Emergence of Anaplasma marginale Antigenic Variants during Persistent Rickettsemia

Abstract

Anaplasma marginale is an ehrlichial pathogen of cattle, in the order Rickettsiales, that establishes persistent cyclic rickettsemia in the infected host. Within each rickettsemic cycle, A. marginale expressing antigenically variant major surface protein 2 (MSP2) emerge. By cloning 17 full-length msp2 transcripts expressed during cyclic rickettsemia, we determined that emergent variants have a single, central hypervariable region encoding variant B-cell epitopes. The N- and C-terminal regions are highly conserved among the expressedA. marginale variants, and similar sequences define the MSP2 homologues in the agent of human granulocytic ehrlichiosis (HGE). This is in contrast to the MSP2 homologues in ehrlichial genogroup I pathogens, Ehrlichia chaffeensis, Ehrlichia canis, and Cowdria ruminantium, that have multiple hypervariable regions. By defining the variable and conserved regions, we were able to show that the single hypervariable region of A. marginale MSP2 encodes epitopes that are immunogenic and induce variant-specific antibody responses during persistent infection. These findings demonstrate that the MSP2 structural variants that emerge during each cycle of persistent rickettsemia are true antigenic variants, consistent with MSP2 antigenic variation as a mechanism ofA. marginale persistence.

Msp2 variation inAnaplasma phagocytophilum in vivodoes not stimulate T cell immune responses or interferon-γ production

“…The studies here provide evidence of the former cross-reactivity hypothesis as an explanation, but do not exclude the latter possibility. This observation is deserving of much more study, as (1) it could be associated with immune control of certain clones (Wang et al, 2004); (2) a similar mechanism for restricting A. marginale clonal growth is demonstrated (French et al, 1999); and (3) passive immunization with antibody partially protects against A. phagocytophilum challenge (Sun et al, 1997).…”

Insights into Mechanisms of Bacterial Antigenic Variation Derived from the Complete Genome Sequence of Anaplasma marginale

“…1). 2,[16][17][18] Although A. marginale is an obligate intracellular parasite, as are all bacteria in the order Rickettsiales, 19 the presence of cyclic bacteremia is clearly reminiscent of the cycles that occur in relapsing fever and African trypanosomiasis, caused by, respectively, an extracellular bacterium (Borrelia hermsii) and an extracellular protozoon (Trypanosoma brucei). 16,20 This similarity among genetically widely disparate organisms reflects the strong evolutionary pressure for development of a mechanism to generate antigenic variants and highlights the importance of antigenic variation in efficient vector-borne transmission.…”

“…20,21 In A. marginale infection, each cycle reflects the emergence of one or, more commonly, multiple clones that express a unique immunodominant outer-membrane protein, designated major surface protein (MSP)-2. 6,17,18 These variants express a unique surface-FIGURE 1. Anaplasma marginale persistent infection is characterized by sequential emergence, replication, and immune control of organisms expressing antigenically variant MSP2.…”

“…6,17,18 These represent true "escape variants" as they are not recognized by antibody present at the time of emergence and are controlled concomitant with the development of IgG2 directed against the specific HVR. 2,18 This cycle of emergence and control continues unabated, allowing life-long persistent A. marginale infection. 2,16,18,22…”

Structural basis for segmental gene conversion in generation of Anaplasma marginale outer membrane protein variants

“…In contrast, Anaplasma marginale and A. phagocytophilum , the causative agents of anaplasmosis in cattle and humans, respectively, do not contain plasmids and use chromosomally encoded pseudogenes as a template for recombination into single-expression sites via gene conversion (Brayton et al ., 2001;2002;Barbet et al ., 2003). In A. marginale , the resulting major surface protein 2 (MSP2) variants express distinct surface domains, encode unique B and T cell epitopes, and evade the immune response to allow lifelong persistence in the mammalian reservoir host -critically important for ixodid ticks to acquire and transmit the pathogen (French et al ., 1998;1999;Palmer et al ., 2000;Brown et al ., 2003).…”