Fasel Nicolas, Full Professor

The human parasite Leishmania is the causative agent of leishmaniasis. After malaria, leishmaniasis stands as the most important protozoan parasitic disease in the world, with 350 million people at risk on 5 continents in 98 countries and steadfastly listed in the top 10 most debilitating infectious diseases in the world (according to DALY units). Cutaneous leishmaniasis (CL) is, by far, the most prevalent form of the disease with symptoms ranging from a single self-healing lesion to chronic metastatic leishmaniasis (ML). In an increasingly immunocompromised population, complicated CL is becoming a more likely outcome, characterised by an allergic hyper-reactivity and resulting in severely inflamed, destructive lesions that are often refractory to current treatment. Despite its staggering prevalence and morbidity, it has been categorised as a ‘neglected disease’, with little clinical research interest, no vaccine and a vastly inadequate therapeutic arsenal.

Considering the high immunogenicity of these parasites and their subsequent instigation of excessive local inflammation, they are surprisingly and notoriously evasive of immune-mediated killing. Indeed, Leishmania drastically change their antigenic make-up between phases of their dimorphic life cycle, switching between the flagellated promastigote in the midgut of a hematophagous sand fly to an obligate-intracellular immotile amastigote, resident and replicating within the oxidative phagolysosomes of macrophages (the very organelles designed to kill them).

Understanding these antigenic shifts and the immune responses to them may help us formulate appropriate immunotherapeutic strategies as well as identify better vaccine candidates.

Moreover, understanding the molecular mechanisms behind their evolutionarily conserved resistance to cell death under oxidative pressure could reveal novel and reliable drug targets.

In the past years, our research has centered around (1) Immunotyping the response to metastatic Leishmania and their endosymbiotic viral antigens and (2) Characterizing and inducing cell death in Leishmania.

(1) Immunotyping the response to metastatic Leishmania and their endosymbiotic viral antigens—Diagnosis, treatment and prevention of complicated cutaneous leishmaniasis.

As veterans of infection, Leishmania guyanensis parasites have been plaguing humankind for centuries, provoking a deleterious hyper-inflammatory immune response, destroying host tissue and forming the ulcerating lesions, which typify most forms of the disease. About 15% of patients develop secondary lesions in the mouth and nose, where parasites metastasise to mucocutaneous tissues creating debilitating and exceptionally disfiguring inflammation.

Our lab has recently shown that a virus within metastatic Leishmania parasites (Leishmania RNA virus: LRV) can act as an independently immunogenic entity, where its RNA-based nucleic acid acted as a potent innate immunogen, triggering a destructive hyper-inflammatory cascade through Toll-Like-Receptor 3 recognition.

Because viral replication depends on the vigor of its host, many viruses have evolved incentives of fitness to pay their keep. When the viral host is a human pathogen, these fitness factors can surface as virulence: creating a Russian doll of pathogenesis where parasites within parasites complicate the disease process.

We dub this process “hyperpathogenism” and have exposed its importance as major clinical consideration in metastatic leishmaniasis.

Interestingly, our preliminary data show this LRV-specific immune response can be rendered protective if primed before the infection as a vaccine, raising the importance of cross talk between the innate responses to the various components of this nested infection.

By appreciating the microbial-virus as a backseat driver of human disease, we may be able to better formulate appropriate clinical intervention or even exploit its presence for clinical benefit. Further, its presence could act as a biomarker, guiding diagnostics, treatment and phylogenetics as well as a having potential as novel molecular target for therapeutic and prophylactic intervention. 

Leishmania RNA virus (Red) in Leishmania guyanensis
infected macrophages (Nuclei of macrophages stained
in blue)

(2) Characterizing and inducing cell death in Leishmania—Metacaspase as a drug target.

Dubbed as the ‘executioner’ enzymes, capases have a well-described role in eukaryotic cell death. Although some caspase-like peptidase activity has been reported in dying Leishmania parasites, a caspase gene is not present in its genome. A single copy of the analogous metacaspase gene is, however, present in Leishmania major. Structurally, L. major metacaspase (LmjMCA) comprises a potential N-terminal mitochondrial localization sequence (MLS), a central domain with the catalytic dyad histidine/cysteine described for caspases and a C-terminal proline-rich sequence. The S. cerevisiae metacaspase YCA1 has been implicated in the death of aging cells, cells defective in some biological functions, and cells exposed to different environmental stresses. In our study of LmjMCA, we demonstrated the functional heterologous complementation of a S. cerevisiae yca1 null mutant with the L. major metacaspase (LmjMCA). Here, cell death was successfully induced in yeast by LmjMCA after oxidative stress. We completed the previous study by investigating LmjMCA activation and cellular localization following its processing. Although the LmjMCA polypeptide precursor form harbors a functional MLS, we demonstrated that LmjMCA is mainly localized in the cytoplasm due to an amino acid sequence in the catalytic domain blocking its transport into the mitochondrion. Finally, in stress conditions, overexpression of the LmjMCA catalytic domain enhanced sensitivity of parasites to ROS. This was the first report describing the activation, release of the catalytic domain (via auto-processing) and cytoplasmic localization of LmjMCA. Further, we demonstrated that LmjMCA increased susceptibility to ROS during stress, which, we predict is through perturbing mitochondrial membrane integrity (Zalila et al., 2011). These results suggest that metacaspases are members of a family of peptidases with an evolutionarily conserved role in cell death function. The identification of an enzyme sensitizing this parasitic pathogen to cell death holds much clinical potential for leishmaniasis: A neglected disease with an aged and problematic therapeutic arsenal, in need of new drug targets. 

N. Fasel: Full list of publications

C. Ronet: Full list of publications

• Weinkopff T, Mariotto A, Simon G, Hauyon-La Torre Y, Auderset F, Schuster S, Zangger H, Fasel N, Barral A, Tacchini-Cottier F. Role of TLR9 Signaling in Experimental Leishmania braziliensis Infection. Infect Immun. 2013 Feb 25. [Epub ahead of print] doi: 10.1128/​IAI.01401-12.
• Zangger H, Ronet C, Desponds C, Kuhlmann FM, Robinson J, Hartley MA, Prével F, Castiglioni P, Pratlong F, Bastien P, Müller N, Parmentier L, Gore Saravia N, Beverley SM, Fasel N. Detection of Leishmania RNA virus in Leishmania parasites. PLoS Negl Trop Dis. 2013 Jan;7(1):e2006. doi: 10.1371/journal.pntd.0002006. Epub 2013 Jan 10.
• Hartley MA, Kohl K, Ronet C, Fasel N. The therapeutic potential of immune cross-talk in leishmaniasis. Clin Microbiol Infect. 2013 Feb;19(2):119-30. doi: 10.1111/1469-0691.12095.
• Hartley MA, Ronet C, Zangger H, Beverley SM, Fasel N. Leishmania RNA virus: when the host pays the toll. Front Cell Infect Microbiol. 2012;2:99. doi: 10.3389/fcimb.2012.00099. Epub 2012 Jul 12. Review 
• Hartley MA, Ronet C, Fasel N. Backseat drivers: the hidden influence of microbial viruses on disease. Curr Opin Microbiol. 2012 Aug;15(4):538-45. doi: 10.1016/j.mib.2012.05.011. Epub 2012 Jun 11.
• Ronet C, Beverley SM, Fasel N. Un virus, hôte indésirable de L. guyanensis, détermine la gravité de la forme mucocutanée de la leishmaniose. m/s n° 11, vol. 27, novembre 2011 DOI : 10.1051/medsci/20112711003.
• Ronet C, Beverley SM, Fasel N. Muco-cutaneous leishmaniasis in the New World: The ultimate subversion. Virulence. 2011 Nov-Dec;2(6):547-52. doi: 10.4161/viru.2.6.17839. Epub 2011 Nov 1.
• Meslin B, Beavogui AH, Fasel N, Picot S. Plasmodium falciparum Metacaspase PfMCA-1 Triggers a z-VAD-fmk Inhibitable Protease to Promote Cell Death. PLoS One. 2011;6(8):e23867. doi: 10.1371/journal.pone.0023867. Epub 2011 Aug 17.
• Fuertes-Marraco SA, Scott CL, Bouillet P, Ives A, Masina S, Vremec D, Jansen ES, O’Reilly LA, Schneider P, Fasel N, Shortman K, Strasser A, Acha-Orbea H. Type I Interferon Drives Dendritic Cell Apoptosis via Multiple BH3-Only Proteins following Activation by PolyIC In Vivo. PLoS One. 2011;6(6):e20189. doi: 10.1371/journal.pone.0020189. Epub 2011 Jun 2.
• Meslin B, Zalila H, Fasel N, Picot S, Bienvenu AL. Are protozoan metacaspases potential parasite killers ? Parasit Vectors. 2011 Feb 28;4:26. doi: 10.1186/1756-3305-4-26.
• Ives A, Ronet C, Prevel F, Ruzzante G, Fuertes-Marraco S, Schutz F, Zangger H, Revaz-Breton M, Lye LF, Hickerson SM, Beverley SM, Acha-Orbea H, Launois P, Fasel N, Masina S. Leishmania RNA Virus Controls the Severity of Mucocutaneous Leishmaniasis. Science. 2011 Feb 11;331(6018):775-8. doi: 10.1126/science.1199326.
• Zalila H, Gonzalez IJ, El-Fadili AK, Delgado MB, Desponds C, Schaff C, Fasel N. Processing of metacaspase into a cytoplasmic catalytic domain mediating cell death in Leishmania major. Mol Microbiol. 2011 Jan;79(1):222-39. doi: 10.1111/j.1365-2958.2010.07443.x. Epub 2010 Nov 9.
• Smirlis D, Duszenko M, Jimenez Ruiz A, Scoulica E, Bastien P, Fasel N, Soteriadou K. Targeting essential pathways in trypanosomatids gives insights into protozoan mechanisms of cell death. Parasit Vectors. 2010 Nov 17;3:107. doi: 10.1186/1756-3305-3-107.
• Jimenez-Ruiz A, Alzate JF, MacLeod ET, Lüder C, Fasel N, Hurd H. Apoptotic markers in protozoan parasites. Parasit Vectors. 2010 Nov 9;3:104. doi: 10.1186/1756-3305-3-104.
• El-Fahdili AK, Zangger H, Desponds C, Gonzalez IJ, Zalila H, Schaff C, Ives A, Masina S, Mottram JC, Fasel N. Cathepsin B-like and cell death in the unicellular human pathogen Leishmania. Cell Death Dis. 2010 Sep 2;1:e71. doi: 10.1038/cddis.2010.51.