Protective Heterologous Immunity against Fatal Ehrlichiosis and Lack of Protection following Homologous Challenge

Abstract

The roles of antibodies and memory T cells in protection against virulent Ehrlichia have not been completely investigated. In this study, we addressed these issues by using murine models of mild and fatal ehrlichiosis caused by related monocytotropic Ehrlichia strains. Mice were primed with either Ehrlichia muris or closely related virulent ehrlichiae transmitted by Ixodes ovatus (IOE) ticks given intraperitoneally or intradermally. All groups were reinfected intraperitoneally, 30 days later, with a lethal high dose of IOE. Priming with E. muris, but not IOE, induced strong CD4؉ and CD8 ؉ memory type 1 T-cell responses, Ehrlichia-specific immunoglobulin G (IgG) antibodies, and persistent infection. Compared to IOE-primed mice, subsequent lethal IOE challenge of E. muris-primed mice, resulted in (i) 100% protection against lethal infection, (ii) strong Ehrlichiaspecific secondary gamma interferon (IFN-␥)-producing effector/effector memory CD4 ؉ and CD8 ؉ T-cell responses, (iii) enhanced secondary anti-ehrlichial antibody response, (iv) accelerated bacterial clearance, and (v) the formation of granulomas in the liver and lung. E. muris-primed mice challenged with IOE had lower levels of serum interleukin-1␣ (IL-1␣), IL-6, and IL-10 compared to unprimed mice challenged with IOE. Interestingly, the fatal secondary response in IOE-primed mice correlated with (i) decline in the Ehrlichiaspecific CD4؉ and CD8 ؉ type 1 responses, (ii) marked hepatic apoptosis and necrosis, and (iii) substantial bacterial clearance, suggesting that fatal secondary response is due to immune-mediated tissue damage. In conclusion, protection against fatal ehrlichial infection correlates with strong expansion of IFN-␥-producing CD4 ؉ and CD8 ؉ effector memory type 1 T cells, which appear to be maintained in the presence of IgG antibodies and persistent infection.

Persistent Infection Contributes to Heterologous Protective Immunity against Fatal Ehrlichiosis

“…Similar to our previous studies (17,33), naive mice challenged with either the lethal low dose or the high dose of IOE had extensive apoptotic cells in the liver parenchyma with few inflammatory cells on day 7 after IOE challenge (Fig. 6A).…”

“…Although comparable bacterial burdens in the organs were found after IOE challenge, E. muris-primed mice treated with doxycycline had greater tissue damage than untreated mice persistently infected with E. muris. The enhanced tissue damage in the absence of higher bacterial burdens after IOE challenge in E. muris-primed mice treated with doxycycline is similar to immune-mediated pathology associated with a complete lack of homologous protection against secondary IOE challenge in mice primed with sublethal doses of IOE (33). The latter does not cause persistent infection (4,33).…”

“…Our previous study had demonstrated that cross-protection of E. muris-primed mice and lack of homologous protection against lethal IOE challenge are associated with the presence and absence of persistent infection, respectively, and distinct memory immune responses (33). In the present study, we addressed the role of persistent infection in maintenance of protective memory immune responses during ehrlichial infections by antibiotic treatment of persistent E. muris infection prior to challenge with IOE.…”

“…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 ϩ and CD8 ϩ 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.…”

“…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.…”