The induction of a T cell-dependent Ab response is mediated by the interaction of the T cell activation Ag, CD40 ligand (CD40L), with CD40. Since this interaction is independent of Ag, coreceptors such as CD4, and MHC molecules, the expression of the CD40L must be strictly regulated or B cell-mediated autoimmunity may be produced. In this study, we examined the requirements for costimulatory signals for induction of CD40L expression in vitro and in vivo on CD4+ T cells from normal and CD28-deficient mice following stimulation with anti-CD3, Con A, or specific peptide Ag. Expression of B7-1 was both necessary and sufficient for induction of the CD40L on normal CD4+ T cells when L cell transfectants were used as APCs. When normal accessory cell populations were used, only partial inhibition of induction of the CD40L was observed with reagents that inhibit B7/CD28 interactions. Furthermore, the CD40L could be induced on CD4+ T cells from CD28-deficient mice. Thus, non-B7/CD28 cellular interactions can also mediate the costimulatory signals needed for induction of CD40L expression.

HLA-DM has been shown in vitro to catalyze the release of invariant chain (Ii) derived peptides from the peptide-binding groove of class II molecules, thereby facilitating the binding of antigenic peptides. Previous studies showed that at steady state, the majority of DM resides in the class II peptide-loading compartment (IIPLC) where Ii dissociates from class II molecules and antigenic peptides are bound. Here we characterize the expression of DM in vivo in subcellular fractions containing the IIPLC. Using quantitative immunoblotting, we show that in the cell as a whole, class II molecules are expressed in 23-fold molar excess of DM. However, DM is concentrated in the IIPLC, where it is present in a considerably higher concentration relative to the class II molecules, in a molar ratio of 5DR:1 DM. This molar ratio of DM to DR in the IIPLC in vivo is consistent with the catalytic function proposed for DM from studies in vitro. We also provide both biochemical and genetic evidence that DM associates with complexes which contain Ii fragments and class II molecules in the IIPLC. Such complexes are only observed in leupeptin-treated cells in which Ii fails to be completely degraded and complexes containing the leupeptin-induced fragment of Ii (LIP) and class II molecules accumulate in the IIPLC. This observation is consistent with LIP-class II complexes being a substrate for DM in vivo and suggests that interactions of DM and LIP-class II are extremely transient under normal conditions.