Geographical Distribution of Cutaneous Leishmaniasis and Pathogenesis

Abstract

Leishmaniasis is still considered to be a global health problem, which spreads in most countries in the world. Leishmania is an intracellular obligate protistan parasite that causes different clinical symptoms in infected humans and other animals. There are clinically different types of the disease including: visceral, cutaneous or muco-cutaneous leishmaniasis. Approximately, two million new infections occurring annually; 0.7 to 1.2 million cases are recorded with cutaneous leishmaniasis and 200,000–400,000 cases return for visceral leishmaniasis. However, Cutaneous leishmaniasis considers one of uncontrolled wobbling endemic diseases, especially in Iraq, which occurs at the skin to cause a dermal lesion. Usually, the lesion is spontaneously healed to leave a colorless depressed scar and permanent immunity.

1. Introduction

Leishmaniasis considers one of the neglected tropical diseases with a wide global distribution (Figure 1) [2]. WHO mentioned that leishmaniasis is from the important tropical diseases and represents a serious health problem. It includes a broad spectrum of clinical symptoms which may led to the fatal [3]. Leishmaniasis is endemic in 98 countries with over 350 million people at risk. There are more than 12 million infections with leishmaniasis and nearly 2 million new infections occurring annually; 0.7 to 1.2 million cases of cutaneous leishmaniasis and 200,000–400,000 cases of visceral leishmaniasis [4, 5]. Overall, two million new cases of cutaneous leishmaniasis are discovered annually in Afghanistan, Iran, Saudi Arabia, Syria, Iraq, Algeria, Brazil and Peru [6].

Figure 1.

Worldwide distribution of cutaneous leishmaniasis [1].

There are approximately 20 species confirmed for Leishmania are pathogenic to humans [7], that may cause different clinical symptoms (Cutaneous Leishmaniasis (CL), Muco-cutaneous Leishmaniasis (MCL) and Visceral Leishmaniasis (VL)) [8]. However, Leishmania parasites infect phagocytic cells. Virulence factors cause an inhibition or modification of host cell signaling pathways (interleukin and chemokine) which effect on other immune cells [9].

1.1 Prevalence of CL and geographical distribution

Although CL is considered as a global health problem, it remains as a neglected disease [10]. Leishmaniasis infections spread in geographically different regions. However, more infections occur in sub- or/and tropics of Africa, the Middle East, Southern and Central America and South Europe and Asia [11]. It is prevailing at a broad global level and there is an increase worldwide in recorded cases number, also is most neglectful diseases. There is a geographical expansion and a global increase in infection of visceral and cutaneous leishmaniasis, which is still associated with vector population expansion [12]. Environment conditions may overlap many biological processes that effect directly on geographically sandfly distribution, reproductive rates and abundance [13]. Further, Leishmania showed as an opportunistic pathogen with HIV-infected adults and even children [14]. Depending to recent reports, leishmaniasis considers an endemic disease in 98 countries and about 1.3 million new cases annually [15]. However, CL is an endemic disease in several countries in the Middle East and North Africa. Foci of L. major (zoonotic cutaneous leishmaniasis) are mainly in Sudan, Egypt, Libya, Tunisia, Jordan and Palestine while in Syria, Saudi Arabia, Iraq and Iran are endemic for both L. tropica (ACL) and L. major (ZCL) [16]. Nearly, there are 500,000 cases of ZCL occur annually, while ACL appear in 400,000 cases. Ninety percent of cutaneous leishmaniasis occur in Afghanistan, Ethiopia, Algeria, Syria, Saudi Arabia, Sudan, Iran, Iraq, Brazil and Peru [17]. In Sri Lanka, leishmaniasis has recorded 8487 cases from 2009 to 2016 [18]. In Pakistan, nearly 21,000–35,000 cases of CL are reported annually [19]. Moroccan Health Ministry mentioned that were 16,852 cases of ACL and 24,804 cases of ZCL in 2004–2013 [13]. Abdellatif et al. [20] observed 140 cases with cutaneous leishmaniasis in one provinces of Libya. Khosravi et al. [21] recorded 869 (4.7%) cases of CL after were examined 18,308 in Kerman province, Iran. In Syria, 22.882 cases via the period 2004–2008 [14], while 53,000 cases in 2012, only in the first half of 2013, were reported 41,000 cases [22]. In Iraq, Alsamarai and Alobaidi [23] are indicated to 107 cases with CL in Alhaweja General Hospital, Kirkuk. Al-Mayali [24] mentioned that infection rate in urban regions population high from rural regions, where most cases was in Diwania center. Al-Obaidi et al. [25] have conducted a survey based on Iraqi CDC database (2008–2015), 17,001 cases recorded in all parts Iraq. Flaih et al., [26] recorded 247 cases of CL in Thi-Qar.

1.2 Pathogenesis

Leishmaniasis is caused by flagellated Leishmania parasites, obligate intracellular protozoa and infect humans and other mammals [27]. The infection is transmitted by infected females sandfly bite and the disease highly spreads in poor populations of tropical and subtropical countries [28]. The putative vectors of the disease are species belonging to the genus Phlebotomus and Lutzomyia [3].

Cutaneous leishmaniasis lesion continues in size and depth, about 4–6 weeks after the sandfly bite, to forms an ulcer that considers primary characteristic of late CL [29]. The clinical appearance of CL is determined by a genetic variation, complex, host immune status and response mechanism, site of the sandfly bite, size and duration of lesion and secondary infections. A combination of these factors is more likely to overlap [19]. Lesions of CL mostly appears on exposed body parts or that are mainly open to the environment as arm, legs, face and neck, but it a little occurs on body parts which are usually covered such as the back and abdomen [30, 31]. In addition, multiple lesions may be as result to the sandflies feeding behaviors which tend to occur multiple bites, prevalence of high numbers of infected insects or rubbing of the lesion to transmit for other areas [6].

CL is often leave a disfiguring scar, especially on visible body sites, causing also social, psychological, and economic problems [30]. Metacyclic promastigotes deposited into the mammalian host skin are engulfed, target host cells are macrophages. Promastigotes differentiate into amastigotes within it. Generally, amastigote replication leads to rupture of the macrophage. The free amastigotes invade other macrophages [7, 32]. Metacyclic promastigotes are highly motile, able to migrate via a collagen matrix, therefore phagocytosis of Leishmania may occur at far locations from the bite site [33]. When metacyclic promastigotes enter the host body, within minutes, the metacyclic promastigotes are engulfed by phagocytic cells, including: macrophages and neutrophils. Neutrophil has very short life span, so macrophages are the main host cells. Metacyclic differentiate into non-motile, small amastigote forms which reproduce by binary fission even rupturing the macrophages to infect other macrophages [32, 34]. However, neutrophils and dendritic cells are act as Trojan horses, which let the parasites to enter macrophages, Leishmania spp. does not activate parasite killing mechanisms (silent entry) [35]. Leishmania parasite can delay neutrophil apoptosis, which allows to the parasite replicates within these cells [36]. Although macrophages are host cells. Neutrophils, dendritic cells, and fibroblasts may be implicated in various stages of the infection [37]. Other cell types, as mast cells, natural killer cells (NK) and Langerhans cells are also involved in leishmaniasis establishment [38]. Both parasitic forms (amastigotes and promastigotes) have different ways, alter the structure of the parasitophorous vacuole and the environment of macrophage, to prevent series of cellular signaling events which lead to the parasite killing within the phagolysosome vacuole that provides an environment for parasite [35].

Leishmania must evade from the innate and adaptive immune responses. It is engulfed by macrophage but resist proteolysis and degradation in the phagolysosome [39, 40]. Macrophages engulf Leishmania directly via interactions with multiple cellular receptors as fibronectin receptor, mannose receptor, Toll-like receptors and CR1 and CR3 complement receptors, also engulf apoptotic neutrophils that contain Leishmanial parasites [7, 41]. Inside host cells, Leishmania parasites to be able to establish and survive. They resist and circumvent anti-parasitic immune response pathways. In some cases, Leishmania spp. smartly invest immune responses or induce cellular signaling pathways to facilitate entry and establishment of parasite [7]. An infection of any type of Leishmania depends on complex, virulence factors and host immunologic response [42].

The first sign of the infection appears as a small erythema around the site of the sandfly bite, amastigotes start to multiply inside dermal macrophages and after a variable incubation period. The erythema gradually converts to an inflammatory papule, usually few millimeters in diameter. It increases in a size and progresses to a nodule that often ulcerates to cause a non-painful ulcer, which appears discoloration of surrounding skin. The lesion may be multiple and/or small lesions surrounding the larger lesion. The development is slow range weeks to moths. The healing occurs after an adaptive cellular immune response that possible to fight the infection [43]. Ulcers become painful if accompanied secondary bacterial or fungal infection [44].

1.3 Essential virulence factors

A virulence is one of the important concepts which researches being on them. Also identification and determination of the virulence factors is a target for researchers in order to provide the road map of a vaccine design, identify of interactions with host defense mechanisms and a role of virulence factors in disease pathogenicity [45]. There are virulence factors have been identified of various Leishmania spp. that are possible at pathogenesis in the host. Some factors help in an engulfment and attachment of the parasite within host cells while other factors act as intracellular survival. Pathogen stops signaling pathways inside the host cell that activate immune response, thereby development of the disease [7].

The outcome of leishmaniasis infections depend both host status and pathogen virulence factors. Leishmania produces various virulence factors to facilitate transmission of parasite and infect mammalian host [46]. All of Leishmania surface coats with a glycocalyx that has a diversity in the composition. Notable, all of the surface-bound molecules share a common structural feature. They all contain a highly conserved glycosylphosphatidylinositol (GPI)-anchor. This type of GPI-lipid anchor is structurally different from those found in mammalian cells. The glycocalyx in the Leishmania promastigote is dominated by GPI-anchored phosphoglycosylated glycans. However, during the life cycle of the parasite, occur changes in composition of the Leishmania surface glycocalyx [47]. Metacyclic promastigote has a thick glycocalyx. The glycocalyx is made of glycoproteins and other glycosylated species anchored into the surface membrane via a glycosylphosphatidylinositol (GPI) linkage [39]. For survival inside macrophages containing microbicidal factors, Leishmania spp. modulate or/and inhibit cell signaling pathways cascades involved in their synthesis [9].

A number of important molecules that had identified as factors in parasite virulence as lipophosphoglycan (LPG), the surface protease (gp63), cysteine proteinases (CPs), proteophosphoglycan (PPG), glicoinositol-phospholipids (GIPLs) and the 11 kDa kinetoplastid membrane protein (KMP-11) [43, 48]. Glycoproteins, LPGs, PPGs, and GIPLs are GPI-anchored molecules in dense surface glycocalyx, Leishmanial parasites also secrete various glycoconjugates especially PPGs. Secreted and surface glycoconjugates for promastigote has importance in an infection, survival, or virulence but are not necessary in order to Leishmania viability inside culture [49].

2. Lipophosphoglycan (LPG)

LPG is most abundant glycolipid that densely coats surface of Leishmania promastigotes. It is composed of a glycan core which is joined a long polymer consist from repeating units of phosphoglycan (PGs) which is terminated with oligosaccharide capping structure. LPG connects to promastigote membrane by a phosphatidylinositol lipid (PI) anchor. The number of repeating units and the cap may differ between Leishmania spp. Also, the amount and structure of LPG varies during parasite life cycle [50, 51]. In all Leishmania species, LPG expresses high amounts on surface of procyclic and metacyclic promastigotes, in contrast with amastigotes [52]. LPG on metacyclic promastigote surface is much longer from procyclic and almost completely absent in amastigotes [39]. Indeed, this molecule has an important role not only in invertebrate and vertebrate host, but too during early steps for establishment of the infection [50]. L. major mutants which lack lpg1 and lpg2 genes are sensitive to host complement system. This leads to unable survival in both intermediate or definitive host [53]. LPG1 is one of the key enzymes in the LPG biosynthesis, that involved in the synthesis of the LPG glycan core [54]. As well as, Leishmania lack lpg1 gene expresses imperfect LPG, not contains PG domain [55]. L. major and Labrus donovani need LPG1 for infection establishment inside macrophages, phagocytosis eliminates LPG1- null mutants, then restoration of LPG expression given capacity to replication of parasite within macrophage [52, 54].

LPG acts as a ligand to attach promastigotes to midgut epithelium of sandfly and also protects promastigotes from destruction by proteolytic enzymes that are destined for blood meal digestion [56]. In mammalian host, leishmanial LPG plays an essential role to avoid the parasite lysis by the host complement system, either by prevent of complement molecules attachment or inactivate the assembly of complement complex on promastigote surface [47]. It has ability to modulation and creating an appropriateness environment in order to parasite survival but the mechanisms of underlying modulation remain completely unknown and not goodly understood [57]. LPGs delay lysosomes formation and allow Leishmania to covert to intracellular amastigote forms [58]. It has been shown to protection of the lysis by complement system, adhesion and entry in macrophages, inhibition of protein kinase C (PKC), inhibition of phagosomal maturation, modulation of nitric oxide (NO) production [59].

LPG interferes with the pro-inflammatory responses for host cell through binding with Toll-like receptor (TLR) 2 and 4 on macrophages and NK cells [57]. LPG-TLR interactions induce ERK phosphorylation, while suppressing of p38 MAP kinase phosphorylation (ERK and p38 MAP kinase are from components of MAP kinases), modulate production of reactive oxygen species (ROS) and nitric oxide and inhibit secretion of pro-inflammatory cytokine [47]. For example, if p38 MAP kinase activate will produced IL-12 and IL-10, that inhibit activation of ERK [57]. LPG binds with complement receptor (CR3) and anchored molecules on macrophage surface to facilitate parasite engulfment [7, 60]. Furthermore, Leishmanial LPG can to impair the nuclear translocation of NF-κB in monocytes which lead to decrease in IL-12 production, and this effects early immune response of the host [43].

3. Major surface protease: (MSP) GP63

GP63 is known leishmanolysin or called major surface protease (MSP) or promastigote surface protease (PSP), also called a zinc- dependent metalloprotease [50, 61, 62]. It is found at Leishmania surface connected through a glycophosphatidylinositol (GPI) anchor, or is direct secreted to an extracellular surrounding [50]. Fracture of GPI anchor by phospholipase C leads up to scattering gp63 in extracellular space, also is secreted directly by the flagellar pocket, as well as the presence of GP63 in intracellular pools [63].

GP63s are encoded by multiple genes that appear vary in their sequence (especially in untranslated regions), array in the Leishmania genome [63, 64]. Differences in the untranslated regions may lead to different gene expression [50]. These genes generate abundant proteins which lead to vary among species and forms of Leishmania that cause a different biological effect [65]. Genes of gp63 are expressed in promastigotes and amastigotes [61]. It is expressed in metacyclic more abundantly than procyclic promastigotes [62]. Also, gp63 has been observed at lower levels through its expression in amastigotes [50].

GP63 is a multifunction enzyme that related to an inhibition of complement components. The recent findings refer to a critical role played by gp63 as an important virulence factor that wide influence cellular signaling mechanisms and related pathways [65]. GP63 is responsible about migration of parasite through extracellular matrix, avoid lysis by the complement system, evasion from macrophagic intracellular hydrolysis [61] and facilitation of promastigotes phagocytosis by macrophage, matrix extracellular degradation, contribute to intracellular migration, NK inhibition and persistence and progression of infection [64, 66]. Also, inhibit nitric oxide production (Leishmanicidal) or macrophages pro-inflammatory cytokines [67].

This protease serves to cleaves complement protein C3b and converts to C3bi (inactivated form of C3b) and protects promastigote from complement-mediated lysis. Further, generation of C3bi leads to uptake promastigote by complement receptors such as CR3 which located at the surface of macrophage. The evidence suggests, promastigote ligates CR3 directly and via opsonized C3bi [7, 62]. C3bi acts as a bridge between complement receptor at surface of macrophage and gp63- bearing promastigote [68]. C3bi generation by enzymatic activity of gp63 bonded to promastigote surface, mediate phagocytosis process by complement receptors (CR1 andCR3) that leading to silent entry of parasite into macrophage [7, 62].

GP63 has been described to is not only to degradation and damage of transcription factors and various kinases, but also it modulates negative regulatory mechanisms of signaling pathways for example protein tyrosine phosphatases (PTP). Activation of PTP together with other signaling molecules leading to inhibition of leishmanicidal and inflammatory functions [63, 65]. GP63 is activate protein tyrosine phosphatases that result alteration of MAP, JAK, STAT1 and IRAK-1kinase pathways. In the nucleus, it is responsible for the inactivation of important transcription factors which activate specific chemokines, such as NF-κB [9, 65].

4. Cysteine proteinases (CPs)

Cysteine proteases (CPs) are degrading enzymes which cleave various proteins. They are act as essential virulence factors in leishmanial infection, as well play an important role in many pathogenic protozoa and other microorganisms [69]. At least from 6 classes proteases classified in the proteolysis: serine, cysteine, glutamate, metallo, threonine and aspartate proteases. Cysteine proteases (CPs) are classified into 72 families, so not all are expressed in parasitic protozoa [70]. In Leishmania genome, there are 154 peptidases include cysteine, aspartic, serine and metalloproteases. Leishmania protease are necessary for continuation and establishment of infection [71]. In several pathogenic organisms, including parasitic protozoa as Entamoeba histolytica, Leishmania and Trypanosoma, found CPs which are enzymes [72, 73]. Leishmania has three types from CPs: CPA, CPB, and CPC [72, 74]. CPs are expressed in higher levels in the mammalian amastigote [70]. Gene expression of CPA is maximal level in amastigote stage, while lower expression is in promastigote [73]. Also, CPB is expressed at high levels in the amastigote, is expressed at a very few level in procyclic-promastigotes [75]. CPB1 and CPB2 are expressed in higher levels of metacyclic promastigote, while CPC in procyclic-promastigote [70].

Although roles of CPs in pathogenesis of Leishmania are unclear, it has been showed that Leishmaniasis cannot continue in macrophages with the presence of CP inhibitors [76]. Cysteine proteases play key roles in Leishmania biology and their inhibition is appearing as an important strategy to the elimination of the disease. They are necessary to metabolism, intracellular survival and reproduction of parasite [74], participate in exsheathing, excystment, also tissue invasion and some parasite immune-evasion [69]. CPs appears an essential role in pathogenicity and leads multi-processes such as modulation or evasion of the host immune response, cell/tissue degradation and damage, catalyze hydrolysis of various host proteins that are responsible of important cellular biological activities in pathways, differentiation of promastigote to amastigote and autophagy [70, 71]. Generally, CPA lead an essential key in the interactions of parasite with host cell [43, 77], while CPB modulates Th1 immune responses, and IFN-γ production via damage of transcription factor (NF- κB) and inhibition ofIL-12 production. Also CPB is modulate levels of parasite proteins as gp63 [70].

The inhibition of CPA and CPB or deletion their genes not only alters autophagy pathway but too prevents transformation into amastigotes, thus support hypothesis of autophagy is required for the differentiation [43, 78]. Parasites evidenced low growth, pathogenicity and efficacy after their treated with CP inhibitors [79]. There are efforts towards make it as vaccine candidates because their potential and their importance. For now, no effective drug or vaccine for leishmaniasis [69, 71, 74].

5. Proteophosphoglycan (PPG)

Leishmania expresses several types from proteophosphoglycans (PPGs). Some of PPGs are secreted and others found on surface of amastigotes and promastigotes [80]. PPGs are a large group from widely glycosylated proteins, have some unique features [81]. It is produce by both two parasite stages, is found on the parasite surface, Golgi, lysosome and flagellar pocket [53]. There are number of PPGs types, include filamentous proteophospho-glycan (fPPG) and GPI-anchored membranous proteophosphoglycan (mPPG), all of that are originally in stages of promastigotes within sandfly vector, there is also non-filamentous proteophosphoglycan (aPPG) which is a major product of amastigotes in mammalian host [82, 83, 84]. The ppg1 gene encodes membrane-bound PPG, and secreted non-filamentous PPG is encoded by ppg2 [85], while ppg3 gene is encode filamentous proteophosphoglycan [86]. Compositional analysis observed PPG contains 67% glycan, 28% protein and 5% phosphate. PPG is composed from repeating units of PGs that attach to protein backbone [81]. PG molecules are either connected to cell surface via GPI-lipid anchor, or secreted as protein-containing phosphoglycans [51]. PPG is proved to bind to macrophage receptors in order to facilitates parasite invasion, this may be explaining that PPG play an early role in infection and prevalence [53, 87, 88] are suggest possibilities of PPG in drug resistant mechanisms and PPG abundance of L. donovani is as evidence for resistant clinical isolates. PPG- null mutant L. major are more sensitive and less virulent to complement-lysis. In infected mice, not develop lesions in PPG- null mutant parasites [82]. Metacyclic promastigotes in sandfly midgut secrete fPPG which condenses to promastigote secretory gel (PSG), to formation a biological plug that block the anterior midgut, this is oblige sandfly to disgorge parasites though blood-feeding [89]. PSG is largest molecule that secretes by promastigotes within sandfly gut. Also, it is support both macrophage assemblage and infection [90]. Inside infected macrophages, amastigotes secrete aPPG which can reach to very high concentrations [90]. The membrane-bound PPG may play a direct important role in host–parasite interactions [90]. It is found on the surface of amastigotes and promastigotes [91]. Within macrophage, believed that aPPG is contribute in formation of parasitophorous vacuole, this protects infection in the mammalian cells, also PPG may contribute binding of Leishmania to host-cells and may be modulate biology of infected macrophage in the early infection [83].

6. Conclusion

CL has a global prevalence and there is an increase worldwide in recorded cases, and also considers from neglectful diseases. The dermal lesion is occurred by an obligate intracellular Leishmania parasite, which transmits via the bite of the female sandfly. The clinical appearances depend on Leishmania species, virulence factors and host immunologic response. Virulence factors have an important role in pathogenicity, so inhibition or absence one of them lead to effecting on parasite.