Loss of Immunization-Induced Epitope-Specific CD4 T-Cell Response following Anaplasma marginale Infection Requires Presence of the T-Cell Epitope on the Pathogen and Is Not Associated with an Increase in Lymphocytes Expressing Known Regulatory Cell Phenotypes

Abstract

We have shown that in cattle previously immunized with outer membrane proteins, infection with Anaplasma marginale induces a functionally exhausted CD4 T-cell response to the A. marginale immunogen. Furthermore, T-cell responses following infection in nonimmunized cattle had a delayed onset and were sporadic and transient during persistent infection. The induction of an exhausted T-cell response following infection presumably facilitates pathogen persistence. In the current study, we hypothesized that the loss of epitope-specific T-cell responses requires the presence of the immunizing epitope on the pathogen, and T-cell dysfunction correlates with the appearance of regulatory T cells. In limited studies in cattle, regulatory T cells have been shown to belong to ␥␦ T-cell subsets rather than be CD4 T cells expressing forkhead box protein P3 (FoxP3). Cattle expressing the DRB3*1101 haplotype were immunized with a truncated A. marginale major surface protein (MSP) 1a that contains a DRB3*1101-restricted CD4 T-cell epitope, F2-5B. Cattle either remained unchallenged or were challenged with A. marginale bacteria that express the epitope or with A. marginale subsp. centrale that do not. Peripheral blood and spleen mononuclear cells were monitored for MSP1a epitope F2-5B-specfic T-cell proliferative responses and were stained for ␥␦ T-cell subsets or CD4 cells before and during infection. As hypothesized, the induction of T-cell exhaustion occurred only following infection with A. marginale, which did not correlate with an increase in either CD4؉ CD25 ؉ FoxP3 ؉ T cells or any ␥␦ T-cell subset examined.A naplasma marginale is a tick-borne intraerythrocytic rickettsial pathogen found in most temperate and tropical regions of the world and causes significant anemia and a mortality rate of up to 30% in naive cattle. Cattle that survive acute disease remain persistently infected for life with cyclic, but microscopically undetectable, levels of bacteremia that do not cause clinical disease (1). Of note, the antigen load in animals during acute and persistent infection is high, reaching 10 9 bacteria per ml of blood during acute infection and 10 7 bacteria per ml of blood in recurrent peaks during persistent infection (2). The mechanisms by which A. marginale is capable of persisting in the immunocompetent host have not been completely elucidated. A. marginale undergoes extensive antigenic variation in immunodominant and abundant major surface protein 2 (MSP2) and MSP3 by gene conversion of whole pseudogenes and segments of pseudogenes into a single expression site (3). Antigenic variation in MSP2, which is rich in T-and B-lymphocyte epitopes, allows the organism to escape specific adaptive immune responses and contributes to persistence (4-7).Our studies have shown that infection of A. marginale in cattle previously immunized with either native MSP2 or recombinant MSP1a resulted in a complete loss of functional CD4 ϩ T-cell responses to the specific immunogen beginning near the peak of acute infection (7,8). The T cells wer...