**3.3 Regulatory T cells**

The role of Treg cells in *Leishmania* infection is still being elucidated, although though they have been shown in rodent models to be involved in disease pathology and parasite persistence depending on the experimental model used. CD4<sup>+</sup> CD25+ Treg cells have been shown to suppress CD4+ T cell activity in *L. major*-infected C57BL/6 mice, thereby favoring parasite persistence [98, 113, 114]. Treg cells influence both primary and secondary infections with *L. major*, as they render otherwise non-susceptible mice susceptible to infection [115]. However, their activity may also be dependent on the infecting *Leishmania* species. For example, Treg cells play a protective role during infection with New World *Leishmania* species, such as *L. amazonensis* [95, 116]. Transferring Treg cells from an *L. amazonensis*-infected mouse to a naïve mouse prior to infection with *L. amazonensis* reduced the development of lesions suggesting that they may also contribute to the control of immunopathogenic responses [116]. Understanding how Treg cells are involved in human *Leishmania*-infections is still being explored, with evidence so far suggesting that these cells play a role at the infection site and contributing directly to parasite persistent as the main source of IL-10 production [95, 98].

#### **3.4 B cells and antibodies**

The function of B cells in CL has not conclusively been shown. During the initial *Leishmania* infection, antibody production by B cells themselves are not believed to play a role, in controlling parasites as *Leishmania* are intracellular. However, some studies indicate that B cells may regulate both protective and pathogenic immune responses during *Leishmania* infection, depending on the infecting species and model used. Production of *L. major* antibodies was shown to be important for DCs to phagocytose parasites, as the absence of antibodies by B cells resulted in larger lesions in B cell−/− mice, higher parasite load, low production of IFNγ and a decreased cell-mediated immune response [117]. Moreover, IgG−/− BALB/c mice infected with *L. major* resulted in larger lesions and higher parasite load compared to IgG<sup>+</sup> BALB/c mice [118]. In contrast, a study using a BALB/c mice deficient in IgM transmembrane domain (μMT), thereby lacking mature B cells, observed that these mice were resistant to *L. major* infection [119]. Other studies using BALB/c mice lacking IL-4Rα expression specifically on B cells, mbicreIL-4Rα−/lox BALB/c mice, resulted in a protective host immunity [29, 119, 120].

There is still a lot of knowledge to gain on B cells' function and whether they play a part in protection or pathology during infection with *Leishmania* parasites.

### **4. Persistent** *Leishmania* **infection and emulating concomitant immunity**

Naturally and experimental infection with cutaneous *Leishmania* species is controlled following the development of an adaptive TH1 immune response. After induction of this response, parasite numbers decline in infected tissues, lesions heal and lifelong immunity against the infecting *Leishmania* species is gained [121]. Though recovery from cutaneous disease has been reached, a small number of *Leishmania* parasites normally remain indefinitely in the host at the initial site of infection; known as persistent parasites [122, 123]. These parasites play an important role in maintaining protective immunity in the event of reinfection by providing a constant source of *Leishmania* antigen for immune stimulation [121, 124]. Both mice and humans who recover from CL maintain chronic subclinical infection at the lesion site and have been shown to be highly resistant to second challenge through sandfly transmitted infections [125]. Though, the immune response is unable to clear the primary infection, the immune system can facilitate concomitant immunity by IFNγ secreting CD4+ TH1 cells [126]. However, reactivation of disease causing infection has been documented for leishmaniasis when the immune system is no longer able to control this low level chronic parasite infection [127, 128]. This is frequently observed when persistently infected individuals become immunosuppressed, such as during infection with the human immunodeficiency virus (HIV) [127, 129].

Currently, vaccine programs have been unsuccessful to emulate the protective responses mediated by concomitant immunity as observed during subclinical infections with persistent parasites (reviewed in [130]). Similar, a sterile cure whereby the parasites are completely eliminated has not been achieved without consequently the loss of long-term immunity [131].

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

In the past the leishmanization live vaccine practice was employed by inoculating virulent *Leishmania* parasites into individuals, however this has since fallen out of practice due to safety concerns regarding development of non-healing lesions [132]. Since then, vaccine-candidates have failed to provide protection against natural exposure even though they demonstrate protective cell-mediated immunity in rodent models. It is thought that this is due to differences in experimental delivery versus the natural route of infection via the bite of a sandfly. Other challenges are observed when using whole killed parasites or subunit protein vaccine candidates only short-term protection in rodent models has been observed [123, 131].

The difference in protective immunity induced following natural infection and inoculation of whole killed parasites is not fully understood but it has been hypothesized that there is a difference in the immunologic memory responses, which is influenced by the presence of live versus killed parasites. Moreover, the adjuvant dose-quantity tested to date may not be sufficient to generate a memory T cell population [123, 128]. It is possible that vaccines utilizing live-attenuated parasites will most closely mimic natural infection, potentially providing long-term protection against infection and disease [131].

Recently, vector-associated factors have been identified to have an important impact on challenge models in vaccine-mediated immunity [130]. Following needle versus infected sandfly challenge in mice showed that various protein/adjuvant-based vaccines provided intermediate protection against needle challenge whereas sandfly challenge failed to provide protection. Despite generating antigen-specific TH1 immune responses prior to and following challenge, vaccines failed to protect against infected sandfly challenge [125, 133]. The sandfly vector challenge model clearly emphasizes important factors induced by the sandfly, such as the impact of recruited inflammatory cells and immune-mediated host cell activation by the vector.
