Persistent infection contributes to heterologous protective immunity against fatal ehrlichiosis.

Human monocytotropic ehrlichiosis (HME), an emerging and often life-threatening tick-transmitted disease, is caused by the obligately intracellular bacterium Ehrlichia chaffeensis. HME is modeled in C57BL/6 mice using Ehrlichia muris, which causes persistent infection, and Ixodes ovatus Ehrlichia (IOE), which is either acutely lethal or sublethal depending on the dose and route of inoculation. A persistent primary E. muris infection, but not a sublethal IOE infection, protects mice against an ordinarily lethal secondary IOE challenge. In the present study, we determined the role of persistent infection in maintenance of protective memory immune responses. E. muris-infected mice were treated with doxycycline or left untreated and then challenged with an ordinarily lethal dose of IOE. Compared to E. muris-primed mice treated with doxycycline, untreated mice persistently infected with E. muris had significantly greater numbers of antigen-specific gamma interferon-producing splenic memory T cells, significant expansion of CD4 CD25 T regulatory cells, and production of transforming growth factor 1 in the spleen. Importantly, E. muris-primed mice treated with doxycycline showed significantly greater susceptibility to challenge infection with IOE compared to untreated mice persistently infected with E. muris. The study indicated that persistent ehrlichial infection contributes to heterologous protection by stimulating the maintenance of memory T-cell responses. Human monocytotropic ehrlilchiosis (HME), an emerging tick-borne disease in the United States, is caused by the obligately intracellular bacterium Ehrlichia chaffeensis, which resides in the host mononuclear phagocytic cells (24, 34). HME manifests initially as a mild flulike illness with nonspecific symptoms (12, 22), and the disease can progress to a severe life-threatening condition resulting from liver dysfunction, multiorgan failure, meningoencephalitis, and adult respiratory distress syndrome (11, 25, 26, 28). Immune-mediated pathology is attributed in severe cases of HME to the severity of inflammation observed in the absence of high numbers of ehrlichiae in the tissues (28). It is unknown whether E. chaffeensis establishes persistent infection in humans, although a few studies reported prolonged infection with E. chaffeensis (9, 27). It is also unknown whether immunocompetent patients recovered from HME are immune to reinfection with a genetically identical or distinct E. chaffeensis strain. However, a case of reinfection with a distinct strain of E. chaffeensis after a 2-year interval in a liver transplant patient who was receiving immunosuppressive therapy has been reported (21). In a previous study, using murine models of persistent ehrlichiosis and fatal ehrlichiosis, we have established that ehrlichial species that differ in their ability to cause acute or chronic persistent infection also differ in stimulation of a longterm protective memory immune response (33). Ehrlichia muris, a mildly virulent Ehrlichia which causes persistent infection in C57BL/6 mice, but not sublethal infection with highly virulent Ixodes ovatus Ehrlichia (IOE), which causes acute infection only, induces strong memory immune responses and confers protection against an ordinarily lethal IOE challenge. The cross-protection induced by persistent E. muris infection is associated with the generation and maintenance of strong Ehrlichia-specific IFN--producing CD4 and CD8 memory Tcell responses in the spleen and production of Ehrlichia-specific serum immunoglobulin G (IgG) responses (33). The correlation between persistent primary E. muris infection and heterologous protection against secondary lethal IOE challenge suggested that persistent infection or antigen persistence could play a role in the maintenance of protective immunity. Antigen persistence or continuous exposure to low levels of antigen is thought to be required for maintenance of protective memory immune responses (3, 14, 37). In the present study we addressed the role of persistent ehrlichial infection in conferring protection against lethal heterologous ehrlichiosis. The results suggested that persistence of E. muris infection in C57BL/6 mice contributes to protective heterologous immunity against IOE infection by stimulating maintenance of memory T-cell and Ehrlichia-specific antibody responses.