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Sexually transmitted HIV-1 strains utilize the chemokine receptor CCR5 for viral entry and inhibitors targeting this coreceptor offer great promise for antiretroviral therapy. They also raise the question, however, whether viral variants exhibiting altered coreceptor interactions and resistance against these antiviral agents might still be pathogenic. In the present study, we analyzed a SIVmac239 envelope (Env) mutant (239DL) containing two mutations in the V3 loop which reduced viral entry via CCR5 by 10- to 20-fold, disrupted utilization of common alternative SIV coreceptors and changed the way Env engaged CCR5. To evaluate its replicative capacity and pathogenic potential in vivo we infected six rhesus macaques with 239DL. We found that 239DL replication was only slightly attenuated early during infection. Thereafter, a D324V change, which restored efficient CCR5 usage and coincided with 239wt-like levels of viral replication, emerged in two animals. In contrast, the viral geno- and phenotype remained stable in the other four rhesus macaques. Although these animals had about 100-fold reduced viral RNA loads relative to 239wt-infected macaques, they showed pronounced CD4 T-cell depletion in the intestinal lamina propria, and one developed opportunistic infections and died with simian AIDS. Thus, changes in the V3 loop that diminished CCR5 usage and altered Env interactions with CCR5 reduced the pathogenic potential of SIVmac in rhesus macaques but did not abolish it entirely.

Certain viruses, bacteria, fungi and parasites target dendritic cells through the interaction with the cellular attachment factor DC-SIGN, making this C-type lectin an attractive target for therapeutic intervention. Studies on DC-SIGN function would be greatly aided by the establishment of a mouse model, however, it is unclear if the murine (m) homologue of human (h) DC-SIGN also binds to pathogens. Here, we investigated the interaction of mDC-SIGN, also termed CIRE, with the Ebolavirus glycoprotein (EBOV-GP), a ligand of hDC-SIGN. We found that mDC-SIGN neither binds EBOV-GP nor enhances infection by reporterviruses pseudotyped with EBOV-GP. Analysis of chimeras between mDC-SIGN and hDC-SIGN provided evidence that determinants in the carbohydrate recognition domain and in the neck domain of mDC-SIGN inhibit a functional interaction with EBOV-GP. Moreover, mDC-SIGN was found be monomeric, suggesting that lack of multimerization, which is believed to be required for efficient pathogen recognition by hDC-SIGN, might be one factor that prevents binding of mDC-SIGN to EBOV-GP. Our results suggest that mDC-SIGN on murine dendritic cells is not an adequate model for pathogen interactions with hDC-SIGN.

Cellular attachment factors like the C-type lectins DC-SIGN and DC-SIGNR (collectively referred to as DC-SIGN/R) can augment viral infection and might promote viral dissemination in and between hosts. The lectin LSECtin is encoded in the same chromosomal locus as DC-SIGN/R and is coexpressed with DC-SIGNR on sinusoidal endothelial cells in liver and lymphnodes. Here, we show that LSECtin enhances infection driven by filovirus glycoproteins (GP) and the S protein of SARS coronavirus, but does not interact with human immunodeficiency virus type-1 and hepatitis C virus envelope proteins. Ligand binding to LSECtin was inhibited by EGTA but not by mannan, suggesting that LSECtin unlike DC-SIGN/R does not recognize high-mannose glycans on viral GPs. Finally, we demonstrate that LSECtin is N-linked glycosylated and that glycosylation is required for cell surface expression. In summary, we identified LSECtin as an attachment factor that in conjunction with DC-SIGNR might concentrate viral pathogens in liver and lymph nodes.

The lectins DC-SIGN and DC-SIGNR augment infection by human immunodeficiency virus (HIV), Ebolavirus (EBOV) and other pathogens. The neck domain of these proteins drives multimerization, which is believed to be required for efficient recognition of multivalent ligands. The neck domain of DC-SIGN consists of seven sequence repeats with rare variations. In contrast, the DC-SIGNR neck domain is polymorphic and, in addition to the wild type (wt) allele with seven repeat units, allelic forms with five and six sequence repeats are frequently found. A potential association of the DC-SIGNR genotype and risk of HIV-1 infection is currently under debate. Therefore, we investigated if DC-SIGNR alleles with five and six repeat units exhibit defects in pathogen capture. Here, we show that wt DC-SIGNR and patient derived alleles with five and six repeats bind viral glycoproteins, augment viral infection and tetramerize with comparable efficiency. Moreover, coexpression of wt DC-SIGNR and alleles with five repeats did not decrease the interaction with pathogens compared to expression of each allele alone, suggesting that potential formation of hetero-oligomers does not appreciably reduce pathogen binding, at least under conditions of high expression. Thus, our results do not provide evidence for diminished pathogen capture by DC-SIGNR alleles with five and six repeat units. Albeit, we cannot exclude that subtle, but in vivo relevant differences remained undetected, our analysis suggests that indirect mechanisms could account for the association of polymorphisms in the DC-SIGNR neck region with reduced risk of HIV-1 infection.