Protective effect of DNA immunization against mycobacterial infection is associated with the early emergence of interferon-gamma (IFN-gamma)-secreting lymphocytes.

The development of more effective anti-tuberculosis (TB) vaccines would contribute to the global control of TB. Understanding the activated/memory T cell response to mycobacterial infection and identifying immunological correlates of protective immunity will facilitate the design and assessment of new candidate vaccines. Therefore, we investigated the kinetics of the CD4+ T cell response and IFN-gamma production in an intravenous challenge model of Mycobacterium bovis bacille Calmette-Guérin (BCG) before and after DNA immunization. Activated/memory CD4+ T cells, defined as CD44hiCD45RBlo, expanded following infection, peaking at 3-4 weeks, and decreased as the bacterial load fell. Activated/memory CD4+ T cells were the major source of IFN-gamma and the level of antigen-specific IFN-gamma-secreting lymphocytes, detected by ELISPOT, paralleled the changes in bacterial load. To examine the effects of a DNA vaccine, we immunized mice with a plasmid expressing the mycobacterial secreted antigen 85B (Ag85B). This led to a significant reduction in mycobacteria in the liver, spleen and lung. This protective effect was associated with the rapid emergence of antigen-specific IFN-gamma-secreting lymphocytes which were detected earlier, at day 4, and at higher levels than in infected animals immunized with a control vector. This early and increased response of IFN-gamma-secreting T cells may serve as a correlate of protective immunity for anti-TB vaccines.