1. Introduction
Cutaneous leishmaniasis (CL) is a neglected disease that mainly affects the poorest populations in developing countries [
1]. Brazil is the country with higher incidence of CL in the American Continent, where the disease is associated to at least seven species, one of the subgenus
Leishmania:
L. (
Leishmania)
amazonensis and the others of the subgenus
Viannia:
L. (
Viannia)
guyanensis,
L. (
Viannia)
braziliensis,
L. (
Viannia)
shawi,
L. (
Viannia)
lainsoni,
L. (
Viannia)
naiffi and
L. (
Viannia)
lindenbergi [
2].
A relevant aspect in the epidemiology of CL in Brazil, but not only observed in this region, is the circulation of parasites carrying a viral endosymbiont denominated
Leishmania RNA Virus 1 (LRV1), found in
L. (
Viannia) sp. [
3,
4]. The outcome and severity of CL might be correlated with several factors, including the
Leishmania species involved and the presence of LRV1 [
4,
5,
6,
7,
8,
9,
10].
It was shown in experimental infections that
Leishmania parasites carrying LRV1 are predisposed to aggravate the course of infection, leading to the mucosal form of leishmaniasis. The role of LRV1 in the exacerbation of the infection relies on Type I interferon (Type I IFNs) production by macrophages and in vivo signaling [
11,
12]. It is known that the virus can subvert innate immunity by acting as a negative regulator of the NLRP3 inflammasome, favoring parasites’ survival and disease chronification [
13]. Furthermore, it was described that the presence of LRV1 increases the risk of mucosal development in human patients by almost three times compared to individuals who present CL without virus detection [
4]. However, the association of LRV1 and mucosal manifestation was not observed in studies evaluating other cohorts, but association with therapeutic failure was demonstrated [
14,
15,
16]. Differences in the parasites circulating in the different regions evaluated and/or in the LRV1 infecting these parasites are possible explanations for the observed differences among some studies.
Previous studies with the complete genome sequences of LRV1 and sequences obtained from a phylogenetically informative region of the viral genome from
L. (
V.)
guyanensis and
L. (
V.)
braziliensis showed evidence of host-specificity in
L. (
Viannia) species-LRV1 interaction, clustering LRV1 sequences according to their
Leishmania species host [
17,
18]. LRV1 from one
L. (
V.)
shawi strain was also analyzed, clustering close to LRV1 from
L. guyanensis, mirroring what is observed in the phylogenetic relationship of these two
Leishmania species [
18]. Although LRV1 was detected already in cultivated strains of
L. naiffi and
L. panamensis [
15,
19,
20,
21], there is no information on phylogenetic relationship of viruses from these species comparing to LRV1 from other species. LRV1 was already associated with human infections caused by
L. lainsoni [
4] and
L.
peruviana [
22], but not in cultivated strains from these species.
Due to the significance of the
Leishmania-LRV1 symbiosis in the epidemiology of cutaneous and mucosal leishmaniasis, it is crucial to gain a comprehensive understanding of the viruses’s diversity and spread within parasite populations. Thus, our study aimed to examine the presence of LRV1 in various strains of Brazilian
L. (
Viannia) species, excluding
L. braziliensis and
L. guyanensis. LRV1 was not detected in different strains of three species with apparently restricted circulation in the Amazon region, and common in areas with reported circulation of LRV1:
L. lainsoni,
L. lindenbergi and
L. utingensis. However, our findings revealed the presence of LRV1 in cultures identified as
L. naiffi and
L. shawi, confirming previous observations. Of note, phylogenetic analyses indicates
L. naiffi, along with
L.
lainsoni, as the most divergent species of the
Viannia subgenus [
23]. Patients infected with
L. naiffi experienced a poor response to antimonial or pentamidine therapy, indicating that the species could resist to first-line treatment. The presence of LRV1 in
L. naiffi raised the possibility that this virus could increase
Leishmania spp virulence and thereby influence therapeutic failure [
15,
24].
L. (
V.)
shawi is a species closely related to
L. guyanensis [
23] dispersed mainly in the Amazon region [
25]. This species is responsible for CL, usually found as a single lesion, but cases of multiple lesions, clearly due to metastases, are occasionally seen [
26]. A recent report showed the first
L. shawi infection in mucosal secretion in Brazil, that represents a warning for the possible association between
L. (V.) shawi and mucosal lesions [
27]. In order to contribute with characterization of LRV1 from different
L. (
Viannia) species and strains and further investigate host-specificity in this combination, we conducted a comparative analyses of publicly available sequences of
L. braziliensis and
L. guyanensis to newly obtained LRV1 from both
L. naiffi and
L. shawi, reported herein.
4. Discussion
Virus-like particles were demonstrated in
Leishmania parasites in the late 1970s [
34] but the first molecular description of
Leishmania RNA Virus was in 1988, for viruses found in the cytoplasm of a
L. (
V.)
guyanensis strain [
35]. Ever since LRV1 has been detected in clinical isolates from Peru [
36], Brazil [
4,
15], Colombia [
37], Bolivia [
16], Costa Rica [
38], French Guiana [
39], and more recently in Panamá [
19]. These viruses were observed not only in
L.
guyanensis, but also in strains identified as
L.
braziliensis,
L.
shawi,
L. naiffi and
L.
panamensis [
4,
18,
19,
20,
21,
40,
41], indicating an old relationship between LRV1 and
L. (
Viannia) subgenus. Furthermore, LRV1 was detected in clinical samples collected from patients infected by
L.
lainsoni [
4] and
L.
peruviana [
22], but no cultivated strains of these two strains positive for LRV1 are available yet, limiting our analyses. Considering the epidemiological and medical importance of the symbiosis between
Leishmania and LRV, there is a distinct requirement to comprehend the variety and spread of the virus within parasite populations.
The coevolution hypothesis for LRV-
Leishmania species emerged in 1995 when Widmer and Dooley performed a phylogenetic analyses and found that genetic distances between LRV types mirror the heterogeneity observed for
Leishmania species based on random amplified polymorphic DNA (RAPD) fingerprints [
42]. More than ten years later a study presenting a genetic characterization and phylogenetic analyses of LRV1 sequences from 27
L.
guyanensis strains and two
L.
braziliensis was published and host-specificity for LRV1 began to be revealed [
17]. A year later a robust phylogenetic analyses was presented including 35 LRV1 sequences from
L.
guyanensis, 11 from
L.
braziliensis and, for the first time, a sequence of LRV1 found in a
L.
shawi strain [
18]. Both studies presented evidence corroborating the hypothesis of the coevolution of LRV1 and
L. (
Viannia) parasites, grouping LRV1 sequences according to the parasite species and geographical clustering.
The mentioned above studies suggest a specific relationship of LRV1 to such
L. (
Viannia) species, combined with the observation of LRV1 in other species, motivate the present study to screen for LRV1 in strains representing
L. (
Viannia) species not analyzed so far and available at the
Leishmania Collection from Fiocruz. To this end, we analyzed all available strains for
L.
lainsoni,
L.
lindenbergi,
L. naiffi,
L.
shawi and
L.
utingensis. As previously demonstrated, LRV1 was detected in
L. naiffi and
L.
shawi strains [
18,
24], but not in
L.
lainsoni, despite the fact that LRV1 was previously detected in clinical samples collected from a patient presenting cutaneous leishmaniasis caused by this species [
4]. Of note,
Leishmania parasites were isolated from this patient and the identified strain was included in our analyses (IOCL 3398), but it was negative for LRV1. There are some possibilities to explain these results, including the possibility of mixed infection by two or more
Leishmania species, but with the isolation and growth in culture medium of
L.
lainsoni to the detriment of another
L. (
Viannia) species that does not grow very well in culture medium as
L.
lainsoni [
43] Loss of LRV1 during the process of cultivation is also another possibility [
44]. Thus, herein we were able to screen LRV1 in different strains from different
L. (
Viannia) species and to analyze nucleotide sequences of LRV1 from
L. naiffi and
L.
shawi. Interestingly was the fact that more than 50% of
L. naiffi strains analyzed were positive for LRV1, but we don't know yet if this symbiotic relationship confers any advance to
L. naiffi parasites, such as the capacity of interacting with different sandfly species and/or dispersion in different geographic regions [
45]. It has been demonstrated that
L. naiffi infection could not have a self-healing nature as it was described years ago [
46,
47]. Patients could experience a poor response to antimonial or pentamidine therapy [
24]. Only in 2019 was first reported a case of a patient infected by
L. naiffi carrying LRV1, raising the possibility that the presence of this virus could increase
Leishmania spp. virulence and thereby influence therapeutic failure [
15], aspects already observed for
L.
braziliensis and
L.
guyanensis, but that must be better investigated.
Although it is still important to investigate the specificity of LRV1 to other
Leishmania species such as
L.
panamensis, since LRV1 was already detected in cultivated strains of this species [
19,
20,
21], our results strongly support this kind of relationship, keeping
L.
shawi in a separate cluster close related to
L.
guyanensis. Here we assumed the groups suggested by Tirera et al 2017 [
17], where
L. guyanensis was divided into five subclusters (A-E). The divergence within
L.
guyanensis is higher than the observed between
L.
guyanensis and
L.
shawi, corroborating the assumption of these as a complex of species [
23]. Following this, LRV1 sequences from
L. shawi formed another subcluster, named here as Group G, closely related to the
L. guyanensis subclusters A and B and C. Although LRV1 sequences were obtained for only two
L. shawi strains so far, the fact that they clustered together, despite the strains being from different geographic regions, is also an important aspect to support host-specificity for LRV1. The diversity of LRV1 from
L. guyanensis, forming several subclusters, must be better explored, but the number of LRV1 sequences from
L.
guyanensis analyzed and the geographic dispersion of these parasites in the Amazon region might be contributing to this result.
Host specificity was also observed for LRV1 genotypes from
L. naiffi. The phylogenetic tree and the NeighborNet (Figs 3 and 4) show that LRV1 sequences from
L. naiffi clustered in a well-supported monophyletic clade. Of note, most of LRV1 sequences from
L. naiffi analyzed were very similar. The diversity observed within LRV1 sequences from the same species must be further investigated, but it is important to consider that all but one
L. naiffi strains presenting highly similar LRV1 were obtained from patients infected in the same endemic region and included in the same study, suggesting a possibility of problems during laboratory manipulation. However, this very similar group contained one strain isolated previously to the mentioned study and was not manipulated together with the other strains, suggesting a homogeneity for the
L. naiffi population circulating in this area and causing human disease, which can represent an epidemic clone. LRV1 sequences obtained for two
L. naiffi strains, IOCL 3316 and IOCL 854 strains, both isolated in Pará, showed a different phylogenetic pattern from the above-mentioned
L. naiffi strains isolated from Manaus, Amazonas. The strain IOCL 854 was obtained from a sandfly species,
Lutzomyia squamiventris, and LRV1 from this strain showed a basal position into the
L. naiffi clade, despite the close relationship of this strain to other
L. naiffi, including IOCL 3007, as previously demonstrated [
23,
48]. Considering
Leishmania (
Viannia) species depicted by microsatellite analyses, it is more expected that parasites from populations circulating in the Amazon basin (POP2 and POP3 after Kuhls et al. [
48]) carries LRV1 and each subpopulation has an association with specific LRV1 genotypes.
By analyzing LRV1 sequences from several strains representing different
L. (Viannia) species we demonstrate that LRV1 genotypes form distinct clusters corresponding to their
Leishmania species host, suggesting that the transfer of viral particles between strains from different species does not occur frequently. Altogether, our results reinforce the concordance between the phylogenetic patterns of LRV1 and
Leishmania (Viannia) species, providing support for the prevailing hypothesis that LRV1 is an ancient virus that has undergone co-evolution with their hosts [
42,
49]. Recently it was shown that parasite hybridization might explain the high occurrence of the symbiotic interaction of LRV1 with
L. braziliensis in Peru and Bolivia [
50]. It is possible that this also explains the high frequency of LRV1 in parasites from the Brazilian Amazon Region, since many possible hybrids were described in the region [
51], and analyses of microsatellite markers have shown extensive diversity in the subgenus
L. (Viannia), with an indication of both clonality and recombination as a strategy of reproduction [
48].