The mechanism by which mycobacteria elicit class I-restricted T-cell respon
ses remains undefined because these organisms have been shown to reside exc
lusively within membrane-bound vesicles in macrophages (M phi), their prima
ry host cells. We studied the interaction of M. avium with dendritic cells
(DC) because they are the most potent antigen-presenting cells and are abun
dant at M. avium infection sites. We observed that both DC and M phi, gener
ated from human peripheral blood monocytes by short-term culture, internali
zed ICI. avium. The onset of programmed cell death and the percentage of ap
optotic cells in infected DC and M phi, were comparable. However, following
infection, DC secreted significantly larger amounts of interleukin-12, but
not interleukin-1 beta, than infected autologous M phi. Further analysis o
f infected cells showed that while phagosomes failed to acidify in both M,
avium-infected DC and M phi, bacilli grew more slowly in DC. Electron micro
scopy studies revealed that M. avium resided within endocytic vacuoles in b
oth cell types. The vacuolar membrane surrounding some bacilli in approxima
tely 10% of the vacuoles in DC possessed several breaks. The importance of
this finding will have to be addressed in future studies.