Neutrophil chemotactic factors secreted from Toxoplasma gondii

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

Toxoplasma gondii infection occurs through the oral route, but we lack important information about how the parasite interacts with the host immune system in the intestine. We used two-photon laser-scanning microscopy in conjunction with a mouse model of oral T. gondii infection to address this issue. T. gondii established discrete foci of infection in the small intestine, eliciting the recruitment and transepithelial migration of neutrophils and inflammatory monocytes. Neutrophils accounted for a high proportion of actively invaded cells, and we provide evidence for a role for transmigrating neutrophils and other immune cells in the spread of T. gondii infection through the lumen of the intestine. Our data identify neutrophils as motile reservoirs of T. gondii infection and suggest a surprising retrograde pathway for parasite spread in the intestine.

Dynamics of neutrophil migration in lymph nodes during infection

Although the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning-laser microscopy to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We found that neutrophils formed both small, transient and large, persistent swarms via a coordinated migration pattern. We provided evidence that cooperative action of neutrophils and parasite egress from host cells could trigger swarm formation. Neutrophil swarm formation coincided in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.

Natural killer cells act as early responders in an experimental infection with Neospora caninum in calves

The intracellular protozoan parasite Neospora caninum is a cause of abortion and congenital disease in cattle worldwide. We have previously shown that natural killer (NK) cells produce IFN-gamma in response to N. caninum tachyzoites in vitro. This study aimed to investigate the role of NK cells and other cellular immune responses in an experimental N. caninum infection model in calves. Phenotyping of peripheral blood lymphocytes showed a drop in the percentage of NK cells at days 4-6 after i.v. inoculation, followed by an increase in the percentage of both NK cells and CD8+ T cells which peaked at days 11-15. A whole blood flow cytometric assay showed that CD4+ T cells were the major IFN-gamma producing cells, but in the early stages of the infection both NK cells and CD8+ T cells contributed to IFN-gamma production. We also compared the ability of two different N. caninum antigen preparations--sonicated soluble antigens and intact heat-inactivated parasites--to induce proliferation and IFN-gamma production in various cell types. Heat-inactivated tachyzoites induced a 3.7 times greater increase in the number of IFN-gamma producing NK cells compared with sonicated soluble antigens. This indicated the presence of some NK cell-stimulating antigens in the intact tachyzoite that were absent from the sonicated soluble antigens. The heat-inactivated whole tachyzoites also inhibited gammadelta T cell proliferation while the soluble antigens from N. caninum did not. We believe this is the first time NK cells have been demonstrated to be early responders in N. caninum infection in calves.

Leishmania promastigotes release a granulocyte chemotactic factor and induce interleukin-8 release but inhibit gamma interferon-inducible protein 10 production by neutrophil granulocytes

Recent data from our laboratory suggest that neutrophil granulocytes (polymorphonuclear leukocytes [PMN]) can serve as host cells for Leishmania major in the early phase of infection. In line with these findings, an early influx of PMN to the infected tissues was shown by others to be associated with susceptibility to infection with L. major. The mechanisms underlying the initial PMN recruitment to the site of infection is poorly understood. In the present study we investigated whether Leishmania can influence PMN migration. Supernatants of Leishmania promastigotes were tested for their chemotactic activity using an in vitro chemotaxis assay. All Leishmania species tested (L. major, L. aethiopica, and L. donovani) displayed a marked chemotactic effect on human PMN. However, no effect on the migration of macrophages and NK cells was observed. Checkerboard analysis revealed that the observed PMN migration was due to chemotaxis rather than chemokinesis. Most of the chemotactic activity was found in fractions containing molecules with sizes between 10 and 50 kDa. Pretreatment of PMN with N-formyl-methionyl-leucyl-phenylalanine blocked the chemotactic activity of Leishmania supernatants up to 75%. In addition, we found that leishmanial contact induced the release of interleukin-8 (IL-8) and inhibited the production of gamma interferon-inducible protein 10 (IP-10) by PMN. These data suggest that infection with Leishmania promastigotes leads to PMN accumulation via the production of a chemotactic factor by the parasites, and this effect is amplified by the induction of IL-8 production in PMN. On the other hand, the inhibition of IP-10 production can lead to prevention of NK cell activation.

CXCR2 deficiency confers impaired neutrophil recruitment and increased susceptibility during Toxoplasma gondii infection

Neutrophil migration to the site of infection is a critical early step in host immunity to microbial pathogens, in which chemokines and their receptors play an important role. In this work, mice deficient in expression of the chemokine receptor CXCR2 were infected with Toxoplasma gondii and the outcome was monitored. Gene-deleted animals displayed completely defective neutrophil recruitment, which was apparent at 4 h and sustained for at least 36 h. Kit(W)/Kit(W-v) animals also displayed defective polymorphonuclear leukocyte migration, suggesting mast cells as one source of chemokines driving the response. Tachyzoite infection and replication were accelerated in CXCR2(-/-) animals, resulting in establishment of higher cyst numbers in the brain relative to wild-type controls. Furthermore, serum and spleen cell IFN-gamma levels in infected, gene-deleted mice were reduced 60-75% relative to infected normal animals, and spleen cell TNF-alpha was likewise reduced by approximately 50%. These results highlight an important role for CXCR2 in neutrophil migration, which may be important for early control of infection and induction of immunity during Toxoplasma infection.

Mice lacking the chemokine receptor CCR1 show increased susceptibility to Toxoplasma gondii infection

Chemokines are critical for the recruitment of effector immune cells to sites of infection. Mice lacking the chemokine receptor CCR1 have defects in neutrophil trafficking and proliferation. In the present study, we tested the susceptibility of CCR1 knockout mice to infection with the obligate intracellular protozoan parasite Toxoplasma gondii. In comparison with parental wild-type mice, CCR1(-/-) mice exhibited dramatically increased mortality to T. gondii in association with an increased tissue parasite load. No differences were observed in Ag-specific T cell proliferation or in cytokine responses between mutant and wild-type mice. However, the influx of PMNs to the peripheral blood and to the liver were reduced in CCR1(-/-) mice during early infection. Our results suggest that CCR1-dependent migration of neutrophils to the blood and tissues may have a significant impact in controlling parasite replication.

Experimental infection of murine splenic lymphocytes and granulocytes with Toxoplasma gondii RH tachyzoites

Toxoplasma gondii, an intracellular protozoan infecting many kinds of eukaryotic cells, has been used to experimentally infect macrophages, epithelial cells, fibroblasts, and various cancer cells, but rarely T and B lymphocytes or granulocytes. The present study was performed to determine the susceptibility of murine (BALB/c or CBA) splenic T and B lymphocytes, and granulocytes to infection with T. gondii RH tachyzoites. The ultrastructure of the infected host cells was observed by TEM, and the degree of intracellular parasite proliferation was quantified using 3H-uracil uptake assay. At 24 hrs post-culture, the host cell cytoplasm was found to contain 1 or 2, or a maximum of 7-8 tachyzoites. Infected T lymphocytes demonstrated a peripherally displaced nucleus, a parasitophorous vacuole enveloping the parasite, and an increased number of mitochondria. In B lymphocytes infected with tachyzoites, RER was not well developed compared to uninfected B lymphocytes. Uninfected granulocytes contained many electron-dense granules, but T. gondii-infected granulocytes demonstrated a decreased number of granules. Based on the 3H-uracil uptake assay, the susceptibility of T and B lymphocytes, and granulocytes, to infection with T. gondii tachyzoites was fairly high irrespective of cell type and strain of mouse. This strongly suggests deterioration in the functioning of infected host immune cells.

Proliferation of Toxoplasma gondii in human neutrophils in vitro

Human neutrophils were infected with tachyzoites of Toxoplasma gondii in vitro. Infection rates after 1 h of incubation were 38.6-52.0% and 16.0-25.8% in the presence or absence of specific antibody respectively. Apparently, neutrophils killed tachyzoites 18 h after infection in the presence of antibody, whereas tachyzoites proliferated in 25.0-35.0% of infected neutrophils in the absence of antibody. Total tachyzoite counts 18 h after infection were approximately 4 times as high as those 1 h after infection in the absence of antibody and complement. Complement also enhanced phagocytosis and killing of tachyzoites by neutrophils, but the effects were less than that of antibody. Superoxide anion was produced intensively within 1 h after infection, depending on the presence of antibody or complement. Neutrophils seem to be responsible for dissemination of tachyzoites in the host during the initial phase of Toxoplasma infection until antibody is produced.