Toll-Like Receptor 2 Targeted Rectification of Impaired CD8⁺ T Cell Functions in Experimental Leishmania donovani Infection Reinstates Host Protection

Leishmania surface molecule lipophosphoglycan-TLR2 interaction moderates TPL2-mediated TLR2 signalling for parasite survival

Leishmania donovani, a protozoan parasite, resides and replicates in macrophages and inflicts the potentially fatal disease visceral leishmaniasis (VL). The parasite-expressed surface lipophosphoglycan (LPG) was implicated in binding TLR2 on NK cells, but the modus operandi of its disease-promoting influence remained unknown. As TPL2, a member of the MAPK module in mammalian macrophages, was implicated in the anti-inflammatory immune response and promoting pathogen survival, we investigated the possibility of TPL2-directed LPG-TLR2 signalling in Leishmania infection. We observed that TLR2 or TPL2 blockade differentially influenced the TLR2 ligand proteoglycan (PGN)-induced p38MAPK and ERK-1/2 activation. TLR2 blockade abrogated the PGN-induced TPL2 activation. L. donovani infection impaired the Akt activation whereas, upon TPL2 inhibition, the infection fails to control Akt phosphorylation. In L. donovani-infected macrophages, TLR2 blocking negatively affected p38, Akt and TPL2 phosphorylation while ERK1/2 phosphorylation increased relative to the infection alone. TPL2 blockade reduced TGF-β, but increased TNF-α expression and diminished amastigote count in macrophages. While exploring stimulation patterns of TLR2 ligands, LPG, unlike PGN, selectively increased TLR2 expression in macrophages. LPG blockade increased p38MAPK and AKT, but slightly affected ERK-1/2 and significantly reduced TPL2 phosphorylation from L. donovani-infected macrophages. Molecular docking and molecular dynamics analysis drew a parallel between LPG's glycan chain lengths with the frequency of interaction with TLR2 which might impact TLR2 signalling. Therefore, the parasite regulates the TLR2 signalling via TPL2 when elicited by LPG-TLR2 interaction for pathogenesis.

Paradoxical immune response in leishmaniasis: the role of toll-like receptors in disease progression

Toll-like receptors (TLRs), members of pattern recognition receptors, are expressed on many cells of the innate immune system and their engagements with antigens regulates specific immune responses. TLRs signalling influences species-specific immune responses during Leishmania infection, thus, TLRs play a decisive role towards elimination or exacerbation of Leishmania infection. To date, there is no single therapeutic or prophylactic approach that fully effective against Leishmaniasis. An in-depth understanding of the mechanisms by which Leishmania species evade, or exploit host immune machinery could lead to the development of novel therapeutic approaches for the prevention and management of leishmaniasis. In this review, the role of TLRs in the induction of a paradoxical immune response in leishmaniasis was discussed. This review focuses on highlighting the novel interplay of TLR2/TLR9 driven resistance or susceptibility to 5 clinically important Leishmania species in human. The activation of TLR2/TLR9 can induce a diverse anti-Leishmania activities depending on the species of infecting Leishmania parasite. Infection with L. infantum and L. mexicana initiate TLR2/9 activation leading to host protective immune response while infection with L. major, L. donovani, and L. amazonensis trigger either a TLR2/9 related protective or non-protective immune responses. These findings suggest that TLR2 and TLR9 are targets worth pursuing either for modulation or blockage to trigger host protective immune response towards leishmaniasis.

Myeloid-Derived Suppressor Cells in Trypanosoma cruzi Infection

Myeloid-derived suppressor cells (MDSCs) are immature heterogeneous myeloid cells that expand in pathologic conditions as cancer, trauma, and infection. Although characterization of MDSCs is continuously revisited, the best feature is their suppressor activity. There are many markers for MDSC identification, it is distinctive that they express inducible nitric oxide synthase (iNOS) and arginase 1, which can mediate immune suppression. MDSCs can have a medullary origin as a result of emergency myelopoiesis, but also can have an extramedullary origin. Early studies on Trypanosoma cruzi infection showed severe immunosuppression, and several mechanisms involving parasite antigens and host cell mediators were described as inhibition of IL-2 and IL-2R. Another mechanism of immunosuppression involving tumor necrosis factor/interferon γ-dependent nitric oxide production by inducible nitric oxide synthase was also described. Moreover, other studies showed that nitric oxide was produced by CD11b⁺ Gr-1⁺ MDSCs in the spleen, and later iNOS and arginase 1 expressed in CD11b⁺Ly6C⁺Ly6G^lo monocytic MDSC were found in spleen and heart of T. cruzi infected mice that suppressed T cell proliferation. Uncontrolled expansion of monocytic MDSCs leads to L-arginine depletion which hinders nitric oxide production leading to death. Supplement of L-arginine partially reverts L-arginine depletion and survival, suggesting that L-arginine could be administered along with anti-parasitical drugs. On the other hand, pharmacological inhibition of MDSCs leads to death in mice, suggesting that some expansion of MDSCs is needed for an efficient immune response. The role of signaling molecules mediating immune suppression as reactive oxygen species, reactive nitrogen species, as well as prostaglandin E2, characteristics of MDSCs, in T. cruzi infection is not fully understood. We review and discuss the role of these reactive species mediators produced by MDSCs. Finally, we discuss the latest results that link the SLAMF1 immune receptor with reactive oxygen species. Interaction of the parasite with the SLAMF1 modulates parasite virulence through myeloid cell infectivity and reactive oxygen species production. We discuss the possible strategies for targeting MDSCs and SLAMF1 receptor in acute Trypanosoma cruzi infection in mice, to evaluate a possible translational application in human acute infections.

Leishmania species-dependent functional duality of toll-like receptor 2

Toll-like receptors (TLRs) are a subset of pattern recognition receptors (PRR) in innate immunity and act as a connecting link between innate and adaptive immune systems. During Leishmania infection, the activation of TLRs influences the pathogen-specific immune responses, which may play a decisive role in determining the outcome of infection, toward elimination or survival of the pathogen. Antigen-presenting cells (APCs) of the innate immune system such as macrophages, dendritic cells (DCs), neutrophils, natural killer (NK) cells, and NKT cells express TLR2, which plays a crucial role in the parasite recognition and elicitation of immune responses in Leishmania infection. Depending on the infecting Leishmania species, the TLR2 pathways may result in a host-protective or a disease-exacerbating response. While Leishmania major and Leishmania donovani infections trigger TLR2-related host-protective and non-protective immune responses, Leishmania mexicana and Leishmania infantum infections are reported to elicit TLR2-mediated host-protective responses and Leishmania amazonensis and Leishmania braziliensis infections are reported to evoke a disease-exacerbating response. These findings illustrate that TLR2-related effector functions are diverse and may be exerted in a species- or strain-dependent manner. TLR2 agonists or antagonists may have therapeutic potentials to trigger the desired immune response during leishmaniasis. In this review, we discuss the TLR2-related immune responses during leishmaniasis and highlight the novel insights into the possible role of TLR2-driven resistance or susceptibility to Leishmania.

Toll-Like Receptor 2 Is Required for Inflammatory Process Development during Leishmania infantum Infection

Visceral leishmaniasis (VL) is a chronic and fatal disease caused by Leishmania infantum in Brazil. Leukocyte recruitment to infected tissue is a crucial event for the control of infections such as VL. Among inflammatory cells, neutrophils are recruited to the site of Leishmania infection, and these cells may control parasite replication through oxidative or non-oxidative mechanisms. The recruitment, activation and functions of the neutrophils are coordinated by pro-inflammatory cytokines and chemokines during recognition of the parasite by pattern recognition receptors (PRRs). Here, we demonstrated that the Toll-like receptor 2 (TLR2) signaling pathway contributes to the development of the innate immune response during L. infantum infection. The protective mechanism is related to the appropriate recruitment of neutrophils to the inflammatory site. Neutrophil migration is coordinated by DCs that produce CXCL1 and provide a prototypal Th1 and Th17 environment when activated via TLR2. Furthermore, infected TLR2^−/− mice failed to induce nitric oxide synthase (iNOS) expression in neutrophils but not in macrophages. In vitro, infected TLR2^−/− neutrophils presented deficient iNOS expression, nitric oxide (NO) and TNF-α production, decreased expression of CD11b and reduced L. infantum uptake capacity. The non-responsive state of neutrophils is associated with increased amounts of IL-10. Taken together, these data clarify new mechanisms by which TLR2 functions in promoting the development of the adaptive immune response and effector mechanisms of neutrophils during L. infantum infection.

Collapse of Cytolytic Potential in SIV-Specific CD8+ T Cells Following Acute SIV Infection in Rhesus Macaques

Poor maintenance of cytotoxic factor expression among HIV-specific CD8+ T cells, in part caused by dysregulated expression of the transcription factor T-bet, is associated with HIV disease progression. However, the precise evolution and context in which CD8+ T cell cytotoxic functions become dysregulated in HIV infection remain unclear. Using the rhesus macaque (RM) SIV infection model, we evaluated the kinetics of SIV-specific CD8+ T cell cytolytic factor expression in peripheral blood, lymph node, spleen, and gut mucosa from early acute infection through chronic infection. We identified rapid acquisition of perforin and granzyme B expression in SIV-specific CD8+ T cells in blood, secondary lymphoid tissues and gut mucosa that collapsed rapidly during the transition to chronic infection. The evolution of this expression profile was linked to low expression of T-bet and occurred independent of epitope specificity, viral escape patterns and tissue origin. Importantly, during acute infection SIV-specific CD8+ T cells that maintained T-bet expression retained the ability to express granzyme B after stimulation, but this relationship was lost in chronic infection. Together, these data demonstrate the loss of cytolytic machinery in SIV-specific CD8+ T cells in blood and at tissue sites of viral reservoir and active replication during the transition from acute to chronic infection. This phenomenon occurs despite persistent high levels of viremia suggesting that an inability to maintain properly regulated cytotoxic T cell responses in all tissue sites enables HIV/SIV to avoid immune clearance, establish persistent viral reservoirs in lymphoid tissues and gut mucosa, and lead ultimately to immunopathogenesis and death.