Tiia Snäkä

Preclinical mouse models are critical for understanding the pathophysiological response to infections and developing treatment strategies for sepsis. In keeping with ethical values, researchers follow guidelines to minimize the suffering of the mice. Weight loss is a criteria used as a humane end point, but there is no official recommendation for a maximum weight loss leading to euthanasia. To evaluate whether the thresholds used in daily practice are optimal, we performed a comprehensive retrospective analysis of data generated over 10 years with > 2300 mice used in models of infection with Listeria monocytogenes, Streptococcus pneumoniae, Candida albicans and H1N1 influenza virus. Weight loss segregated mice that survived from those that did not. Statistical analyses revealed that lowering the weight loss thresholds used (none, 30% or 20%) would have increased mortality rates due to the sacrifice of mice that survived infections (p < 0.01–0.001). Power calculations showed high variability and reduction of power as weight loss thresholds approached 20% for S. pneumoniae and L. monocytogenes models. Hence, weight loss thresholds need to be adapted to each model of infection used in a laboratory. Overall, weight loss is a valuable predictor of mortality that contributes to the robustness of composite scores. To our knowledge, this is the most extensive study exploring the relationship between weight loss threshold and sepsis outcome. It underscores the importance of the infection-model-specific evaluation of weight loss for use in clinical scores defining humane endpoints to minimize mouse suffering without compromising statistical power and scientific objectives.

Leishmania parasites preferentially invade macrophages, the professional phagocytic cells, at the site of infection. Macrophages play conflicting roles in Leishmania infection either by the destruction of internalized parasites or by providing a safe shelter for parasite replication. In response to invading pathogens, however, macrophages induce an oxidative burst as a mechanism of defense to promote pathogen removal and contribute to signaling pathways involving inflammation and the immune response. Thus, oxidative stress plays a dual role in infection whereby free radicals protect against invading pathogens but can also cause inflammation resulting in tissue damage. The induced oxidative stress in parasitic infections triggers the activation in the host of the antioxidant response to counteract the damaging oxidative burst. Consequently, macrophages are crucial for disease progression or control. The ultimate outcome depends on dangerous liaisons between the infecting Leishmania spp. and the type and strength of the host immune response.

The oxidative burst generated by the host immune system can restrict intracellular parasite entry and growth. While this burst leads to the induction of antioxidative enzymes, the molecular mechanisms and the consequences of this counter-response on the life of intracellular human parasites are largely unknown. The transcription factor NF-E2-related factor (NRF2) could be a key mediator of antioxidant signaling during infection due to the entry of parasites. Here, we showed that NRF2 was strongly upregulated in infection with the human Leishmania protozoan parasites, its activation was dependent on a NADPH oxidase 2 (NOX2) and SRC family of protein tyrosine kinases (SFKs) signaling pathway and it reprogrammed host cell metabolism. In inflammatory leishmaniasis caused by a viral endosymbiont inducing TNF-α in chronic leishmaniasis, NRF2 activation promoted parasite persistence but limited TNF-α production and tissue destruction. These data provided evidence of the dual role of NRF2 in protecting both the invading pathogen from reactive oxygen species and the host from an excess of the TNF-α destructive pro-inflammatory cytokine.

Candida albicans is a major cause of fungal diseases in humans, and its resistance to available drugs is of concern. In an attempt to identify novel antifungal agents, we initiated a small-scale screening of a library of 199 natural plant compounds (i.e., natural products [NPs]). In vitro susceptibility profiling experiments identified 33 NPs with activity against C. albicans (MIC ₅₀ s ≤ 32 μg/ml). Among the selected NPs, the sterol alkaloid tomatidine was further investigated. Tomatidine originates from the tomato ( Solanum lycopersicum ) and exhibited high levels of fungistatic activity against Candida species (MIC ₅₀ s ≤ 1 μg/ml) but no cytotoxicity against mammalian cells. Genome-wide transcriptional analysis of tomatidine-treated C. albicans cells revealed a major alteration (upregulation) in the expression of ergosterol genes, suggesting that the ergosterol pathway is targeted by this NP. Consistent with this transcriptional response, analysis of the sterol content of tomatidine-treated cells showed not only inhibition of Erg6 (C-24 sterol methyltransferase) activity but also of Erg4 (C-24 sterol reductase) activity. A forward genetic approach in Saccharomyces cerevisiae coupled with whole-genome sequencing identified 2 nonsynonymous mutations in ERG6 (amino acids D249G and G132D) responsible for tomatidine resistance. Our results therefore unambiguously identified Erg6, a C-24 sterol methyltransferase absent in mammals, to be the main direct target of tomatidine. We tested the in vivo efficacy of tomatidine in a mouse model of C. albicans systemic infection. Treatment with a nanocrystal pharmacological formulation successfully decreased the fungal burden in infected kidneys compared to the fungal burden achieved by the use of placebo and thus confirmed the potential of tomatidine as a therapeutic agent.

More than 35 million people remain infected with HIV-1. Upon antiretroviral therapy cessation, HIV-1-positive individuals systematically fail to achieve sustained virological remission, revealing the presence of a reservoir. This reservoir takes into account anatomical sanctuaries where HIV-1 continues to replicate, and latently infected cells also known as the latent reservoir (LR). A better understanding of the nature and features of the LR and its quantification are crucial to evaluate the efficiency of therapeutic strategies aiming at purging HIV-1. Culture- and PCR-based assays have already been implemented to measure the LR, and new assays are continuously being developed. In this review, we will discuss these methods highlighting the difficulties to accurately measure the LR, one main obstacle in curing HIV-1.