Leishmaniavirus Type 1 Enhances In Vitro Infectivity and Modulates the Immune Response to Leishmania (Viannia) Isolates

<i>Leishmania RNA virus</i> 1 (<i>LRV-1</i>) is a double-stranded RNA virus identified in several <i>Leishmania</i> spp. <i>LRV-1</i> has been associated with increased disease severity and therapeutic failure in cutaneous leishmaniasis (CL). Although <i>LRV-1</i> has been reported in the Americas, its influence on parasite infectivity and host immune responses remains poorly characterized in Panamanian isolates. In this study, we investigate the in vitro infectivity and immunomodulatory effects of <i>LRV-1</i>-positive (<i>LRV-1</i>⁺) versus <i>LRV</i>-1-negative (<i>LRV-1</i>^-) isolates of <i>Leishmania (Viannia)</i>, including clinical strains of <i>L. (V.) panamensis</i> and <i>L. (V.) guyanensis</i>. A total of 21 isolates (nine <i>LRV-1</i>⁺, nine <i>LRV-1</i>^-, and three reference strains) were used to infect human U937 macrophages. The infectivity index (II) was measured at 24, 48, and 72 h post-infection. Cytokine levels of TNF-α, IFN-γ, IL-4, IL-6, IL-10, and IL-17 were quantified by flow cytometry, and IL-1β by ELISA at 24 and 48 h. <i>LRV-1</i>⁺ isolates exhibited significantly higher infectivity at 48 h (mean II = 1386.2) and 72 h (mean II = 1316.8) compared to <i>LRV-1</i>^- isolates (mean II = 714.4 and 571.0, respectively; <i>p</i> < 0.001). Two <i>L. (V.) panamensis LRV-1</i>⁺ isolates associated with complicated CL cases displayed the highest II values. Cytokine analysis revealed that <i>LRV-1</i>⁺ isolates induced elevated TNF-α (<i>p</i> < 0.01) and IL-1β (<i>p</i> < 0.001), along with reduced IFN-γ (<i>p</i> < 0.01), while no significant differences were observed for IL-4, IL-6, IL-10, or IL-17. These findings indicate that <i>LRV-1</i> enhances parasite infectivity and promotes a pro-inflammatory cytokine profile, which may contribute to disease persistence and treatment failure.