Prolonged intervals during Mycobacterium tuberculosis subunit vaccine boosting contributes to eliciting immunity mediated by central memory-like T cells

It is believed that central memory T cells (TCM) provide long-term protection against tuberculosis (TB). However, the effects of TB subunit vaccine immunization schedule, especially the vaccination intervals, on T cell immune memory is still unclear. In this study, mice were immunized with fusion protein ESAT6-Ag85B-MPT64 (190-198)-Mtb8.4-Rv2626c (LT70) based subunit vaccine three times according to the following schedules: ① 0, 3rd and 6th week respectively (0-3-6w), ② 0, 4th and 12th week (0-4-12w), and ③ 0, 4th and 24th week (0-4-24w). We found that both schedules of 0-4-12w and 0-4-24w induced higher level of antigen specific IL-2, IFN-γ and TNF-α than 0-3-6w immunization. Among them, 0-4-12w induced the highest level of IL-2, which is a key cytokine mainly produced by TCM. Moreover, by cultured IFN-γ ELISPOT and cell proliferation assay etc., we found that the vaccination schedule of 0-4-12w elicited higher numbers of TCM like cells, stronger TCM - mediated immune responses and higher protective efficacy against M. bovis BCG challenge than 0-3-6w did. It suggests that prolonging the vaccination interval of TB subunit vaccine to some extent contributes to inducing more abundant TCM like cells and providing stronger immune protection against mycobacteria infection.