Identification and Expression of Human Cytomegalovirus Transcription Units Coding for Two Distinct Fcγ Receptor Homologs

ABSTRACT Cellular receptors for the Fc domain of immunoglobulin G (IgG) (FcγRs) comprise a family of surface receptors on immune cells connecting humoral and cellular immune responses. Several herpesviruses induce FcγR activities in infected cells. Here we identify two distinct human cytomegalovirus (HCMV)-encoded vFcγR glycoproteins of 34 and 68 kDa. A panel of HCMV strains exhibited a slight molecular microheterogeneity between Fcγ-binding proteins, suggesting their viral origin. To locate the responsible genes within the HCMV genome, a large set of targeted HCMV deletion mutants was constructed. The mutant analysis allowed the identification of a spliced UL119-UL118 mRNA to encode vFcγR gp68 and TRL11/IRL11 to encode vFcγR gp34. Both vFcγRs are surface resident type I transmembrane glycoproteins. Significant relatedness of sequences in the extracellular chain of gpUL119-118 and gpTRL11 with particular immunoglobulin supergene family domains present in FcγR I and FcγRs II/III, respectively, indicates a different ancestry and function of gpUL119-118 and gpTRL11. The HCMV-encoded vFcγRs highlight an impressive diversification and redundancy of FcγR structures.

Interactions of CD4 with MHC class II molecules, T cell receptors and p56 lck

CD4 and CD8 are members of the immunoglobulin supergene family of proteins, and function as co-receptors with the T cell receptor (TCR) in binding MHC class II or class I molecules, respectively. Within this multimeric complex, CD4 interacts with three distinct ligands. CD4 interacts through its D1 and D2 domains with MHC class II proteins, through its D3 and D4 domains with T cell receptors, and through its cytoplasmic tail with p56 lck , a src -related, protein tyrosine kinase. Each of these interactions is important in the function of CD4 and will be discussed in turn.

Homophilic adhesion between Ig superfamily carcinoembryonic antigen molecules involves double reciprocal bonds

Both carcinoembryonic antigen (CEA) and neural cell adhesion molecule (NCAM) belong to the immunoglobulin supergene family and have been demonstrated to function as homotypic Ca(++)-independent intercellular adhesion molecules. CEA and NCAM cannot associate heterotypically indicating that they have different binding specificities. To define the domains of CEA involved in homotypic interaction, hybrid cDNAs consisting of various domains from CEA and NCAM were constructed and were transfected into a CHO-derived cell line; stable transfectant clones showing cell surface expression of CEA/NCAM chimeric-proteins were assessed for their adhesive properties by homotypic and heterotypic aggregation assays. The results indicate that all five of the Ig(C)-like domains of NCAM are required for intercellular adhesion while the COOH-terminal domain containing the fibronectin-like repeats is dispensable. The results also show that adhesion mediated by CEA involves binding between the Ig(V)-like amino-terminal domain and one of the Ig(C)-like internal repeat domains: thus while transfectants expressing constructs containing either the N domain or the internal domains alone were incapable of homotypic adhesion, they formed heterotypic aggregates when mixed. Furthermore, peptides consisting of both the N domain and the third internal repeat domain of CEA blocked CEA-mediated cell aggregation, thus providing direct evidence for the involvement of the two domains in adhesion. We therefore propose a novel model for interactions between immunoglobulin supergene family members in which especially strong binding is effected by double reciprocal interactions between the V-like domains and C-like domains of antiparallel CEA molecules on apposing cell surfaces.

Introduction to the CEA Family: Structure, Function and Secretion

Due to the phenomenal progress in the field of tumor immunology that took place during the last twenty years, we dispose today of highly specific and sensitive techniques and reagents like monoclonal antibodies (MAbs). In this context the discovery in human carcinomas of tumor-associated antigens, such as CEA, was of primary importance, especially since the latter was found to have clinical relevance as a tumor marker. Based on animal models, a new in vivo technology for the detection of tumors and metastases was developed in recent years, that uses anti-CEA MAbs, or fragments of them, coupled to radio-isotopes. This technique, called radioimmunodetection (RAID), also paved the way for immunotherapeutic procedures, where again CEA served as the target-antigen. This new technique holds great promise, provided the epitope-specificity of the MAbs is well-controlled: it has been shown that CEA belongs to a large gene-family of at least 22 members, which can be subdivided into two subgroups (i.e., the CEA- and the PSG-subgroup) and which in turn belongs to the immunoglobulin-supergene family. Great structural similarities render the distinction of the various cross-reactive molecules by immunological means rather difficult.

Characterization of a new T-lineage glycoprotein expressed in mature T- cell leukemias and lymphomas

We identified a new human, T-lineage restricted glycoprotein of molecular weight 120 Kd that is expressed primarily in mature T-cell malignancies. The antigen, named TCA-1 (T-cell cytoplasmic antigen), is an intracellular glycoprotein found mainly in the Golgi stacks, although a few cell lines also display surface membrane TCA-1. Many but not all T-cell neoplasms express this antigen. The antigen is absent from neoplastic and normal human tissue outside the T-lymphocyte lineage. TCA-1 was identified by murine monoclonal antibodies produced after immunization of mice with T-cell chronic lymphocytic leukemia cells. The glycoprotein is a monomer containing approximately 4% N- linked carbohydrate with terminal D-galactose residues. Partial amino acid sequence analysis of TCA-1 shows homology with an immunoglobulin heavy chain region, which suggests that TCA-1 may belong to the immunoglobulin supergene family of receptor and adhesion molecules.

Characterization of a new T-lineage glycoprotein expressed in mature T- cell leukemias and lymphomas

We identified a new human, T-lineage restricted glycoprotein of molecular weight 120 Kd that is expressed primarily in mature T-cell malignancies. The antigen, named TCA-1 (T-cell cytoplasmic antigen), is an intracellular glycoprotein found mainly in the Golgi stacks, although a few cell lines also display surface membrane TCA-1. Many but not all T-cell neoplasms express this antigen. The antigen is absent from neoplastic and normal human tissue outside the T-lymphocyte lineage. TCA-1 was identified by murine monoclonal antibodies produced after immunization of mice with T-cell chronic lymphocytic leukemia cells. The glycoprotein is a monomer containing approximately 4% N- linked carbohydrate with terminal D-galactose residues. Partial amino acid sequence analysis of TCA-1 shows homology with an immunoglobulin heavy chain region, which suggests that TCA-1 may belong to the immunoglobulin supergene family of receptor and adhesion molecules.