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Infection and Immunity, October 2007, p. 4909-4916, Vol. 75, No. 10
0019-9567/07/$08.00+0     doi:10.1128/IAI.00710-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Influence of Host Immunity on Parasite Diversity in Theileria parva{triangledown}

Frank Katzer,1 Daniel Ngugi,1 Christian Schnier,2 Alan R. Walker,1 and Declan J. McKeever1,2*

Centre for Tropical Veterinary Medicine, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG, United Kingdom,1 Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, Midlothian EH26 0PZ, United Kingdom2

Received 25 May 2007/ Returned for modification 26 June 2007/ Accepted 9 July 2007

We examined the influence of host immunity on the genotypic diversity of the intracellular transforming cattle parasite Theileria parva. By tracking the emergence of discrete parasite genotypes in an animal challenged with a bulk stabilate following immunization with its major component clone, we observed a profound modulation of genotypic frequencies in the breakthrough schizont population. In particular, no incidences of the immunizing clone were observed and a progressive decline was apparent in the relatedness of breakthrough genotypes to it. These observations were reflected in the genotypic profile of transmissible parasite stages that emerged in the erythrocyte fraction of the animal and in parasite progeny generated by tick pickup. In a separate experiment, genotypic profiles of breakthrough parasite populations were observed to vary between unrelated immune animals selected on the basis of the major histocompatibility complex (MHC) class I phenotype, a known determinant of the specificity of the immune response. Furthermore, immunization and challenge of calves with molecularly distinct but cross-protective parasite populations revealed that infection results in transmissible erythrocyte forms in spite of a protective immune response. These observations suggest that immunity does not prevent transmission of challenge parasites and that its impact on the parasite at a population level is influenced by herd MHC diversity.

* Corresponding author. Present address: Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, United Kingdom. Phone: 44 1707 666318. Fax: 44 1707 666402. E-mail: dmckeever{at}rvc.ac.uk

{triangledown} Published ahead of print on 16 July 2007.

Editor: W. A. Petri, Jr.

Infection and Immunity, October 2007, p. 4909-4916, Vol. 75, No. 10
0019-9567/07/$08.00+0     doi:10.1128/IAI.00710-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.





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