tofluorescent macrophages and myeloid DCs contain cell- associated beads. This indicates that uptake of cell-associ- ated antigen is not restricted to lymphoid DCs. This con- trasts with the study by Fossum and Rolstad, in which allogeneic lymphocyte cell debris was found mainly in lymphoid DCs (60). However, differences in the in vivo model systems used may explain this discrepancy. Second, myeloid and lymphoid DCs do not seem to differ in their capacity to stimulate T cells, as both DC subsets have been shown to stimulate CD81 T cells efficiently in vitro and in vivo (39). This leaves differences in antigen processing pathways between myeloid and lymphoid DCs as the most likely explanation. A selective transport of internalized an- tigens to the cytosol has been shown to occur in DCs and to be absent in macrophages (20). Both bone marrow– derived DCs and a splenic-derived DC line exhibited this ability, but ex vivo DCs were not evaluated. Our results suggest that this endosome to cytosol transport will be lim- ited to lymphoid DCs. Cross-presentation of cell-associated antigens to CD81 T cells has been shown to result in either T cell activation or tolerance (1, 2, 8–10, 61, 62). In both cases, bone marrow– derived cells are necessary (8–10). Lymphoid DCs have been speculated to mediate peripheral tolerance, whereas myeloid DCs have been suggested to be essential for T cell stimulation (49, 63). Our studies do not specifically address the outcome of the DC–T cell interaction, as T cell pro- liferation is involved in both processes. However, our method of intravenous immunization with OVA-loaded cells has repeatedly been shown to lead to T cell activation and memory (10, 61, 64). Therefore, we are convinced that we have isolated the APC required for CD81 T cell cross-priming. An attractive alternative explanation for the different outcomes of DC–T cell interaction is offered by several studies indicating that the activation state of the DCs is an important factor. DCs involved in cross-priming are acti- vated via CD40–CD40L interaction provided by CD41 T cells (64–66). CD81 T cell cross-priming does not occur in the absence of CD41 T cells, illustrating the crucial role of CD41 T cells in this process (10). Likewise, CD41 T cells play an important role in preventing the induction of CD81 T cell tolerance to self-antigens, as the addition of antigen-specific CD41 T cells converts CD81 T cell toler- ance induction into CD81 T cell activation (67). Further- more, in several other tolerance induction systems, in vivo CD40 activation resulted in abrogation of tolerance and in T cell activation (68–71). Taken together, these studies strongly suggest that the activation state of the DCs is an important factor in determining the outcome of the DC–T cell interaction and that lymphoid DCs may be implicated in both activation and tolerization of antigen-specific CD81 T cells. Recent studies suggest different functions for lymphoid and myeloid DCs in the activation of CD41 T cells, prefer- entially inducing Th1 and Th2 responses, respectively (40– 42). The capacity of lymphoid DCs to drive Th1 responses is explained by their production of IL-12 and IFN-g (27, 52, 72). Furthermore, the localization of the different DC and Th subsets appears to be similar. Th1 cells primarily re- side in proximity to the lymphoid DCs in the central T cell zone of the PALS, whereas Th2 cells are found in the outer PALS near the B cell follicles and the marginal zone where myeloid DCs are located (73). Differential chemokine re- ceptor expression patterns in both Th and DC subsets are essential in establishing this homing pattern (73–76). Our data add a new role for the lymphoid DCs as the main stimulator of CD81 T cell responses specific for cell-associ- ated antigens. This points to an interaction between lym- phoid DCs, CD81 T cells, and Th1 cells. It remains to be investigated whether CD81 T cell cross-priming is specifi- cally associated with and dependent on Th1 responses. In summary, our data demonstrate that lymphoid DCs play a key role in cross-presentation of cell-associated anti- gens to CD81 T cells in vivo. Because cross-presentation enables the activation of naive T cells specific for antigens that are not expressed by the APCs themselves, we envis- age that lymphoid DCs will be essential for the induction of a large spectrum of CD81 T cell responses. We thank Drs. A. Norment, S. Clarke, K. Urdahl, and P. Fink for critical reading of the manuscript. 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