Reduced replication of Toxoplasma gondii is necessary for induction of bradyzoite-specific antigens: a possible role for nitric oxide in triggering stage conversion

Abstract

Stage conversion between tachyzoites and bradyzoites of Toxoplasma gondii was investigated in vitro by using murine bone marrow-derived macrophages (BMMs) as host cells. Following infection of untreated BMMs with tachyzoites, spontaneous expression of bradyzoite-specific antigens (Bsa) occurred at low frequency with Toxoplasma strain-dependent ratios from 0.03 to 2%. As previously described for peritoneal macrophages, activation of tachyzoite-infected BMMs with gamma interferon (IFN-gamma) or lipopolysaccharide resulted in the induction of Bsa. When bradyzoites were used for infection, prolonged expression of Bsa could be observed in IFN-gamma-activated BMMs. The induction of Bsa expression seemed to be closely linked to parasite multiplication and increased to maximal values of 50 to 70% in intermediately activated macrophages with nitric oxide (NO) levels that allowed reduced parasite replication. Identical results in stage conversion were obtained when sodium nitroprusside was used as a source of exogenous NO, indicating that NO might be a molecular trigger of stage conversion. NO is reactive with iron-sulfur centers in proteins, thereby inhibiting proteins involved in the mitochondrial respiratory chain. Using oligomycin and antimycin A as inhibitors of mitochondrial function, growth inhibition of parasites and induction of Bsa were obtained. Since microglia are the functional correlates of macrophages in the central nervous system and inhibit T. gondii upon activation with IFN-gamma, a similar mechanism might be involved during cyst development in the brain.

Disruption of the Toxoplasma gondii bradyzoite‐specific gene BAG1 decreases in vivo cyst formation

“…Y8 grew more slowly than both H7 and PLK, despite behaving similarly to PLK in vivo. When parasites were grown in sodium nitroprusside, a nitric oxide donor that induces bradyzoite formation (Bohne et al, 1994), all three strains grew at a similar rate ( Fig. 6B).…”

Withdrawal of skeletal muscle cells from cell cycle progression triggers differentiation ofToxoplasma gondiitowards the bradyzoite stage

“…The differentiation of T. gondii fast-replicating tachyzoites to dormant bradyzoites is accompanied by various morphological, metabolic, epigenetic and transcriptomic adaptations (Dubey et al, 1998;Dzierszinski et al, 2004;Radke et al, 2005;Bougdour et al, 2009). It is also closely linked to a growth shift towards a prolonged division time prior to the onset of cyst formation (Bohne et al, 1994;Radke et al, 2003). The environmental factors that trigger bradyzoite formation during in vivo infection have however remained elusive.…”

Chitinase Dependent Control of Protozoan Cyst Burden in the Brain

“…Toxoplasma gondii is a common intracellular protozoan parasite that forms a chronic infection in the brain for the lifetime of the host. The infection is controlled, in part, through the effector mechanisms of macrophages that result in the conversion of fast replicating tachyzoites to the slow replicating, cyst forming bradyzoites [1][2][3]. Cysts can form in all tissues but exist predominantly in the brain for the lifetime of the host requiring a continuous immune response to prevent cyst reactivation and Toxoplasmic encephalitis, a common cause of AIDS related fatalities [4,5].…”