CD40LG and influenza: The use of CD154 as an immune adjuvant to enhance the immune efficacy of vaccines has been progressively demonstrated in human respiratory syncytial virus (RSV) gene vaccines, bovine herpesvirus gene vaccines, swine fever E2 subunit vaccines, influenza vaccines and some avian gene vaccines, but its use in PCV2 subunit vaccines has rarely been reported [21,23,24,30,34].