| Infectivity of Anaplasma spp. develops when infected ticks feed on a mammalian host (transmission feed). Anaplasma marginale Major Surface Protein (MSP) 2 is expressed from a 3.5kb operon that contains, in a 5′ to 3′ direction, four open reading frames, opag3, opag2, opag1, and msp2. Restriction and expression of MSP2 in the tick occurs early in the midgut within the first 48h of the blood meal, when ticks acquire infection. A. marginale is present in the tick salivary glands before transmission feeding is initiated, but the msp2 mRNA and MSP2 protein levels per A. marginale increase only minimally and transiently in salivary glands of transmission-feeding ticks, when compared to unfed ticks. A. marginale numbers per tick increase gradually in salivary glands both of transmission-fed and unfed ticks. The operon structure is conserved among five genotypically and phenotypically distinct A. marginale strains, A. ovis, and A. eentrale. The individual OpAG amino acid sequences are highly conserved among A. marginale strains. OpAG2 and OpAG3 are expressed by all examined A. marginale strains during the acute rickettsemia in the mammalian host and, similar to MSP2, localize to the bacterial surface. OpAG2 and OpAG3 were also expressed in an infected Ixodes scapularis tick cell line. In contrast, using the same A. marginale strains and two different Dermacentor spp. only OpAG2 was expressed in transmission feeding ticks. OpAG1 expression was not detected in the infected mammalian host, the infected tick cell line, or within infected Dermacentor spp. ticks. MSP2 variant selection is an early event in the tick and the specific MSP2 variants SGV1 and SGV2 are expressed both in the midgut and salivary glands. In addition, the differential expression of outer membrane proteins from within an operon is a novel finding in tick-transmitted bacteria and dissecting how this expression is regulated may be broadly applicable to understanding how the pathogen adapts to the transition between the mammalian host and the tick vector. |
