The role of the P28 multigene family in the pathogenesis of Ehrlichia chaffeensis infection.

Ehrlichia chaffeensis is a gram-negative obligate intracellular bacterium which parasitizes cells of the monocyte/macrophage lineage in mammals. E. chaffeensis possesses a unique cell wall that lacks LPS and the classical peptidoglycan found in other gram-negative bacteria. Ehrlichia reside inside unique membrane-bound vacuoles in the cytoplasm of host cells, known as morulae. E. chaffeensis is transmitted by lone star ticks, Amblyomma americanum, and is not transmitted transovarially, but is maintained in the environment by persistent infection of animal hosts. E. chaffeensis causes the emerging infectious disease, human monocytic ehrlichiosis (HME), a potentially fatal illness. Although easily treated with tetracycline, due to its obligate intracellular parasitism it is difficult to diagnose by routine clinical methods. E. chaffeensis expresses a 28 kDa outer membrane protein from a multigene family known as P28/OMP-1. Different alleles from this multigene family are expressed in different host cell types, but the role of this membrane protein remains unknown. Also, few host cell receptors, as well as host-pathogen protein interactions, have been identified for E. chaffeensis. Genetic manipulation of the family Anaplasmataceae has not been previously achieved, significantly hampering ehrlichial research. In this work, we describe a method for the transformation of the genus Ehrlichia, and apply this method to study the role of the P28 in ehrlichial pathogenesis by development of a knockout strain. Furthermore, we identify a novel host cell receptor implicated in ehrlichial binding and infection of host cells.