Rapid loss of early antigen-presenting activity of lymph node dendritic cells against Ag85A protein following Mycobacterium bovis BCG infection

Control of Mycobacterium tuberculosis (Mtb) infection requires CD4+ T-cell responses and major histocompatibility complex class II (MHC II) presentation of Mtb antigens (Ags). Dendritic cells (DCs) are the most potent of the Ag-presenting cells and are central to the initiation of T-cell immune responses. Much research has indicated that DCs play an important role in anti-mycobacterial immune responses at early infection time points, but the kinetics of Ag presentation by these cells during these events are incompletely understood. In the present study, we evaluated in vivo dynamics of early Ag presentation by murine lymph-node (LN) DCs in response to Mycobacterium bovis bacillus Calmette–Guérin (BCG) Ag85A protein. Results showed that the early Ag-presenting activity of murine DCs induced by M. bovis BCG Ag85A protein in vivo was transient, appearing at 4 h and being barely detectable at 72 h. The transcription levels of CIITA, MHC II and the expression of MHC II molecule on the cell surface increased following BCG infection. Moreover, BCG was found to survive within the inguinal LN DC pool, representing a continuing source of mycobacterial Ag85A protein, with which LN DCs formed Ag85A peptide-MHCII complexes in vivo. Our results demonstrate that a decrease in Ag85A peptide production as a result of the inhibition of Ag processing to is largely responsible for the short duration of Ag presentation by LN DCs during BCG infection in vivo.