Recombinant expression and evaluation of a- and b- tubulin from Trypanosoma congolense as vaccine candidates for African trypanosomiasis.

Abstract

African trypanosomiasis is caused by protozoan parasites known as trypanosomes, which are transmitted by the tsetse fly, affecting both humans and animals. Trypanosoma congolense is one of the main trypanosome species affecting cattle and causes the disease known as nagana. Control of animal African trypanosomiasis currently relies on chemotherapy and vector control methods, neither of which has proven satisfactory. An effective vaccine against trypanosomiasis would be the most cost effective solution to control the disease; however, due to the phenomenon of antigenic variation, intrinsic to the parasite’s outer coat of variable surface glycoprotein, this has not yet been achieved. Recent vaccine efforts have been centred on identification of invariant parasite antigens for use as vaccine candidates. Trypanosome cytoskeleton components have in recent years been shown to be capable of providing a protective immune response against trypanosome infection. These include tubulin proteins, which form the main components of the cytoskeleton, as well as microtubule associated proteins (MAPs) and paraflagellar rod proteins. In the present study α- and β-tubulin from T. congolense were recombinantly expressed and their immuno-protective potential in mice assessed. Amplification of both α- and β-tubulin ORFs from T. congolense genomic DNA was followed by cloning of the amplicons into the T-vector pTZ57R/T, and thereafter sub-cloning into the bacterial expression vector, pET238a and the yeast expression vector pPICZαA28. Only the α-tubulin amplicon was successfully sub-cloned into pICZAαA28; however, no protein expression was achieved upon transfection of the methylotrophic yeast, Pichia pastoris, with this construct. Subcloning of both α- and β-tubulin inserts into pET28a was successful. Expression of recombinant α- and β-tubulin as fusion proteins with a histidine tag, both at a size of 55 kDa, was achieved in Escherichia coli host BL21 (DE3). Recombinant proteins were successfully purified using nickel chelate chromatography under denaturing conditions. Refolding was first attempted by dilution of purified denatured proteins in a refolding buffer followed by reconcentration, but was largely unsuccessful. A second, more successful refolding method was performed wherein denatured proteins were refolded by application of a decreasing gradient of urea, while bound to a nickel chelate column. Native tubulin from cultured T.congolense procyclics was successfully purified and renatured using a polymerisation/depolymerisation method for use as a control for immunisation. Mice were immunised separately with refolded recombinant α- and β-tubulin, native tubulin or an irrelevant protein VP4AA expressed in the same way as the tubulins. ELISA analysis confirmed the production of antibodies against each protein. Parasitaemia developed in all mice following challenge with T. congolense. Only the group immunised with β-tubulin recorded no deaths during the monitoring period despite the presence of parasitaemia, with 60% of mice immunised with α-tubulin or VP4AA and the no antigen control and no mice from the native tubulin immunised group surviving. The results showed that partial protection against trypanosomiasis caused by T. congolense infection was achieved in the group immunised with β-tubulin and suggest that β-tubulin may have vaccine potential.