Chimeric CD46/DAF molecules reveal a cryptic functional role for SCR1 of DAF in regulating complement activation

Ayu (Plecoglossus altivelis) CD46 isoforms protect the cells from autologous complement attack

CD46 is an important immune regulatory receptor with multiple functions. However, studies on the function of teleost CD46, especially the different CD46 isoforms are limited. In this study, we identified three membrane cofactor protein (MCP, CD46) gene isoforms from ayu (Plecoglossus altivelis) and tentatively named as PaCD46 isoforms. PaCD46 isoforms were generated by alternative splicing and all consisted of four conserved short consensus repeats (SCRs), and the variable serine-threonine-proline-rich domain, transmembrane hydrophobic domain, and cytoplasmic tail. Phylogenetic analysis showed that the isoforms clustered together with other fish CD46 and then with higher animal CD46. Western blotting analysis of peripheral blood mononuclear cells (PBMC) revealed three bands, all of which had much larger molecular weights than the theoretical values of the three PaCD46 isoforms. Moreover, three PaCD46 isoforms were individually expressed on HEK293 cells, and Western blotting showed the similar band profile to that of PBMC. The recombinant extracellular domain of the PaCD46 isoforms, obtained by expression in Pichia pastoris, significantly reduced hemolysis activity of ayu sera. Furthermore, each of the three PaCD46 isoforms respectively protected the HEK293 cells expressing the isoform. The isoforms were also identified for their protection of autologous PBMC from complement activation. These results provided the first evidence that PaCD46 isoforms may be complement regulatory proteins to prevent complement-induced damage to self-tissue.

Cell surface activation of the alternative complement pathway by the fusion protein of measles virus

Measles virus (MV)-infected cells are activators of the alternative human complement pathway, resulting in high deposition of C3b on the cell surface. Activation was observed independent of whether CD46 was used as a cellular receptor and did not correlate with CD46 down-regulation. The virus itself was an activator of the alternative pathway and was covered by C3b/C3bi, resulting in some loss in infectivity without loss of virus binding to target cells. The cell surface expression of MV fusion (F), but not haemagglutinin, envelope protein resulted in complement activation of the Factor B-dependent alternative pathway in a dose-dependent manner and F-C3b complexes were formed. The underlying activation mechanism was not related to any decrease in cell surface expression of the complement regulators CD46 and CD55. The C3b/C3bi coating of MV-infected cells and virus should ensure enhanced targeting of MV antigens to the immune system, through binding to complement receptors.

Human Herpesvirus 6 and Measles Virus Employ Distinct CD46 Domains for Receptor Function

We employed a quantitative cell fusion assay to identify structural domains of CD46 required for its function as a receptor for human herpesvirus 6 (HHV-6). We examined the activities of recombinant variants of CD46, including different isoforms as well as engineered truncations and molecular chimeras with decay-accelerating factor, a related protein in the family of regulators of complement activation (RCA). We observed strong receptor activity for all four CD46 isoforms, which differ in the membrane-proximal extracellular and cytoplasmic domains, indicating that the critical determinants for HHV-6 receptor activity reside outside the C-terminal portion of CD46. Analysis of the short consensus repeat (SCR) regions that comprise most of the extracellular portion of CD46 indicated a strong dependence on SCRs 2 and 3 and no requirement for SCRs 1 or 4. Fusion-inhibition studies with SCR-specific monoclonal antibodies supported the essential role of SCRs 2 and 3 in HHV-6 receptor activity. These findings contrast markedly with fusion mediated by measles virus glycoproteins for which we observed a strict dependence on SCRs 1 and 2, consistent with previous reports. These results expand the emerging notion that CD46 and other members of the RCA family are co-opted in distinct manners by different infectious pathogens.

A CD46CD[55-46] chimeric receptor, eight short consensus repeats long, acts as an inhibitor of both CD46 (MCP)- and CD150 (SLAM)-mediated cell-cell fusion induced by CD46-using measles virus

According to their cellular receptor use, measles virus (MV) strains can be separated into two phenotypes, CD46-using and CD46-non-using. A long chimeric receptor, CD46CD[55-46], was generated from the CD46 backbone, encompassing the four short consensus repeat (SCR) domains of CD46 linked via a flexible glycine hinge to SCR1 and SCR2 of CD55, SCR3 and SCR4 of CD46 and the STP, transmembrane and cytoplasmic tail of CD46. This chimeric receptor was proficient for MV binding but deficient in mediating MV-induced cell-to-cell fusion and virus replication, possibly due to the extended distance between the MV haemagglutinin (H) binding site (CD46 SCR1-SCR2) and the cell membrane. When coexpressed with either wild-type CD46 or CD150, this fusion-incompetent receptor exerted a dominant negative effect and inhibited both cell-to-cell fusion and entry of MV with CD46-using, but not CD46-non-using, phenotype. A soluble octameric CD46-C4bpalpha exhibited similar CD46- and CD150-mediated fusion inhibition properties only against CD46-using MV. This suggests that the long CD46CD[55-46] receptor acts by sequestering incoming MV prior to its binding to the shorter functional CD46 or CD150 receptor.