Composite reinforcer (R-2) that is self-adhesive, by curing, to a diene rubber matrix, which can be used as reinforcing element for a pneumatic tire, comprising: one or more reinforcing thread(s) (20), for example a carbon steel cord; a first layer (21) of a thermoplastic polymer, the glass transition temperature of which is positive, for example a polyamide, covering said thread, individually each thread or collectively several threads; a second layer (22) of a composition comprising a poly(p-phenylene ether) (“PPE”) and a functionalized diene elastomer bearing functional groups selected from epoxide, carboxyl, acid anhydride and acid ester groups, in particular an epoxidized SBR, covering the first layer (21). Process for manufacturing such a composite reinforcer and rubber article or semi-finished product, especially a pneumatic tire, incorporating such a composite reinforcer.