In vivo MHC class II presentation of cytosolic proteins revealed by rapid automated tandem mass spectrometry and functional analyses

We report a strategy for high through-put sequence analyses of large MHC class II-bound peptide repertoires which combines automated electrospray ionization tandem mass-spectrometry with computer-assisted interpretation of the tandem mass spectra using the algorithm SEQUEST. This powerful approach discerned 128 peptide sequences displayed by the murine MHC class II molecule I-Ab in activated B cells and macrophages, including a surprisingly large number of peptides derived from self cytosolic proteins. Mice lacking the chaperone molecule H-2M were used to generate T cells specific for selected self peptides. Functional T cell analyses of ex vivo antigen-presenting cells indicated that peptides originating from cytosolic proteins are efficiently presented by splenic and thymic dendritic cells, but less so by resting B cells or thymic cortical epithelial cells. These results suggest that central tolerance to at least some MHC class II-bound self peptides derived from cytosolic proteins exists in vivo.

Invariant chain-independent function of H-2M in the formation of endogenous peptide-major histocompatibility complex class II complexes in vivo

Efficient loading of major histocompatibility complex class II molecules with peptides requires the invariant chain (Ii) and the class II-like molecule H-2M. Recent in vitro biochemical studies suggest that H2-M may function as a chaperone to rescue empty class II dimers. To test this hypothesis in vivo, we generated mice lacking both Ii and H-2M (Ii-/-M-/-). Antigen presenting cells (APCs) from Ii-/-M-/- mice, as compared with APCs from Ii-/- mice, exhibit a significant reduction in their ability to present self-peptides to a panel of class II I-Ab-restricted T cells. As a consequence of this defect in the loading of self peptides, CD4(+) thymocyte development is profoundly impaired in Ii-/-M-/- mice, resulting in a peripheral CD4(+) T cell population with low levels of T cell receptor expression. These findings are consistent with the idea that H-2M functions as a chaperone in the peptide loading of class II molecules in vivo.

Deficient positive selection of CD4 T cells in mice displaying altered repertoires of MHC class II-bound self-peptides

The role of self-peptides in positive selection of CD4+ T cells has been controversial. We show that some self-peptides are presented by the MHC class II molecule I-A(b) in mice lacking Ii or H-2M but not in mice expressing a transgene-encoded peptide fused to I-A(b). In experiments using specific antibodies to block selection, these low-abundance self-peptides were implicated in the positive selection of some CD4+ T cells in H-2M-/- mice. However, all three mutant backgrounds failed to positively select two class II-restricted transgenic T cell receptors. Our findings suggest that minor components of the self-peptide repertoire can contribute to positive selection of a significant number of CD4+ T cells. In addition, the data suggest that T cell receptor repertoires selected in wild-type mice and in mice displaying limited spectra of self-peptides are distinct.

Enhanced hematopoietic activity of a human granulocyte/macrophage colony-stimulating factor-interleukin 3 fusion protein

Granulocyte/macrophage colony-stimulating factor-interleukin 3 (GM-CSF-IL-3) fusion proteins were generated by construction of a plasmid in which the coding regions of human GM-CSF and IL-3 cDNAs were connected by a synthetic linker sequence followed by subsequent expression in yeast. Both GM-CSF-IL-3 and IL-3-GM-CSF fusion proteins were purified to homogeneity and shown to bind to cell-surface receptors through either their GM-CSF or IL-3 domains. The fusion proteins exhibited enhanced receptor affinity, proliferative activity, and hematopoietic colony-stimulating activity compared with either IL-3 and/or GM-CSF alone. This suggests that GM-CSF-IL-3 fusion proteins may hold future promise as therapeutic agents.

IL-1 and its receptor are translocated to the nucleus

The internalization and intracellular transport of IL-1 and its receptor were examined in the murine T cell line EL-4. For 4 h after internalization intracellular 125I-IL-1 alpha remains bound to its receptor without degradation. Electron microscope autoradiography demonstrates that internalized IL-1 accumulates in purified nuclei. The IL-1 extracted from these nuclei is still bound to receptor. As no receptors for IL-1 were detected in untreated nuclei, these results suggest IL-1 driven translocation of the cell surface IL-1R complex to the nucleus. IL-1R internalization was correlated with IL-1 signal transduction events required to induce growth factor production from several subclones of EL-4 cells. The subsequent transport of the internalized IL-1R complex to the nucleus suggests the possibility for a nuclear site for IL-1R signaling.

IL-1 and its receptor are translocated to the nucleus

The internalization and intracellular transport of IL-1 and its receptor were examined in the murine T cell line EL-4. For 4 h after internalization intracellular 125I-IL-1 alpha remains bound to its receptor without degradation. Electron microscope autoradiography demonstrates that internalized IL-1 accumulates in purified nuclei. The IL-1 extracted from these nuclei is still bound to receptor. As no receptors for IL-1 were detected in untreated nuclei, these results suggest IL-1 driven translocation of the cell surface IL-1R complex to the nucleus. IL-1R internalization was correlated with IL-1 signal transduction events required to induce growth factor production from several subclones of EL-4 cells. The subsequent transport of the internalized IL-1R complex to the nucleus suggests the possibility for a nuclear site for IL-1R signaling.