Search

An antigen, BMA7, which induced partial immunity against tick infestation has been isolated from Boophilus microplus using two different protein fractionation protocols, accompanied by vaccination and parasite challenge trails. The antigen is a 63 kDa glycoprotein isolated from semi-engorged adult female ticks. Though significant, the induced immunity is less striking than that previously reported for antigen Bm86 from the same parasite. However, co-vaccination with Bm86 and BMA7 can enhance immunity over that seen with a commercial vaccine based on Bm86 alone. Limited peptide sequence information shows significant variation in the BMA7 protein occurs. The antigen has approximately 36 kDa of glycosylation, in both N-linked and O-linked oligosaccharides. There is evidence that both polypeptide and oligosaccharide are antigenic, but the chemical nature of the protective antigenic sites is not clear. There is little or no immunological response to the antigen during natural infestation with parasites, suggesting the antigen is 'concealed' and protective immunity dependent on artificial vaccination. The antigen has some similarities with the vertebrate mucins. It is widely distributed in tick tissues and membrane bound but its function is currently unknown.

A protein, Bm91, which was first identified as a protective vaccine antigen from the tick Boophilus microplus, has regions of very strong amino acid sequence similarity to mammalian carboxydipeptidases or angiotensin converting enzymes (ACE; E.C. 3.4.15.1). This protein is now shown to share many biochemical and enzymatic properties with mammalian carboxydipeptidases. It is enzymatically active in a conventional assay for ACE using hippuryl-Gly-Gly as substrate. The hydrolysis of the C-terminal nonapeptide of the insulin B chain proceeds by sequential removal of carboxy-terminal dipeptides. The similarities extend to the dependence of activity on pH and added salt. Bm91 is inhibited by two well-characterized inhibitors of the mammalian enzymes, the drug Captopril and a nonapeptide, and the inhibition occurs in similar concentration ranges to those effective with the mammalian enzymes. However, the natural substrates of the tick enzyme are unknown. Angiotensin I itself is a poor substrate and the enzyme's natural substrates are likely to be one or more of the pharmacologically active peptides occurring in insects and arthropods.

A membrane protein that can be used successfully to vaccinate cattle against the tick Boophilus microplus has been purified and characterized. The mature protein, which is referred to as Bm91, has an apparent m.w. of 86,000, an isoelectric point of 4.8 to 5.2, and is glycosylated, with an affinity for lentil lectin. Bm91 is of relatively low abundance, with approximately 300 to 400 micrograms being recovered from 1 kg of semiengorged adult female ticks. The protein is located largely in the salivary gland and gut of these ticks. Partial amino acid sequence data for the protein show striking similarities to that of mammalian angiotensin-converting enzyme, suggesting that the Ag may have an enzymatic function. The protein seems not to be recognized by sera from cattle with extensive exposure to ticks under natural conditions. The immunity induced by vaccination, therefore, represents another example of vaccination against a hematophagous parasite with "concealed" Ags.

Boophilus microplus contains a nucleotidase-like enzyme which is able to hydrolyze a range of nucleoside 5'-mono-, di- and triphosphates to the nucleoside. Its relationship to several other nucleotide hydrolyzing enzymes has been explored. Limited peptide sequencing shows similarities to both mammalian nucleotidases and the Escherichia coli uridine diphosphate sugar hydrolase. The tick enzyme also hydrolyzes UDP-glucose, though by a mechanism different to that of the bacterial enzyme. On the other hand, it resembles the mammalian nucleotidases in that there is evidence that it is attached to the cell membrane by a glycosyl-phosphatidylinositol (GPI) anchor.