Expression of the cytolethal distending toxin (Cdt) operon in Actinobacillus actinomycetemcomitans: Evidence that the CdtB protein is responsible forG(2) arrest of the cell cycle in human T cells

We have previously shown that Actinobacillus actinomycetemcomitans produces an immunosuppressive factor that is encoded by the cdtB gene, which is hom ologous to a family of cytolethal distending toxins (Cdt) expressed by seve ral Grant-negative bacteria. In this study, we report that the cdt locus in A. actinomycetemcomitans is composed of five open reading frames, designat ed orf1, orf2, cdtA, cdtB, and cdtC, The deduced amino acid sequences of th e five open reading frames are highly conserved among A. actinomycetemcomit ans strains 652, Y4, 29522, and HK1651. There is also strong homology with the Cdt proteins of Haemophilus ducreyi (87-91%), but only partial homology with that of Campylobacter jejuni and Escherichia coli (29-48%). Analysis of A. actinomycetemcomitans mRNA by RT-PCR suggests that the two small open reading frames upstream of cdtA are coexpressed with cdtA, cdtB, and cdtC, We next utilized a series of plasmids that express various combinations of the cdt genes to determine their requirement for expression of immunoinhib itory activity. Cell extracts of E. call transformed with each of the plasm ids were tested for their capacity to induce G(2) arrest in the cell cycle of PHA-activated human T cells. These experiments suggest that expression o f cdtB alone is sufficient to induce G(2) arrest in human T cells, but do n ot exclude the possibility that cdtC also contributes to cell cycle arrest, The implications of our results with respect to the function of the indivi dual Cdt proteins are discussed.