Af. Barbet et al., Antigenic variation of Anaplasma marginale: Major surface protein 2 diversity during cyclic transmission between ticks and cattle, INFEC IMMUN, 69(5), 2001, pp. 3057-3066
The rickettsial pathogen Anaplasma marginale expresses a variable immunodom
inant outer membrane protein, major surface protein 2 (MSP2), involved in a
ntigenic variation and long-term persistence of the organism in carrier ani
mals, MSP2 contains a central hypervariable region of about 100 amino acids
that encodes immunogenic B-cell epitopes that induce variant-specific anti
bodies during infection. Previously, we have shown that MSP2 is encoded on
a polycistronic mRNA transcript in erythrocyte stages of A. marginale and d
efined the structure of the genomic expression site for this transcript. In
this study, we show that the same expression site is utilized in stages of
R marginale infecting tick salivary glands, We also analyzed the variabili
ty of this genomic expression site in Oklahoma strain A, marginale transmit
ted from in vitro cultures to cattle and between cattle and ticks. The stru
cture of the expression site and flanking regions was conserved except far
sequence that encoded the MSP2 hypervariable region. At least three differe
nt MSP2 variants were encoded in each A. marginale population. The major se
quence variants did not change on passage of A, marginale between culture,
acute erythrocyte stage infections, and tick salivary glands but did change
during persistent infections of cattle. The variant types found in tick sa
livary glands most closely resembled those present in bovine blood at the t
ime of acquisition of infection, whether infection was acquired from an acu
te or from a persistent rickettsemia, These variations in structure of an e
xpression site for a major, immunoprotective outer membrane protein have im
portant implications for vaccine development and for obtaining an improved
understanding of the mechanisms of persistence of ehrlichial infections in
humans, domestic animals, and reservoir hosts.