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Rapid and Long-Term Disappearance of CD4⁺ T Lymphocyte Responses Specific for Anaplasma Marginale Major Surface Protein-2 (MSP2) in MSP2 Vaccinates following Challenge with Live A. marginale¹

Jeffrey R. Abbott^*, Guy H. Palmer^*, Kimberly A. Kegerreis^*, Peter F. Hetrick^*, Chris J. Howard, Jayne C. Hope and Wendy C. Brown^2,*

^* Program in Vector-Borne Disease, Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA 99164; and Compton Laboratory, Institute of Animal Health, Compton, Newbury, United Kingdom

In humans and ruminants infected with Anaplasma, the major surface protein 2 (MSP2) is immunodominant. Numerous CD4⁺ T cell epitopes in the hypervariable and conserved regions of MSP2 contribute to this immunodominance. Antigenic variation in MSP2 occurs throughout acute and persistent infection, and sequentially emerging variants are thought to be controlled by variant-specific Ab. This study tested the hypothesis that challenge of cattle with Anaplasma marginale expressing MSP2 variants to which the animals had been immunized, would stimulate variant epitope-specific recall CD4⁺ T cell and IgG responses and organism clearance. MSP2-specific T lymphocyte responses, determined by IFN- ELISPOT and proliferation assays, were strong before and for 3 wk postchallenge. Surprisingly, these responses became undetectable by the peak of rickettsemia, composed predominantly of organisms expressing the same MSP2 variants used for immunization. Immune responsiveness remained insignificant during subsequent persistent A. marginale infection up to 1 year. The suppressed response was specific for A. marginale, as responses to Clostridium vaccine Ag were consistently observed. CD4⁺CD25⁺ T cells and cytokines IL-10 and TGF-1 did not increase after challenge. Furthermore, a suppressive effect of nonresponding cells was not observed. Lymphocyte proliferation and viability were lost in vitro in the presence of physiologically relevant numbers of A. marginale organisms. These results suggest that loss of memory T cell responses following A. marginale infection is due to a mechanism other than induction of T regulatory cells, such as peripheral deletion of MSP2-specific T cells.

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