Production of IFN-γ by CD4 T Cells Is Essential for Resolving Ehrlichia Infection

Abstract

To address the role of cellular immunity during ehrlichia infection, we have used a newly described model of monocytic ehrlichiosis that results from infection of mice by an ehrlichia that was isolated from an Ixodes ovatus tick (Ixodes ovatus ehrlichia, IOE). Immunocompetent C57BL/6 and BALB/c mice exhibited a dose-dependent susceptibility to IOE infection. Mice infected with a high dose inoculum (∼1000 organisms) exhibited pronounced thrombocytopenia, lymphopenia, anemia, and morbidity within 12 days postinfection. Infection was associated with bacterial colonization of a number of tissues. In contrast, mice infected with a low dose inoculum (∼100 organisms) exhibited only transient disease and were able to resolve the infection. SCID mice were highly susceptible to low-dose infection, indicating that adaptive immunity was required. Resistance to sublethal challenge in both C57BL/6 and BALB/c mice was CD4-, but not CD8-, dependent and required IL-12p40-dependent cytokines, IFN-γ, and TNF-α, but not IL-4. CD4 T cells purified from infected mice proliferated in vitro in response to IOE Ags. T cell proliferation was associated with production of IFN-γ, and the production of this cytokine by CD4 T cells rescued IFN-γ-deficient mice from fatal infection. Exogenous IFN-γ was capable of inducing microbiocidal activity in infected macrophages. The data suggest that classical immune mechanisms involving CD4 cells and type 1 cytokines are responsible for macrophage activation and for elimination of this intracellular bacterial pathogen.

Fatal Recall Responses Mediated by CD8 T cells during Intracellular Bacterial Challenge Infection

“…Although exposure to a pathogen typically generates protective immunity, recall responses to low-dose IOE infection were found to be ineffective, because low-dose IOE-challenged mice were not protected against a high-dose challenge infection (19). This was not due to an inability of mice to generate protective memory responses, given that it has been possible to generate protection against high-dose IOE infection after heterologous prior infection with a closely related ehrlichia, Ehrlichia muris (21).…”

“…Other studies from our laboratory support an essential role for Abs during IOE infection, a requirement that is not typical of intracellular bacterial infections (20). In contrast, CD8 T cells were not essential for protection against a low-dose IOE infection (19).…”

“…Our previous studies demonstrated that low-dose IOE infection (0.5 ϫ LD 50 ; ϳ100 bacteria) did not provide protection against a subsequent high-dose (2 ϫ LD 50 ) fatal challenge infection (19). In an attempt to boost protective responses in IOE-immunized mice, we administered a secondary low-dose inoculum ϳ4 wks following primary low-dose IOE challenge.…”

“…Spleens were harvested and processed as described previously (19). CD8 T cells were purified from spleen tissue by magnetic bead negative selection using a murine T cell subset purification kit, according to the protocol provided by the manufacturer (R&D Systems).…”

“…Studies of host resistance and susceptibility in the mouse have used E. chaffeensis, as well as closely related monocytotropic ehrlichia isolates that cause a range of different experimental outcomes, including bacterial clearance, persistent infection, and fatal disease (15)(16)(17)(18). Using one such agent, Ixodes ovatus ehrlichia (IOE), 3 we previously demonstrated that CD4 ϩ T cell-mediated IFN-␥-dependent production is critical for host defenses (19). Other studies from our laboratory support an essential role for Abs during IOE infection, a requirement that is not typical of intracellular bacterial infections (20).…”