This work was supported by grants from Ministerio de Economia y Competitividad-Spain (to C. L.-O., P. M. and C.B.), Gobierno del Principado de Asturias and Instituto de Salud Carlos III (RTICC), Spain. The Instituto Universitario de Oncologia is supported by Fundacion Bancaria Caja de Ahorros de Asturias. C.B. is a recipient of a 'Miguel Servet' fellowship from the ISCIII-FIS. P.M. is a member of the Spanish Cell Therapy Network (Tercel). C.L.-O. is an Investigator of the Botin Foundation supported by Banco Santander through its Santander Universities Global Division. This paper is dedicated to the memory of Nestor M.O.