The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface.

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

The human cytomegalovirus Fc receptor gp68 binds the Fc CH2-CH3 interface of immunoglobulin G.

Recognition of immunoglobulin G (IgG) by surface receptors for the Fc domain of immunoglobulin G (Fcgamma), FcgammaRs, can trigger both humoral and cellular immune responses. Two human cytomegalovirus (HCMV)-encoded type I transmembrane receptors with Fcgamma-binding properties (vFcgammaRs), gp34 and gp68, have been identified on the surface of HCMV-infected cells and are assumed to confer protection against IgG-mediated immunity. Here we show that Fcgamma recognition by both vFcgammaRs occurs independently of N-linked glycosylation of Fcgamma, in contrast with the properties of host FcgammaRs. To gain further insight into the interaction with Fcgamma, truncation mutants of the vFcgammaR gp68 ectodomain were probed for Fcgamma binding, resulting in localization of the Fcgamma binding site on gp68 to residues 71 to 289, a region including an immunoglobulin-like domain. Gel filtration and biosensor binding experiments revealed that, unlike host FcgammaRs but similar to the herpes simplex virus type 1 (HSV-1) Fc receptor gE-gI, gp68 binds to the C(H)2-C(H)3 interdomain interface of the Fcgamma dimer with a nanomolar affinity and a 2:1 stoichiometry. Unlike gE-gI, which binds Fcgamma at the slightly basic pH of the extracellular milieu but not at the acidic pH of endosomes, the gp68/Fcgamma complex is stable at pH values from 5.6 to pH 8.1. These data indicate that the mechanistic details of Fc binding by HCMV gp68 differ from those of host FcgammaRs and from that of HSV-1 gE-gI, suggesting distinct functional and recognition properties.

More active human L1 retrotransposons produce longer insertions.

The vast majority of L1 insertions are 5' truncated and thus inactive. Yet, the mechanism of 5' truncation is unknown. To examine whether the frequency of L1 retrotransposition is directly correlated with the length of genomic L1 insertions, we used a cell culture assay to measure retrotransposition frequency and a PCR-based assay to measure L1 insertion length. We tested five full-length human L1 elements that retrotranspose at different frequencies: LRE3, L1(RP), L1.3, L1.2A and L1.2B. Our data suggest that L1 insertion length correlates with L1 retrotransposition frequency for insertions >1 kb in length. For two elements, L1(RP) and L1.2A, we found that swapping the reverse transcriptase domains had little effect. Instead, we found that genomic insertion length and retrotransposition frequency are substantially affected by amino acid substitutions at positions 363, 1220 and 1259 in ORF2. We suggest that the region containing residues 1220 and 1259 may be important in the binding of ORF2p to L1 RNA to facilitate reverse transcription.

Hot L1s account for the bulk of retrotransposition in the human population.

Although LINE-1 (long interspersed nucleotide element-1, L1) retrotransposons comprise 17% of the human genome, an exhaustive search of the December 2001 "freeze" of the haploid human genome working draft sequence (95% complete) yielded only 90 L1s with intact ORFs. We demonstrate that 38 of 86 (44%) L1s are polymorphic as to their presence in human populations. We cloned 82 (91%) of the 90 L1s and found that 40 of the 82 (49%) are active in a cultured cell retrotransposition assay. From these data, we predict that there are 80-100 retrotransposition-competent L1s in an average human being. Remarkably, 84% of assayed retrotransposition capability was present in six highly active L1s (hot L1s). By comparison, four of five full-length L1s involved in recent human insertions had retrotransposition activity comparable to the six hot L1s in the human genome working draft sequence. Thus, our data indicate that most L1 retrotransposition in the human population stems from hot L1s, with the remaining elements playing a lesser role in genome plasticity.