Exacerbated Leishmaniasis Caused by a Viral Endosymbiont can be Prevented by Immunization with Its Viral Capsid

Recent studies have shown that a cytoplasmic virus called Leishmaniavirus (LRV) is present in some Leishmania species and acts as a potent innate immunogen, aggravating lesional inflammation and development in mice. In humans, the presence of LRV in Leishmania guyanensis and in L. braziliensis was significantly correlated with poor treatment response and symptomatic relapse. So far, no clinical effort has used LRV for prophylactic purposes. In this context, we designed an original vaccine strategy that targeted LRV nested in Leishmania parasites to prevent virus-related complications. To this end, C57BL/6 mice were immunized with a recombinant LRV1 Leishmania guyanensis viral capsid polypeptide formulated with a T helper 1-polarizing adjuvant. LRV1-vaccinated mice had significant reduction in lesion size and parasite load when subsequently challenged with LRV1+ Leishmania guyanensis parasites. The protection conferred by this immunization could be reproduced in naïve mice via T-cell transfer from vaccinated mice but not by serum transfer. The induction of LRV1 specific T cells secreting IFN-γ was confirmed in vaccinated mice and provided strong evidence that LRV1-specific protection arose via a cell mediated immune response against the LRV1 capsid. Our studies suggest that immunization with LRV1 capsid could be of a preventive benefit in mitigating the elevated pathology associated with LRV1 bearing Leishmania infections and possibly avoiding symptomatic relapses after an initial treatment. This novel anti-endosymbiotic vaccine strategy could be exploited to control other infectious diseases, as similar viral infections are largely prevalent across pathogenic pathogens and could consequently open new vaccine opportunities.

About 80% of leishmaniasis infections result in cutaneous manifestations with a broad symptomatic spectrum, ranging from self-healing localized to disseminated lesions. The mechanism behind these latter aggravated forms of leishmaniasis is still poorly understood. One possible factor is an endosymbiotic RNA virus identified in the cytoplasm of several Leishmania species (Leishmania RNA virus 1). LRV1 acts as a virulence factor, causing a destructive hyper-inflammatory response. In this study, we tested the prophylactic potential of a vaccine formulated with a recombinant LRV1 capsid and a T helper 1-polarizing adjuvant. Our approach conferred significant protection against LRV1+ Leishmania guyanensis infection, decreasing lesional inflammation and parasite burden. Further analysis demonstrated that this vaccine induced a potent T helper 1 response. Consequently, we propose that the LRV1-capsid is a promising vaccine component in order to reduce clinical complications (e.g. symptomatic relapses) in areas endemic to LRV1 co-infected Leishmania species. Taken together, we present an original strategy, whereby targeting the pervasive intracellular viruses within pathogens may reduce pathologic inflammation and offer an extra-genetic candidate that may circumvent escape mutations or poor response to drug treatment.

Generation of the short RNA transcript in Leishmaniavirus correlates with the growth of its parasite host, Leishmania

Leishmaniavirus 1 is a double-stranded RNA virus that infects the New World kinetoplastid parasites, Leishmania braziliensis, and Leishmania guyanensis. The isolated virus particles contain an RNA-dependent RNA polymerase which exhibits both transcriptase activity for genome-length plus-strand synthesis and replicase activity for genome-length minus-strand synthesis. Recently, we identified a 320 nucleotide short RNA transcript of Leishmaniavirus 1-4, derived from the 5' end of the viral plus-strand, which is generated by the virus capsid via site-specific cleavage of the full-length positive single-stranded RNA. We have hypothesized that this short RNA transcript functions to regulate the virus life cycle during the growth of its parasite host, Leishmania guyanensis. To address this hypothesis, we measured the relative amount of short RNA transcripts and the absolute number of viral genomes per infected cell from log through stationary phase of the parasite growth cycle. In vitro assays of the viral polymerase showed an overall increase in viral polymerase activity from log growth into stationary phase which mirrored an in vivo increase in the quantity of double-stranded genome as measured by agarose gel electrophoresis. We have developed competitive reverse transcription-polymerase chain reaction (RT-PCR) assays to measure the relative amounts of viral transcripts in infected cells as well as the number of viral genomes per infected cell. The results of these assays show that the amount of full-length virus transcripts peaks in the parasite stationary phase (132 transcripts per cell), and that the short transcript is most abundant in the early stationary phase cells (24 transcripts per cell).

Short report: detection of Leishmaniavirus in human biopsy samples of leishmaniasis from Peru

Leishmaniavirus is a double-stranded RNA virus that persistently infects some strains of the protozoan parasite Leishmania. There is considerable interest in the possibility that the presence of this virus alters parasite phenotype and may affect disease pathogenesis. If so, the virus marker could provide a valuable prognostic indicator for human leishmaniasis, particularly in those cases caused by New World parasite strains. The virus has been detected in cultured L. braziliensis, L. b. guyanensis, and L. major. To date there has been no information as to the extent of infection in samples prior to culturing in the laboratory. This study demonstrates, through the reverse transcription-polymerase chain reaction, that Leishmaniavirus exists in human biopsy samples of leishmaniasis prior to manipulation in culture.

Hygromycin B resistance mediates elimination of Leishmania virus from persistently infected parasites

A series of pX63-HYG derivatives encoding Leishmania RNA virus 1-4 (LRV1-4) sequences were electroporated into cells of Leishmania strain M4147, a virus-infected strain of L. guyanensis. After 6 weeks of drug selection (hygromycin B), transfected parasites lacked detectable quantities of viral genomic double-stranded RNA, viral capsid protein, and RNA-dependent RNA polymerase (RDRP) activity. Evidence of viral infection was not recovered upon removal of the drug. While viral RNA transcripts were produced from electroporated expression vectors, as determined by reverse transcription-PCR, viral antigens were not detected, suggesting that the antiviral effects of hygromycin B are mediated through translation inhibition. A short-term selection study suggests that the LRV1-4 elimination may not only be a function of hygromycin B as a protein synthesis inhibitor but also possibly related to the mechanism of hygromycin B resistance in Leishmania strains.