Oxygen and immunity to Leishmania infection

Abstract

Oxygen availability plays a fundamental role in shaping immune responses to infections. Leishmaniasis, a disease caused by protozoan parasites of the genus Leishmania, manifests in a spectrum of clinical outcomes, ranging from localized cutaneous lesions to life-threatening visceral infections. Like many infections and chronic diseases, Leishmania-infected tissues are characterized by hypoxia. Despite the recognized importance of oxygen in immune regulation, our understanding of how hypoxia shapes the immune landscape in leishmaniasis remains in its early stages. Collectively, the published studies in leishmaniasis highlight the critical role of oxygen availability and hypoxia-inducible factor (HIF) in orchestrating immune responses, particularly within myeloid cells. Here, we review the literature on how oxygen availability and HIF signaling influence the immune response in leishmaniasis. By consolidating existing findings and identifying gaps in knowledge, we aim to inspire further research into the interplay between oxygen availability, immune function, disease progression, and therapeutic potential in leishmaniasis.

Keywords: hypoxia; immune responses; leishmaniasis; oxygen; parasites.

Fig 1

Regulation of HIF-α under normoxic and hypoxic conditions. (a) Normoxia: In the presence of sufficient oxygen, the oxygen-dependent enzymes, prolyl hydroxylase domain proteins and factor inhibiting HIF-1 have access to their substrate to facilitate the hydroxylation of HIF-α, rendering it inactive. When PHDs hydroxylate HIF-α, it is recognized by the VHL E3 ubiquitin ligase complex, which ubiquitinates HIF-α, targeting it for proteasomal degradation. Additionally, when FIHs hydroxylate HIF-α, it inhibits the interaction with a transcriptional co-activator, p300. (b) Hypoxia: Under low oxygen conditions, PHDs and FIH are inhibited due to reduced availability of oxygen as a substrate. This allows HIF-α to become stabilized, where it will translocate to the nucleus to dimerize with HIF-1β. The HIF-α/HIF-1β heterodimer recruits co-activators, such as p300, to bind HREs in the promoter regions of target genes. This interaction drives the transcription of hypoxia-responsive genes involved in processes such as angiogenesis, metabolism, and cell survival, adapting the cellular response to low oxygen levels.

Fig 2

Mechanisms of HIF-α stabilization in Leishmania infection through hypoxia-dependent and -independent conditions. (a) Development of hypoxia: hypoxia can result as a by-product of immune responses to infection that require oxygen to fulfill. One major contributor to the development of hypoxia in Leishmania infection is through the production of ROS by neutrophils. (b) HIF-α stabilization independent of hypoxia: while the canonical signaling of HIF-α occurs through hypoxia, HIF-α stabilization can occur in the presence of adequate oxygen. This noncanonical signaling can be mediated through intracellular iron dysregulation facilitated by the inherent need for Leishmania to scavenge host cell iron to survive.

Fig 3

Role of HIF-α in host defense to Leishmania infection under hypoxic, physioxic, and normoxic conditions. In vivo studies (potentially hypoxia-dependent HIF stabilization): mice with myeloid-specific HIF-1α deletion (Lysm^CreHif1a^fl/fl) infected with L. major subcutaneously (s.c.) exhibit larger lesions and increased parasite burdens due to reduced iNOS expression. Additionally, mice with myeloid-specific HIF-1β deletion (Lysm^CreHif1b^fl/fl) infected intradermally (i.d.) with L. major develop larger lesions, but the parasite burden remains unchanged. This is attributed to increased VEGF-A production, which exacerbates lesion growth through changes in vascularization but does not affect parasite replication. Physioxia: in Leishmania-infected cells cultured at physiological oxygen levels (5% oxygen), HIF-1α/HIF-1β stabilization enhances NADPH oxidase production, leading to increased ROS production and enhances iNOS expression, but this does not confer to higher NO levels as infected macrophages show diminished NO production. Normoxia: macrophages with HIF-1α deficiency (Lysm^CreHif1a^fl/fl) exhibit increased lipogenesis, which allows for a greater survival advantage to support parasite replication, suggesting that HIF-1α plays a protective role in restricting parasite growth under normal oxygen levels.

Fig 4

Effects of oxygen levels and HIF signaling on host susceptibility during Leishmania infection. Systemic hypoxia: wild-type (WT) mice infected subcutaneously (s.c.) with L. major exposed to systemic hypoxia exhibit a reduction in NO due to a decrease in oxygen levels needed for iNOS to turnover NO production. This dampening of NO leads to an increase in parasite burdens, as it is a critical effector molecule for Leishmania control. Hypoxia: in vitro studies demonstrate that hypoxic conditions in macrophages reduce NO production, impairing the ability to control L. major infection. Additionally, intradermal (i.d.) infection of WT mice with L. major creates highly inflamed and hypoxic regions. CD8 T cells that have newly migrated from the draining lymph node to the hypoxic skin upregulate granzyme B production. These granzyme B-positive CD8 T cells do not control parasites and only exacerbate inflammation and delay wound healing. In vivo studies (potentially hypoxia-dependent HIF stabilization): mice with dendritic cell-specific deletion of HIF-1α (CD11c^Cre Hif1a^fl/fl) infected intravenously (i.v.) with L. donovani exhibit altered cytokine profiles, including increased IL-12 and decreased IL-10 production. This cytokine shift promotes the generation of short-lived effector (Eff) CD8 T cell and CD4 Th1 cell responses, which are both necessary for the control of visceral disease. This study highlights the impact of HIF-1α signaling in dendritic cells, where it controls the production of IL-10 while dampening IL-12 production, which benefits Leishmania survival and pro-inflammatory immune evasion. Hyperoxia: in vitro studies of Leishmania-infected macrophages cultured under hyperoxic conditions show increased ROS production, leading to enhanced parasite killing. These data suggest that high oxygen levels favor parasite clearance, whereas low oxygen levels provide a greater opportunity for parasite survival.