Ly6Chigh Monocytes Control Cerebral Toxoplasmosis

Abstract

Cerebral infection with the parasite Toxoplasma gondii is followed by activation of resident cells and recruitment of immune cells from the periphery to the CNS. In this study, we show that a subset of myeloid cells, namely Ly6ChighCCR2+ inflammatory monocytes that infiltrate the brain upon chronic T. gondii infection, plays a decisive role in host defense. Depletion of this monocyte subset resulted in elevated parasite load and decreased survival of infected mice, suggesting their crucial role. Notably, Ly6ChighCCR2+ monocytes governed parasite control due to production of proinflammatory mediators, such as IL-1α, IL-1β, IL-6, inducible NO synthase, TNF, and reactive oxygen intermediate. Interestingly, Ly6ChighCCR2+ monocytes were also able to produce the regulatory cytokine IL-10, revealing their dual feature. Moreover, we confirmed by adoptive transfer that the recruited monocytes further develop into two distinct subpopulations contributing to parasite control and profound host defense. The differentiated Ly6CintCCR2+F4/80int subset upregulated MHC I and MHC II molecules, suggesting dendritic cell properties such as interaction with T cells, whereas the Ly6CnegF4/80high cell subset displayed elevated phagocytic capacity while upregulating triggering receptor expressed on myeloid cells-2. Finally, we have shown that the recruitment of Ly6Chigh monocytes to the CNS is regulated by P-selectin glycoprotein ligand-1. These results indicate the critical importance of recruited Ly6Chigh monocytes upon cerebral toxoplasmosis and reveal the behavior of further differentiated myeloid-derived mononuclear cell subsets in parasite control and immune regulation of the CNS.

Behavior of Neutrophil Granulocytes during Toxoplasma gondii Infection in the Central Nervous System

“…In the chronic stage of the infection, brain resident microglia cells displayed an activated phenotype, and peripheral immune cells including CD11b − lymphoid and CD11b + myeloid cells infiltrated the brain. In line with previous reports, characterization of brain immune cells revealed that recruited CD11b + myeloid cells contained Ly6G − inflammatory monocytes and Ly6G + neutrophil granulocytes (Möhle et al, 2014; Biswas et al, 2015). Despite low absolute numbers of neutrophils in the chronic stage, they form a substantial portion of the myeloid cell compartment in the CNS in the acute stage of the infection.…”

“…Alongside the enhanced egress of immune cells in the periphery, cerebral toxoplasmosis leads to the activation of brain resident cells and infiltration of circulating immune cells to the CNS (Möhle et al, 2014; Biswas et al, 2015). Peripheral immune cell influx into the infected brain is characterized by initial invasion of neutrophils, followed by the recruitment of BM-derived monocytes and the lymphocytes.…”

“…Peripheral blood was obtained from posterior vena cava and lysed with erythrocyte (RBC) lysis buffer (eBioscience). Subsequently, these cells were stained with the desired fluorescent conjugated antibodies (Biswas et al, 2015). …”

“…Cells were permeabilized using Permeabilization Buffer (Biolegend). To measure the cytokine expression, the staining was performed with the following antibodies: IL-1α (ALF-161), TNF (MP6-XT22), NOS2 (CXNFT), IFN-γ (XMG1.2), IL-1β (NJTEN3), and IL-10 (JES5-16E3) from eBioscience in Permeabilization Buffer (Biolegend) (Biswas et al, 2015). …”

“…In addition, peripheral immune cell subsets of myeloid and lymphoid origin are recruited to the inflamed CNS (John et al, 2011). Recently, we characterized the behavior of infiltrating CD11b + Ly6C hi myeloid derived cells, which carry out a pivotal role upon cerebral toxoplasmosis (Möhle et al, 2014, 2016; Biswas et al, 2015). Several studies have highlighted the role of brain-resident cells, lymphocytes, DCs, and macrophages (Strack et al, 2002; John et al, 2011), however, the function of neutrophil granulocytes in host defense in the CNS is still uncertain.…”