To identify the genes encoding novel immunodominant antigens of
Theileria parva a λgt11 library of piroplasm genomic
DNA was immunoscreened with bovine recovery serum and a gene encoding
a 150 kDa antigen (p150) was identified. The
predicted polypeptide contains an N-terminal secretory signal sequence
and a proline-rich region of repeated amino acid
motifs. The repeat region is polymorphic between stocks of
T. parva in both copy number and sequence, and analysis of
the repeat region from 10 stocks of T. parva revealed 5 p150 variants.
A monoclonal antibody (mAb) against the T. parva
polymorphic immunodominant molecule (PIM) cross-reacted with the recombinant
p150. The p150 has sequence
homology with a PIM peptide sequence containing the anti-PIM mAb epitope.
Immunoelectron microscopy demonstrated that the p150 antigen, like PIM,
is located in the microspheres of the sporozoites and is exocytosed
following sporozoite invasion of the host lymphocyte. By
immunoelectron microscopy p150 was subsequently transiently
detectable on the sporozoite surface and in the lymphocyte cytosol.
Immunoblotting showed that p150 is also expressed
by the schizont stage, but at much lower levels compared to the sporozoite.
These results suggest a major role for p150
in the early events of host–sporozoite interaction.