Dendritic cells (DCs) process and present self and foreign antigens to induce tolerance or immunity. In vitro models suggest that induction of immunity is controlled by regulating the presentation of antigen, but little is known about how DCs control antigen presentation in vivo. To examine antigen processing and presentation in vivo, we specifically targeted antigens to two major subsets of DCs by using chimeric monoclonal antibodies. Unlike CD8 ⁺ DCs that express the cell surface protein CD205, CD8 ^– DCs, which are positive for the 33D1 antigen, are specialized for presentation on major histocompatibility complex (MHC) class II. This difference in antigen processing is intrinsic to the DC subsets and is associated with increased expression of proteins involved in MHC processing.

The maturation status of dendritic cells (DCs) determines whether they prime or tolerize T cells. We targeted ovalbumin peptide exclusively to DCs in situ using an antibody to DEC-205 and studied the interaction of DCs with naive CD4⁺ T cells in tolerizing or priming conditions. We used two-photon microscopy to simultaneously track antigen-specific OT-II T cells, nonspecific T cells and DCs in lymph nodes of living mice. In both tolerance and immunity, OT-II cells arrested on DCs near high endothelial venules beginning shortly after extravasation and regained their baseline speed by 18 h. Thus, early antigen-dependent T cell arrest on DCs is a shared feature of tolerance and priming associated with activation and proliferation.

Interferon (IFN)-γ, a cytokine critical for resistance to infection and tumors, is produced by CD4+ helper T lymphocytes after stimulation by cultured dendritic cells (DCs) that secrete a cofactor, interleukin (IL)-12. We have identified a major IL-12–independent pathway whereby DCs induce IFN-γ–secreting T helper (Th)1 CD4+ T cells in vivo. This pathway requires the membrane-associated tumor necrosis family member CD70 and was identified by targeting the LACK antigen from Leishmania major within an antibody to CD205 (DEC-205), an uptake receptor on a subset of DCs. Another major DC subset, targeted with 33D1 anti-DCIR2 antibody, also induced IFN-γ in vivo but required IL-12, not CD70. Isolated CD205+ DCs expressed cell surface CD70 when presenting antigen to T cell receptor transgenic T cells, and this distinction was independent of maturation stimuli. CD70 was also essential for CD205+ DC function in vivo. Detection of the IL-12–independent IFN-γ pathway was obscured with nontargeted LACK, which was presented by both DC subsets. This in situ analysis points to CD70 as a decision maker for Th1 differentiation by CD205+ DCs, even in Th2-prone BALB/c animals and potentially in vaccine design. The results indicate that two DC subsets have innate propensities to differentially affect the Th1/Th2 balance in vivo and by distinct mechanisms.

Cellular Fcγ receptors are essential for IgG-dependent effector functions in vivo. There is convincing evidence that selective activating Fcγ receptors are responsible for the activity of individual IgG subclasses. Thus, IgG1 activity is absent in FcγRIII-deficient mice, and several studies suggest that the activity of the most potent IgG subclasses, IgG2a and IgG2b, might be dependent on either individual or a combination of activating FcγRs. To study the role of individual activating FcγRs for IgG subclass activity, we generated an FcγRIV-deficient mouse and showed that a variety of IgG2a- and IgG2b-dependent effector functions are impaired in the absence of this activating Fc receptor in models of autoimmunity and antibody-dependent cellular cytotoxicity.