**3. Adaptive immune system in** *Leishmania* **infection and disease**

Following the involvement of innate immune cells in targeting *Leishmania* parasites and antigen presentation, immune cells of the adaptive immune system are activated to induce a *Leishmania*-specific response. The adaptive immune system plays a pivotal role in *Leishmania* infection through the interplay between T cell-mediated and antibody-mediated immune responses and the induction of immune memory. The complexity of these immune responses, which facilitate the resolution of CL is also reflected by the various phenotypes of clinical CL presentations observed in individuals [66]. On one end of the immune spectrum, a strong T cell response is observed. Although the high levels of IFNγ lead to parasite control, an exacerbated T helper (TH) type-1 response and increased number of CD8+ cytotoxic T cells may also lead to the development of MCL. In contrast, the other end of the immune spectrum is characterized by a high level of *Leishmania*-specific antibodies and a limited T cellmediated response. Individuals have an uncontrolled parasite load (as parasites are not neutralized by antibodies), which is a consequence of low levels of TH1 cytokines and this results in DCL manifestations [16, 67, 68]. An intermediate level of both T cell and antibody responses will lead to a form of CL that will normally self-heal over time.

#### **3.1 CD4<sup>+</sup> T cells**

The generation of *Leishmania*-specific CD4+ T cells is required for protective immunity, and they play a major role in shaping the adaptive immune response. CD4+ TH cells are essential in determining disease outcome by driving the differentiation and activation of different CD4+ TH cell subsets through the production of cytokines, which either mediate host protection or promote disease pathogenesis (**Figure 4**).

#### **Figure 4.**

*Interaction between immune cells and Leishmania parasites. (A) Infected macrophages produce proinflammatory cytokines such as interleukin-12 (IL-12), IL-18 and tumor necrosis factor-*α *(TNF-*α*). These cytokines (B) recruit natural killer (NK) cells to the infection site and (C) promote CD4+ TH cell differentiation into CD4+ TH1. (B) NK cells and (C) CD4+ TH1 cells activate infected macrophages by producing interferon-*γ *(IFN*γ*). (D) Activated infected macrophages then release reactive oxygen species (ROS) and inducible nitric oxide synthase (iNOS), which results in parasite killing. (E) Infected monocytes kill Leishmania parasites through the release of ROS and migrate to the lymph node. Here they promote CD4<sup>+</sup> TH1 differentiation by producing IL-12. CD4+ TH1 migrates to the skin where they (D) activate infected macrophages. In contrast (F) CD4<sup>+</sup> TH cells produce IL-4 (an anti-inflammatory cytokine) which drives the differentiation of CD4+ TH2 cells. (G) Secretion of anti-inflammatory cytokines (such as transforming growth factor-*β*; TGF*β*) by CD4+ TH2 suppresses the production of iNOS and NO by macrophages leading to parasite survival. (H) TGF*β *drives the differentiation into CD4+ TH9 cells, which downregulates the release of IFN*γ *and TNF-*α *from infected macrophages, thereby promoting disease. (I) TGF*β *and IL-6 drives differentiation into CD4<sup>+</sup> TH17 cells that stimulates the secretion of IL-1*β *and infiltration of neutrophils that are believed to aggravate the disease. Adapted from [69]. Created with Biorender.com*

Previously, it was widely believed that the induction of either a CD4<sup>+</sup> TH1 or TH2 response determined the outcome of infection i.e., induction of protection versus pathology. Subsequent studies have shown that there are a multitude of factors that contribute to the outcome of *Leishmania* infection, thus the TH1/TH2 model oversimplifies a complex interaction between host and parasite. Factors such as the genetic background of the model (or host) and the *Leishmania* parasite (species and strain) studied, contribute to differential disease outcomes. It is acknowledged that several CD4<sup>+</sup> T cell subsets are implicated in disease outcome, such as CD4+ regulatory T (Treg) cells, CD4<sup>+</sup> T helper populations (TH1, TH2, TH9 and TH17 effector) and T follicular helper (TFH) cells [58, 70–72].

Cytokines produced by CD4<sup>+</sup> T cell subsets and other infected immune cells are generally classified as pro-inflammatory or anti-inflammatory and have been shown to be differentially associated with disease protection or progression, respectively (**Table 2**). Their role in activating and recruiting immune cells to the infection site shapes the adaptive immune response.


*Protective and Pathogenic Immune Responses to Cutaneous Leishmaniasis DOI: http://dx.doi.org/10.5772/intechopen.101160*





−*/*−*, deficient; DCs, dendritic cells; IL, interleukin; IFN, Interferon; MIP, macrophage inflammatory protein; NK, natural killer; NKT, natural killer T cells; NO, nitric oxide; TFH, T follicular helper cells; TGF, transforming growth factor; TH, T helper cell; TNF, tumor necrosis factor; Treg, T regulatory cell; WT, wild type.*

#### **Table 2.**

*Selection of cytokines and their role in cutaneous leishmaniasis.*

It is recognized that the development of CD4+ TH1 immune responses promotes host protection against CL and is associated with the production of pro-inflammatory cytokines (such as IFNγ and IL-12). CD4+ TH1 cells are key producers of IFNγ, which has been shown in resistant and susceptible mouse models to be vital in controlling *L. major* parasites [92, 93]. In human and mice, the production of IFNγ activates infected macrophages to enhance the respiratory burst (as discussed above), which eliminates parasites residing and replicating within the phagolysosome, as explained earlier [59].

In contrast to the protective role of CD4+ TH1 cells, susceptibility to *Leishmania* infection and CL progression is influenced by the induction of an IL-4-driven TH2 type immune response as well as the production of the anti-inflammatory cytokines, IL-10, IL-13 and TGFβ [94]. Rodent studies have shown that IL-4-secreting CD4+ TH2 cells and IL-10 secreting Treg cells promote parasite growth and disease susceptibility [95]. For example, the CD4+ TH2-secreting cytokines, IL-4 [96] and IL-10 [97], was identified as having important roles in BALB/c mice' susceptibility to infection. In the absence of IL-4 or IL-10, BALB/c mice, were able to control parasite growth and resolve lesions resulting in a protective CD4+ TH1 response. Likewise, IL-10 likewise plays a role in disease self-healing C57BL/6 mice. When lacking IL-10, C57BL/6 mice exhibited a faster lesion healing time compared to WT [98]. The roles of IL-4 and IL-10 in promoting susceptibility in human patients with CL are less clear, although elevated IL-10 has been linked to uncontrolled parasite growth in VL [99].

Some cytokines are also considered to have a dual role in relation to disease outcome [100]. The production of the CD4+ TH1 cytokines IFNγ and TNF-α is critical in controlling *Leishmania* infection, however an aggravated production of these two cytokines have been affiliated with severe disease with lesion chronicity [91].

#### **3.2 CD8+ T cells**

The role of CD8+ T cells in *Leishmania* infection is still poorly understood. They have both a protective and a pathological role depending on whether the cells are producers of cytokines or are acting as cytolytic T cells, respectively (reviewed in [101]). The contribution and effectiveness of CD8+ T cells in relation to parasite control is determined by the *Leishmania* species and experimental model (infective dose and host genetics).

RAG knockout (KO) mice (deficient in both B and T cells) developed lesions at a slower rate (*L. major* infection) compared to WT mice or not at all (*L. braziliensis* and *L. amazonensis* infection) [102–104]. When reconstituted with CD8+ T cells, RAG KO mice developed severe pathology with lesions [102, 103]. In BALB/c mice infected with *L. braziliensis*, depletion of CD8+ T cells resulted in reduced lesion size despite having a similar level of parasites in the skin compared with control mice [103].

Mimicking a natural low-dose infection with *L. major*, studies revealed that CD8+ T cells play a role in protection, associated with high production of IFNγ, which activates macrophages leading to parasite control [102, 105]. Furthermore, IFNγ stimulates DCs to produce IL-12 which promote the development and differentiation of CD4+ TH1 cells. This suggests that CD8<sup>+</sup> T cells are important in skewing towards TH1 response through the production of IFNγ and in eliminating the majority of parasites before lesion development. The role that CD8+ T cells play in infection may be associated with their location in the host [106]. When located in the draining lymph node, CD8<sup>+</sup> T cells produce IFNγ and are protective [107]. In contrast, when migrating to the lesion site during infection, CD8+ T cells produce lower levels of IFNγ and exhibit cytolytic activity, leading to cell death and an exaggerated inflammatory response that further promotes tissue damage [108]. This is supported by findings from a mouse model showing CD8<sup>+</sup> T cells that had migrated to the skin, produced lower levels of IFNγ and instead exhibited cytolytic activity promoting disease progression [103]. There is substantial evidence for a pathogenic role of CD8<sup>+</sup> T cells in patients infected with *L. braziliensis* [109–111]. As the disease progresses from small nodules to larger skin lesions, an increase in CD8+ T cells and a decrease in CD4+ T cells was observed in the histopathological analysis of human skin lesions [112]. In CL patients a link between CD8<sup>+</sup> T cell mediated cytotoxicity and IL-1β inflammasome activation was observed [111]. This activation of NLRP3 inflammasome pathway and its promotion of disease inflammation is currently targeted for host-directed therapy [88, 106].
