Cathelicidin contributes to prompt protective inflammation against Toxoplasma gondii
Date
2019-07-10
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Abstract
Toxoplasma gondii is an intracellular parasite infecting all warm-blooded animals, including humans. Although infected immunocompetent individuals are usually asymptomatic, in immunocompromised individuals, T. gondii can affect the central nervous system and may cause congenital toxoplasmosis and death. Cathelicidins are short cationic peptides secreted by leukocytes and epithelial cells with antimicrobial and immunomodulatory activities. It has been proposed that cathelicidin might be a key defense against intracellular pathogens. For instance, human cathelicidin (LL-37), either endogenous or synthetic exogenous, reduced survival of intracellular Mycobacterium tuberculosis in macrophages. However, the role of cathelicidin in toxoplasmosis has been barely investigated. The objective was to elucidate the contributions of cathelicidin during acute systemic and long-term infection with T. gondii. In an acute generalized model of toxoplasmosis, C57BL/6 cathelicidin-deficient (Camp-/-) and wild type (Camp+/+) mice were challenged with luciferase-green fluorescence protein tagged T. gondii (high virulence Type 1 RH, 1 x 105, intra-peritoneal). Although all mice had succumbed by day 5 post infection, Camp-/- mice failed to initiate pro-inflammatory responses in vital organs (ileum, colon, liver, spleen and brain; p<0.05) at early infection (1 d). Consequently, more parasite load was detected in those vital organs (p<0.05). In long-term toxoplasmosis, we determined that Camp-/- mice were more susceptible to oral challenge with T. gondii cysts (low-virulence Type 2 ME 49); all Camp-/- infected mice died by day 12 post infection (p<0.05), whereas Camp+/+ infected and PBS treated mice survived throughout the 14 d study. These Camp-/- mice had more severe hepatitis, with evident liver necrosis and increased inflammatory cytokines, Ifn-γ and Tnf-α gene expression, than Camp+/+ mice (p<0.05). In Camp-/- infected mice, there was pronounced inflammation and Ifn-γ gene synthesis in their spleen (p<0.05) and in their ileum, more severe epithelial disruption, with fewer goblet cells. In in vitro studies, human macrophages (THP-1) infected by T. gondii (ME 49) expressed elevated endogenous gene transcriptional cathelicidin (p<0.05) together with induced gene transcriptional expression of IFN-γ and TNF-α (p<0.05). Camp-/- bone marrow derived macrophages (BMMs) challenged with T. gondii (RH or ME 49) secreted more Tnf-α (p<0.05) compared to infected Camp+/+ BMMs. The T. gondii burdens (both RH and ME 49 strains) were similar between Camp+/+ and Camp-/- BMMs. However, when THP-1 cells macrophages were pre-stimulated with recombinant human LL-37 cathelicidin (2 uM), T. gondii burden was reduced (p<0.05). In summary, this study identified the critical role of cathelicidin in initiating host immune responses, promptly during toxoplasmosis. In the absence of cathelicidin, inflammatory responses in vital organs (liver and spleen) were exaggerated at later infection (3 and 14 d) and they were detrimental to the host. Immunomodulatory roles of cathelicidin were manifested in infected macrophages, where the peptide supressed exaggerated inflammatory responses and aided macrophage killing capabilities. Cathelicidin secreted by macrophages during T. gondii infection has a vital role in host-pathogen balance, controlling parasite burden and preventing overwhelming inflammatory responses.
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Keywords
Toxoplasma gondii, Cathelicidin, innate immunity, macrophages, infection, inflammation
Citation
Tan, Y. L. (2019). Cathelicidin contributes to prompt protective inflammation against Toxoplasma gondii (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.