Theileria parva and the bovine CTL response: down but not out?

Abstract

SUMMARYTheileria parva is a tick-borne intracellular protozoan of cattle, with obligate sequential differentiation stages in lymphocytes and erythrocytes. Immunity is mediated by cytotoxic T lymphocytes (CTL) that target and clear parasitized lymphocytes but allow persistence of infected erythrocytes, which are required for transmission to the tick. The life cycle of T. parva is haploid with the exception of a brief diploid stage in the tick vector during which sexual recombination occurs. There is evidence for antigenic diversity in field parasite populations, although broad immunity can be acquired following exposure to a limited number of strains. The CTL response in individual animals is tightly focused and its specificity is strongly influenced by major histocompatibility complex (MHC) phenotype. This review discusses the issue of how CTL immunity is likely to impact on parasite population structure in the light of available information on diversity of the parasite and its ability to recombine. Keywordscattle; immune selection; population structure; Theileria parva REVIEWIn host-parasite systems that involve the reciprocal application of evolutionary pressure, or co-evolution, it has been argued that an equilibrium establishes over time between host resistance and pathogen virulence (1). This equilibrium may be based on avirulent parasites and low investment in resistance by the host, or high levels of host resistance and highly virulent parasites. Where host resistance arises from adaptive immunity, co-evolution is driven by maintenance of immune recognition by the host on the one hand and avoidance of recognition by the parasite on the other (2).Like other vector-borne pathogens, Theileria parasites face selective pressure in both ruminant host and tick vector. Much of this pressure derives from immune mechanisms involving polymorphic recognition and/or effector molecules (3,4). The genus incorporates a number of species with the capability of transforming nucleated cells to a state of uncontrolled proliferation -a trait unique among eukaryotes. One of these parasites, T. parva, causes a severe and often fatal lymphoproliferative disease of cattle known as East Coast fever (ECF) in eastern, central and southern Africa. Transmitted by Rhipicephalus appendiculatus ticks, the disease exerts substantial economic impact in the region. This arises not only from mortalities, but also from losses in production and the costs of acaricides for tick control, which is the principal means of prevention. In addition, the disease constrains dairy and beef sectors in endemic areas by limiting the introduction of Increasing prevalence of acaricide resistance in tick populations has intensified the search for alternative control measures. Available vaccination options rely on infection with live parasites and simultaneous treatment with oxytetracycline -the so-called infection and treatment method (6). Widespread uptake of this approach has been constrained by its reliance on a cold chain and, because it gives rise to a ca...

Genotypic diversity, a survival strategy for the apicomplexan parasite Theileria parva

“…This is in line with the reported tight focus of the CTL response (see McKeever, 2006). This has very important implications for the sustained use of a given infection and treatment vaccine in the same geographical area.…”

East Coast Fever Caused by Theileria parva Is Characterized by Macrophage Activation Associated with Vasculitis and Respiratory Failure

“…Theileria parva is an intracellular apicomplexan parasite of Cape buffalo ( Syncerus caffer ) and Bos indicus and Bos taurus cattle in sub-Saharan Africa. Mortality rates are high in most imported Bos taurus breeds and indigenous breeds raised in non-endemic areas [ 1 ]. T .…”

The kinomes of apicomplexan parasites

“…The rodent malaria parasites P. berghei, P. chabaudi and P. yoelii are model organisms for the study of pathogenesis and parasite biology [20,21]. Both Theileria species are tick-borne haemoparasites of cattle, T. parva causing East Coast fever [22,23], and T. annulata causing tropical theileriosis [24]. Besides their economic importance, Theileria species are of great biological interest in the context of mammalian cell transformation, because they are the only intracellular eukaryotic pathogens known to reversibly transform their host cells through the hijacking of a number of its signalling pathways [25][26][27].…”