CD4 and infection: Heterologous prime-boost strategy has been used in many models of pathogenic infections [15], and some studies demonstrate that prime-boost strategies using BCG as prime and heterologous constructs such as recombinant DNA, recombinant adenovirus, and recombinant poxviruses as boosting immunogens can enhance CD4+ and CD8+ T-cell responses against TB [6, 7, 16–18].