Covalent attachment of activated products of the third component of complement to antigen enhances its immunogenicity, but the mechanism is not clear. This effect is mediated by specific receptors, mCR1 (CD35) and mCR2 (CD21), expressed primarily on B cells and follicular dendritic cells in mice. To dissect the role of mCR1 and mCR2 in the humoral response, we have disrupted the Cr2 locus to generate mice deficient in both receptors. The deficient mice (Cr2-/-) were found to have a reduction in the CD5+ population of peritoneal B-1 cells, although their serum IgM levels were within the range of normal mice. Moreover, Cr2-/- mice had a severe defect in their humoral response to T-dependent antigens that was characterized by a reduction in serum antibody titers and in the number and size of germinal centers within splenic follicles. Reconstitution of the deficient mice with bone marrow from MHC-matched Cr2+/+ donors corrected the defect, demonstrating that the defect was due to B cells themselves. These results indicate an obligatory role of B cell complement receptors in responses of the B cells to protein antigens.

The mechanisms underlying the profound suppression of cell-mediated immunity (CMI) accompanying measles are unclear. Interleukin-12 (IL-12), derived principally from monocytes and macrophages, is critical for the generation of CMI. Measles virus (MV) infection of primary human monocytes specifically down-regulated IL-12 production. Cross-linking of CD46, a complement regulatory protein that is the cellular receptor for MV, with antibody or with the complement activation product C3b similarly inhibited monocyte IL-12 production, providing a plausible mechanism for MV-induced immunosuppression. CD46 provides a regulatory link between the complement system and cellular immune responses.

No abstract

Purified eukaryotic nuclear RNA polymerase II consists of three subspecies that differ in the apparent molecular masses of their largest subunit, designated Ho, Ha, and HIb for polymerase species HO, HA, and BIB, respectively. Subunits Ho, Ha, and IHb are the products of a single gene. We present here the amino acid composition of calf thymus subunits Ha and lIb and the C-terminal amino acid sequence of subunit Ha (Ho) inferred from the nucleotide sequence of part of the mouse gene encoding this RNA polymerase subunit. The calculated amino acid composition of the peptide unique to subunit Ha indicates that subunit Ha contains a domain rich in serine, proline, threonine, and tyrosine. The sequence at the 3' end of the mouse RNA polymerase H largest subunit gene reveals that the C-terminal domain consists of 52 repeats of a seven amino acid block with the consensus sequence Tyr-SerPro-Thr-Ser-Pro-Ser. This sequence is also unusual in that it contains a high percentage of potential phosphorylation sites.Eukaryotic nuclear RNA polymerases have been purified from a number of species and their complex subunit structure has been determined (1, 2). The three major classes, designated I, II, and III, differ in subunit structure and transcriptive function. Each enzyme contains from 9 to 14 subunits and has a total molecular mass of 5-6 x 105. RNA polymerases I, II, and III each contain two nonidentical large subunits (>100 kDa) and 7-12 smaller subunits. The large subunits of one class are not shared by other classes but are highly conserved within each class among different species (3-6).The only subunit that has been assigned a tentative functional role is the largest subunit of RNA polymerase II. Greenleaf and collaborators have isolated a strain of Drosophila resistant to the polymerase II inhibitor aamanitin (7). This mutation (C4) was localized to band lOC on the X chromosome and this region was cloned by P-element tagging (8,9). A 7-kilobase (kb) transcript from this region was shown to encode the largest subunit of RNA polymerase II (10). The Drosophila largest subunit gene has been crosshybridized to a number of eukaryotic DNAs (11) and has been used to clone the homologous genes from several species, including yeast (12) and mouse (unpublished data). Sequence analyses of the mouse and Drosophila clones have shown that the genes are highly conserved and are homologous to the ' subunit of Escherichia coli RNA polymerase (unpublished data), implicating the largest subunit in the processes of DNA binding and RNA chain elongation.Three different forms of RNA polymerase II have been described in a number of species. These enzymes, designated IIO, IIA, and IIB, differ in the apparent molecular masses of their largest subunit (IIo, Ha, and IIb) and appear to be related in part by limited proteolysis (13). However, anomalous mobility in NaDodSO4 gels due to postsynthetic modification cannot be ruled out. The three forms of the largest subunit Ho (240 kDa), Iha (210-220 kDa), and lIb (170-180 kDa) also differ in ...

No abstract

Complement protein C1q is required to maintain immune tolerance. The molecular mechanism responsible for this link has not been determined. We have previously demonstrated that C1q binds directly and specifically to surface blebs of apoptotic human keratinocytes, suggesting that it may participate in clearance of self Ags generated during programmed cell death. Here, we demonstrate that C1q also binds directly to apoptotic blebs of vascular endothelial cells and PBMC. These apoptotic cells are recognized by the globular heads of C1q, which bind specifically to the surface blebs, and deposition increases as the blebs mature on the cell surface. These observations suggest that C1q may participate in the clearance of apoptotic cells from the circulation and from the walls of the vascular lumen. The interaction of surface blebs with the globular heads of C1q suggests that surface blebs may be capable of directly activating the classical pathway of complement under certain circumstances, generating C4- and C3-derived ligands for receptors such as CR1, CR2, CR3, and CR4. Appropriate recognition of apoptotic cells by C1q and targeted clearance of the molecular contents of surface blebs to complement receptors may be critical for the maintenance of immune tolerance.

SummaryImmune context is an essential determinant of the host response to potential autoantigens. The clustering of the autoantigens targeted in systemic lupus erythematosus within surface blebs of apoptotic cells generates high concentrations of autoantigen within discrete subceUular packages. We demonstrate here that when apoptosis is induced by Sindbis virus infection, viral antigens and autoantigens cocluster exclusively in small surface blebs of apoptotic cells. The surface of these blebs is rich in viral glycoproteins, and virions can be seen blebbing from their surface. We propose that these blebs of mixed foreign and self-origin define a novel immune context that may challenge self-tolerance.

SummaryThe CD21/CD19/TAPA-1 complex of B lymphocytes amplifies signal transduction through membrane immunoglobulin (mIg), recruits phosphatidylinositol 3-kinase (PI3-kinase), and induces homotypic cellular aggregation. The complex is unique among known membrane protein complexes of the immune system because its components represent different protein families, and can be expressed individually. By constructing chimeric molecules replacing the extraceUular, transmembrane, and cytoplasmic regions of CD19 and CD21 with those of HLA-A2 and CD4, we have determined that CD19 and TAPA-1 interact through their extraceUular domains, CD19 and CD21 through their extracellular and transmembrane domains, and, in a separate complex, CD21 and CD35 through their extracellular domains. A chimeric form of CD19 that does not interact with CD21 or TAPA-1 was expressed in Daudi B lymphoblastoid cells and was shown to replicate two functions of wild-type CD19 contained within the complex: synergistic interaction with mIgM to increase intracellular free calcium and tyrosine phosphorylation and association with the p85 subunit of PI3-kinase after ligation of mlgM. The chimeric CD19 lacked the capacity of the wild-type CD19 to induce homotypic cellular aggregation, a function of the complex that can be ascribed to the TAPA-1 component. The CD21/CD19/TAPA-1 complex brings together independently functioning subunits to enable the B cell to respond to low concentrations of antigen.