Antigen-specific CD4+T cells in Anaplasma marginale infection of calves

Acquired T-cell immunity is central for protection against intracellular infection. However, the role of antigen-specific T cells in persistent systemic disease is poorly understood. We investigated this with the ruminant bacterium Anaplasma marginale, which replicates to 109 bacteria/ml of blood during acute infection and 106 bacteria/ml throughout persistent anaplasmosis. We established that immunization-specific CD4+ T cell responses were rapidly and permanently lost following challenge. To determine the fate of antigen-specific CD4+ T cells we enumerated specific T cells with DRB3*1101 MHC class II tetramers and FACS. Immunization produced antigen-specific T cell frequencies of 0.025% of CD4+ T cells that rapidly declined near peak bacteremia to 0.0003%. Low frequencies of tetramer+ CD4+ T cells in spleen, liver, and lymph nodes 9-12 weeks post-challenge indicated lack of sustainable antigen-specific responses in these lymphoid compartments. Infection of cattle with A. marginale, therefore, caused rapid, permanent loss of antigen-specific T cells, which may be a strategy of immune modulation for A. marginale. We next examined the role of specific-CD4+ T cells primed by infection by observing serum, PBMC, and spleen for antigen-specific humoral and CD4+ T cell responses during acute and persistent infection of na ve calves. Calves developed high titers of antigen-specific IgG day 9 post-infection, but specific T cells appeared in PBMC day 39 and as late as day 112 post-infection. Responding T cells were predominantly CD4+ T cells that did not produce IFN-[gamma], and responses were transient, disappearing rapidly and reappearing sporadically 8 or less times in one year. Splenocytes developed initial antigen-specific T cell responses days 48 and 89 post-infection, and responses were similarly transiently with no evidence of sequestration during infection. In conclusion, A. marginale-specific CD4+ T cells primed by infection occur in PBMC and spleen during acute and persistent infection well after emergence of high titers of specific IgG, and cells are continuously down regulated similar to immunization-specific T cells, in the face of persistently high level bacteremia