Background

Cutaneous and mucocutaneous leishmaniasis (CL/ML) cause significant morbidity globally and are vulnerable to changes from environmental events and conflict. In this ecological study, we aim to measure the associations between annual CL/ML cases, conflict intensity, and environmental factors between 2005 and 2022 globally.

Methods

We pulled annual case data from the WHO for 52 nations that had conflict intensity scores (ranging from 1-10) from the Bertelsmann Transformation Index. Using Earth observation tools, we gathered temperature, precipitation, vegetation, and humidity data, in addition to data on annual estimates of population, internal displacement, and GDP. We fit a negative binomial generalized additive model with a random nation-level intercept.

Results

Conflict was positively associated with increased CL/ML across the studied nations (IRR: 1.09, 95% CI: 1.01-1.16, p = 0.02). Given this, intense conflict (a score of ten) was associated with over double the risk of CL/ML compared to the lowest conflict levels (score of one). We also identified a curvilinear relationship between mean temperature and cases, as well as between vegetation level and cases. Each had small pockets of significant increased and decreased risk, respectively. Larger mean humidity ranges were negatively associated with cases. Importantly, the relationship between conflict intensity and cases was mediated by displacement.

Discussion

Conflict is significantly associated with increased CL/ML cases. This is especially true at higher conflict levels, marking when conflict turns violent. The destruction of critical infrastructure (e.g., that related to healthcare, water, and sanitation) often seen during conflict could drive this association. Such environments can be hospitable to sandflies and can heighten individuals' vulnerability through increased malnutrition, poverty, and displacement. Understanding this relationship is crucial for public health preparedness and response, especially as conflicts become increasingly violent and protracted.

Host-directed therapies (HDTs) constitute promising alternatives to traditional therapy that directly targets the pathogen but is often hampered by pathogen resistance. HDT could represent a new treatment strategy for leishmaniasis, a neglected tropical disease caused by the obligate intracellular parasite Leishmania. This protozoan develops exclusively within phagocytic cells, where infection relies on a complex molecular interplay potentially exploitable for drug targets. We previously identified naloxonazine, a compound specifically active against intracellular but not axenic Leishmania donovani. We evaluated here whether this compound could present a host cell-dependent mechanism of action. Microarray profiling of THP-1 macrophages treated with naloxonazine showed upregulation of vATPases, which was further linked to an increased volume of intracellular acidic vacuoles. Treatment of Leishmania-infected macrophages with the vATPase inhibitor concanamycin A abolished naloxonazine effects, functionally demonstrating that naloxonazine affects Leishmania amastigotes indirectly, through host cell vacuolar remodeling. These results validate amastigote-specific screening approaches as a powerful way to identify alternative host-encoded targets. Although the therapeutic value of naloxonazine itself is unproven, our results further demonstrate the importance of intracellular acidic compartments for host defense against Leishmania, highlighting the possibility of targeting this host cell compartment for anti-leishmanial therapy.

CXCR3, expressed mainly on activated T and NK cells, is implicated in a host of immunological conditions and can contribute either to disease resolution or pathology. We report the generation and characterization of a novel CXCR3 internal ribosome entry site bicistronic enhanced GFP reporter (CIBER) mouse in which enhanced GFP expression correlates with surface levels of CXCR3. Using CIBER mice, we identified two distinct populations of innate CD8(+) T cells based on constitutive expression of CXCR3. We demonstrate that CXCR3(+) innate CD8(+) T cells preferentially express higher levels of Ly6C and CD122, but lower levels of CCR9 compared with CXCR3(-) innate CD8(+) T cells. Furthermore, we show that CXCR3(+) innate CD8(+) T cells express higher transcript levels of antiapoptotic but lower levels of proapoptotic factors, respond more robustly to IL-2 and IL-15, and produce significantly more IFN-γ and granzyme B. Interestingly, CXCR3(+) innate CD8(+) T cells do not respond to IL-12 or IL-18 alone, but produce significant amounts of IFN-γ on stimulation with a combination of these cytokines. Taken together, these findings demonstrate that CXCR3(+) and CXCR3(-) innate CD8(+) T cells are phenotypically and functionally distinct. These newly generated CIBER mice provide a novel tool for studying the role of CXCR3 and CXCR3-expressing cells in vivo.