A further investigation of the leishmaniosis outbreak in Madrid (Spain): low-infectivity phenotype of the Leishmania infantum BOS1FL1 isolate to establish infection in canine cells

Human leishmaniosis caused by Leishmania infantum is a zoonotic disease, with dogs as the main reservoir in Mediterranean Basin countries. The largest European outbreak of human leishmaniosis declared in the southwestern Madrid region (Spain) is characterized by unusual epidemiological and clinical features, such as the emergence of new wild reservoirs (hares and rabbits), whereas the seroprevalence, infection, and severity of canine leishmaniosis have not substantially changed since the first studies conducted in Madrid before the outbreak. Previous studies reported that L. infantum isolates from the Madrid leishmaniosis focus displayed elevated virulence in in vivo models of infection and increased infectivity in murine target cells. With the aim of studying whether changes in the host-parasite interaction and virulence profile have developed, we first assessed the behaviour of one circulating isolate of the outbreak, IPER/ES/2012/BOS1FL1 (BOS1FL1), compared to that of a well-characterized strain from canine leishmaniosis, MCAN/ES/1996/BCN150 (BCN150), in terms of infection capacity (percentage of infected cells, representing infectivity, and number of amastigotes per infected cell, representing the intensity of infection) in canine monocytes and macrophages. BCN150 displayed significantly higher infectivity (76.82 ± 4.40 vs 38.58 ± 2.19; P 0.0001) and intensity of infection (3.64 ± 0.13 vs 1.83 ± 0.12; P 0.0001) than BOS1FL1 when interacting with canine cells. Our ROS induction results did not differ significantly between the two isolates or with the responses previously described for other L. infantum isolates. Paradoxically, increased resilience to hydrogen peroxide exposure was observed for BOS1FL1 (% viability 40.62 ± 5.54 vs 26.37 ± 2.93; P = 0.039). Finally, we demonstrated that a decreased intracellular load of BOS1FL1 was associated with increased IFN-γ (261.21 ± 26.29 vs 69.80 ± 9.02; P = 0.0151) and decreased IL-10 production (165.06 ± 23.87 vs 264.41 ± 30.58; P = 0.0002). In this study, we provide the first detailed insight into the differences between the isolate BOS1FL1 from the outbreak in Madrid and the well-characterized strain BCN150 MON-1 obtained from a dog in their response to interacting with canine cells. However, further studies are necessary to shed light on the immune mechanisms resulting in BOS1FL1 exhibiting less virulent behaviour in canine cells than in cells derived from other host species.