Efficient use of a small genome to generate antigenic diversity in tick-borne ehrlichial pathogens

Ehrlichiae are responsible for important tick-transmitted diseases, includi ng anaplasmosis, the most prevalent tick-borne infection of livestock world wide, and the emerging human diseases monocytic and granulocytic ehrlichios is. Antigenic variation of major surface proteins is a key feature of these pathogens that allows persistence in the mammalian host, a requisite for s ubsequent tick transmission. In Anaplasma marginale pseudogenes for two ant igenically variable gene families, msp2 and msp3, appear in concert. These pseudogenes can be recombined into the functional expression site to genera te new antigenic variants. Coordinated control of the recombination of thes e genes would allow these two gene families to act synergistically to evade the host immune response.