ABSTRACTDespite the widespread use ofMycobacterium bovisBCG, the only licensed vaccine against tuberculosis (TB), TB remains a global epidemic. To assess whether more direct targeting of the lung mucosa by respiratory immunization would enhance the potency and longevity of BCG-induced anti-TB protective immunity, the long-term impact of intranasal (i.n.) BCG vaccination was compared to conventional subcutaneous (s.c.) immunization by using a mouse model of pulmonary tuberculosis. Although significantly improved protection in the lung was seen at early time points (2 and 4 months postvaccination) in i.n. BCG-immunized mice, no differences in pulmonary protection were seen 8 and 10 months postvaccination. In contrast, in all of the study periods, i.n. BCG vaccination induced significantly elevated protective splenic responses relative to s.c. immunization. At five of nine time points, we observed a splenic protective response exceeding 1.9 log₁₀protection relative to the s.c. route. Furthermore, higher frequencies of CD4 T cells expressing gamma interferon (IFN-γ) and IFN-γ/tumor necrosis factor alpha, as well as CD8 T cells expressing IFN-γ, were detected in the spleens of i.n. vaccinated mice. Using PCR arrays, significantly elevated levels of IFN-γ, interleukin-9 (IL-9), IL-11, and IL-21 expression were also seen in the spleen at 8 months after respiratory BCG immunization. Overall, while i.n. BCG vaccination provided short-term enhancement of protection in the lung relative to s.c. immunization, potent and extremely persistent splenic protective responses were seen for at least 10 months following respiratory immunization.