Following the recognition and internalization of pathogens, DCs migrate to secondary lymphoid organs to present processed antigens to naïve T cells (66). Subsequently, the adaptive immune response becomes initiated via the presentation of small peptides through either MHC class I or class II molecules. Basically, the former class mediates the recognition of endogenous peptides by cytotoxic CD8+ T cells, while the latter is involved in the presentation of exogenous peptides to CD4+ T helper cells. Notably, the process of antigen fragmentation is of paramount importance to allow for proper antigen presentation, since MHCII molecules only present peptides with a specific number of amino acids (67). Alternatively, DCs are also capable of mobilizing MHC I molecules in order to display exogenously derived-antigens, a process known as cross-presentation (68). Additionally, co-stimulatory molecules (such as CD40, CD80, and CD86) are essential to effective antigen presentation, by providing secondary signals for T cell expansion and differentiation (69).

Several species of Leishmania employ distinct strategies to regulate the expression of co-stimulatory molecules, which dampens the process of antigen presentation (70). Accordingly, the modulation of co-stimulatory molecules can be associated not only with enhanced parasites survival and growth, but also with subsequent disease outcome.

Figueiredo et al. experimentally demonstrated that L. amazonensis induced lower levels of MHCII, CD86 and CD40 expression in BMDCs (bone marrow -derived DCs) from C57BL/6 mice, resulting in a decline in T-cell proliferation (65). Furthermore, the adoptive transfer of BMDCs expressing low levels of CD40 was associated not only with T regulatory cell expansion, but also with an increase in L. donovani burden in BALB/c mice (71). It has been also shown that CD40 and its ligand are important for the development of resistance against L. major infection (72, 73). Hai Qi et al. reported that L. amazonensis amastigotes mitigated IL-12 production in a CD40-dependent manner in a BALB/c infection model, which was followed by an increase in levels of IL-4 (74). Subsequently, amastigote-infected DCs were observed to be able to activate pathogenic CD4+ T cells, which could potentially lead to exacerbated Leishmania proliferation and the progression of pathogenesis (72). Thus, emerging evidence suggests that reduced CD40 expression could possibly facilitate Leishmania infection.

The importance of CD80 and CD86 expression has been highlighted in the establishment of early immune responses. For instance, the infection of human moDCs with L. amazonensis downregulates the expression of CD80 and upregulates the expression of CD86, which is followed by a decrease in IL-6 production during DC differentiation (75). Although CD86 possibly takes center stage in this context, the equivalent expression of other costimulatory molecules can lead to the early production of IFN-γ or IL-4 during infection by L. major depending on the experimental model (76). Together, these results reinforce the contribution of these molecules in the production of different cytokines by properly stimulated T cells.

Several species of Leishmania can modulate antigen processing and the expression of MHC II molecules (77). DCs infected with L. major amastigotes not only upregulate the expression of several molecules involved in antigen presentation, such as MHC class II, CD40, CD54, CD80, and CD86, but also exhibit elevated rates of IL-12 production. It should be noted that only the amastigote forms of these parasites were capable of inducing this increase in L. major-infected DCs (78). Conversely, L. mexicana amastigotes do not promote increased expression of CD80, CD54, and MHC II molecules in BMDCs, suggesting that these discrepancies in the immune response by DCs occur in a species-specific manner (79).

Interestingly, a recent study by Resende et al. reported a dichotomic response between L. infantum-infected and non-infected DCs. In this study, the authors observed that uninfected DCs expressed higher levels of IL-12p40 and other co-stimulatory molecules, which enabled DCs to elicit appropriate CD4⁺ T cell immunoprotective responses, whereas infected DCs expressed lower levels of co-stimulatory molecules and high IL-10 production (80). This finding suggests that L.infantum-infected DCs and their uninfected counterparts exert antagonistic roles in the activation and polarization of T cells, mechanistically revealing a novel evasion strategy employed by this species. Along the same lines, a study carried out by Carvalho et al demonstrated that, in contrast to L-braziliensis-infected DCs, only uninfected DCs upregulate the expression of MHC II, CD80, and CD86. Interestingly, it was also observed that despite enhancing the expression of such molecules, L-braziliensis-infected DCs produced higher levels of TNF-α in response to stimulation with LPS. These findings corroborate the hypothesis that uninfected and Leishmania-infected DCs can act conjointly, yet distinctly, to promote immune responses against the parasite, since uninfected DCs can lead to enhanced T cell activation, while the production of TNF-α by infected DCs may contribute to the control of parasite proliferation at the site of infection (81).

Paradoxically, DCs are also able to present exogenous antigens through MHC-I, with significant consequences on the activation of CD8 T cells (82). It has been reported that this phenomenon, also known as cross-presentation, is of great importance to the expansion of antigen-specific cytotoxic CD8 T cells, which are responsible for eliciting an effective immune responses against Leishmania. Accordingly, in this context, DC figure as the most potent inducers of IFN-γ production by CD8+ T lymphocytes. Brewig et al. demonstrated that, in experimental leishmaniasis, the priming of CD4⁺ and CD8⁺ T cell relies essentially on the activity of distinct DC subtypes (83). Indeed, it was reported that the depletion of Langerin⁺ DCs was associated with the reduced proliferation of L. major-specific CD8+ T cells. As a consequence, the amount of primed CD8 T cells found at the site of infection and in lymph nodes was significantly reduced (83). In a similar vein, a study conducted by Ashok et al revealed the importance of cross-priming DCs in the effective constraint of L. major infection. It was shown that Batf3^−/− mice (which lack CD8⁺/CD103⁺ DCs) exhibited increased susceptibility to L. major (84). Furthermore, a study by Lemos et al explored the function of CD8⁺ DCs in antigen presentation during L. major infection in a murine model that restricted the expression of MHC-II to CD8a⁺/CD11b⁺ DCs. Notably, it was observed that CD8a⁺/CD11b⁺ DCs could efficiently restrain L. major infection by eliciting the effective constraint of parasites by CD4 T cells (85).

Numerous studies have revealed that the underlying mechanisms of antigen processing depend not only on the constitutive proteasome or the immunoproteosome, but also on the involvement of alternative molecular machineries of cytosolic degradation, such as tripeptidyl peptidase II (TPPII) and nardilysin (86–88). TPPII is a known eukaryotic peptidase related to several cellular processes, such as antigen processing, apoptosis and cell division. However, it should be noted that TPPII activation occurs mainly when proteasome function becomes compromised (89).

Although little is known about the detailed mechanisms of cytosolic endopeptidases, such as Nardilysin, their role seems to be indispensable in the generation of some specific epitopes (90). The importance of alternative antigen processing machinery should be further investigated in the context of leishmaniasis.

Although the participation of CD8⁺ T cells in Leishmania infection is still controversial, growing evidence indicates that protective responses rely substantially on the effective dendritic cell-mediated activation of cytotoxic lymphocytes (86, 87, 91, 92).

Immature DCs strategically reside in peripheral tissues, where they exercise their main function as immune guards. As discussed previously, these sentinels specialize in antigen uptake via their apparatus to internalize foreign particles. In peripheral tissues, PAMP-mediated activation confers an immunostimulatory phenotype to DCs, characterized by the upregulation of molecules also associated with an enhanced migratory ability. Subsequently, DCs migrate toward lymph nodes, where they exchange information with naïve T cells via the antigen presentation process (93).

In order to ensure precise mobilization, the migration of DCs needs to be highly coordinated and regulated by particular recruitment signals (94). The primary mechanisms of DC migration involve the cooperative action of chemokines and their receptors. Chemokine receptors are typically transmembrane proteins associated with G-proteins whose activation triggers signaling pathways responsible for the promotion of cell mobilization (95). Some evidence seems to suggest that DC subtypes exhibit diverse chemokine receptors, conferring subtype-specific migration dynamics. Commonly, immature DCs express CCR2, CCR5, CCR6, CXCR2, and CXCR4 in a predominant fashion. Upon pathogen-mediated activation, DCs undergo a maturation processes that culminates in the crucial upregulation of CCR7 (Figure 2A) (96).

By way of evolution, protozoan parasites developed strategies to mitigate DC functioning by inhibiting access to T cells, thusly restricting the establishment of efficient adaptive immune responses. In the context of flagellate protozoan infection, successful DC migration to draining lymph nodes (dLNs) is substantially dependent on both CCR2 and CCR7 expression (97). Several studies have reported that Leishmania can induce a reduction in rates of DC migration (98, 99). In vitro studies have elucidated the roles of both soluble products and membrane constituents, such as L. major LPG, in the inhibition of DC motility (98, 100). The underlying molecular mechanisms of Leishmania-induced mitigation of DC motility remain elusive. A recent study suggested that L. major exploits the junction adhesion molecule C (JAM-C) to reduce DC migration rates, and demonstrated that the experimental blockage of this molecule enhanced the immunological response against this parasite (101). An investigation in an animal model exhibited an L. major- susceptible phenotype, suggesting that the depletion of CCR2 culminated in poor DC migration and a skewed Th2 immune response (102). Furthermore, infection with L. donovani, a viscerotropic species, promotes an inhibition in the expression of CCR7 mediated by IL-10 production, which ensures that DCs will not be able to reach splenic regions, thereby contributing to the progression of visceral leishmaniasis (Figure 2B) (103). Another in vivo study associated CCL19/CCL21 deficiency with a reduction in both DC mobility and resistance to L. donovani infection (103).

Collectively, the current data suggest that efficacious DC migration is essential to the establishment of effective responses against Leishmania parasites. The literature clearly indicates that these parasites employ a plenitude of strategies to prevent DCs from activating T cells during the course of several clinical forms of leishmaniasis. Deciphering the complex dynamics surrounding the Leishmania-mediated impairment of DC mobilization will provide new insights into the evasion mechanisms employed by these parasites and elucidate their effects on the immunopathogenesis of leishmaniasis.

Faced with infection and inflammation, DCs must cope with increasing catabolic and anabolic demands via the redirection of a plethora of metabolic pathways to support their major immune functions (104). Typically, the metabolism of inactive DCs is characterized by the central roles of oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO), for energy supply and biomolecule synthesis, respectively (105). New evidence suggests that, after the initiation of PAMP-mediated activation, DCs undergo metabolic reprogramming, relying substantially on anaerobic glycolysis for ATP production, a process characterized by the conversion of pyruvate into lactate. Despite being ineffective in the generation of ATP, glycolysis can be coupled with several anabolic pathways, such as fatty acid synthesis and the pentose phosphate pathway, allowing for the biosynthesis of other macromolecules, namely lipids and nucleotides, respectively (106, 107). In this scenario, DCs exhibit low rates of oxidative phosphorylation. These deviations in the metabolic repertoire of DCs are prominent regulators of immune responses, as metabolic enzymes and their products can influence the establishment of inflammation (108).

In this inflammatory milieu, immune cells are poorly supplied with oxygen and nutrients for their metabolic processes, leading to the activation of hypoxia-inducible transcription factor 1α (HIF-1α) (109). Recently, HIF-1α was recognized as a major player in the induction of glycolysis, since it promotes the transcription of several enzymes involved in glucose metabolism (110). Nevertheless, its expression was shown to favor L. donovani infection in a model of chronic visceral leishmaniasis, as increased HIF-1a expression in murine splenic DCs was correlated with decreased IL-12 production, allowing parasite survival through limited Th1 cell expansion (111). In consonance with these observations, Hammani et al. demonstrated the importance of the IRF-5/HIF1α transcription factor axis in the impairment of DCs to promote the expansion of CD8⁺ T cells (112). Conversely, in vitro experiments showed that HIF1α enhanced both Leishmania major elimination and levels of NO production in macrophages (Figure 3) (113). Together, while these observations suggest that HIF1α downregulates some DC functions against Leishmania, this effect may be cell-specific. Moreover, a recent study highlighted the contribution of two energetic sensors, Sirtuin 1 (SIRT1) and AMP-activated protein kinase (AMPK), to L. infantum survival and replication in macrophages (114).

Given the relevance of the metabolic processes of DCs in supporting the immunobiological functioning of these cells, it is unsurprising that an increasing number of studies have contemplated this interesting topic in recent years. Nevertheless, few studies have attempted to investigate the role of DC immunometabolism in Leishmania infection. Currently, the molecular players involved in metabolic reprogramming and the mechanistic basis of immunometabolism continue to remain elusive in the context of leishmaniasis.

Interactions between host cells and parasites prompt several alterations in a range of biological processes occurring in DCs, including epigenetic alteration via modified gene expression. This phenomenon is not dependent on DNA sequence modifications and includes DNA methylation, histone modifications, chromatin remodeling and regulation by non-coding RNAs (115–117).

The activation of transcription factors is one of the major regulatory elements occurring in epigenetic alterations (118, 119). PU.1 transcription factor has been described as an essential TF for the development and functioning of DCs, as evidenced by the expression of FLT3, granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) and macrophage colony stimulating factor receptor (M-GSFR) (119, 120). It has been demonstrated that PU.1 is also involved in the regulation of basal expression of DC-SIGN, which in turn influences the repertoire of antigen uptake in DCs (121). PU.1 can also regulate the promoter region of genes CD80 and CD86 in murine bone marrow–derived DCs, leading to the overexpression of these co-stimulatory molecules, thereby enhancing DC migration and the activation of T cells (122).

In face of tissue damage or infection, several modifications in the histones alter chromatin conformation, leading to changes in the expression profile of critical genes in specific DC subsets (123, 124). Tserel et al. showed by GWAS (Genome-wide Association Study) that histone modifications can influence the processes of differentiation, phagocytosis and antigen presentation in moDCs through the upregulation of surface marker expression and chemokine production. Similar findings have been reported in macrophages, reinforcing the similarity of epigenetic mechanisms in the development of both cell types (125).

The importance of epigenetic changes in DCs infected by Leishmania remains unclear. However, L. donovani infection in macrophages was shown to lead to changes in the methylation of CpGs sites via parasite exosome secretion, which may enhance parasite replication and survival (126). Furthermore, L. amazonensis infection promoted epigenetic modifications at the IL-10 locus in murine macrophages, which activated ERK1/2 pathways and promoted parasite survival (127). In addition, this parasite species can upregulate histone deacetylases (HDACs), which enhances iNOS promoters in macrophages, thusly favoring infection (128).

Taken together, in addition to playing a crucial role in the development of DCs, these findings seem to suggest that interactions between Leishmania and immune cells can trigger epigenetic modifications that may alter the course of the infection. However, much remains to be elucidated with regard to this topic.

DCs are relevant immunological agents in the concatenation of innate and adaptative branches of immunity. Here, we have attempted to integrate recent advances in molecular aspects of the immunobiological functioning of DCs with the current state of understanding regarding the pathogenic mechanisms of leishmaniasis. Although a large body of evidence supports the central role of DC activation in the establishment of responses against Leishmania parasites, the overwhelming complexity of Leishmania-DC interactions impedes the attainment of a comprehensive understanding of the molecular processes involved in DC activation. Further clarification is required to unravel the interplay between different DC subtypes and different species and life cycle stages of Leishmania as well as how parasites subvert particular aspects of DC activation in the effort to successfully establish infection. Finally, an enhanced understanding of the fundamental molecular events underlying DC activation will lead to the expansion of our current base of knowledge surrounding leishmaniasis as well as offer new therapeutic targets.

RT and SN designed the review and wrote the manuscript. IN, MR, IS, and RL assembled the review and wrote the manuscript. NM and CB supervised the work, designed the review, and wrote the manuscript.