clinical forms such as active VL or diffuse CL, development of Leishmania-specific TH1 response is inhibited and there is no evidence of cutaneous DTHresponse to leishmanial antigens. These progressive cases initially were thought to be associated with a TH2 type immune response whereby downregulation of macrophage activity follows the production of cytokines such as IL-4, IL-10, and IL-13 (TH2 associated cytokines) and transforming growth factor-b (TGF-b)3. However, most studies have not corroborated that there is a clear TH2skewing in human VL5. Typically VL is associated with increased production of multiple and primarily pro-inflammatory cytokines and chemokines. VL patients have been found to have elevated plasma protein levels of IL-1, IL-6, IL-8, IL-12, IL-15, IFN-g inducible protein 10, monokine induced by INF-g and TNF-a5. These observations suggest that development of VL is not driven by TH2 skewing per se, but that other mechanisms contribute to the pathogenesis of VL.

            Although polarised TH1 and TH2 responses can be produced in animals and related to resistance and susceptibility to infection, in patients with clinically apparent infection, TH1 and TH2 type responses are not characteristically polarised, as both activating and suppressive cytokines are detected3. In any way, why TH1 response arises and dominate in some persons and not in others remains unknown.

            Both experimental and clinical data support a relevant pathogenic role for interleukin 10, especially in visceral leishmaniasis and PKDL, and cytokine balance (e.g., IFN-g to IL-10 ratio) can affect both clinical outcome and responses to treatment3,5. Patients with active VL show elevated levels of IL-10 in serum as well as enhanced IL-10 mRNA levels in spleen, lymph nodes, and bone marrow5. Similarly, PKDL, a rare sequela to cure from VL, is associated with high level of IL-10 in blood and skin during clinical disease5. IL-10 is known to suppress the development of TH1 responses.Experimentally targeting IL-10 for therapeutic inhibition allows activation of TH1 cell responses, promote parasite killing and synergy with chemotherapy in acute infection6.TGF-b may act in synergy with IL-10 in VL. TGF-bhas down-modulatory effects on macrophages and its blockade has been found to limit parasite replication in these cells5.

            A study carried out in the Sudan suggested a protective role for TH17 cells in human VL,  showing a correlation between the presence of L. donovani-specific T cells, secreting IL-17 and IL-22, and protection against developing VL7. TH17 cells are primarily pro-inflammatory CD4+ T cells that have the potential to secrete the prototypic cytokine IL-17. IL-27 is a cytokine central in the regulation of TH17 cells, which mainly is produced by antigen-presenting cells. Studies in mice have demonstrated that IL-27 can inhibit the differentiation of TH17 cells involved in autoimmunity and pathogenetic responses to infection5. Kumar and Nylén have suggested that IL-27 promote the differentiation and expansion of antigen-specific IL-10 producing T cells and inhibit the potentially protective TH17 lineage and thereby facilitate parasite survival5.

            The role of the anti-leishmanial antibody response seen in VL patients is unclear. In patients with progressive VL, high titers of Leishmania-specific antibodies of the class IgE, IgM and IgG are observed, whereas CL patients lack Leishmania-specific antibodies or mount a very weak response2,3,5. After cure and presumed immunity to VL, antibodies against Leishmania persist for a long time, even 15 years or more5. Detection of antileishmanial antibodies has traditionally proven useful in the diagnosis of VL disease, although they cannot distinguish between active and past infection. However, this response of anti-leishmanial antibodies in patients with active VL appears neither protective in acute disease nor prevent reactivation after cure2,3,5

            Persons with cutaneous or mucosal leishmaniasis (ML)have evidence of both TH1 and TH2 lymphocytes in their lesions but the systemic response is predominantly TH12. Their peripheral blood mononuclear cells proliferate and produce IFN-g and IL-2 in response to leishmanial antigens in vitro, and those infected exhibit cutaneous DTH responses, as evidenced by positive skin tests in vivo2.Complex chemokine and cytokine responses govern the tissue localization of effector cells and the resulting immune responses, but the precise sequence of events that results in skin necrosis and eventual healing has not yet been characterized2.

            Mucosal involvement in leishmaniasis has been observed both in the Old and New World, but ML has different pathogenesis and clinical manifestations related to the geographic presentation2,3,8-10. In America, a small percentage of persons with CL due to Leishmania (Viannia) Braziliansis (and related species) develop mucous membrane involvement of the nose, oral cavity, pharynx, or larynx months to years after their skin lesions have healed. ML in Latin America, called Espundia, is a severe form of leishmaniasis that may result in mutilating and destructive mucosal lesions. It is thought that the development of this form of ML does not stem from a weak immune response but is instead due to an unregulated inflammatory response that leads to a massive infiltration of inflammatory cells2,3. The infiltration results in the severe destruction of the nasopharyngeal mucosa, despite the effective reduction of the number of parasites. At present, there is no way to predict in whom this more severe form of the infection will develop.

            Almost 20 years ago, Stuart et al. described that some South American leishmania parasites harbor a double-stranded RNA virus, the endosymbiont Leishmania virus type 1 (LRV1)11.  To date, the distribution of leishmania strains containing LRV1 has been limited to a few strains in specific regions in South America12. Recently, a study by Ives et al.13 showed that LRV1 may greatly amplify the responses of macrophages after interaction with leishmania parasite. These authors observed that macrophages infected with strains of L. (V.) guayanensis produced much higher levels of cytokines and chemokines if the clones of leishmania had caused mucosal disease than if the leishmania clones had not caused it. They also showed that such leishmania clones contained LRV1, which promoted the production of proinflammatory molecules (TNF-a, IL-6, CXCL10, and CCL5). Using various toll-like receptor (TLR) pathway-mutant mice, the investigators demonstrated that this response was dependent on the presence of the endosomally TLR3 gene product (TLR3), which recognizes double-stranded RNA, and its downstream adapter protein, toll-interleukin-1-receptor domain-containing adapter inducing interferon-beta.  The investigators elegantly hypothesize that viral RNA is released by dead parasites soon after infection; the binding of viral RNA to TLR3 results in the production of cytokines and chemokines that enhance inflammatory response and thus exacerbate disease13. However, the role of this virus in causing ML remains to be determined, since leishmania molecules are known to bind at least five different TLRs and ML caused by L. (V.) braziliensis without LRV1 in Brazil has been observed14.

            Several studies from areas endemic for VL have demonstrated that most people infected with Leishmania species never develop disease2,3,15. Asymptomatic leishmaniasis infection is not well defined but is usually ascertained by a positive serological test, polymerase chain reaction (PCR), or leishmanin skin test in individuals who are otherwise in a healthy condition15. Ratios of asymptomatic infection to incident clinical cases varies widely, depending on the geographic region studied and the method used15. The understanding immune mechanism that leads to control of infection is key questions for research in this field. Peripheral blood mononuclear cells from some (but not all) individuals with asymptomatic infection respond to stimulation with leishmanial antigen and produce IL-2, IFN-g and IL-125,15.Interestingly, numbers of CD4+T cells are increased in persons with asymptomatic infection having positive DTH, and CD8+T cells isolated from asymptomatic subjects produce high amounts of IFN-g, which strongly suggests a role of CD8+T cells in human resistance to Leishmania infection15.

            A better understanding of the disease-causing roles of immunologically active cells and their cytokine products, along with genetics and correlations with clinical subsets will lead to more focused treatment modalities and the avoidance of the need for long-term conventional chemotherapy which is associated with toxicity and relapse of the disease. Immunotherapeutic strategies aimed to strengthen Leishmania-specific immune response, prior to or in synergy with conventional therapy, may lower the required dose of the treatment regimen and thus toxicity. Moreover, immunotherapy may also improve the drug efficacy, reduce the emergence of drug-resistant strains, and circumvent the problems of treatment in immunocompromised hosts. Attempts to identify cytokines, which may be useful in leishmaniasis therapy, have focused on the balance between TH1 and TH2 cells. Targeting (or boosting) regulatory pathways and cytokines for the treatment of leishmaniasis may be a rational treatment tool. Research in this challenging area is essential to achieve a more effective control of  Leishmaniasis in the near future.

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Published: 10 April 2017


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