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. marginale

Abstract

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-γ 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-β1 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.

Maintenance of Antibody to Pathogen Epitopes Generated by Segmental Gene Conversion Is Highly Dynamic during Long-Term Persistent Infection

“…However, antigen-specific CD4 ϩ T lymphocytes clearly provide some of the signals for this transition. Consistent with this hypothesis is the observa- tion with A. marginale that, although robust CD4 ϩ T-cell responses specific to both the HVR and conserved domains can be induced by MSP2 immunization (1), these cannot be consistently detected during actual infection (2). Dissecting the mechanism underlying the short-lived antibody responses, both from a structural basis of the HVR and the immunologic basis, will further enhance our understanding of how highly antigenically variant pathogens persist within the infected host.…”

Identification of Novel Antigenic Proteins in a ComplexAnaplasma marginaleOuter Membrane Immunogen by Mass Spectrometry and Genomic Mapping

“…This protection is associated with high IgG2 titers against the immunodominant major surface protein 2 (MSP2) (16) and CD4 ϩ T-lymphocyte proliferative responses to MSP1, MSP2, and MSP3 (16) as well as MSP4 (W. C. Brown, unpublished observations). However, none of these individual proteins has induced protection equivalent to that of the complex outer membrane immunogen (1,16,43,44,48,49,58). This discrepancy in ability to induce protection suggests at least two possible explanations.…”

Unraveling the immune regulatory mechanisms imposed by Anaplasma

“…This experiment confirmed that the infection-induced loss of response was not restricted to the variable MSP2 antigen, but occurred using a different immunization protocol and a conserved protein epitope. Furthermore, for both studies, Anaplasma-specific T cell responses were not detected in peripheral blood lymphocytes from control sham immunized animals following challenge (Abbott et al, 2005; W. C. Brown, unpublished data).…”

“…These results suggest the sudden loss of MSP2-specific T cell responses in the peripheral blood was not simply due to an increase in regulatory T cells in peripheral blood in response to infection (Abbott et al, 2005). However, the spleens were not evaluated for functional suppressor T cells, which if present, could explain these results, as the spleen is important for removing infected erythrocytes during hemoparasite infections.…”

“…However, immunization models using A. marginale MSPs have recently revealed infection-induced regulation of a strong pre-challenge CD4 + T cell response. In the first model, cattle were hyperimmunized by subcutaneous inoculation of gel-purified native MSP2 in Th1 adjuvants and challenged 6 months later with organisms expressing the same major MSP2 variant that was present in the immunogen (Abbott et al, 2005). The predicted protection against infection with organisms expressing this major MSP2 variant was not realized; rather the lack of protection against anemia and rickettsemia was accompanied by a dramatic loss in MSP2-specific CD4 + T cell responses.…”