A CONDITIONED DENDRITIC CELL CAN BE A TEMPORAL BRIDGE BETWEEN A CD4(-HELPER AND A T-KILLER CELL() T)

To generate an immune response, antigen-specific T-helper and T-killer cells must find each other and, because they cannot detect each other 's presence, they are brought together by an antigen-loaded dendritic cell that displays antigens to both(1-3). This three-cell interaction, however, seems nearly impossible because all three cell types are rar e and migratory. Here we provide a potential solution to this conundru m, We found that the three cells need not meet simultaneously but that the helper cell can first engage and 'condition' the dendritic cell, which then becomes empowered to stimulate a killer cell. The first ste p (help) can be bypassed by modulation of the surface molecule CD40, o r by viral infection of dendritic cells. These results may explain the longstanding paradoxical observation that responses to some viruses a re helper-independent, and they evoke the possibility that dendritic c ells may take on different functions in response to different conditio ning signals.