Many experimental vaccines have been developed that show promise in animal models of cutaneous or visceral forms of leishmaniasis, but an effective human vaccine still does not exist. Our prior studies demonstrated that defined or whole cell, non-living vaccines that protect mice against needle challenge with L. major fail to protect against infected sand fly challenge. Importantly, mice with a healed primary infection are solidly protected against infected sand fly challenge that is associated with the speed with which effector cells appear at a site of challenge. In 2014, we carried out a systematic study of the tissue homing properties, functionality, and life span of subsets of memory and effector CD4 T cells activated in the setting of chronic Leishmania major infection in resistant C57Bl/6 mice. We found that pre-existing, CD44+CD62L-T-bet+Ly6C+ effector (TEFF) cells that are short-lived in the absence of infection and are not derived from memory cells reactivated by secondary challenge, mediate concomitant immunity. Upon adoptive transfer and challenge, non-dividing Ly6C+ TEFF cells preferentially homed to the skin, released IFN-g, and conferred protection as compared to CD44+CD62L-Ly6C- effector memory or CD44+CD62L+Ly6C- central memory cells. During chronic infection, Ly6C+ TEFF cells were maintained at high frequencies via reactivation of TCM and the TEFF themselves. The lack of effective vaccines for many chronic diseases may be because protection against infectious challenge requires the maintenance of pre-existing TEFF cells, and is therefore not amenable to conventional, memory inducing, vaccination strategies. We continue to study the early inflammatory events in the skin at the site of inoculation by sand fly bite or needle. The route of pathogen inoculation by needle has been shown to influence the outcome of infection. Employing needle inoculation with L. major, we identified striking differences in the preexisting and acute cellular response following intra-dermal (i.d.) inoculation of the ear, sub-cutaneous (s.c.) inoculation of the footpad, or inoculation of the peritoneal (i.p.) cavity. Initiation of infection at different sites was associated with vastly different phagocytic populations. Neutrophils were the dominant infected cells following i.d., but not s.c. or i.p. inoculation. Inoculation of the ear dermis resulted in higher frequencies of total and infected neutrophils versus the footpad and this was associated with a 10-fold increase in early parasite loads. Following inoculation of the ear in the absence of neutrophils, parasite phagocytosis by other cell types did not increase and fewer parasites were able to establish infection. Regardless of inoculation site, the frequency of infected neutrophils within the total infected CD11b+ population was higher than the frequency of total neutrophils within the total CD11b+ population, demonstrating that neutrophils are overrepresented as a proportion of infected cells. Employing i.d. inoculation of the ear to model natural sand fly transmission of parasites has significant consequences for infection outcome when compared to the s.c. or i.p. route, including the phenotype of infected cells and the number of parasites that are able to establish infection. Mouse models are also being used to study non-healing forms of disease. Infection of C57BL/6 mice with most L. major strains, such as L. major Friedlin (LmFn) from Israel, results in a healing lesion and clearance of most parasites from the site of inoculation in the skin. By contrast, infection of C57BL/6 mice with the L. major Seidman strain (LmSd) isolated from a patient with chronic cutaneous lesions acquired in Senegal, despite eliciting a strong Th1 response, results in a non-healing lesion, poor parasite clearance, and complete destruction of the ear dermis. We observed that in comparison to LmFn, infection with LmSd elicited an early recruitment of CD4+ and CD8+ T cells, and prominently neutrophils to the site of inoculation in the skin. LmSd infection also induced early expression of IL-1a mRNA and IL-1b staining dermal cells, and mice deficient in IL-1R, IL-1b, ASC and caspase-1/11 developed healing lesions, minimal pathology and cleared LmSd parasites from the site. Resistance to LmSd infection in IL-1R-/- mice was associated with a significant upregulation of the antigen-specific Th1 response in the lesion and draining lymph node. IL-1b maturation and secretion by infected bone marrow derived macrophages were shown to be dependent on caspase-1/11 activation in vitro. These data reveal that inflammasome dependent IL-1b, induced early after infection in a Th1 polarized setting, plays a crucial role in the development of severe pathology and the non-healing response to L. major. These studies are unique in the innate immunity field in identifying inflammasome dependent IL-1b as preventing rather than promoting host defense against a microbial pathogen. The mechanisms underlying the failure to control the growth and systemic spread of Leishmania parasites in human visceral leishmaniasis (VL) are not well understood. A key immunological feature of VL is the inability of peripheral blood mononuclear cells (PBMCs) to proliferate or to produce IFNg in response to leishmanial antigens. Unexpectedly, it was recently shown that Leishmania specific IFNg that can readily be detected when a whole blood stimulation assay (WBA) is used. We sought to define the conditions that permit whole blood cells to respond to antigen stimulation, and clarify the biological role of the IFNg found to be released by cells from VL patients. CD4+ T cells were found to be crucial for and the main source of the IFNg production in Leishmania stimulated WB cultures. Complement, antibodies and red blood cells present in whole blood do not play a significant role in the IFNg response. However, removal of CD15+ or CD56+ cells reduced IFNg production, suggesting that neutrophils and NK cells, or other cells expressing these markers, contribute to the response in WB. Most importantly, blockade of IFNg in ex-vivo splenic aspirate cultures demonstrated that despite the progressive nature of their disease, the endogenous IFNg produced in patients with active VL serves to limit parasite growth. Little is known about CD8 T cells in human VL and it is unclear if these cells have a protective, pathological and/or suppressive function. To better understand the role of CD8 T cells in human VL, we examined molecules associated with anergy and cytotoxic T lymphocytes (CTL) in peripheral blood mononuclear cells (PBMC) and splenic aspirates (SA), and in CD8 cells derived from these tissues. Gene and surface marker expression suggest that splenic CD8 cell predominantly display an anergic phenotype, whereas CD8-PBMC have features of both anergic cells and CTLs. CD8 cells contribute to the baseline IFNg levels in whole blood and SA cultures, but not to the Leishmania induced IFNg release that is revealed using WB cultures. Blockade of CTLA-4 or PD1 had no effect on IFNg production or parasite survival in SA cultures. Following cure, CD8 T cells contribute to the Leishmania induced IFNg production observed in Leishmania stimulated cell cultures. We suggest CD8 T cells are driven to anergy/exhaustion in human VL, which affect their ability to contribute to protective immune responses.
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