WPP-owned ad tech firm Xaxis has sold its premium ad serving platform Open AdStream to AppNexus for an undisclosed sum.

The technology, which was originally acquired by Xaxis in 2007 when it bought 24/7 Real Media, will be rebranded as Xaxis For Publishers and aligned with AppNexus own offering.

“Publishers want a technology partner they can trust,” said AppNexus CEO Brian O’Kelley. “Open AdStream has been one of the most effective and trusted ad serving platforms in the world for years. We’re going to make it #1 in programmatic.”

The deal is expected to close by the end of the year, and is part of a wider agreement with WPP. The marketing giant is investing $25m (£15.3m) in AppNexus, giving WPP a 15 per cent stake in the company – a huge jump from its previous stake of less than one per cent.

“Our clients want world-class solutions from independent ad technology companies,” said WPP CEO Martin Sorrell. “We’re delighted to deepen our partnership with AppNexus, which has proven itself as a clear leader in ad tech and a company with the scale and ingenuity to continue to transform our industry.”