Co-infection with Leptomonas seymouri enhances macrophage survival and promotes intracellular parasite persistence during Leishmania donovani infection

Abstract

BackgroundLeishmania donovani (LD) is an obligate intracellular parasite that survives and replicates within macrophages. Leptomonas seymouri (LS), a traditionally monoxenous trypanosomatid, has been repeatedly co-isolated with LD from kala-azar cases in India, often together with Leptomonas seymouri narna-like virus 1 (Lepsey NLV1). Whether LS can survive and replicate within mammalian macrophages, and how co-infection influences parasite and virus dynamics, remain unresolved. Methods and FindingsIntracellular survival, replication, and revival of LS alone and during co-infection with LD were systematically evaluated using murine (RAW 264.7) and human (THP-1) macrophages. Quantitative ITS1 PCR demonstrated significant increases in intracellular parasite DNA over 48-168 hours post-infection (h.p.i), indicating active replication rather than persistence. Reduced extracellular count suggested restricted cell lysis and enhanced macrophage survival in co-infection compared to mono-infections. Giemsa staining confirmed intracellular localization of LS. Amastigotes from infected macrophages revived as motile promastigotes upon transformation, whereas extracellular parasites failed to revive beyond 48 hours, confirming macrophages as the exclusive niche for prolonged viability. Co-infection dampened macrophage IL-12 (p40) production, augmenting macrophage survival. MTT assay confirmed the same. In THP-1, co-infection resulted in a marked increase in intracellular LS count and viral load than LS mono-infection at 96 and 168 h.p.i. relative to 48 h.p.i., suggesting that LD co-infection favors higher intracellular virus titres. ConclusionsOur findings demonstrated that LS could replicate within mammalian macrophages and persist during mono-or LD co-infection. The identification of a stable LD-LS-virus interaction highlights a previously underappreciated "triple-pathogen" biology with potential implications for LD pathogenesis.