CD4 and tuberculosis: TB immunity relies heavily on components of the cellular immune system.3 Moreover, it has long been appreciated that CD4+ T cell immunity is critical in host defense to TB.4 Yet, a randomized, placebo-controlled phase 2b trial of modified vaccinia Ankara (MVA) virus expressing antigen 85 A used as a BCG booster in infants demonstrated no efficacy against TB despite the induction of modest IFN-γ, TNF-α, IL-2 producing CD4+ T cell responses.5 Of note, while MVA characteristically induces robust CD8+ T cell responses, this TB vaccine did not induce CD8+ T cell responses.