Beyond memory T cells: mechanisms of protective immunity to tuberculosis infection.

Tuberculosis (TB) is a serious infectious disease caused by infection with Mycobacterium tuberculosis, and kills more people annually than any other single infectious agent. Although a vaccine is available, it is only moderately effective and an improved vaccine is urgently needed. The ability to develop a more effective vaccine has been thwarted by a lack of understanding of the mechanism of vaccine-induced immune protection. Over recent decades, many novel TB vaccines have been developed and almost all have aimed to generate memory CD4 T cells. In this review, we critically evaluate evidence in the literature that supports the contention that memory CD4 T cells are the prime mediators of vaccine-induced protection against TB. Because of the lack of robust evidence supporting memory CD4 T cells in this role, the potential for B-cell antibody and "trained" innate cells as alternative mediators of protective immunity is explored.