Bernard Krust

Pleiotrophin inhibits HIV infection by binding the cell surface‐expressed nucleolin

The growth factor pleiotrophin (PTN) has been reported to bind heparan sulfate and nucleolin, two components of the cell surface implicated in the attachment of HIV‐1 particles to cells. Here we show that PTN inhibits HIV‐1 infection by its capacity to inhibit HIV‐1 particle attachment to the surface of permissive cells. The β‐sheet domains of PTN appear to be implicated in this inhibitory effect on the HIV infection, in particular the domain containing amino acids 60–110. PTN binding to the cell surface is mediated by high and low affinity binding sites. Other inhibitors of HIV attachment known to bind specifically surface expressed nucleolin, such as the pseudopeptide HB‐19 and the cytokine midkine prevent the binding of PTN to its low affinity‐binding site. Confocal immunofluorescence laser microscopy revealed that the cross‐linking of surface‐bound PTN with a specific antibody results in the clustering of cell surface‐expressed nucleolin and the colocalization of both PTN and nucleolin signals. Following its binding to surface‐nucleolin, PTN is internalized by a temperature sensitive mechanism, a process which is inhibited by HB‐19 and is independent of heparan and chondroitin sulfate proteoglycans. Nevertheless, proteoglycans might play a role in the concentration of PTN on the cell surface for a more efficient interaction with nucleolin. Our results demonstrate for the first time that PTN inhibits HIV infection and suggest that the cell surface‐expressed nucleolin is a low affinity receptor for PTN binding to cells and it is also implicated in PTN entry into cells by an active process.

HIV‐1 neutralizing antibodies elicited by the candidate CBD1 epitope vaccine react with the conserved caveolin‐1 binding motif of viral glycoprotein gp41

To date, candidate HIV‐1 vaccines that have been tested in clinical trials have failed to induce broadly neutralizing activities and/or antibodies that inhibit infection by primary isolates of HIV‐1. We recently identified a conserved caveolin‐1 binding motif, WNNMTWMQW, in the ectodomain of HIV‐1 transmembrane envelope glycoprotein gp41. We designed the synthetic CBD1 peptide SLEQIWNNMTWMQWDK, corresponding to the consensus caveolin‐1 binding domain (CBD) in gp41, and showed that it elicits in rabbits the production of antibodies that inhibit infection of primary CD4+ T lymphocytes by various primary HIV‐1 isolates. Although a conserved and highly homologous caveolin‐1 binding motif is present in the transmembrane envelope glycoprotein of different HIV‐2 isolates, anti‐CBD1 immune sera do not inhibit HIV‐2 infection. Here we show that anti‐CBD1 antibodies are directed against the conserved caveolin‐1 binding motif WNNMTWMQW in the CBD1 epitope. In spite of this, anti‐CBD1 antibodies do not react with the CBD2 peptide SLTPDWNNMTWQEWER, corresponding to the potential consensus caveolin‐1 binding domain in HIV‐2. The presence of a conserved proline residue upstream of the caveolin‐1 binding motif in CBD2 might affect the presentation of this motif, and thus account for the lack of reactivity of the immune sera. Anti‐CBD1 antibodies therefore appear to be directed against a conformational epitope mimicked by the synthetic CBD1 peptide. In accordance with this, anti‐CBD1 immune sera react with the native but not denatured gp41. The reactivity of anti‐CBD1 immune sera with a highly conserved conformational epitope could explain the broad inhibitory activity of such antipeptide antibodies against HIV‐1 isolates of various clades.