Differential Infectivity and Division ofToxoplasma gondiiin Human Peripheral Blood Leukocytes

Abstract

When tachyzoites were incubated with human peripheral blood leukocytes in vitro, more monocytes and dendritic cells than neutrophils or lymphocytes were infected. Although tachyzoites were able to divide in each of these cell types, monocytes and dendritic cells were more permissive to rapid tachyzoite division than neutrophils or lymphocytes.Toxoplasma gondii is an obligate intracellular parasite that can infect any nucleated host cell by a process of active penetration (7). Acute toxoplasmosis is characterized by dissemination and intracellular growth of the tachyzoite in a variety of host organs. In the innate phase of the immune response, nonspecific inflammatory cells, including neutrophils, monocytes, and dendritic cells, are elicited to the site of infection. The ensuing response of these cells allows for the establishment of the specific cell-mediated immune response that provides for long-term protection in the host against recurrent infection (13). Previous studies have reported that human neutrophils and monocytes become infected by tachyzoites. However, for each cell type there are conflicting reports of the subsequent fate of these intracellular parasites; both parasite stasis (14,16,30) and parasite division (15, 19) are described. In a recent study it was shown that there was markedly less uptake and division of tachyzoites in adherent monocytes than in nonadherent monocytes (9). In the present study the response of human peripheral blood leukocytes to tachyzoites was evaluated.Infection of human leukocytes is parasite dose dependent. Dendritic cells, monocytes, lymphocytes, and neutrophils were isolated from peripheral blood from toxoplasma-seronegative donors under endotoxin-free conditions. To obtain dendritic cells, mononuclear cells were isolated from blood by density gradient centrifugation (2:1 [vol/vol] ratio of blood to 1.07 g of Ficoll-Hypaque per ml; 500 ϫ g for 20 min). Washed cells were plated for 2 h at 37°C in tissue culture flasks, and then nonadherent cells were removed. The adherent cells were cultured in medium supplemented with 2,000 U of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; generously supplied by Immunex, Seattle, Wash.) and 20 ng of recombinant human interleukin-4 (rhIL-4; PeproTech Inc., Rocky Hill, N.J.) per ml. Every 3 days the medium in these flasks was supplemented with rhGM-CSF and rhIL-4. At day 8, the cells demonstrating dendritic cell morphology (nonadherent with projections) were harvested, and their phenotype was characterized by flow cytometry as described previously (27).Monocytes and lymphocytes were isolated from peripheral blood mononuclear cells obtained by cytophoresis carried out with volunteers as described previously (2). Briefly, monocytes were enriched to 85 to 95% by aggregation of washed cytophoresed cells at 4°C. Supernatants containing enriched lymphocytes were removed and purified further (see below). Platelets were removed from monocytes by washing cells twice in Versene buffer (0.2 g of EDTA per ml in phos...

Motile invaded neutrophils in the small intestine ofToxoplasma gondii-infected mice reveal a potential mechanism for parasite spread

“…Nevertheless, we demonstrate a striking overrepresentation of neutrophils among T. gondii-invaded leukocytes in the small intestine. Consistent with this finding, T. gondii has been shown to survive and replicate in human neutrophils, and a high proportion of neutrophils in T. gondii-infected lymph nodes contain parasites (13,38,39). These findings may reflect direct targeting of neutrophils by the parasite, for example through release of parasite-derived neutrophil chemotactic factors (40).…”

Avirulent strains of Toxoplasma gondii infect macrophages by active invasion from the phagosome

“…This interplay could then underlie a more robust innate and adaptive immune activation, leading to effective control of avirulent parasites. Additionally, avirulent T. gondii strains preferentially infect mononuclear cells, including naive resident macrophages (this report), monocytes (29,30) and neutrophils (31), and also differentially induce hypermigratory phenotype in infected macrophages and dendritic cells (10,32). It is tempting to speculate that these maneuvers together represent a constellation of adaptive responses characteristic of avirulent T. gondii.…”

Host cell manipulation by the human pathogen Toxoplasma gondii

“…The molecular mechanism of T. gondii dissemination through the host is unclear. However, T. gondii appears to preferentially invade DCs in vitro rather than other blood cells [115]. Building on this observation, Lambert and colleagues recently tested the possibility that circulating capacity of DCs is exploited by T. gondii as a means of promoting dissemination during acute infection.…”