Leishmania, the causative agent of leishmaniases, is an intracellular parasite of macrophages, transmitted to humans via the bite of its sand fly vector. This protozoan organism has evolved strategies for efficient uptake into macrophages and is able to regulate phagosome maturation in order to make the phagosome more hospitable for parasite growth and to avoid destruction. As a result, macrophage defenses such as oxidative damage, antigen presentation, immune activation and apoptosis are compromised whereas nutrient availability is improved. Many Leishmania survival factors are involved in shaping the phagosome and reprogramming the macrophage to promote infection. This review details the complexity of the host–parasite interactions and summarizes our latest understanding of key events that make Leishmania such a successful intracellular parasite.

The phagosome is a central mediator of both the homeostatic and microbicidal functions of a macrophage. Following phagocytosis, Mycobacterium tuberculosis (Mtb) is able to establish infection through arresting phagosome maturation and avoiding the consequences of delivery to the lysosome. The infection of a macrophage by Mtb leads to marked changes in the behavior of both the macrophage and the surrounding tissue as the bacterium modulates its environment to promote its survival. In this study, we use functional physiological assays to probe the biology of the phagosomal network in Mtb-infected macrophages. The resulting data demonstrate that Mtb modifies phagosomal function in a TLR2/TLR4-dependent manner, and that most of these modifications are consistent with an increase in the activation status of the cell. Specifically, superoxide burst is enhanced and lipolytic activity is decreased upon infection. There are some species- or cell type-specific differences between human and murine macrophages in the rates of acidification and the degree of proteolysis. However, the most significant modification is the marked reduction in intra-phagosomal lipolysis because this correlates with the marked increase in the retention of host lipids in the infected macrophage, which provides a potential source of nutrients that can be accessed by Mtb.

All natural Leishmania infections start in the skin; however, little is known of the contribution made by the sand fly vector to the earliest events in mammalian infection, especially in inflamed skin that can rapidly kill invading parasites. During transmission sand flies regurgitate a proteophosphoglycan gel synthesized by the parasites inside the fly midgut, termed promastigote secretory gel (PSG). Regurgitated PSG can exacerbate cutaneous leishmaniasis. Here, we show that the amount of Leishmania mexicana PSG regurgitated by Lutzomyia longipalpis sand flies is proportional to the size of its original midgut infection and the number of parasites transmitted. Furthermore, PSG could exacerbate cutaneous L. mexicana infection for a wide range of doses (10–10,000 parasites) and enhance infection by as early as 48 hours in inflamed dermal air pouches. This early exacerbation was attributed to two fundamental properties of PSG: Firstly, PSG powerfully recruited macrophages to the dermal site of infection within 24 hours. Secondly, PSG enhanced alternative activation and arginase activity of host macrophages, thereby increasing L-arginine catabolism and the synthesis of polyamines essential for intracellular parasite growth. The increase in arginase activity promoted the intracellular growth of L. mexicana within classically activated macrophages, and inhibition of macrophage arginase completely ablated the early exacerbatory properties of PSG in vitro and in vivo. Thus, PSG is an essential component of the infectious sand fly bite for the early establishment of Leishmania in skin, which should be considered when designing and screening therapies against leishmaniasis.

Iron-sensitive fluorescent chemosensors in combination with digital fluorescence spectroscopy have led to the identification of a distinct subcellular compartmentation of intracellular redox-active "labile" iron. To investigate the distribution of labile iron, our research has been focused on the development of fluorescent iron sensors targeting the endosomal/lysosomal system. Following the recent introduction of a series of 3-hydroxypyridin-4-one (HPO) based fluorescent probes we present here two novel HPO sensors capable of accumulating and monitoring iron exclusively in endosomal/lysosomal compartments. Flow cytometric and confocal microscopy studies in murine macrophages revealed endosomal/lysosomal sequestration of the probes and high responsiveness toward alterations of vesicular labile iron concentrations. This allowed assessment of cellular iron status with high sensitivity in response to the clinically applied medications desferrioxamine, deferiprone, and deferasirox. The probes represent a powerful class of sensors for quantitative iron detection and clinical real-time monitoring of subcellular labile iron levels in health and disease.