African trypanosomiasis is one of the most important hemoprotozoan diseases affecting both humans and livestock in sub-Saharan Africa. A blunted erythropoietin (Epo) response has been proposed as one possible mechanism that contributes to the nonresponsive anemia in trypanosome-infected cattle. The overall objective of this study was to examine the pathophysiologic role of Epo in the anemia associated with Trypanosoma congolense infection in cattle at the molecular and cellular levels. Bovine Epo cDNA from N’Dama cattle (Bos taurus) and Boran cattle (Bos indicus) was isolated by polymerase chain reaction (PCR) amplification and by screening of a bovine kidney cDNA library. Using in situ hybridization, Epo mRNA was detected in interstitial fibroblast-like cells in the kidney of an anemic calf (created by phlebotomy). Epo mRNA was also detected in the spleen from the anemic calf and the spleen of a 19 week old bovine fetus using quantitative PCR assay. The bovine Epo cDNA was expressed in three different expression systems: bacterial, vaccinia virus (VV), and Chinese Hamster Ovary (CHO) cells. The recombinant bovine Epo (rbovEpo) expressed in CHO cells was more biologically active than that expressed in the other systems. The purified rbovEpo was used to develop a hybridoma secreting high affinity monoclonal antibodies to rbovEpo. Competitive reverse transcription and the PCR (RT/PCR) were used to compare the concentrations of Epo mRNA in the kidney during chronic and acute infection with T. congolense in trypanotolerant (N’Dama) and trypanosusceptible (Boran) cattle. The Boran cattle were more anemic, with packed cell volumes (PCV) ranging between 14-21% while N'Dama cattle had PCV values between 19-36%. Surprisingly, the increases in Epo mRNA did not vary significantly in the kidneys from the two breeds. However, the amount of Epo receptor transcripts in bone marrow during acute infection of N’Damas was significantly (P <0.05) higher than that detected in the marrows of Borans. These findings suggest that Boran cattle are incapable of eliciting an appropriate Epo response for their degree of anemia. Negative regulators of Epo gene expression, such as IL-1α and β, TNFα and IFNγ were expressed in the kidneys from chronically infected Boran cattle at significantly (p < 0.05) higher concentration than in the kidneys from chronically infected N’Dama cattle. Furthermore, during acute infection with 7. congolense the concentration of IFNγ mRNA in the kidney from Borans was significantly higher - as were the mRNAs of IL1α and β, and IFNγ - in the bone marrow from Boran cattle compared to that from N’Dama cattle (p < 0.05). Nucleotide sequence analysis of the 3’ untranslated region (UTR) of the N’Dama and Boran Epo cDNA sequences revealed a polymorphism. Also a single position mutation of Try ——> His close to a unique phosphoserine motif was identified on the predicted peptide sequence of Epo receptor in Boran cattle. These may contribute to the differences in the erythropoietic response observed in the two breeds of cattle. These are the first observations that link a genetic marker (Epo and EpoR polymorphism) to a phenotypic criterion (PCV) for trypanotolerance of cattle. / Ph. D.