Contributed by Guest Blogger: Joseph Zaino ‘11

Recent research has found a unique relationship between the intracellular parasite, Leishmania, and it’s corresponding Leishmania RNA virus-1 (LRV1). Ives et. al. concluded that Leishmania parasites, in the presences of LRV1, suppressed the host immune response and strengthened the pathogen’s persistence. Leishmania infects the human immune system by attacking macrophages. The parasite causes the infection known as leishmaniasis, which is typically transmitted by sand files. This is a serious infection, affecting an estimated 12 million people in the Mediterranean basin, Africa, the Middle East, Asia, Central and South America. The strain of parasite investigated by this study was mucocutaneous leishmaniasis (MCL). MCL destroys the soft tissues of the face and nasopharyngeal regions, as well as damages host immune responses.

Leishmania parasites are dependent on proinflammatory protein mediators called Toll-like receptors (TLRs). TLRs are found in intracellular vesicles of the macrophage- presumably the same vesicles that host Leishmania. Ives et. al. confirmed that TLR3-TRIF dependent pathways are essential for macrophage infection by Leishmania. The unusual part is that TLRs usually help the mammalian immune system to eliminate pathogens. Specifically, TLR-3 recognizes the double stranded RNA of many viruses that are released from dead parasites, unable to survive within their host. Observations found that between virally infected and non-virally infected Leishmania, the virally infected ones were more likely to successfully infect a host. Similarly, metastasizing parasites had greater levels of the LRVI virus than non-metastasizing parasites. The authors verified this finding by treating macrophages with purified LRVI, and observing the same phenotypic infection as the viral-infected Leishmania. Further models concluded that when TLR3 is deleted from macrophages, parasitic persistence was diminished.

This apparent mutualism seems to benefit both Leishmania and the virus by allowing a more successful rate of host infection. Many Leishmania species have lost RNAi interference pathways, allowing viruses to inhibit and replicate within them. In this case, the virally infected parasite is more persistent against macrophages, and more damaging to the mammalian immune system. Thus, it is advantageous for the parasite to coexist with the LRV1 virus. If severe MCL infections are contingent on LRV1 for infection, then future research can perhaps focus on this relationship in order to better understand and cure leishmaniasis.