Leishmania Infection-Induced Proteolytic Processing of SIRPα in Macrophages

Oxidative stress-driven enhanced iron production and scavenging through Ferroportin reorientation worsens anemia in antimony-resistant Leishmania donovani infection

Despite the withdrawal of pentavalent-antimonials in treating Visceral leishmaniasis from India, recent clinical isolates of Leishmania donovani (LD) exhibit unresponsiveness towards pentavalent-antimony (LD-R). This antimony-unresponsiveness points towards a genetic adaptation that underpins LD-R's evolutionary persistence and dominance over sensitive counterparts (LD-S). This study highlights how LD evolutionarily tackled antimony exposure and gained increased potential of scavenging host-iron within its parasitophorous vacuoles (PV) to support its aggressive proliferation. Even though anti-leishmanial activity of pentavalent antimonials relies on triggering oxidative outburst, LD-R exhibits a surprising strategy of promoting reactive oxygen species (ROS) generation in infected macrophages. An inherent metabolic shift from glycolysis to Pentose Phosphate shunt allows LD-R to withstand elevated ROS by sustaining heightened levels of NADPH. Elevated ROS levels on the other hand trigger excess iron production, and LD-R capitalizes on this surplus iron by selectively reshuffling macrophage-surface iron exporter, Ferroportin, around its PV thereby gaining a survival edge as a heme-auxotroph. Higher iron utilization by LD-R leads to subsequent iron insufficiency, compensated by increased erythrophagocytosis through the breakdown of SIRPα-CD47 surveillance, orchestrated by a complex interplay of two proteases, Furin and ADAM10. Understanding these mechanisms is crucial for managing LD-R-infections and their associated complications like severe anemia, and may also provide valuable mechanistic insights into understanding drug unresponsiveness developed in other intracellular pathogens that rely on host iron.

The expression of signal regulatory protein alpha (SIRPα) in periodontal cells and tissue

Signal regulatory protein alpha (SIRPα) is mainly expressed by cells of myeloid origin. This membrane glycoprotein is shown to be involved in regulation of different inflammatory conditions, such as colitis and arthritis. However, SIRPα has not been investigated in relationship to periodontitis, an inflammatory condition affecting the tooth supporting tissues. We aim to investigate if resident cells in the periodontium express SIRPα and whether a possible expression is affected by inflammatory conditions. Primary human keratinocytes, fibroblasts, periodontal ligament cells, and osteoblasts were cultured with or without the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-α) or interleukin-1-beta (IL-1β). All different periodontal cell types showed a basal mRNA expression of SIRPα. Pro-inflammatory cytokines induced a 2-3-fold significant increase in SIRPα expression in both cultured human gingival fibroblasts and osteoblasts but neither in keratinocytes nor in periodontal ligament cells. Tissue sections from human gingival tissue biopsies were histochemically stained for SIRPα. Epithelial keratinocytes and gingival fibroblasts stained positive in sections from periodontally healthy as well as in sections from periodontitis. In periodontitis sections, infiltrating leukocytes stained positive for SIRPα. We highlight our finding that oral keratinocytes, gingival fibroblasts, and periodontal ligament cells do express SIRPα, as this has not been presented before. The fact that inflammatory stimulation of gingival fibroblasts increased the expression of SIRPα, while an increased expression by gingival fibroblasts in periodontitis tissue in situ could not be detected, is indeed contradictory.

Access and utilization of host-derived iron by Leishmania parasites

Iron is involved in many biochemical processes including oxygen transport, ATP production, DNA synthesis and antioxidant defense. The importance of iron also applies to Leishmania parasites, an intracellular protozoan pathogen causing leishmaniasis. Leishmania are heme-auxotrophs, devoid of iron storage proteins and the heme synthesis pathway. Acquisition of iron and heme from the surrounding niche is thus critical for the intracellular survival of Leishmania inside the host macrophages. Moreover, Leishmania parasites are also exposed to oxidative stress within phagolysosomes of macrophages in mammalian hosts, and they need iron superoxide dismutase for overcoming this stress. Therefore, untangling the strategy adopted by these parasites for iron acquisition and utilization can be good targets for the development of antileishmanial drugs. Here, in this review, we will address how Leishmania parasites acquire and utilize iron and heme during infection to macrophages.